METHODS AND COMPOSITION FOR DIAGNOSING AND TREATING <i>PSEUDOXANTHOMA ELASTICUM</i> AND RELATED CONDITIONS

Abstract: Methods and compositions are provided for diagnosing and treating <i>Pseudoxanthoma elasticum</i> (PXE) patinet and PXE carriers. Methods and compositions are based on the discovery that PXE mutations are located in the MRP6 (ABCC6) gene.

Description: METHODS AND COMPOSITION FOR DIAGNOSING AND TREATING PSEUDOXANTHOMA ELASTICUM AND RELATED CONDITIONS GOVERNMENT SUPPORT Work described herein was supported, in part, by Federal Grant No. EY 13019. The U.S. Government has certain rights in the invention. RELATED APPLICATIONS This application claims priority to, and the benefit of U.S.S.N. 60/184,269, filed February 23, 2000, the disclosure of which is incorporated by reference herein. FIELD OF THE INVENTION The present invention relates generally to the field of physiological dysfunctions associated with Pseudoxanthoma elasticum. More particularly, the invention is concerned with the identification of a gene associated with Pseudoxanthoma elasticum, as well as mutations in the gene that cause the disease. The present invention also relates to methods for detecting and diagnosing Pseudoxanthoma elasticum, to methods for identifying carriers of mutant and normal alleles of the gene associated with Pseudoxanthoma elasticum, to methods for screening compounds to identify potential therapeutics for Pseudoxanthoma elasticum, to treatment methods for Pseudoxanthoma elasticum. and to useful cell lines and animal models of the disease. BACKGROUND OF THE INVENTION Pseudoxanthoma elasticum (PXE) is a heritable disorder characterized by mineralization of elastic fibers in skin, arteries and the retina, that result in dermal lesions with associated laxity and loss of elasticity, arterial insufficiency, cardiovascular disease and retinal hemorrhages leading to macular degeneration. The skin manifestations are among the most common characteristics of PXE, but the ocular and cardiovascular symptoms are responsible for the morbidity of the disease. Characteristic skin lesions are generally an early sign of PXE and were first described by a French dermatologist in 1896. Skin lesions are usually detected during childhood or adolescence and progress slowly and often unpredictably. Therefore, the initial diagnosis of PXE is sometimes made by a dermatologist. The skin lesions consist of yellowish papules and plaques and laxity with loss of elasticity, and result from an accumulation of abnormal mineralized elastic fibers in the mid-dermis. Lesions are typically seen on the face, neck, axilla, antecubital fossa, popliteal fossa, groin and periumbilical areas. A PXE diagnosis can be confirmed by a skin biopsy that shows calcification of fragmented elastic fibers in the mid- and lower dermis. Another characteristic of PXE is the presence of ocular lesions due to the accumulation of abnormal elastic fibers in the Bruch' s membrane, resulting in angioid streaks. Doyne was the first to describe these ocular streaks in 1889, and Knapp introduced the term "angioid streaks" for their resemblance to blood vessels. The combination of PXE and ocular manifestations was initially referred to as the Gronblad-Strandberg syndrome, after the names of two ophthalmologists who independently related the occurrence of angioid streaks to PXE in 1929. The majority of PXE patients (approximately 85%) develop ocular manifestations during their second decade of life. Bilateral angioid streaks are normally seen as linear gray or dark red lines with irregular serrated edges lying beneath normal retinal blood vessels and represent breaks in the Bruch's membrane. The Bruch's membrane is not in a true sense a "membrane" but rather a heterogeneous elastin-rich layer separating the chorioid from the retina. The elastic laminae of the Bruch's membrane is located between two layers of collagen (type I, III and IV) which lie in direct contact with the basement membranes of the retinal pigmented epithelium (RPE) and the capillaries in the choriocapillary layer of the chorioid. As a consequence of angioid streaks, a PXE patient progressively develops a chorioidal neovascularization with a subsequent hemorrhagic detachment of the fovea and later scarring. Optic nerve drusen may also be associated with angioid streaks and results in visual field deficits and even advanced visual impairment. Common cardiovascular complications of PXE are due to the presence of abnormal calcified elastic fibers in the internal elastic lamina of medium-sized arteries. The broad spectrum of phenotypes includes premature atherosclerotic changes, intimal fibroplasia causing angina or intermittent claudication or both, early myocardial infarction and hypertension. Fibrous thickening of the endocardium and atrioventricular valves can also result in restrictive cardiomyopathy. Approximately 10% of PXE patients also develop gastrointestinal bleeding and central nervous system complications (such as stroke and dementia) as a consequence of systemic arterial wall mineralization. In addition, renovascular hypertension and atrial septal aneurysm can be seen in PXE patients. Strikingly, lung abnormalities are not a significant phenotypic feature of PXE, even though pulmonary tissues are rich in elastic fibers. Mineralization of pulmonary elastic fibers has only been noted in a few patients. PXE is usually found as a sporadic disorder but examples of both autosomal recessive and autosomal dominant forms of PXE have been reported. Partial manifestations of the PXE phenotype have also been described in presumed carriers in PXE families. Recent reports have linked both the dominant and recessive forms of PXE to a 5 cM domain on chromosome 16P13.1 However, the mechanisms underlying the physiological defects characteristic of PXE are not understood. Therefore, there is a need in the art for methods and compositions for diagnosing and treating PXE and PXE associated phenotypes. SUMMARY OF THE INVENTION The invention provides methods and compositions for diagnosing and treating PXE and PXE associated physiological dysfunctions. According to the invention, mutations associated with PXE are located in the (MRP6) ABCC6 gene. Therefore, methods for detecting the presence of a mutation associated with PXE involve interrogating the (MRP6) ABCC6 gene, or a portion thereof, for the presence of one or more mutations that are associated with PXE. Accordingly, one aspect of the invention provides methods for identifying individuals that have one or two mutant alleles at the PXE locus. PXE is most often an autosomal recessive disease. Therefore, an individual with two mutant (MRP6) ABCC6 alleles associated with PXE will develop symptoms characteristic of the disease. In contrast, an individual with one mutant (MRP6) ABCC6 allele associated with PXE is a carrier of the disease and does not develop fullblown PXE. However, according to one embodiment of the invention, a PXE carrier may develop mild forms of the characteristic manifestations. Accordingly, a PXE carrier status can be indicative of a predisposition to PXE related symptoms such as eye, skin, or cardiovascular problems. In a preferred embodiment of the invention, genetic counseling is provided to an individual identified as having a mutation associated with PXE in one or both alleles of the PXE ((MRP6) ABCC6) locus. In another aspect, the invention provides compositions for detecting the presence of a mutation associated with PXE at the (MRP6) ABCC6 locus. In a preferred embodiment, an oligonucleotide that hybridizes to the (MRP6) ABCC6 locus is used in a diagnostic assay. In a more preferred embodiment, the oligonucleotide includes a sequence complementary to a mutation that is associated with PXE. Alternatively, an antibody-based diagnostic assay is used to detect the presence of a mutation associated with PXE at the (MRP6) ABCC6 locus. Other aspects of the invention include therapeutic uses of the (MRP6) ABCC6 gene or protein, drug screening, the identification of (MRP6) ABCC6 homologues in other organisms (including mammalian organisms), cellular and animal models of PXE, the identification of (MRP6) ABCC6 functional domains related to the PXE phenotype, the identification of regulators of (MRP6) ABCC6 expression (mutations in these regulators can also result in PXE related symptoms), the identification of genes/proteins that interact with (MRP6) ABCC6 (alterations in these interacting molecules can also cause PXE related symptoms). Thus, in one series of embodiments the invention provides methods for screening for the presence of a PXE mutation by interrogating an MRP6 nucleic acid obtained from a patient for the presence of a PXE mutation. The screen is positive is the presence of a PXE associated mutation is detected. A PXE associated mutation is a mutation that causes the PXE phenotype in an individual that is homozygous for the mutation. PXE associated mutations also causes the PXE phenotype in an individual that is a compound heterozygote with two different mutant alleles at the MRP6 locus, wherein each allele is a PXE associated allele. Nucleic acid is isolated from a patient biological sample, and the biological sample is preferably blood, saliva, amniotic fluid, or tissue such as a biopsy tissue. According to the invention, an MRP6 nucleic acid is a nucleic acid obtained from the MRP6 locus. An MRP6 nucleic acid can be mRNA, genomic DNA or cDNA from the MRP6 locus, or a PCR product of any of the above. According to the invention, the MRP6 locus includes the MRP6 exons, introns, and associated promoter and regulatory sequences in the genome surrounding the MRP6 exons. In one series of embodiments, a PXE associated mutation is detected in MRP6 using a nucleic acid based detection assay. Preferred nucleic acid based detection assays include hybridization assays, primer extension assays, SSCP, DGGE, RFLP, LCR, DHPLC, and enzymatic cleavage assays. In another series of embodiments, a PXE associated mutation is detected in a protein based detection assay. Preferred protein based detection assays include ELISA and a Western blot assays. In one embodiment of the invention, mutation detection assays are provided to screen the MRP6 locus or a portion thereof to determine whether a mutation is present. The lack of MRP6 expression or the expression of a physically aberrant form of MRP6 may be sufficient to determine that an individual has a PXE associated mutation at the MRP6 locus. Alternatively, the determination that a mutation is present in the MRP6 locus may not be sufficient to determine the PXE status of an individual in the absence of information concerning the specific identity of the mutation. If such a mutation is present, it may be identified according to methods of the invention, for example by sequencing the region of the MRP6 locus that contains the mutation. Once a mutation is identified in a patient sample, the PXE status of the patient can be determined according to methods of the invention. In an alternative embodiment of the invention, specific mutation detection assays are provided to detect a known PXE associated MRP6 mutation in a patient sample. In another series of embodiments, the invention provides oligonucleotide probes or primers and antibodies for use in mutation detection assays or screens according to the invention. In another series of embodiments, the invention provides methods for screening candidate drug compounds to identify therapeutic compounds for treating PXE patients (individuals that have PXE due to the presence of two recessive PXE associated MRP6 alleles, or one apparently dominant PXE allele) or PXE carriers (individuals with one normal MRP6 allele and one allele with a PXE associated mutation). In another series of embodiments, the invention provides methods for treating PXE patients or carriers using a normal MRP6 nucleic acid or protein to restore normal MRP6 function to the individual or to specific cells or tissues or the individual. In another series of embodiments, the invention provides methods for creating transgenic or knockout cell lines and animals in order to provide a model system for PXE. In another series of embodiments, the invention provides methods for identifying compounds such as other intracellular proteins that interact with MRP6 thereby to identify additional therapeutic targets for PXE treatment. Accordingly, the invention provides methods and compositions for unambiguously determining the PXE status of an individual. The invention provides methods for detecting deletions, substitutions, insertions, and rearrangements in the MRP6 locus that are associated with PXE. In preferred embodiments, the invention provides methods for identifying mutations known to be associated with PXE. Preferred mutations include mutations that affect one or more of the bases in codons 1114, 1 138, 1141, 1298, 1302, 1303, 1314, 1321 and other codons identified herein as being important for normal MRP6 function. Alternatively, the invention provides methods to identify mutations that result in non-conservative substitutions in the MRP6 locus. In a further embodiment, the invention provides assays to detect PXE associated mutations at intron/exon splice sites of the MRP6 gene. The invention also provides methods to detect mutations that affect one or more regulatory elements of the MRP6 gene, including the promoter, the polyA site and other transcriptional or translational control sequences. Methods of the invention are also useful to screen a population in order to identify individuals with one or more PXE associated MRP6 alleles. According to the invention, these individuals are provided with appropriate genetic counseling in view of their PXE status. