DIAGNOSTIC METHOD AND KIT FOR DETERMINING Rh BLOOD GROUP GENOTYPE

Abstract: The invention provides a diagnostic method of determining Rh genotypes by the identification of the molecular basis of Rh polymorphisms. Specifically, the invention provides a method for directly determining Dd and associated CcEe genotypes with great accuracy, overcoming problems associated with traditional serologic typing methods and leading to a direct discrimination of D/D, D/d, and d/d genetic status. The diagnostic method allows genotyping of fetuses to assess the risk of hemolytic diseases caused by Rh alloimmunization and genetic counseling and/or testing of couples to predict the outcome of pregnancies in relation to Rh incompatibilities. The method of the invention preferably employs amplification of Rh nucleic acid sequences, and employs differential cleavage of RhD-, RhCc- and/or RhEe-specific nucleic acid sequences by a restriction enzyme. Furthermore, diagnostic kits for the determination of Rh genotypes are provided.

Description: DIAGNOSTIC METHOD AND KIT FOR DETERMINING Rh BLOOD GROUP GENOTYPE BACKGROUND OF THE INVENTION This invention relates to a method for determining Rh blood group genotypes. More particularly, the invention relates to a molecular genetic method for the direct determination of Rh zygosities related to D/D , D/d, and did status and the associated CcEe genotypes. The human red cell Rh (Rhesus) antigens form a complex blood group system of major importance in clinical medicine (Ref. 1). Of the Rh antigens, the D antigen is the most immunogenic and best known to stimulate alloimmunization. Individuals are phenotypically divided into RhD-positive and RhD-negative (also referred to as d) subgroups, based on the presence or absence of the D antigen on their erythrocyte membranes. Rh D/d incompatibilities and maternal alloimmunization to the D antigen are the most common and severe cause of hemolytic disease of the newborn (Ref. 2). Severely affected (anemic) fetuses may even need blood transfusion in utero , a dangerous invasive treatment. Other common and clinically important Rh antigens include the C, c, E and e (together also designated "non-D") antigens. Although they are not considered in the classification of Rh positive or negative status, the C, c, E, and e antigens have become relatively more common causes of fetal/neonatal hemolytic disease encountered in the United States. This is because immigrant families with prior Rh sensitization make up a sizable percentage of current Rh cases. All of these Rh antigens or specificities reside in a family of nonglycosylated but palmitylated transmembrane proteins referred to as RhD, C/c and E/e polypeptides (Refs. 3-5). Two related structural genes, D and CcEe , encode the red cell membrane proteins carrying these antigens. While C/c and E/e are considered antigenic products of allelic genes, the D antigen has no serologically detectable antithetical "d" antigen (Ref. 6). In most cases, the D-negative phenotype has been associated with a complete absence of the D gene. In rare cases, the d phenotype may result from a partially deleted D gene or a silent D gene defective in expression. Genetically, both the D and non-D antigens are expressed in autosomal codominant fashion and they are inherited en bloc through eight different Rh haplotypes, i.e., DCe , DcE, Dee , DCE, dCe , dcE, dee and dCE (Ref. 6). The frequency of these organizational frameworks is not random, but varies considerably among different ethnic populations. (See Table 1, described in detail elsewhere herein). The structural basis for Rh haplotypes was not immediately apparent after Fisher's classic synthesis of genetic evidence gathered from population studies (Ref. 6). Over the years, several conceptual models, variously evoking the composition of the RH locus to include from one to three structural genes, have been put forward to account for the inheritance of Rh antigens (Refs. 6-7). The isolation of two different forms of Rh cDNA (Refs. 8-12) provides information necessary for the analysis of these models at the molecular level. Southern blotting and transcript sequencing analyses have indicated the presence of two genes, D and CcEe (non-D), in Rh-positive subjects, but only one gene { CcEe ) in Rh-negative subjects (Refs. 12-14). This finding constitutes strong evidence supporting the two-gene model of the Rh locus (Ref. 7). Nevertheless, no correlation of the genomic maps with the Rh haplotype frameworks has been possible because of an inability to separate the extremely homologous D and non-D genes (Refs. 12, 13). Historically, the determination of Rh genotypes has been made indirectly, being confined principally to methods of detecting and identifying Rh antigen expression on erythrocyte membranes. These serological methods employ antibodies or other compounds which identify and interact with the Rh proteins or portions thereof. For example, various antibodies specific for particular Rh antigens have been identified. Agglutination methods for detecting Rh protein and other blood group antigens are described in U.S. Patent Nos. 5,324,479, 5,302,512, 5,213,963, 4,560,647, 4,403,042, 4,358,436, and 4,148,607. Numerous publications describing such conventional agglutination-type tests are available, such as Walker RH et al., eds.. Technical Manual, 11th ed., American Association of Blood Banks, Arlington, Virginia (1993), the entire disclosure of which is incorporated herein by reference (Ref. 15). These conventional serological methods are limited in that they provide information about expressed protein profiles, but are not capable of defining Rh antigen molecular structure or the molecular genetic makeup of the subject. To infer the inheritance of Rh genes, these methods generally require the acquisition of blood samples from not only the subjects being examined but also from other family members. Moreover, in the case of phenotyping a fetus, obtaining a blood sample involves a potentially dangerous procedure in which the fetus is susceptible to hemorrhage and possibly death. In any case, all of these conventional methods are deficient as