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the structure of the MRP6 gene and the surrounding genomic regions. Two (MRP6) ABCC6 mutations that cause PXE are indicated. Figure 2 shows the predicted topology of the MRP6 protein and the location often mutations causing PXE. Figure 3 shows conserved amino acids in the human MRP6 protein. Figure 4 shows co-segregation of the PXE phenotype with the Rl 141X mutation in exon 24 of the (MRP6) ABCC6 gene. Figure 5 shows segregation of the PPXE phenotype for an apparent autosomal dominant mutation. Figure 6 shows a construct for deleting exon 28 in a mouse. DETAILED DESCRIPTION OF THE INVENTION The invention provides methods and compositions for diagnosing and treating PXE and PXE related symptoms. Methods and compositions of the invention rely in part on the discovery that mutations associated with PXE map to the (MRP6) ABCC6 gene locus on chromosome 16. Accordingly, the invention provides useful PXE related diagnostic and therapeutic methods and compositions by exploiting wild-type and mutant (MRP6) ABCC6 genes and proteins. I. PXE Associated Mutations in the (MRP6) ABCC6 gene a) Mapping of PXE associated mutations to the ( RP6) ABCC6 genetic locus Although the first case of PXE was reported by Darier in 1896, most PXE cases have been reported since the 1970s. In most reports, PXE is inherited as an autosomal recessive (AR) phenotype or appears as a sporadic phenotype. However, kindreds showing apparent autosomal dominant (AD) inheritance have also been reported. Using DNA from patients and unaffected family members from 21 unrelated PXE families, the PXE phenotype was linked to the short arm of chromosome 16. A very significant linkage with an 8 cM region was demonstrated with a maximum lod score of 8.07. A subsequent haplotype analysis and recombination mapping reduced the locus from 8 cM to 820 kb where six candidate genes were identified. The locus was later reduced to less than 600 kb and one candidate gene was excluded. All 109 exons of the five remaining candidate genes were screened by a combination of single-strand conformation polymorphism (SSCP), heteroduplex analysis (HA) or direct sequencing using genomic DNA from a cohort of 17 unrelated PXE patients and three unrelated normal individuals. The first six mutations, clearly associated with the PXE phenotype, were found in the (MRP6) ABCC6 gene (also known as the ABCC-6 gene). This analysis is described in further detail in Example 2. According to the invention, the MRP6 gene has 31 exons as shown in Figure 1. A 107.7 kb genomic sequence that includes the MRP6 locus is shown in SEQ ID NO: 1. The sequence of SEQ ID NO: 1 shows the complementary strand of the MRP6 gene. The intron/exon boundaries are as follows (on the complementary strand of SEQ ID NO: 1): Exl: 102783-102748; Ex2: 101180-100998; Ex3: 99296-99171 ; Ex4: 99031-98903; Ex5: 93798-93676; Ex6: 91594-91533; Ex7: 88207-88076; Ex8: 82954-82757; Ex9: 81524-81347; ExlO: 77528-77367; Exl l : 72268-72176; Exl2 69718-69515; Exl3: 68325-68182; Exl4: 66562-66475; Exl5: 64385-64310; Exl6 62282-62156; Exl 7: 61940-61764; Ex 18 58324-58157; Exl9: 56985-56811 ; Ex20 55345-55270; Ex21 : 52757-52637; Ex22 49588-49381 ; Ex23: 45578-45268; Ex24 42837-42638; Ex25: 41209-41083; Ex26 39226-39125; Ex27: 37453-37307; Ex28 34674-34516; Ex29: 34437-34271 ; Ex30 30412-30218; Ex31 : 29881-29773. The mRNA coding sequence for human MRP6 is shown in SEQ ID NO: 2, and the encoded protein sequence is shown in SEQ ID NO: 3. b) Identifying PXE associated mutations in the (MRP6) ABCC6 locus According to methods of the invention, additional PXE associated mutations were identified in the (MRP6) ABCC6 locus using a combination of single strand conformation polymorphism (SSCP), heteroduplex analysis (HA) and direct sequencing. Single nucleotide mutations in the (MRP6) ABCC6 gene were identified in several cohorts of individuals originating from the United States, South Africa and several European countries (Belgium, Germany, Holland, Italy and United Kingdom). To confirm the causative or polymorphic nature of new variants, a control panel of 300 alleles (150 normal individuals) was screened and the co-segregation of the identified variant and the PXE phenotype was verified. It is noteworthy that two single-allele mutations (R1141X, R1339C) were found in control panels of normal individuals indicating that heterozygote mutant (MRP6) ABCC6 alleles can be found in the normal population. However, the missense mutation (R1339C) was identified in the genetically distinct Afrikaners of South Africa. The frequency of heterozygote carriers deduced only from the appearance of these heterozygote mutations is 1.3 percent and is consistent with the commonly accepted figures of 0.6 to 2.5%). Indeed, while most mutations appeared to be private, a few have been clearly identified as recurrent (R1141X, R518Q, 3775delT, 16.5 kb deletion between exon 22 and 29. Most of the mutations (63%) were missense substitutions, 17%) were nonsense mutations (5), 13% were frameshift mutations (4 deletions or an insertions of a single nucleotide) and 7% were likely to affect splicing (2). Twenty-seven of the mutations (90%) affected the C-terminal half of the (MRP6) ABCC6 protein and particularly the various domains of the C-terminal ATP-binding site, which are encoded by exons 28 to 30, where 12 (40%) mutations were clustered. Remarkably, 10 mutations (33%) affected arginyl residues. Eight of these were missense substitutions, suggesting an essential structural or functional role for these arginyl residues in (MRP6) ABCC6. Large deletions, which are not detected by SSCP or HA, can be identified by the loss of heterozygosity of informative polymorphic markers. Seven highly informative microsatellites present in a 300 kb region encompassing both ABCCI and ABCC6, have been successfully used to detect large deletions involving parts or the entire ABCC6 gene. The loss of heterozygosity can also be efficiently implemented by using several highly polymoφhic variants present in the ABCC6 gene. The latter approach was used to detect a partial deletion of the (MRP6) ABCC6 gene, in a compound heterozygous state, in a family with an apparent dominant form of PXE, as discussed in Example 3. A non- limiting list of known PXE associated mutations at the MRP6 locus are shown in Table 1. ch= compound heterozygote; ht= heterozygote; hm= homozygote; ivs= intervening sequence Table 1. Known PXE associated mutations at the human MRP6 locus. According to methods of the invention, additional PXE associated mutations can be identified in the (MRP6) ABCC6 locus according to methods of the invention. For example, single strand conformation polymoφhism (SSCP), heteroduplex analysis (HA), or direct sequence analysis can be used to identify additional mutations at the MRP6 locus. In one embodiment, the analysis is performed on genomic DNA. Alternatively, the analysis is performed on cDNA or on exon containing DNA amplification products such as exon containing PCR products. Deletion mutations are preferably detected using diagnostic PCR assays of genomic DNA and by Southern hybridization according to methods known in the art. In addition, fluorescent in situ hybridization (FISH) analysis of human chromosome preparations can be used to identify a deletion at the MRP6 locus or a deletion that encompasses all or part of the MRP6 locus. Specific mutations are preferably identified using DNA arrays including mutation specific oligonucleotide probes. Alternatively, mutation-specific antibodies can be used to detect mutations that alter an existing epitope or create a new specific epitope on the MRP6 protein. Preferably, specific antibodies are used on proteomic chips to detect protein altering mutations in the MRP6 gene. Mutations can also be detected using mass spectrometry, and mutation-specific mass spectrometer profiles can be generated for MRP6 nucleic acid or protein analysis according to methods known in the art. c) PXE Associated Mutations at the (MRP6) ABCC6 Locus i) The (MRP6) ABCC6 gene and protein The (MRP6) ABCC6 gene, also known as the ABBC6 gene, encodes an ATP- binding cassette transporter (an ABC transporter) belonging to sub-family "C" which includes genes involved in drug-resistance such as MRP1 to 6 (ABCC1-6). (MRP6) ABCC6/ABCC6 encodes a 165 kDa protein that is located in the plasma membrane and has 17 membrane-spanning helices grouped into three transmembrane domains. MRP6 is highly homologous to MRP1 and may act as an efflux pump of amphipathic anion conjugates. Accordingly, in one aspect of the invention, MRP6 transports glutathione anion conjugates and also anionic drugs. Therefore, an individual that is a PXE carrier or a PXE homozygote or compound heterozygote may have reduced transport of anionic drugs and may be more receptive to chemotherapy using such drugs. The ABCC family of genes also includes the cystic fibrosis transmembrane conductance regulator gene (ABCC7 or CFTR) and the sulfonylurea receptor genes (ABCC8 and 9 or SUR). Therefore, in contrast to genetic changes involved in other elastic fiber diseases such as Supravalvular Aortic Stenosis (SVAS), Marfan syndrome, and Cutis laxa, PXE associated mutations in the (MRP6) ABCC6 gene are not directly related to elastic fibers. The (MRP6) ABCC6/ABCC6 gene is expressed at relatively high levels in a limited range of tissues, notably in kidney and liver. However, low levels of expression are also observed in smooth muscle cells and macrophages. According to the invention, this tissue distribution suggests that (MRP6) ABCC6 has a function related to cellular detoxification which may affect the calcification of elastic fibers in skin, arteries and the retina. Alternatively, calcification of elastic fibers in skin, arteries, and the retina may result from MRP6 functional deficiencies in those tissues. The predicted structure of the MRP6 protein is shown in Fig. 2. Transmembrane domains (unshaded rectangles), nucleotide-binding fold regions (NBF) and Walker motifs are indicated and were identified by amino acid sequence homology with similar transporters. Arrows indicate the positions of several PXE associated mutations. The large shaded rectangle represents the cell membrane. According to the invention, the transmembrane domains of the MRP6 protein shown in Fig. 2 are hydrophobic stretches of amino acids identified via transmembrane domain predictions (SOSUI and DAS transmembrane prediction programs, http://www.biokemi.su.se/-server/DAS/: http://azusa.proteome.bio.tuat.ac.ip/sosui/). Regions of MRP6/ABCC6 with a high degree of conservation when compared with similar proteins (ABC transporters) include the regions involved in the binding and hydrolysis of ATP also known as nucleotide binding folds (NBF). According to the invention, the MRP6 protein has two nucleotide-binding fold regions (NBF1 and NBF2) as shown in Fig. 2. These regions correspond to the following amino acid segments of the human MRP6 protein: NBF1 residues 656-679, 747-768, and 775-784 of SEQ ID NO: 3; and NBF2 residues 1292-1307, 1321-1327, and 1403-1433 of SEQ ID NO: 3. According to one embodiment of the invention, conserved amino acids in the MRP6 protein are amino acids identified by comparing 12 ABC transporter proteins from Human, Rat, Mouse, C. elegans, Yeast (S. cerevisiae) and A. thaliana. Preferred conserved amino acids are shown in Fig. 3 (conserved amino acids are underlined). According to the invention, conserved domains are concentrated in the C-terminal portion of the protein, where over 90%> of the PXE causing mutations have been identified. ii) Mutations in the (MRP6) ABCC6 gene According to one aspect of the invention, PXE is caused by a mutation at the (MRP6) ABCC6 locus that results in reduced MRP6 protein function. PXE associated mutations include mutations that affect the level of MRP6 protein expression in addition to mutations that alter the functional properties of an expressed MRP6 protein. PXE associated mutations at the (MRP6) ABCC6 locus include chain-terminating mutations. Such mutations are typically recessive and account for the autosomal recessive nature of the associated PXE phenotype. However, PXE associated mutations identified at the (MRP6) ABCC6 locus include chain terminating mutations at different positions in the (MRP6) ABCC6 gene, and several substitution, deletion and insertion mutations. According to the invention, the C-terminal half of the MRP6 protein is functionally important. Indeed, many of the PXE associated mutations were identified in exons 23- 29. However, even a I to T substitution at position 1424 (out of 1503 amino acid residues) results in a PXE associated phenotype. Accordingly, a chain terminating or frameshift mutation in any one of exons 1-29, even up to position 1424 in exon 30, and maybe even beyond is expected to be associated with PXE. According to the invention, the PXE phenotype associated with different mutations in the (MRP6) ABCC6 gene varies in relation to the functional properties of the mutant (MRP6) ABCC6 protein product. Therefore, individuals with different PXE associated mutations can have PXE symptoms of differing severity. In addition, different individuals having the same PXE mutations, but in different genetic backgrounds, can also develop PXE symptoms of differing severity. Accordingly, different mutations at the PXE locus are expected to result in PXE phenotypes of differing severity. For example, in one embodiment of the invention, a mutation that results in the absence of MRP6 protein expression (for example a deletion of part or all of the gene, a chain terminating mutation, a mutation that prevents mRNA production, or a mutation that prevents translation of the mRNA) is expected to have a more severe PXE phenotype than a mutation that interferes with normal MRP6 protein function without destroying the function (for example an amino acid substitution that alters the structure and function of the protein without inactivating it. In particular, an individual that is a homozygote for a mutation that prevents MRP6 protein expression, or that is a compound heterozygote with two different mutations each of which prevents MRP6 protein expression, is expected to have a more severe phenotype than an individual that has a mutation with less severe effects on MRP6 protein function at one or both alleles of the MRP6 locus. In a further embodiment of the invention, a heterozygote carrier of a PXE mutation can exhibit characteristic manifestations of PXE. In particular, a carrier of a recessive mutation can show partial skin, eye or cardiovascular symptoms. According to the invention, heterozygote carriers of different (MRP6) ABCC6 mutations can develop different subsets of PXE related symptoms and can have symptoms of varying severity. Indeed, there are numerous examples of dermal "elastic fibers changes" or cardiovascular abnormalities ranging from hypertension to myocardial infarction, in family members of severely affected individuals. According to the invention, cases of partial expression of PXE symptoms in heterozygote carriers are cases that had been assumed to be examples of dominant inheritance with for example 10 to 20% penetrance. The various subtypes of a disorder or a dual mode of inheritance of a disease are frequently due either to mutations in different genes or different mutations in the same gene. Epidermolysis buflosa (EB) is an excellent example of a disorder characterized by several clinical types caused by distinct mutations in the same gene or mutations in different genes. EB is viewed as a group of heritable mechano-bullous skin diseases classified into three major categories of simplex, junctional and dystrophic forms. EB simplex is due to mutations in the genes encoding keratins 5 and 14, the junctional form is associated with mutations in the kalininU aminin 5 genes; and the dystrophic disorder result from mutations in the type VII collagen gene (COL7AI). The dystrophic EB presents clinical sub-types: the Hallopeau-Siemens type is autosomal recessive and caused by nonsense mutations and glycine substitutions result in the autosomal dominant form. In contrast to EB, no locus heterogeneity has been shown for PXE. According to the invention, most cases of PXE, if not all, are due to (MRP6) ABCC6 mutations. While the clinical heterogeneity in PXE patients may be caused by different types of (MRP6) ABCC6 mutations, the different PXE lesions (vascular, ocular, and dermal) observed for different autosomal recessive and seemingly dominant PXE mutations are clinically indistinguishable. Furthermore, identical PXE mutations can be either recessive or apparently dominant in unrelated pedigrees. Accordingly, different PXE mutations in different genetic backgrounds are associated with different severities of PXE symptoms. Furthermore, a PXE mutation can result in a partial PXE phenotype in a carrier individual (thereby accounting for observations of apparent dominant forms of PXE). iii) Population Distributions of (MRP6) ABCC6 mutations According to the invention, different PXE associated (MRP6) ABCC6 mutations exist in the population, and new (MRP6) ABCC6 mutations arise sporadically. Based on current estimations of the prevalence of PXE in the United States (between 1 : 100,000 and 1 :25,000), the frequency of appearance of heterozygote individuals with PXE mutations should be between 0.6 and 2.5 percent of the general population (1.5 to 6.0 million individuals). Given the risk of heterozygote individuals having children with PXE, an important aspect of the invention is to provide a genetic screen to identify heterozygote carriers of PXE mutations. According to the invention, a PXE carrier is an individual with one mutant allele of the (MRP6) ABCC6 gene, wherein the mutant allele is an allele that results in a PXE phenotype in an individual that is homozygous for that allele (or in an individual that is heterozygous with two different (MRP6) ABCC6 mutant alleles, each of which is associated with PXE). According to a further embodiment of the invention, a significant factor in the complex phenotype of the PXE multi-organ disorder is partial expression of the full range of the PXE symptoms in heterozygote carriers in recessive pedigrees. For example, a single mutant-ABCC6 allele, for example Rl 141X, within heterozygote carriers can manifest a partial, mostly vascular-related phenotype. Indeed, cardiovascular abnormalities are frequently seen in obligate carriers but ocular and dermal lesions have also been diagnosed. The PXE phenotype, as observed in several heterozygous carriers, range from sub-clinical manifestations to visible lesions. The spectrum of these partial phenotypes overlaps with that of the less severely affected PXE patients. There is, therefore, a continuum in the PXE phenotype between heterozygous carriers and PXE patients, which make the clinical diagnosis of the less severe forms of PXE equivocal. According to the invention, cardiovascular symptoms associated with PXE mutations at the MRP6 gene include atherosclerosis, hypertension, stroke, gastrointestinal bleeding, intermittent claudication. Ocular symptoms include macular or retinal degeneration and skin related symptoms include premature aging and solar elastosis. According to the invention, the identification of the PXE gene provides methods for an unambiguous molecular diagnosis of patients and the identification of heterozygous carriers in families with autosomal recessive PXE or apparent autosomal dominant PXE, and the identification of homozygous PXE individuals or PXE carriers in the general population. According to the invention, different populations can contain different characteristic PXE associated MRP6 mutations or different frequencies of PXE associated MRP6 mutations due to factors such as founder effects. For example, a founder effect in the South African Afrikaner population is thought to have caused the observed higher frequency of PXE in Afrikaners. According to the invention, a higher frequency of PXE in a population correlates with a higher frequency of PXE associated MRP6 mutations. Intra- familial variation of the phenotype is a well known characteristic of PXE. These variations may be due to genetic and/or environmental causes. A few environmental factors are thought to influence the PXE phenotype. Among these, calcium and Vitamin D have been reported to contribute to the severity of the phenotype in some cases. Life style, smoking, diet, sun-exposure and obesity are also likely to modulate the penetrance of the phenotype. Indeed, remarkably dissimilar PXE phenotypes have been observed recently in identical twins. According to the invention, non-genetic factors contributing to the development of PXE symptoms in heterozygote carriers can be identified. Studies involving large cohorts of twins for example, such as those used by the Queensland Institute of Medical Research of Australia (http://genel2i.qimr.ed are also useful to identify both genetic and environmental factors related to the development of the PXE phenotype. II. Diagnostic Applications (MRP6) ABCC6 genes and gene products, including mutant genes and gene products, as well as probes, primers, and antibodies, are useful for identifying carriers of PXE associated mutations. According to the invention, PXE associated mutations can be identified in families with a PXE pedigree or in individuals not previously known to be at risk of carrying a PXE related mutation. PXE associated mutations can be routinely screened using probes to detect the presence of a mutant (MRP6) ABCC6 gene or protein by a variety of methods. In preferred embodiments of the invention, individuals are screened for the presence of a recurrent mutation that is known to be present at a relatively high frequency in the population. For example, a preferred method of the invention screens an individual from a population for the presence of an MRP6 mutation that accounts for about 30%, and preferably 50%, and more preferably over 50%, of known incidences of PXE in the population. An alternative method of the invention screens an individual for the presence of two or more, preferably about five, more preferably about ten, and even more preferably over ten PXE associated MRP6 mutations. In methods that include assays for a plurality of PXE associated MRP6 mutations, the plurality of mutations preferably account for about 30%, and more preferably 50%o, and even more preferably over 50%, of known incidences of PXE in the population. In one aspect of the invention, the identification of a specific mutation is not necessary. A diagnostic assay may be based on the detection of an MRP6 protein expression defect resulting from, for example, reduced levels of mRNA expression. Indeed, the analysis of steady state levels of (MRP6) ABCC6 mRNA in skin fibroblasts from a PXE patient carrying a homozygous Rl 141X mutation showed that MRP6 mRNA levels were lower than in skin fibroblasts from a normal individual. Accordingly, low levels MRP6 mNRA can result from a mutation within the coding sequence, such as a nonsense mutation that results in nonsense mediated decay. In addition, low mRNA levels can be caused by mutations either an intron or an exon that destabilizes the RNA, or by a mutation in a regulatory region (including a promoter region) that reduces transcription of the MRP6 gene. Furthermore, the presence of a truncated MRPό mRNA can be used as a diagnostic indicator for the presence of a PXE associated mutation. Alternatively, the presence of a mutation that affects the amount, size, or other physical properties of the MRP6 protein can be detected without knowing the identity of the mutation. For example a decreased level of MRP6 protein or a the presence of a truncation in the MRP6 protein can be used as a diagnostic indicator for the presence of a PXE associated mutation. In addition, the presence of a larger than expected MRP6 protein (that may result for example, from a gene fusion or from one or more frameshift mutations that produce a larger and possibly non-functional protein) can be used as a diagnostic inidicator for the presence of a PXE associated mutation. Accordingly the invention provides a method for screening for the presence of a PXE associated mutation at the MRP6 locus without specifically identifying the mutation. Such methods are useful to identify homozyogtes, compound heterozygotes, or carriers. According to the invention, the identification of the presence of any PXE associated mutation at the MRP6 locus can be used as a positive diagnosis of PXE in an individual with PXE symptoms or to diagnose a PXE patient who has not yet developed PXE symptoms but who is identified as a homozygote or a compound heterozygote for PXE associated MRP6 mutations. Alternatively, the detection of the presence of a PXE associated MRP6 mutation according to the invention provides a method for screening a population to identify individuals who are carriers of a PXE associated mutation. In general, a PXE carrier is distinguished from a PXE homozygote by the presence of both a normal allele and a PXE mutant allele in the carrier and the presence of two PXE mutant alleles in the homozygote. According to the invention, a normal allele can contain a neutral polymoφhism as disclosed herein. a) Nucleic acid based diagnostics When a diagnostic assay is to be based upon nucleic acids from a sample, the assay may be based upon mRNA, cDNA or genomic DNA. If mRNA is used from a sample, there may be little or no expression of transcripts unless appropriate tissue sources are chosen or available. Preferred tissue sources are biopsies of full thickness skin or skin fibroblasts cultured from dermal biopsies. Whether mRNA, cDNA or genomic DNA is assayed, standard methods well known in the art may be used to detect the presence of a particular sequence either in situ or in vitro (see, e.g., Sambrook et al., (1989) Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Press, Cold Spring Harbor, NY). As a general matter, however, any tissue with nucleated cells may be examined. Genomic DNA used for the diagnosis may be obtained from body cells, such as those present in the blood, tissue biopsy, surgical specimen, or autopsy material. The DNA may be isolated and used directly for detection of a specific sequence or may be amplified by the polymerase chain reaction (PCR) prior to analysis. Similarly, RNA or cDNA may also be used, with or without PCR amplification. To detect a specific nucleic acid sequence, direct nucleotide sequencing, hybridization using specific oligonucleotides, restriction enzyme digest and mapping, PCR mapping, RNase protection, chemical mismatch cleavage, ligase-mediated detection, and various other methods may be employed. Oligonucleotides specific to particular sequences can be chemically synthesized and labeled radioactively or non-radioactively (e.g., biotin tags, ethidium bromide), and hybridized to individual samples immobilized on membranes or other solid-supports (e.g., by dot-blot or transfer from gels after electrophoresis), or in solution. The presence or absence of the target sequences may then be visualized using methods such as autoradiography, fluorometry, or colorimetry. These procedures can be automated using redundant, short oligonucleotides of known sequence fixed in high density to silicon chips, or in other oligonucleotide array formats. Whether for hybridization, RNase protection, ligase-mediated detection, PCR amplification or any other standards methods described herein and well known in the art, a variety of subsequences of the MRP6 sequences disclosed or otherwise enabled herein will be useful as probes and/or primers. These sequences or subsequences will include both normal MRP6 sequences and PXE associated MRP6 mutant sequences. In general, useful oligonucleotide probes or primer sequences will include at least 8-9, more preferably 10-50, and most preferably 18-24 consecutive nucleotides from the MRP6 introns, exons or intron/exon boundaries. Depending upon the target sequence, the specificity required, and future technological developments, shorter sequences may also have utility. Therefore, any MRP6 derived sequence which is employed in a diagnostic assay may be regarded as an appropriate probe or primer. Particularly useful sequences include nucleotide positions from the MRP6 gene for which PXE associated mutations are known, or sequences which flank these positions. As discussed above, a variety of PXE causing mutations have now been identified at the human MRP6 gene locus. Detection of these and other PXE associated mutations is now enabled using isolated nucleic acid probes or primers derived from normal or mutant MRP6 genes. According to the invention, useful oligonucleotide probes or primers are derived from sequences encoding the C-terminal half of the MRP6 protein, the conserved NBF sequences, and conserved amino acid sequence shown in Fig. 3. Particularly useful oligonucleotides are derived from sequences known to have PXE associated mutations, such as the sequences including the mutations shown in Table 1. As disclosed above, a number of PXE associated MRP6 mutations have already identified, and it is expected that more will be identified according to the compositions and methods disclosed herein. Therefore, the present invention provides isolated nucleic acid probes and primers corresponding to normal and mutant sequences from any portion of the MRP6 gene, including exons. introns, and 5' and 3' UTRs, which may be shown to be associated with the development of PXE. Merely as an example, and without limiting the invention, useful diagnostic probes and primers derived from the MRP6 DNA are disclosed in Example 5. For in situ hybridization-based detection of a normal or mutant MRP6, a sample of tissue may be prepared by standard techniques and then contacted with one or more of the nucleic acids described herein, preferably one which is labeled to facilitate detection, and an assay for nucleic acid hybridization is conducted under stringent conditions which permit hybridization only between the probe and highly or perfectly complementary sequences. For the single nucleotide substitutions associated with PXE, high stringency hybridization conditions will be required to distinguish most mutant sequences from normal sequences. When the MRP6 genotypes of an individual's parents are known, probes may be chosen accordingly. Alternatively, probes to a variety of mutants may be employed sequentially or in combination. Because PXE carriers will be heterozygous, probes to normal sequences also may be employed and homozygous normal individuals may be distinguished from mutant heterozygotes by the amount of binding (e.g., by intensity of radioactive signal). In another variation, competitive binding assays may be employed in which both normal and mutant probes are used but only one is labeled. In addition to oligonucleotide-based hybridization assays, methods of the invention include direct sequencing, loss of heterozygosity, SSCP, HA, and Conformation-Sensitive Gel Electrophoresis (CSGE) to detect a PXE associated MRP6 mutation. As discussed above, preferred mutations to be screened for are those shown in Table 1. However, additional mutations identified according to the invention are also useful as markers of PXE, including deletions in the (MRP6) ABCC6 locus. According to one embodiment of the invention, a diagnostic test can be a nucleic scanning test where the assay detects the presence of a mutation in the nucleic acid being interogated. In an alternative embodiment, a diagnostic test can interrogate a nucleic acid for the presence of a specific mutation. According to this invention, base pair deletions or alterations leading to the omission of amino acid residues in the gene product are determined. Nucleic acid primers and probes are used in a variety of PCR-based amplification and hybridization assays to screen for and detect the presence of defective ABCC6 gene or mRNA in a patient. The genetic information derived from the intron/exon boundaries is also very useful in various screening and diagnosis procedures. Various nucleic acid scanning methods are used for scanning the MRP6 genomic. mRNA or cDNA sequence obtained from a patient for detecting, for example, large deletions and substitutions in the sequence that would be indicative of the disease. These nucleic acid scanning techniques include PCR-based techniques and using oligonucleotide probes that hybridize to specific regions of the gene. In one embodiment of the invention, preferred mutations for nucleic acid scanning techniques include large deletions in the genomic sequence for the ABCC6 gene for example a 16.5 kb deletion spanning from exon 22 to exon 29. Primers are designed to various regions of the ABCC6 gene which are used for PCR-based detection of large deletions in the gene. In another embodiment of the invention, primers are designed to the ABCC6 gene that are able to differentiate between the size of the amplified wild-type sequence and sequence containing a specific mutation, for example a deletion. In a preferred embodiment of the invention, nucleic acid probes are provided which comprise either ribonucleic or deoxyribonucleic acids. Typically, the size of the probes varies from approximately 18 to 22 nucleotides. Functionally, the probe is long enough to bind specifically to the homologous region of the ABCC6 gene, but short enough such that a difference of one nucleotide between the probe and the DNA being tested disrupts hybridization. Thus the nucleic acid probes of the present invention are capable of detecting single nucleotide changes in the ABCC6 gene. In a preferred embodiment of the invention, nucleic acid probes are 100 % homologous to a mutant allele of the ABCC6 gene, but not to the wild-type gene. In another embodiment of the invention, the nucleic acid probes are 100 % homologous to the wild-type allele. Accordingly, the invention provides methods for determining whether an individual is homozygous or heterozygous for a particular allele using both a wild-type and an allele-specific probe. According to one method of the invention, mutations are detected by sequencing specific regions of the ABCC6 gene. In a preferred embodiment, the specific regions encompass one or more mutations presented in Table 1. In an alternative embodiment, a specific region being interrogated includes one of exons 1 -31. Preferred exons include exons 25-29, and more preferably exon 28 in which many PXE associated mutations have been identified. According to still other methods of the present invention rapid screening techniques are used to determine whether exons of the ABCC6 gene carry any mutations. Such techniques can be followed by one of the techniques already described above which are specific for a particular allele or mutation. One such rapid screening technique involves the determination of the conformation of single strands of DNA which have been amplified from exon sequences that are known to carry mutations, including the mutations presented in Table 1. The single strands are run in non-denaturing electrophoretic gels, such as are typically used for sequencing DNA. The mobility of single stranded DNA on such gels is sensitive to the conformation of the DNA fragments. The conformation of the single stranded DNA is dependent on its base sequence, alterations in even one base affecting the conformation. Thus the presence of a wild-type or mutant allele described herein can be detected by amplifying an exon sequence, denaturing the duplex molecules, and separating them on the basis of their conformation on non-denaturing polyacrylamide gels. If mutant alleles are present, they will have a different mobility than wild-type sequences amplified with the same primers. Most conveniently, the amplified sequences will be radiolabeled to facilitate visualization on gels. This can be readily accomplished using labeled primers or a labeled nucleotide. For a general reference on this technique see Orira, et al., Genomics vol. 5, pp. 874-879 (1989). A preferred nucleic acid amplification product for SSCP analysis is between about 100 and 500 bp, and more preferably between about 140 and 300 bp. According to another rapid screening technique of the present invention, an amplified fragment containing a mutation is detected using denaturing gradient gel electrophoresis (DGGE). For a general reference on this technique see Sheffield, et al.. Proc. Natl. Acad. Sci. vol. 86, pp. 232-236 (1989). Briefly, double stranded fragments which are generated by amplification (PCR) can be subjected to DGGE. "DGGE is a gel system that separates DNA fragments according to their melting properties. When a DNA fragment is electrophoresed through a linearly increasing gradient of denaturants. the fragment remains double stranded until it reaches the concentration of denaturants equivalent to a melting temperature (Tm) that causes the lower-temperature melting domains of the fragment to melt. At this point, the branching of the molecule caused by partial melting sharply decreases the mobility of the fragment in the gel. The lower- temperature melting domains of DNA fragments differing by as little as a single-base substitution will melt at slightly different denaturant concentrations because of differences in stacking interactions between adjacent bases in each DNA strand. These differences in melting cause two DNA fragments to begin slowing down at different levels in the gel, resulting in their separation from each other." Sheffield, et al., ibid. Use of a GC clamp as taught in Myers et al., Nucleic Acids Res. vol. 13, pp. 3111-3146 (1985) increases the sensitivity of detection of this method from about 40% to about 100%. If mismatches are present, which would be the case if the DNA sample amplified was heterozygous for an ABCC6 allele, they will be visible on these DGGE gels. Double stranded fragments containing one wild-type strand and one mutant strand will have a different mobility on these gels than will double stranded fragments which contain two wild-type or two mutant strands, due to the different melting temperatures of these species. Thus, the melting temperature of fragments amplified from different regions of the ABCC6 gene can be determined by DGGE and can be used to indicate whether a mutant allele is present. In one embodiment, a region of the (MRP6) ABCC6 gene that encodes an important functional domain of the (MRP6) ABCC6 protein is screened for the presence of any mutation. For example, a preferred diagnostic assay interrogates the region of the (MRP6) ABCC6 gene that encodes an ATP binding site of the (MRP6) ABCC6 protein, a region that encodes a hydrophobic transmembrane domain, or a region that encodes a conserved amino acid, preferably in the C-terminal half of the MRP6 protein. One major application of the nucleic acid based diagnostics is in the area of genetic testing, carrier detection and prenatal diagnosis. Individuals carrying mutations in the ABCC6 gene (disease carrier or patients) may be detected at the DNA level with the use of a variety of techniques. The genomic DNA used for the diagnosis may be used directly for detecting specific sequences or may be amplified enzymatically in vitro, for example by PCR. The detection of specific DNA sequence may be achieved by methods such as hybridization using specific oligonucleotides (Wallace et al. Cold Spring Harbour Symp. Quant. Biol. 51 : 257-261 (1986)), direct DNA sequencing (Church and Gilbert, Proc. Nat. Acad. Sci. U. S. A. 81 : 1991 -1995 (1988)). the use of restriction enzymes (Flavell et al. Cell 15: 25 (1978), Geever et al Proc. Nat. Acad. Sci. U. S. A. 78: 5081 (1981)), discrimination on the basis of electrophoretic mobility in gels with denaturing reagent (Myers and Maniatis, Cold Spring Harbour Sym. Quant. Biol. 51 : 275-284 (1986)), RNase protection (Myers, R. M., Larin, J.. and T. Maniatis Science 230: 1242 (1985)), chemical cleavage (Cotton et al Proc. Nat. Acad. Sci. U. S. A. 85: 4397-4401 , (1985)) and the ligase-mediated detection procedure (Landegren et al Science 241 : 1077 (1988)). Oligonucleotides specific to normal or mutant sequences are chemically synthesized using commercially available machines, labelled radioactively with isotopes or non-radioactively (with tags such as biotin (Ward and Langer et al. Proc. Nat. Acad. Sci. U. S. A. 78: 6633-6657 (1981)), and hybridized to individual DNA samples immobilized on membranes or other solid supports by dot-blot or transfer from gels after electrophoresis. The presence or absence of these specific sequences are visualized by methods such as autoradiography or fluorometric (Landegren et al, 1989) or colorimetric reactions (Gebeyshu et al. Nucleic Acids Research 15: 4513-4534 (1987)). Sequence differences between normal and mutants may be revealed by the direct DNA sequencing method of Church and Gilbert. Cloned DNA segments may be used as probes to detect specific DNA segments. The sensitivity of this method is greatly enhanced when combined with PCR (Wrichnik et al, Nucleic Acids Res. 15:529-542 (1987); Wong et al, Nature 330:384-386 (1987); Stoflet et al, Science 239:491-494 ( 1988)). In the latter procedure, a sequencing primer which lies within the amplified sequence is used with double-stranded PCR product or single-stranded template generated by a modified PCR. The sequence determination is performed by conventional procedures with radiolabeled nucleotides or by automatic sequencing procedures with fluorescent-tags. Genetic testing based on DNA sequence differences may be achieved by detection of alteration in electrophoretic mobility of DNA fragments in gels with or without denaturing reagent. Small sequence deletions and insertions can be visualized by high resolution gel electrophoresis. For example, a PCR product with a small deletion is clearly distinguishable from the normal sequence on an 8% non-denaturing polyacrylamide gel. DNA fragments of different sequence compositions may be distinguished on denaturing formamide gradient gel in which the mobilities of different DNA fragments are retarded in the gel at different positions according to their specific "partial-melting" temperature (Myers, supra). In addition, sequence alterations, in particular small deletions, may be detected as changes in the migration pattern of DNA heteroduplexes in non-denaturing gel electrophoresis, as have been detected for the 3 dp (1507) mutation and in other experimental systems (Nagamine et al. Am. J. Hum. Genet, 45:337-339 (1989)). Alternatively, a method of detecting a mutation comprising a single base substitution or other small change could be based on differential primer length in a PCR. For example, one invariant primer could be used in addition to a primer specific for a mutation. The PCR products of the normal and mutant genes can then be differentially detected in acrylamide gels. Sequence alterations may occasionally generate fortuitous restriction enzyme recognition sites which are revealed by the use of appropriate enzyme digestion followed by conventional gel-blot hybridization (Southern, J. Mol. Biol 98: 503 ( 1975)). DNA fragments carrying the site (either normal or mutant) are detected by their reduction in size or increase of corresponding restriction fragment numbers. Genomic DNA samples may also be amplified by PCR prior to treatment with the appropriate restriction enzyme; fragments of different sizes are then visualized under UV light in the presence of ethidium bromide after gel electrophoresis. In another embodiment of the invention, sequence changes at specific locations may also be revealed by nuclease protection assays, such as RNase (Myers, supra) and S I protection (Berk, A. J., and P. A. Sharpe Proc. Nat. Acad. Sci. U. S. A. 75: 1274 (1978)), the chemical cleavage method (Cotton, supra) or the ligase-mediated detection procedure (Landegren supra). In addition to conventional gel-electrophoresis and blot-hybridization methods, DNA fragments may also be visualized by methods where the individual DNA samples are not immobilized on membranes. The probe and target sequences may be both in solution or the probe sequence may be immobilized (Saiki et al. Proc. Natl. Acad. Sci USA, 86:6230-6234 (1989)). A variety of detection methods, such as autoradiography involving radioisotopes, direct detection of radioactive decay (in the presence or absence of scintillant), spectrophotometry involving colorigenic reactions and fluorometry involving fluorogenic reactions, may be used to identify specific individual genotypes. In a preferred embodiment of the invention, for example, a PCR with multiple, specific oligonucleotide primers and hybridization probes, may be used to identify a plurality of possible mutations at the same time (Chamberlain et al. Nucleic Acids Research 16: 1 141 -1 155 ( 1988)). The procedure may involve immobilized sequence- specific oligonucleotides probes (Saiki et al, supra). According to the invention, assays are performed to detect a deletion within or including the MRP6 gene using Southern hybridization, FISH analysis, or diagnostic PCR. It is expected that most deletions will occur between repetitive Alu sequences that are common within the introns of the MRP6 gene. Preferred PCR primers for detecting these deletions are primers that flank intron Alu sequences. According to one aspect of the invention, many of the PXE associated MRP6 mutations are found in exons 22-30 Accordingly, preferred assays of the invention interrogate any one of exons 22-30, taken alone or in combination, for the presence of a PXE associated MRP6 mutation. In a preferred embodiment of the invention, a diagnostic assay interrogates the entire (MRP6) ABCC6 locus for the presence of a mutation, using for example SSCP, HA, or CSGE, and direct sequencing. In a more preferred embodiment, an assay interrogates a portion of the ABCC6 locus for the presence of a mutation. If a mutation is detected, it is first compared to known mutations associated with PXE (Table 1 ) and known neutral polymoφhisms (Table 2) that are not associated with PXE. If the mutation has not yet been observed as either a PXE associated mutation or as a neutral polymoφhism, the nature of the mutation is considered. If the mutation is a deletion, nonsense, frameshift or other mutation that affects expression of a normal MRP6 protein, the mutation is considered to be a PXE mutation. Similarly, if the mutation results in a nonconservative amino acid change or an amino acid change in a conserved sequence such as an NBF, a transmembrane sequence, or a change in a conserved amino acid shown in Fig. 3, the mutation is considered to be a PXE mutation. In addition, if the mutation results in low levels of MRP6 expression, the mutation is considered to be a PXE mutation. However, if the mutation results in a conservative amino acid change in a non-conserved part of the MRP6 protein the mutation is considered to be a neutral polymoφhism. Nonetheless, a patient identified with a previously unknown neutral polymoφhism according to this analysis should be subjected to additional diagnostic tests to look for known PXE associated symptoms or subclinical symptoms. According to one aspect of the invention, the detection of PXE carriers and PXE patients is determined through the identification of mutant MRP6 alleles in DNA from patients, family members and apparently unrelated and normal individuals. A single allele, with no evidence of a second mutant allele and the presence of a normal allele will be considered a carrier. Patients may be identified as either compound heterozygotes (having two different mutant alleles) or homozygotes (two identical mutant alleles). b) Protein based diagnostics Different approaches to a MRP6 protein-based diagnostic assay can be used to detect the presence of a PXE related mutation in a patient. Preferred assays include detecting a mutant electrophoretic mobility, the presence of a mutant epitope, the absence of a normal epitope, or by identifying altered biological activity, for example altered ATP binding or altered transport of a synthetic, preferably radiolabeled molecule. In one embodiment, diagnosis can be achieved by monitoring differences in the electrophoretic mobility of normal and mutant proteins. Such an approach will be particularly useful in identifying mutants in which charge substitutions are present, or in which insertions, deletions or substitutions have resulted in a significant change in the electrophoretic migration of the resultant protein. Alternatively, diagnosis may be based upon differences in the proteolytic cleavage patterns of normal and mutant proteins, differences in molar ratios of the various amino acid residues, or by functional assays demonstrating altered function of the gene products. In preferred embodiments, protein-based diagnostics will employ differences in the ability of antibodies to bind to normal and mutant MRP6 proteins. Such diagnostic tests may employ antibodies which bind to the normal proteins but not to mutant proteins, or vice versa. In particular, an assay in which a plurality of monoclonal antibodies, each capable of binding to a mutant epitope, may be employed. The levels of anti-mutant antibody binding in a sample obtained from a test subject (visualized by, for example, radiolabelling, ELISA or chemiluminescence) may be compared to the levels of binding to a control sample. Alternatively, antibodies which bind to normal but not to mutant MRP6 protein may be employed, and decreases in the level of antibody binding may be used to distinguish homozygous normal individuals from mutant heterozygotes or homozygotes. Such antibody diagnostics may be used for in situ immunohistochemistry using biopsy samples of tissues obtained from patients. c) Genetic counseling According to one embodiment of the invention, genetic counseling is provided to an individual identified as a PXE carrier, a PXE homozygote, or a PXE compound heterozygote (an individual with two different PXE mutant alleles). According to the invention, individuals carrying two PXE mutant alleles are provided information about ameliorating treatments for some of the symptoms of PXE. For example, a person who inherits PXE recessively is cautioned with regard to diet and activity. A low fat, high fibre, heart healthy diet is critical for maintaining cardiovascular health. Regular exercise appears to alleviate some of the symptoms of peripheral vascular disease. Medications to allow the passage of blood through narrowed arteries may be recommended. Individuals exhibiting eye manifestations should not engage in activities that put them at risk for injury to the eye that could subsequently lead to hemorrhage and vision loss. Smoking should be avoided at all costs since it appears to increase the rate and severity of eye disease. In one embodiment of the invention, a patient identified as being a carrier of a PXE associated mutation or as being a homozygote or a compound heterozygote for PXE associated mutations should be advised to reduce calcium intake or to use drugs that reduce calcium intake in order to reduce the severity of the phenotype. HI. Therapeutic applications The present invention provides a basis for therapeutic treatments of PXE related symptoms caused by mutations at the (MRP6) ABCC6 locus. According to the invention, normal (MRP6) ABCC6 nucleic acid or protein is provided to cells and/or a patient having a PXE associated mutation at the (MRP6) ABCC6 locus. Preferred target tissues include the kidney and liver, but also other tissues where low levels of MRP6 expression have been observed, such as smooth muscle cells and macrophages. Preferred target tissues also include tissues or cells that exhibit PXE related symptoms, such as a blood vessel, the gastrointestinal tract, occular tissue, the urinary tract, and skin. a) Nucleic acid-based therapeutics According to the invention, PXE or PXE associated symptoms can be prevented or treated by providing a normal PXE gene or cDNA to a patient that is diagnosed as having on or more PXE associated mutations at the (MRP6) ABCC6 locus. The fact that PXE is a recessive disease makes it particularly amenable to gene therapy, because it is expected that most, if not all, PXE associated MRP6 mutations reduce the amount of functional MRP6 protein in a cell and can be compensated for by providing normal MRP6 to the cell. In one series of embodiments, normal copies of the MRP6 gene are introduced into patients to code successfully for normal protein in one or more different affected cell types. The gene must be delivered to those cells in a form in which it can be taken up and code for sufficient protein to provide effective function. Thus, it is preferred that the recombinant gene be operably joined to a strong promoter so as to provide a high level of expression which will compensate for the absence of sufficient amounts of normal MRP6. As noted above, the recombinant construct may contain endogenous or exogenous regulatory elements, inducible or repressible regulatory elements, or tissue- specific regulatory elements. Preferred vectors for introducing an MRP6 gene to a cell or a patient include retroviral vectors, because of their high efficiency of infection and stable integration and expression. Other viral vectors which can be used include adeno-associated virus, vaccinia virus, bovine papilloma virus, or a herpes virus such as Epstein-Barr virus. Alternative vectors include plasmids that are replicated in human cells. In another series of embodiments, a mutant MRP6 gene may be replaced by homologous recombination with a recombinant construct. The recombinant construct preferably contains a normal copy of the MRP6 gene. Alternatively, a regulatory region of a normal MRP6 gene in a PXE carrier may be altered to increase expression of normal MRP6. i) Wild type genes In one series of embodiments, a normal human (MRP6) ABCC6 gene is introduced to cells or a patient. A normal (MRP6) ABCC6 gene includes a gene with one or more polymoφhic variations that are not associated with PXE. In one embodiment, an MRP6 genomic sequence is used. In an alternative embodiment an MRP6 cDNA sequence is used. ii) Related genes In an alternative series of embodiments, an (MRP6) ABCC6 related gene is provided to a cell or tissue having a PXE associated mutation. According to the invention, an (MRP6) ABCC6 related gene encodes a protein that has similar functional properties as a normal human MRP6 protein and can compensate for the absence of sufficient amounts of normal human MRP6 protein in a patient cell or tissue. Preferably, an (MRP6) ABCC6 homologue from another mammalian species is used. For example the mouse or rat MRP6 genes or cDNAs could be used. In one embodiment of the invention, a homologue from a non-mammalian species is used. Alternatively, a nucleic acid encoding a different ABC protein is used, for example an MRP1 encoding nucleic acid. The present invention also provides for cells or cell lines, both prokaryotic and eukaryotic, which have been transformed or transfected with the nucleic acids of the present invention so as to cause clonal propagation of those nucleic acids and/or expression of the proteins or peptides encoded thereby. Such cells or cell lines will have utility both in the propagation and production of the nucleic acids and proteins of the present invention but also, as further described herein, as model systems for diagnostic and therapeutic assays. As used herein, the term "transformed cell" is intended to embrace any cell, or the descendant of any cell, into which has been introduced any of the nucleic acids of the invention, whether by transformation, transfection, infection, or other means. Methods of producing appropriate vectors, transforming cells with those vectors, and identifying transformants are well known in the art. b) Protein based therapeutics Treatment of PXE symptoms may be performed by directly providing normal protein to a patient cell or tissue. Sufficient amounts of substantially pure MRP6 protein can be obtained from cultured cell systems which express the protein. Delivery of the protein to the affected tissue can then be accomplished using appropriate packaging or administrating systems including, for example, liposome mediated protein delivery to the target cells. c) Drug Therapies In one embodiment of the invention, a drug identified according to methods of the invention is administered to a patient diagnosed with PXE or a PXE carrier with PXE related symptoms . Alternatively, a drug is administered to prevent or minimize the development of PXE or PXE associated symptoms in individuals identified as having one or more PXE mutations at the ABCC6 locus. IV. Drug Discovery applications The present invention provides a basis for screening drug candidates to identify useful therapeutic compositions to treat or alleviate the symptoms of PXE. In a series of embodiments, the invention provides screens based on MRP6 activity. As used with respect to this series of embodiments, the term "activity" broadly includes gene and protein expression, protein post-translation processing, trafficking and localization, and any functional activity (e.g., enzymatic, receptor-effector, binding, channel), as well as downstream effects of any of these. MRP6 appears to be an integral membrane protein and may have transport related functions, and it also has ATP binding cassettes. Accordingly, these functional properties can be used as a basis for a screen to identify compounds that increase MRP6 function. In one embodiment, a drug candidate is screened for its ability to increase expression of the MRP6 gene. A preferred screen monitors the level of normal MRP6 mRNA in cells grown in culture in the presence and absence of the candidate compound. Alternatively, normal MRP6 protein levels are monitored. Useful cells for these assays are preferably normal cells or PXE carrier cells. However, a PXE cell can also be used and the levels of mutant MRP6 expression can also be monitored. A compound that increases the level of MRP6 expression is particularly useful to treat a PXE carrier in order to increase the level of MRP6 expressed from the normal allele. However, a compound that increases the level of MRP6 expression can also be useful to treat a PXE homozygote or compound heterozygote if the PXE associated MRP6 allele(s) encodes an MRP6 protein that retains some normal MRP6 function or if the allele is a mutation that reduces the level of normal MRP6 function. Other assays are useful for screening candidate compounds to identify a compound that increases normal MRP6 function. In one embodiment, an assay screens a compound for the ability to restore normal phenotype to dermal fibroblasts isolated from a PXE patient. Dermal fibroblasts isolated from patients with PXE exhibit abnormal phenotype when grown in vitro (Quaglino et al., Biochimica et Biophysica Acta 1501 (2000) 51-62). These phenotypes include an increased proliferation index compared to normal fibroblasts when grown in monolayer. PXE fibroblasts also have lower adhesion properties to collagen type I and to plasma fibronectin when compared to normal fibroblasts. Accordingly, these phenotypes provide a basis for an assay to identify a compound that restores normal MRP6 function to dermal fibroblasts isolated from a patient that was identified as having a PXE associated MRP6 mutation. In another embodiment of the invention, an assay is used to screen candidate compounds for their ability to increase the ATPase activity of an MRP6 proteins. In a preferred embodiment, the assay monitors the ATPase activity of an MRP6 protein encoded by an MRP6 gene with a PXE associated mutation in the presence and absence of the candidate compound. ATPase activity of purified MRP6 can be assayed according to methods known in the art (see, for example, Mao et al., Biochimica et Biophysica Acta 1461, 69-82 (1999). According to the invention, a compound that increases the ATPase activity of a PXE associated MRP6 protein variant is useful to treat a patient that is heterozygous or homozygous for the PXE allele that encodes the protein variant used in the assay. In a similar embodiment of the invention, an assay is used to screen candidate compounds for their ability to increase the transport activities of an MRP6 protein, in particular a PXE associated MRP6 protein variant. A useful transport assay is provided in Oude et al, Biochim Biophys Acta, 1241(2), 215-68, 1995. A compound identified according to this screen is useful to treat PXE patients and PXE carriers as described above. V. Disease Models The invention provides a basis for designing cellular and animal models of PXE. Such models are useful to study the development of the PXE disease in PXE homozygotes and compound heterozygotes and to identify potential PXE associated physiological dysfunctions in PXE carriers. Such models are also useful in screens to identify therapeutic compounds to prevent or treat PXE symptoms. a) Cellular models According to the invention, cellular models can be made by deleting one or both MRP6 alleles, or by introducing one or more PXE associated MRP6 alleles into a cell line grown in vitro, using methods known in the art. Preferred cell lines include renal and hepatic cell lines. Other useful cell lines include those derived from skin (keratinocytes and fibroblasts) and ocular tissue (ganglioma cells). b) Animal models The present invention also provides for the production of transgenic non- human animal models for the study of PXE, for the screening of candidate pharmaceutical compounds, and for the evaluation of potential therapeutic interventions. Animal species which suitable for use in the animal models of the present invention include, but are not limited to, rats, mice, hamsters, guinea pigs, rabbits, dogs, cats, goats, sheep, pigs, and non-human primates (e.g., Rhesus monkeys, chimpanzees). For initial studies, transgenic rodents (e.g., mice) are preferred due to their relative ease of maintenance and shorter life spans. Transgenic yeast or invertebrates (e.g., nematodes. insects) may be preferred for some studies because they will allow for even more rapid and inexpensive screening. Transgenic non-human primates, however, may be preferred for longer term studies due to their greater similarity to humans. Based on the identification of MRP6 as the gene associated with PXE, there are now several available approaches for the creation of a transgenic animal models for PXE, including animal models with one or both MRP6 alleles deleted and animal models with one or two MRP6 alleles with mutations similar to known PXE associated human MRP6 mutations. To create an animal model (e.g., a transgenic mouse), a mutant MRP6 gene can be inserted into a germ line or stem cell using standard techniques of oocyte microinjection, or transfection or microinjection into embryonic stem cells. Animals produced by these or similar processes are referred to as transgenic. If the mutation knocks out the MRP6 gene or a portion thereof, the animals are referred to as knockouts. For oocyte injection, one or more copies of the recombinant DNA constructs of the present invention may be inserted into the pronucleus of a just-fertilized oocyte. This oocyte is then reimplanted into a pseudo-pregnant foster mother. The liveborn animals are screened for integrants using analysis of DNA (e.g., from the tail veins of offspring mice) for the presence of the inserted recombinant transgene sequences. The transgene may be either a complete genomic sequence injected as a YAC, BAC, PAC or other chromosome DNA fragment, a cDNA with either the natural promoter or a heterologous promoter, or a minigene containing all of the coding region and other elements found to be necessary for optimum expression. Retroviral infection of early embryos can also be done to insert the recombinant DNA constructs of the invention. In this method, the transgene is inserted into a retroviral vector which is used to infect embryos (e.g., mouse or non-human primate embryos) directly during the early stages of development to generate chimeras, some of which will lead to germline transmission. Homologous recombination using stem cells allows for the screening of gene transfer cells to identify the rare homologous recombination events. Once identified, these can be used to generate chimeras by injection of blastocysts, and a proportion of the resulting animals will show germline transmission from the recombinant line. In a preferred embodiment, inactivation of the MRP6 gene in mice may be accomplished by designing a DNA fragment which contains sequences from an MRP6 exon flanking a selectable marker. Homologous recombination leads to the insertion of the marker sequences in the middle of an exon, causing inactivation of the MRP6 gene and/or deletion of internal sequences. DNA analysis of individual clones can then be used to recognize the homologous recombination events. The techniques of generating transgenic animals, as well as the techniques for homologous recombination or gene targeting, are now widely accepted and practiced. A laboratory manual on the manipulation of the mouse embryo, for example, is available detailing standard laboratory techniques for the production of transgenic mice (Hogan et al., 1986). A large number vectors are available to accomplish this and appropriate sources of genomic DNA for mouse and other animal genomes to be targeted are commercially available from companies such as GenomeSystems Inc. (St. Louis, Missouri, USA). The typical feature of these targeting vector constructs is that 2 to 4 kb of genomic DNA is ligated 5' to a selectable marker (e.g., a bacterial neomycin resistance gene under its own promoter element termed a "neomycin cassette"). A second DNA fragment from the gene of interest is then ligated downstream of the neomycin cassette but upstream of a second selectable marker (e.g., thymidine kinase). The DNA fragments are chosen such that mutant sequences can be introduced into the germ line of the targeted animal by homologous replacement of the endogenous sequences by either one of the sequences included in the vector. Alternatively, the sequences can be chosen to cause deletion of sequences that would normally reside between the left and right arms of the vector surrounding the neomycin cassette. The former is known as a knock-in, the latter is known as a knock-out. Example 5 describes a knockout of most of exons 28 and 29 in mouse MRP6. VI. (MRP6) ABCC6 Interacting Molecules According to the invention, molecules that interact with a normal MRP6 gene product gene product provide candidates 1) for identifying additional types of mutations that result in a PXE phenotype and 2) for additional levels of therapeutic intervention to overcome or minimize the effect of a mutant PXE gene product. For example, the identification of a protein that interacts with a normal MRP6 protein but not with a PXE mutant protein provides a potential target for therapeutic intervention if the function of the interacting protein can be modified to compensate for the absence of normal MRP6 protein. According to the invention, (MRP6) ABCC6 interacting molecules can be identified according to a number of biochemical and genetic methods known in the art, including affinity chromatography, mutational analysis, and yeast two hybrid analysis. As will be obvious to one of ordinary skill in the art, there are numerous other methods of screening individual proteins or other compounds, as well as large libraries of proteins or other compounds (e.g., phage display libraries and cloning systems from Stratagene, La Jolla, CA) to identify molecules which bind to normal or mutant MRP6 proteins. All of these methods comprise the step of mixing a normal or mutant MRP6 protein or protein fragment with test compounds, allowing for binding (if any), and assaying for bound complexes. The invention is further illustrated by the following non-limiting examples. EXAMPLES Example 1 : Materials and Methods a) Sources of Patient Samples PXE International, Inc. To date, PXE International has assembled a database of over 2100 PXE patients from 1400 families from 31 countries including North America, several European and South American countries and South Africa. From this cohort of patients and family members, genomic DNA has been prepared from whole blood samples obtained from over 1200 PXE patients and family members. Honolulu Heart Program In the early 1950's, studies around the world were reporting geographic differences in coronary heart disease (CHD) mortality, pathology, prevalence, and incidence. Among these reports were those of significant differences in the CHD and cerebrovascular disease rates in Japan and in the United States. The overall mortality for men in Japan and in the United States was similar, but the rates for CHD were strikingly different. Reported CHD mortality among Japanese was approximately twenty percent of that among U.S. Caucasians. At about the same time, Japanese living in Hawaii and California were reported to have a lower overall mortality than either U.S. Caucasians or Japanese living in Japan. The reasons for these differences were not apparent. However, it was felt that the study of these populations might offer important clues to the etiology of heart and vascular disease. The compared populations, living in Japan, Hawaii and California were of Japanese ancestry to limit genetic variation between study groups. The Honolulu cohort of the Ni-Hon-San (Nippon-Honolulu, San Francisco) study formed the basis for the Honolulu Heart Program (HHP). That study has now been underway for 35 years, providing extensive information about the role of lifestyle, diet, and other risk factors to development of chronic diseases of major public health importance in 8,000 Japanese-American men. Although several studies have suggested that PXE is more frequent in females there is no evidence that the observed gender difference is caused by genetic factors. Indeed, men would often report skin lesions, usually the first signs of PXE, later in life, than women. Therefore, a study of an exclusively male cohort from the HHP should not compromise the general applicability of the conclusions. PXE has also no particular predilection for any ethnic or racial group. PXE has indeed largely been described in Caucasians but also in African and Asian populations. PXE cases reported in Japan have shown no phenotypic or prevalence differences when compared to those observed in Caucasian populations. Unaffected control subjects DNA has been prepared from 150 unrelated individuals with no evidence of PXE. These samples have been aliquoted and are currently stored at -80°C. They are routinely used as control DNA samples and will be used to confirm that any new and potential mutation detected in a PXE patient or relative is indeed a mutation and not a neutral polymoφhism. The donors of these DNA samples were adults of either sex from various ethnic backgrounds . b) Mutation Detection Methods Detection of single nucleotide mutations Single strand conformation polymorphism (SSCP) analysis is based on the observation that single stranded DNA will adopt, in non-denaturing conditions, a secondary structure that is strictly sequence-dependant. Slight variations in sequence, such as a single nucleotide change can alter the conformation of a DNA fragment, which can be resolved on a non-denaturing polyacrylamide gel. Heteroduplex analysis (HA) is based on the observation that heteroduplexes formed between two DNA strands with one or more mismatches have electrophoretical mobility distinctly different from homoduplexes. While both methods (SSCP and HA) can detect point mutations, some sequence variants are more readily detected by one procedure than the other. Accordingly, a preferred screening method, uses a combination of SSCP and modified HA called Conformation-Sensitive Gel Electrophoresis or CSGE. In a preferred assay, each characterized PCR primer pair is radioactively labeled using T4 polynucleotide kinase and γ-[P J-ATP. For SSCP analysis, radiolabeled PCR products are mixed with denaturing loading buffer and loaded onto a 0.5 x MDE (MDE is a mutation detection enhancement polyacrylamide-derived matrix provided by FMC products), 0.6 x TBE native polyacrylamide gel and electrophoresed overnight at 8 watts in a sequencing gel apparatus. Separated, radiolabeled conformers are visualized by autoradiography. For CSGE, EDTA is added to the incubated PCR reaction mix to a final concentration of 1 mM and the reaction will be heat-denatured and incubated for 60 minutes at 68°C to allow heteroduplex formation. Heteroduplex products are analyzed on a 6% polyacrylamide gel (29:1 ratio of acrylamide/bisacrylamide), 10% (v/v) ethylene glycol and 15%) (w/v) formamide in O.5X TTE buffer (1 X TTE is 89 mM Tris, 15 mM taurine, 0.5 mM EDTA, pH 9.0). A solution of 20% (v/v) ethylene glycol, 30% (w/v) formamide and 0.05% xylene cyanol and bromophenol blue is mixed equally with the samples. The gel is run at 35 to 45 watts for 2 to 4 hours at room temperature. As for SSCP, CSGE conformers are revealed by autoradiography. When an abnormal conformer or heteroduplex is detected, the segregation of the variant is analyzed for DNA samples from the entire family. Subsequently, the DNA sequence of the variant is determined by eluting normal and altered DNA conformers directly from the electrophoresis gel. These PCR fragments are eluted in water, re- amplified and directly used as a template for sequencing using an ABI 310 automated sequencer (Perking Elmer). A panel of 150 DNA samples of normal unrelated individuals is used to identify abnormal variants that are common polymoφhisms in the ABCC6 locus. Mutation detection by enzymatic cleavage Single nucleotide substitutions often modify the recognition site of a restriction enzyme. Polymoφhisms and mutations can, therefore, be detected in a rapid and convenient manner by the enzymatic cleavage of a PCR fragment containing the nucleotide change. This method is frequently employed to verify the presence of previously characterized mutations or polymoφhisms in DNA samples for control, study or diagnostic puφoses. It can also be used for screening a large number of samples. Out of the ABCC6 mutations listed in Table 1, 10 mutations were identified with a unique restriction pattern. For example, three possible Hhal restriction profiles for one of these mutations, R1339C (4015C to T) can be visualized by electrophoresis. According to the invention, single nucleotide mutations in ABCC6 are detectable by enzymatic cleavage. Accordingly, this method is useful as an initial step to appropriately complement the screening of large cohorts with more traditional mutations detection techniques. PCR mapping A single base substitution mutation may be detected based on differential PCR product length or production in PCR. Thus, primers which span mutant sites or which, preferably, have 3' termini at mutation sites, may be employed to amplify a sample of genomic DNA, mRNA or cDNA from a subject. A mismatch at a mutational site may be expected to alter the ability of the normal or mutant primers to promote the polymerase reaction and, thereby, result in product profiles which differ between normal subjects and heterozygous and/or homozygous MRP6 mutants. The PCR products of the normal and mutant gene may be differentially separated and detected by standard techniques, such as polyacrylamide or agarose gel electrophoresis and visualization with labeled probes, ethidium bromide or the like. Because of possible non-specific priming or readthrough of mutation sites, as well as the fact that most carriers of mutant alleles will be heterozygous, the power of this technique may be low. Electrophoretic mobility Genetic testing based on DNA sequence differences also may be achieved by detection of alterations in electrophoretic mobility of DNA, mRNA or cDNA fragments in gels. Small sequence deletions and insertions, for example, can be visualized by high resolution gel electrophoresis of single or double stranded DNA, or as changes in the migration pattern of DNA heteroduplexes in non-denaturing gel electrophoresis. MRP6 mutations or polymoφhisms may also be detected by methods which exploit mobility shifts due to single-stranded conformational polymoφhisms (SSCP) associated with mRNA or single-stranded DNA secondary structures. Chemical cleavage of mismatches Mutations in MRP6 may also be detected by employing the chemical cleavage of mismatch (CCM) method. In this technique, probes (up to ~ 1 kb) may be mixed with a sample of genomic DNA, cDNA or mRNA obtained from a subject. The sample and probes are mixed and subjected to conditions which allow for heteroduplex formation (if any). Preferably, both the probe and sample nucleic acids are double-stranded, or the probe and sample may be PCR amplified together, to ensure creation of all possible mismatch heteroduplexes. Mismatched T residues are reactive to osmium tetroxide and mismatched C residues are reactive to hydroxylamine. Because each mismatched A will be accompanied by a mismatched T, and each mismatched G will be accompanied by a mismatched C, any nucleotide differences between the probe and sample (including small insertions or deletions) will lead to the formation of at least one reactive heteroduplex. After treatment with osmium tetroxide and/or hydroxylamine to modify any mismatch sites, the mixture is subjected to chemical cleavage at any modified mismatch sites by. for example, reaction with piperidine. The mixture may then be analyzed by standard techniques such as gel electrophoresis to detect cleavage products which would indicate mismatches between the probe and sample. Other methods Various other methods of detecting MRP6 mutations, based upon the MRP6 sequences disclosed and otherwise enabled herein, will be apparent to those of ordinary skill in the art. Any of these may be employed in accordance with the present invention. These include, but are not limited to, nuclease protection assays (SI or ligase-mediated), ligated PCR, denaturing gradient gel electrophoresis (DGGE), restriction endonuclease fingeφrinting combined with SSCP (REF-SSCP), and the like. Methods for analyzing MRP 6 mRNA levels: The steady state levels of (MRP6) ABCC6 mRNA was analyzed in skin fibroblasts from a PXE patient carrying a homozygous Rl 141X mutation. Total skin fibroblast RNA from an unaffected control individual and a PXE patient was used to synthesize single stranded cDNA using oligo(dT). PCR primers derived from (MRP6) ABCC6 mRNA sequence were then used in two consecutive rounds of 25 cycles of PCR. Poly(A)+ RNA from normal human kidney (obtained from Clontech) was used as a positive control for detection of (MRP6) ABCC6 mRNA. MRP-1 mRNA was detected in the same cDNA samples used for ABCC6 mRNA with 30 cycles of PCR. The 390 bp and 180 bp DNA fragments detected correspond to the expected size of (MRP6) ABCC6 and MRP-1 mRNA domains encoded within exons 6-9 and 2-3 of the (MRP6) ABCC6 and MRP-1 genes respectively. No reverse transcriptase (No RT) controls were included to confirm that no PCR products were obtained in the absence of cDNA synthesis. Stringent hybridization conditions High stringency conditions are at least equivalent to a temperature in the range of about 40-70 degrees C, and between about 0.05 and 0.5 M sodium ion. High stringency hybridization conditions are well known in the art and can be optimized for a specific oligonucleotide based on the length and GC content of the oligonucleotide as described in, for example, Sambrook et al., Molecular Cloning, A Laboratory Manual (Cold Spring Harbor, N.Y.. 1982). An oligonucleotide selected for hybridizing to the target nucleic acid, whether synthesized chemically or by recombinant DNA methodologies, is isolated and purified using standard techniques and then preferably labeled (e.g., with 35s or 32p) using standard labeling protocols. A sample containing the target nucleic acid then is run on an electrophoresis gel, the dispersed nucleic acids transferred to a nitrocellulose filter and the labeled oligonucleotide exposed to the filter under stringent hybridizing conditions, e.g. 50% formamide, 5 X SSPE, 2 X Denhardt's solution, 0.1% SDS at 42°C, as described in Sambrook et al. (1989) supra. The filter may then be washed using 2 X SSPE, 0.1% SDS at 68°C, and more preferably using 0.1 X SSPE, 0.1% SDS at 68°C. Other useful procedures known in the art include solution hybridization, and dot and slot RNA hybridization. Optionally, the amount of the target nucleic acid present in a sample is then quantitated by measuring the radioactivity of hybridized fragments, using standard procedures known in the art. Example 2: The positional cloning of the PXE gene Blood samples and skin biopsies from PXE patients and unaffected relatives in the United States were collected by PXE International Inc., by Dr. Ivonne Pasquali- Ronchetti in Italy, by Dr. F. Michael Pope in the United Kingdom and by Dr. Anne de Paepe in Belgium. Blood samples were obtained from 100 Caucasian control individuals with no family history of PXE. Genomic DNA was isolated from aliquots of blood. Low passage and confluent skin fibroblasts were obtained from 3 mm full thickness skin biopsies using known procedures. To identify the gene that contains mutations responsible for PXE, the disease locus was confined to a region of about 8 cM between markers D16S500 and- D16S3041. Recombination mapping reduced this large critical region to an 820 kb domain containing six candidate genes. These genes encode an isoform of Myosin Heavy Chain (MYH11), two Multidrug Resistance-associated Proteins (MRP-1 and (MRP6) ABCC6), an unknown protein called pM5 and two identical unknown proteins referred to as UNK. Using a polymoφhic microsatellite repeat (GAAAp) located at the 5' end of the MRP-1 gene (Fig. 1) an informative meiotic recombination in one PXE patient was identified and this permitted the exclusion of the MYH11 as a candidate gene and reduced the size of the PXE region to 570 kb and 5 candidate genes. Figure 1 shows the previously defined PXE locus covering 820 kb between markers D16S3060 and D16S79 at 16pl3.1. The BAC contig that covers this region is shown along with the identity of the BACs. Figure lb shows the gene content of the PXE locus represented from the telomere (left) to the centromere (right). The transcriptional orientation of the genes is indicated by arrows. A flag represents the position of a polymoφhic marker (GAAA / 7) used to identify an additional meiotic recombination in one PXE patient that excluded the MYHII gene as the PXE gene. The 109 exons within the five candidate genes were then screened for mutations by Single-Strand Conformation Polymoφhism (SSCP) and Heteroduplex Analysis (HA) using genomic DNA from a cohort of 20 unrelated PXE patients. Mutation detection, sequence analysis and RT-PCR, SSCP, and Heteroduplex Analysis (HA) were carried out as previously described. Intron-derived primers for PCR amplification of exons present in the genes encoding MRP-1, (MRP6) ABCC6, pM5 and both UNK gene were synthesized using intron sequences available in the TIGR database (http://www.tigr.org). PCR products were typically 150-350 bp in length and included complete intron exon boundaries. Typical PCR reactions, were performed in the presence of j2 P-labelled primers in a 9700 thermocycler (Perkin Elmer). Radioactive PCR products were analyzed either by SSCP or HA using MDE polyacrylamide gel (FMC) according to the manufacturer's instructions. DNA conformers were eluted in water from gel slices, re-amplified and sequenced utilizing the same primers used to generate these PCR products. DNA sequence analysis was performed using ABI BigDye terminator cycle sequencing with an ABI310 automated DNA sequencer. The sequence information generated by the sequencer was analyzed using the ABI software. The Sequencher™ 3.1 program was used to identify variation between the sequence of putative mutations and control sequences. RT-PCR was performed on total RNA from cultured human skin fibroblasts and human kidney poly(A)+ RNA. The sequences of the PCR primers used are: (MRP6) ABCC6: 5'-AGCCACGTTCTGGTGGGTTT-3' (SEQ ID NO: 4); 5'- GGAGGCTTGGGATCACCAAT-3' (SEQ ID NO: 5); MRP-1: 5- CAACTGCATCGTTCTGTTTG-3' (SEQ ID NO: 6); and 5'- ATACTCCTTGAGCCTCTCCA-3' (SEQ ID NO: 7). Following synthesis, PCR products were separated by electrophoresis through 1.2% agarose and visualized by staining with ethidium bromide. DNA sequence analysis of two conformers detected in PCR products containing exons 19 and 28 of the pM5 gene revealed two private single nucleotide polymoφhisms (SNPs) within the intronic sequence flanking these exons (Table 2). These were the only sequence variants detected in the 31 exons of the pM5 gene using a cohort of 20 PXE patients. Screening all eight exons of each of the two UNK genes revealed only one SNP in the first exon of either one or both UNK genes in 5 PXE patients. This was a silent nucleotide change (C33T) within the 11 th codon (SI 1) of the open reading frame of either one or both unknown genes and therefore unrelated to PXE. Screening all 31 exons of the MRP-1 gene in a panel of PXE patients identified several sequence variants (Table 2) that are not functionally related to PXE as they either occurred in intronic sequences or did not encode changes in amino acids. In addition, two missense variants (R633Q and G671 V) were seen in exons- 14 and 16 in two unrelated PXE patients but these substitutions were unlikely to be responsible for PXE as they were also found in a panel of 200 alleles from unaffected, ethnically matched control individuals. Finally, in screening the 31 exons of the (MRP6) ABCC6 gene, the first mutations that are clearly responsible for PXE were identified. A C->T substitution within exon 24 (C3421T) of the (MRP6) ABCC6 gene generated a stop codon at position 1141 (R1141X; Fig. 1 and Table 2). Figure lc shows the intron exon structure of the (MRP6) ABCC6 gene. Intron sizes are drawn approximately to scale and the exons are numbered from the 5' end of the (MRP6) ABCC6 gene. Figure Id shows chromatograms of partial DNA sequence from two unrelated PXE patients containing a nonsense and a splice site mutation in exon 24 and intron 21 respectively. In exon 24, the sequence shows a 3421C>T substitution (arrowhead), which would generate a stop codon at position 1141 (Rl 141X). PXE patients in a consanguineous Italian pedigree were found to be homozygous for this stop codon mutation. In intron 21, a G to T substitution (IVS21+1G>T) was observed within the invariant GT sequence of the donor splice site. This mutation would influence constitutive splicing of (MRP6) ABCC6 pre-mRNA and was found in two unrelated PXE patients as a compound heterozygote in association with either R1141X or R1 138Q. Firgure lc shows the sequence of the normal and mutant nucleotide and amino acid sequences for the nonsense mutation in exon 24 and the splice site variant within intron 21. The C3421T variant in exon 24, which was not found in the control panel of 200 normal alleles, co-segregated in a homozygous form with a recessive PXE phenotype in an Italian family in which all unaffected individuals but one were heterozygote carriers (Fig. 4). Figure 4 shows a large consanguineous Italian pedigree, SSCP conformers for a homozygous variant (Rl 141X) in exon 24 were noted in all four PXE patients (shaded symbols). All other unaffected family members were heterozygote for this nonsense mutation except one unaffected family member, indicated by an arrow. SSCP conformers from normal unrelated control DNA have been included. Total RNA from the PXE patient indicated by was used for an RT PCR analysis of (MRP6) ABCC6 mRNA and shown to have low levels of MRP6 mRNA. This Rl 141X mutation results either in an (MRP6) ABCC6 protein lacking 362 amino acids at the C-terminal domain (Fig. 3) or a null allele, produced through nonsense mediated decay of a truncated (MRP6) ABCC6 mRNA. Indeed, an analysis of steady state (MRP6) ABCC6 mRNA levels in skin fibroblasts from PXE patients of this Italian pedigree indicated the absence of detectable (MRP6) ABCC6 mRNA, suggesting that the homozygous Rl 141X mutation results in the total loss of MRP6 gene product rather than the production of a truncated protein. Rl 14 IX was also found in a homozygous state in unrelated patients with autosomal recessive PXE from the United Kingdom and Belgium. Haplotype analysis of the PXE locus in families with the Rl 14 IX mutation revealed that this mutation is travelling within different haplotypes, suggesting that Rl 141X may be a recurrent mutation. In two families with a recessive form of PXE from the United Kingdom and the United States, PXE patients were found to be compound heterozygotes. Affected individuals carried a substitution (TVS21 + 1G>T) affecting the donor- splice site of exon 21 of (MRP6) ABCC6, in association with either the nonsense RI141X substitution in exon 24 or a missense mutation, Rl 138Q also in exon 24. The splice site mutation occurred at the donor invariant dinucleotide and lowered the splice potential score from 72.1 to 53.8. Several other missense variants (Table 2) were also found within exon 24 and 28 of the (MRP6) ABCC6 gene. These single nucleotide substitutions, none of which were detected in the control panel of 200 alleles, occurred within highly conserved coding domains, particularly the domain in exon 28 encoding the Walker A region of the second ATP binding fold (Fig. 2). All the detected homozygous or compound heterozygous mutations were found to be associated with autosomal recessive PXE. One missense mutation (3961G>A) was observed in a family with an apparently dominant form of PXE. All the other heterozygous alterations were detected in individuals with sporadic PXE. The mode of inheritance of these sporadic PXE cases is presently unknown. Elastic fibers within elastic tissues such as skin and the arterial wall are fragmented and calcified in PXE patients. Dermal and vascular elastic fiber calcification is patchy and does not involve all elastic fibers in these tissues. Therefore, without wishing to be bound by any particular theory, calcification of elastic fibers in PXE is probably therefore, a secondary consequence of a primary defect of either elastic fiber assembly or the interaction of elastic fibers with other extracellular matrix components. Accordingly, MRP6 function is more likely to be related to fiber assembly or matrix interactions than calcium transport. Another possibility is that the maintenance of the integrity of normal elastic fibers, extracellular matrix polymers subject to constant mechanical stress, is modulated by (MRP6) ABCC6 in a way that has yet to be explained. Polymoφhic markers in genes encoding known elastic fiber proteins (tropoelastin, lysyl oxidase, fibrillin 1 and 2) were used in a linkage and sib pair analysis, performed with families with both autosomal recessive (AR) and dominant (AD) forms of PXE. No obvious linkage between these markers and the PXE phenotype was found. Table 2 nt change Codon # Effect Location Status UNK gene polymorphisms 33C>T 11 Ser to Ser Exon 1 Hetero pM5 gene polymorphisms 21870T 729 Gly to Gly Exon 19 Hetero 3241 G>A 1081 Glu to Lys Exon 28 Hetero MRP-1 gene polymorphisms 1062T>C 354 Asn to Asn Exon 9 Hetero 1898G>A 633 Arg to Gin Exon 14 Hetero 200 lOT 667 Ser to Ser Exon 16 Hetero 2012G>T 671 Gly to Val Exon 16 Both 4002G>A 1334 Ser to Ser Exon 28 Hetero IVS29-18delT - - Intron 29 Hetero (V1RP6 ABCC6 gene polymorphisms 549G>A 183 Leu to Leu Exon 5 Hetero IVS11-41A>G _ - Intron 11 Both 1841T>C 614 Val to Ala Exon 14 Both 2490OT 830 Ala to Ala Exonl9 Both IVS25+90G>A - - Intron25 Both IVS27-46A>G - - Intron 27 Hetero IVS28+490T - - Intron 28 Hetero 3"UTR+17G>A _ _ 3' UTR Hetero (MRP6) ABCC6 gene mutations IVS21 +1 G>T - mRNA Intron 21 Compound splicing 3341 G>C 1 1 14 Arg to Pro Exon 24 Homo 3413G>A 1138 Arg to Gin Exon 24 Compound 342 l OT 1 141 Arg to X Exon 24 Compound+Both 3775delT 1259 Fram Shift Exon 27 Hetero 3892G>T 1298 Val to Phe Exon 28 Hetero 3904G>A 1302 Gly to Arg Exon 28 Homo 3907G>C 1303 Ala to Pro Exon 28 Hetero 3940OT 1314 Arg to Tφ Exon 28 Homo 3961G>A 1321 Gly to He Exon 28 Hetero Table 2: A summary of all variants identified in the PXE locus in a cohort of 20 umelated PXE patients. Nucleotide (nt) numbering was derived either from full length published cDNA sequences or from putative cDNA deduced from genomic DNA sequence. Hetero indicates that a variant was identified in a heterozygous state. Homo indicates that a variant was found in a homozygous state. Both, indicates that a variant was seen in both heterozygous and homozygous states. Compound, indicates that a variant was characterized as a compound heterozygote. Example 3 : Mutation detection in dominant pedigrees The segregation of ABCC '6 mutations with the PXE phenotype was studied in three pedigrees with an apparent dominant inheritance. Two of the dominant families (families 1 and 3) presented three generations of individuals, while the remaining pedigree contained only two generations. In all 3 families, a heterozygous mutation, Rl 14 IX in exon 24, was found to segregate with the PXE phenotype. In the two- generation family (family 2), an apparent loss of heterozygosity of the Rl 141X allele was detected in the second generation of affected individuals (II-l to -3). Several polymorphic variants in the surrounding exons and introns were subsequently analyzed by SSCP. Only one variant, V614A in exon 14, was found to be informative. These results suggested a heterozygous sub-microscopic deletion, which was paternally inherited. This deletion, with a breakpoint between exon 14 and 24, extended beyond exon 24, probably corresponds to a recurrent deletion recently characterized in 4 unrelated families. The latter deletion, confined to a region of the gene between intron 22 and 29 eliminated 16.5 kb of genomic DNA. Therefore, individuals II-l, II-2 and II-3 of family 2 have inherited compound heterozygote mutations, clearly indicating the recessive nature of PXE in this pedigree. Moreover, the phenotype displayed by the mother (Individual 1-2) carrying a heterozygous allele Rl 14 IX, suggested the partial expression of the phenotype in an obligate carrier. Indeed, individual 1-2 showed discreet skin lesions on the neck associated with a positive von Kossa staining of a skin biopsy- (from lesional skin) indicating the presence of calcium salt precipitates typical of PXE. No angioid streaks were reported for this obligate carrier and no cardiovascular examination has been performed yet. In the remaining families (family 1 and 3) no other mutations were found. However, the PXE phenotype of the family members dramatically varied with the generations, clearly suggesting either pseudo-dominance or partial penetrance in obligate carriers. In family 1 , the paternal grandmother and the father presented discreet skin lesions on the neck region associated with a positive von Kossa staining of a skin biopsy (no other manifestation were diagnosed), while both children, although very young, had already visible signs of plaques of coalesced papules on the neck and angioids streaks following ocular examination. In family 3, the paternal grandfather was severely affected with lax and redundant skin, disciform scaring of the retina (the vision is severely impaired at this stage) in addition to active gastrointestinal bleeding and intermittent claudication. The father was only diagnosed with a positive von Kossa staining of a skin biopsy while 3 of his children presented with the characteristic PXE skin lesions and angioid streaks. Accordingly, heterozygote carriers of PXE mutations can develop PXE related phenotypes including sub-clinical manifestations of PXE. According to the invention, the penetrance of PXE lesions associated with a single mutant (MRP6) ABCC6 allele is between about 10 to 20% of all carriers, based on the frequency of described AD PXE cases. Therefore, a pedigree with AR PXE presents sub-clinical manifestations of PXE in 10 to 20%) of the obligate carriers. These carriers will be parents of an affected individual and 25% of the unaffected siblings. Example 4: PXE Heterozygote Frequencies Upon screening a small sample of the general population (150 normal individuals) as part of a control panel to verify whether nucleotide substitutions found in the (MRP6) ABCC6 gene from PXE patients were indeed mutations, two heterozygote mutations were found in unrelated subjects. The first of these variants was a founder mutation (R1339C) only present in South African Afrikaners while the second substitution is a recurrent nonsense mutation (Rl 141X). Rl 141X is one of the four recurrent mutations that have been identified. These mutations are far more likely to be found in the general population than private mutations, which, in principle, can only be found in related individuals. The frequency of heterozygote carriers deduced from the presence of one recurrent mutation in the relatively small sample of the general population is 0.7%). However, four recurrent mutations have thus far been identified. Although each of the recurrent mutations is likely to have a different frequency, the frequency of carriers can be as high as 2.8%, which is consistent with the commonly accepted prevalence of heterozygote carriers in the general population (0.6 to 2.5%). Based upon these frequency of heterozygotes and the predicted penetrance of the PXE phenotype in heterozygote carriers (10-20%), heterozygote carriers with PXE symptoms are expected at a frequency of about 0.25% of the general population. In a cohort of about 3000 individuals between 8 and 15 persons presenting cardiovascular, ocular or dermal symptoms would be expected. These numbers provide a basis for a statistical analysis of the correlation between single (MRP6) ABCC6 mutations and partial manifestations of PXE. Additional cohorts with clinically defined cardiovascular abnormalities such as the 1200 sib-pairs group from the Family Blood Pressure Program with hypertension, or the NHLBI Framingham study (http://www.nhlbi.nih.gov/about/framingham/) from which an appropriate cohort of 2400 to 4500 individuals is available, can be used to provide additional statistical significance. Example 5 : Creating a mouse knockout To create a knock-out mouse for ABCC6 a neomycin resistance cassette is introduced between exons 28 and 29 as shown in Fig. 6. This results in the destruction of the second ATP binding domain whose Walker A domain is encoded by exon 28 and which is essential for the function of any ABC transporter. Based on the cDNA sequence for the mouse ABCC6 gene (SEQ ID NO: 8), primers with restriction sites were designed to amplify genomic DNA and allow cloning into vector pPNT described in Tybulewicz et al., Cell vol. 65, 1153-1163, 1991. Specifically, a 2.2 kb DNA fragment from exon 26 to exon 28 is cloned into the unique BamHI of pPNT, and a 2.3 kb DNA fragment containing exon 29 to 30 was cloned in the Xhol site of pPNT. In the resulting construct, the neomycin cassette from the vector interrupts the reading frame of ABCC6 in exon 28 after the conserved Lysine in the walker A domain. This construct will be linearized by Notl digestion and transfected into mouse 129 stem cells. The two resistance cassettes provided by the vector (TK and Neo) will allow screening for homologous recombination and knock out of an ABCC6 locus according to methods known in the art (see, for example, Tybulewicz et al., Cell vol. 65, 1153-1 163, 1991). The deletion construct shown in Fig. 6 will be transfected into E. coli and amounts of DNA sufficient for the targeted mutagenesis process will be produced. This construct will be inserted into embryonic cell lines and cells that incoφorate the construct will be implanted into surrogate mothers and MRP6 null mice will be obtained according to methods known in the art. According to the invention, the production of MRP6 null mice with symptoms resembling those of human PXE would provide further proof that mutations at the MRP6 locus are responsible for PXE. However, a more important use for MRP6 null mice, or mice that are carriers of an MRP6 deletion (heterozygotes having an allele with the MRP6 deletion and a wild-type MRP6 allele) is to provide an animal model to study the development and progression of PXE, and to provide an animal model useful in the development of therapeutic approaches (including identifying therapeutic drugs) to treat existing PXE or to prevent or reduce the symptoms of PXE before they develop. Example 6: Examples of oligonucleotide probes and probes useful to detect PXE associated MRP mutations. Various probes corresponding to regions of specific mutations in ABCC6 are used in standard oligonucleotide hybridization, in oligonucleotide array or nucleic acid chip assays (see www.brownlab.stanford.edu), and in PCR-based techniques for the detection of PXE. Each of the mutations shown below are indicative of a mutation in the ABCC6 gene that leads to PXE. In a preferred embodiment, the probe shown in SEQ ID NO: 10 is used for the detection of a G to A mutation in Exon 24 of the ABCC6 gene. CAGTGGTCCAGGGCATTCCGA (SEQ ID NO: 10) In another embodiment, the probe shown in SEQ ID NO: 11 is used for the detection of a C to T mutation in Exon 24 of the ABCC6 gene. CAGTGGTCCGGGCATTCTGA (SEQ ID NO: 11) In yet another embodiment of the invention, the probe in SEQ ID NO: 12 is used for the detection of a G to C mutation in Exon 24 of the ABCC6 gene. GACCGTTGGAGTCAGCCAGCTACTCG (SEQ ID NO: 12). In another embodiment of the invention, the probe in SEQ ID NO: 13 is used for the detection of a C to G mutation in Exon 24 of the ABCC6 gene. GACCCTTGGAGTCAGCCAGCTACTGG (SEQ ID NO: 13) In another embodiment, the following probes are used for the detection of specific mutations in Exon 26 of the ABCC6 gene. In a preferred embodiment of the invention, the probe in SEQ ID NO: 14 is used for the detection of a C to T mutation in Exon 26 of the ABCC6 gene. GGATGTAGGACTATGCCTGGACGCCC (SEQ ID NO: 14) In a preferred embodiment of the invention, the probe in SEQ ID NO: 15 is used for the detection of a G to C mutation in Exon 26 of the ABCC6 gene. GGATGCAGGACTATGCCTGCACGCCC (SEQ ID NO: 15) In yet another preferred embodiment of the invention, specific mutations in Exon 27 of the ABCC6 gene are detected using the probes shown below. In a preferred embodiment of the invention, the probe in SEQ ID NO: 16 is used for the detection of a C to A substitution in Exon 27 of the ABCC6 gene TGCAGCTAAGCCCCCCTGGC (SEQ ID NO: 16) The probe sequence in SEQ ID NO: 17 is used for the detection of a deletion in Exon 27 of the ABCC6 gene. TGCAGCTCAGCCCCCCGGC (SEQ ID NO: 17) In yet another embodiment of the invention, the probe in SEQ ID NO: 18 is used for the detection of a G to A mutation in Exon 27 of the ABCC6 gene. GCTCCAAGCTCCCTGGAGGC (SEQ ID NO: 18) Mutations in Exon 28 of the ABCC6 gene in patients are detected using the probes shown in SEQ ID NOs. 19, 20, 21, 22, 23, 24 and 25. In a preferred embodiment of the invention, the probe in SEQ ID. 19 is used for the detection of a C to T mutation in Exon 28 of the ABCC6 gene. CTGTGGCTCCAGGAGGCAGCTGAGGGTGGG (SEQ ID NO: 19) In yet another preferred embodiment of the invention, the probe in SEQ ID NO: 20 is used for the detection of a G to A mutation in Exon 28 of the ABCC6 gene. CTGCAGCTCCAGGAGGCAGCTGAGGGTGGG (SEQ ID NO: 20) Similarly, in a preferred embodiment of the invention, the probe in SEQ ID NO: 21 is used for the detection of a G to A mutation in a different region of Exon 28 of the ABCC6 gene. CTGCGGCTCCAGGAGGCAGCTGAGAGTGGG (SEQ ID NO: 21) Probes in SEQ ID NOs. 22, 23, 24 and 25 are used for the detection of additional specific mutations in Exon 28 of the ABCC6 gene. GTGGGCATCTTTGGCAGGACCGGGG (SEQ ID NO: 22) GTGGGCATCGTTGGCAGGACTGGGG (SEQ ID NO: 23) GTGGGCATCTTTGGCAGGACCAGGG (SEQ ID NO: 24) GTGGGCATCTTTGGCAGGACCGGGC (SEQ ID NO: 25) EQUIVALENTS The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. INCORPORATION BY REFERENCE Each of the patent documents and scientific publications disclosed herein is incoφorated by reference into this application in its entirety.

Claims: What is claimed is: CLAIMS L A method for screening for the presence of a PXE mutation, the method comprising the steps of: interrogating an MRP6 nucleic acid in a patient sample for the presence of a PXE mutation; and, identifying a positive screen as the presence of said mutation, 2. The method according to claim 1 , wherein the said patient sample is selected from the group consisting of blood, saliva, amniotic fluid, and tissue. 3. The method according to claim 2, wherein the said patient sample is blood. 4. The method according to claim 1, wherein said interrogating step is a nucleic acid sequence scanning assay. 5. The method according to claim 4, wherein said scanning assay is selected from the group consisting of SSCP, DGGE, RFLP, LCR, DHPLC, and enzymatic cleavage. 6. The method according to claim 1, wherein said interrogating step is a specific mutation detection assay. 7. The method according to claim 6, wherein said detection assay is selected from the group consisting of oligonucleotide hybridization and primer extension assays. 8. The method according to claim 1, wherein said interrogating step is a nucleic acid sequencing assay. ^ 9. The method according to claim 1 , wherein said assay detects the presence of a mutation selected from the group consisting of a deletion, a substitution, an insertion, and a rearrangement. 1 10. The method according to claim 1 , wherein said mutation is a mutation in codon 1 141. 1 1 1. The method according to claim 1 , wherein said mutation is a deletion of base 3775. 1 12. The method according to claim 1 , wherein said mutation is a non- conserved amino acid substitution, l 13. The method according to claim 1 , wherein said mutation is in a codon 3 selected from the group consisting of 1 1 14, 1 138, 1 141 , 1298, 1302, 1303, 1314, and 4 1321. 1 2 14. The method according to claim 1 , wherein said mutation is in a splice site 3 in an intron. l 2 15. The method according to claim 1 , wherein said mutation is in the promoter 3 region of the MRP6 gene. 1 2 16. The method according to claim 1, wherein said mutation is in a polyA site 3 of the MRPό gene 1 2 17. The method according to claim 1, wherein said nucleic acid is selected 3 from the group consisting of mRNA, genomic DNA, and cDNA. 18. A method for screening for the presence of a PXE mutation, the method comprising the steps of: detecting MRP6 expression; and, identifying a positive screen as one that detects a level of MRP6 expression that is lower than normal MRP6 expression. 19. The method of claim 18, wherein said detecting step detects MRP6 mRNA. 20. The method of claim 18, wherein said detecting step detects MRP6 protein. 21. The method according to claim 20, wherein said detecting step is an antibody-based assay. 22. The method according to claim 21 , wherein said assay is selected from the group consisting of an ELISA and a Western blot. 23. A method for screening for the presence of a PXE mutation, the method comprising the steps of: interrogating an MRP6 protein in a patient for the presence of a PXE protein change; and, identifying a positive screen as the presence of said change. 24. The method according to claim 23, wherein said interrogating step measures the size of the MRP6 protein. 25. The method according to claim 1 , wherein said assay detects the presence of a mutation in an exon of the MRP6 gene. 26. The method according to claim 25, wherein said exon is selected from exons 1-31 of the MRP6 gene 27. The method according to claim 1, wherein said interrogating step is a hybridization assay. 1 28. An oligonucleotide for use in an assay to detect a PXE associated mutation, wherein said oligonucleotide hybridizes to a PXE mutation in an MRP6 nucleic acid under stringent conditions. 1 29. An oligonucleotide of claim 32, wherein said oligonucleotide is selected from the group consisting of the oligonucleotides of SEQ ID NO: 10 through SEQ ID NO: 25. l 30. A method for identifying a patient at risk of having children with PXE, the 3 method comprising the step of interrogating a patient sample for the presence of a PXE 4 mutant MRP6 allele. l 2 31. A method for identifying a patient at risk of developing a PXE associated 3 symptom, the method comprising the step of interrogating a patient sample for the 4 presence of a PXE mutant MRP6 allele. 1 2 32. A method for diagnosing PXE in a patient, the method comprising the step 3 of interrogating a patient sample for the presence of two PXE mutant MRP6 alleles. 1 2 33. The method of claim 32, wherein said patient is a homozygous PXE 3 patient.

Inventors: BOYD CHARLES D, CSISZAR KATALIN, LESAUX OLIVIER, TERRY SHARON, URBAN ZSOLT, BOYD, CHARLES, D., CSISZAR, KATALIN, LESAUX, OLIVIER, TERRY, SHARON, URBAN, ZSOLT
Applicants: PXE INTERNATIONAL INC, UNIV HAWAII, PXE INTERNATIONAL, INC., UNIVERSITY OF HAWAII
Application year: 2001
Main classification: C12Q1
IPC codes: C12Q1