diagnostic tools, because none of these methods provides any means by which Rh genotype might be determined. It has been suggested to use restriction fragment length polymorphisms (RFLPs) in conjunction with serology testing to improve the understanding of blood group genes and their inheritance (Ref. 16). This proposal is severely limited in that it requires studying DNA markers which are not blood group genes themselves, but which are closely linked with particular blood group genes. Generally such yet-to-be-identified flanking DNA markers do not show a complete linkage disequilibrium with the structural genes. Thus, they cannot be used to determine Rh genotypes with precision, but might be developed as molecular tools for linkage analyses. The molecular bases of certain polymorphisms which occur in the Rh locus have been elucidated (See, e.g., Refs. 10, 17-18). Various techniques have been used in these investigations, including the polymerase chain reaction (PCR), molecular cloning procedures, RFLPs, etc. None of the reported studies, however, teaches or suggests any method by which both Dd and CcEe genotypes can be simultaneously determined, and Rh zygosities related to DID, Did, and did status directly determined. Indeed, many of these investigations have recognized that the Rh system exhibits extreme polymorphism at the antigenic level which, in turn, is associated with great heterogeneity at the mRNA level (Ref. 18). As a result, it is increasingly apparent that current understanding of the Rh system is fundamentally incomplete. Previous knowledge has simply not been sufficient to enable an accurate and reliable method of determining Rh genotypes. For example, previous studies have led to a method for the determination of RhD genotype by means of DNA amplification (Refs. 19-20). This method employs the PCR technique and a set of unique primers to amplify exon 10 of the RhD gene. The primer set, however, limits this method to a determination of DID or did genotype, and is not capable of distinguishing the Did and the associated CcEe genotypes. Moreover, this method has been found to be unreliable in certain situations, because only a small portion of the Rh genes, namely exon 10, is assessed by genomic amplification. Specifically, the use of the method in certain subjects results in anomalous DNA amplification fragments, causing either false negative or false positive results (Refs. 21-23). This method is, therefore, neither complete with respect to the major Rh antigens, nor is it sufficiently accurate to permit its routine use for patient counseling and management. As a result, there exists a need for a method of safely, conveniently, and accurately detecting Rh genotypes, including both D and non-D genotypes. It would also be especially desirable to provide a method for determining Rh genotypes in a fetus without a requirement for obtaining blood samples. A test based on DNA samples taken from amniotic cells would allow the clinician to avoid the risk of harm to the fetus and to more accurately predict the potential of anti-Rh-associated hemolytic disease of the newborn. SUMMARY OF THE INVENTION The present invention provides a diagnostic method for the direct determination of Rh blood group genotypes. More particularly, the present invention provides a method for the differential determination of RhDd and associated CcEe genotypes. In one embodiment, the invention provides a diagnostic method for the determination of Rh genotype of a subject. The method involves selectively cleaving genomic DNA to provide Rh gene fragments in an amount sufficient to provide information characterizing an Rh genotype. Preferably, the diagnostic method of the invention includes selectively cleaving genomic DNA to provide fragments of Rh genes which characterize the eight common Rh haplotypes. More preferably, the method enables the provision of fragments of Rh genes in an amount sufficient to distinguish RhD from RhC/c and E/e genes . Still more preferably, the method of the invention provides fragments which specifically characterize or identify one or more aspects of Rh genotypes associated with the RhD and/or RhCc.Ee genes. The method of the invention also permits identification of an Rh genotype, or provides information which can be employed with other tests or assays to more fully characterize the genetic blueprint underlying the expression of Rh antigens. Preferably, the method of the invention involves the analysis of Rh gene(s) by means of a restriction enzyme which selectively or differentially cleaves a DNA sample (preferably including genomic DNA) to produce fragments unique to the D gene and/or the CcEe genes. The restriction enzyme is preferably selected to digest a DNA sample to provide Rh gene fragments which characterize or differentiate the Did genotype and/or the Clc and Ele genotypes. More preferably, the restriction enzyme digestion cleaves genomic or other DNA differentially based on polymorphisms in the intronic regions of the Rh structural genes. Most preferably, the restriction enzyme will produce Rh DNA fragments which specifically characterize or differentiate individual Rh genes and allow precise determination of their combinations, i.e., the Rh genotypes. A highly preferred restriction enzyme for use according to the invention is Sphl . Informative combinations of Sphl with other restriction enzymes may also be employed. in another embodiment, the diagnostic method of the invention includes amplifying DNA from a subject to produce multiple copies of the DNA of interest, in this case Rh DNA. Preferably, such amplification is accomplished prior to the selective cleavage of the Rh DNA. Amplification is achieved by methods known in the art such as polymerase chain reaction or ligase chain reaction, or a combination thereof. Typically, DNA amplification is performed by means of an oligonucleotide primer or set of primers which amplifies Rh DNA. The amplified Rh DNA is then treated, i.e., digested, with a restriction enzyme, to produce Rh DNA fragments in an amount sufficient to provide information concerning Rh genotype of the subject. In the method of the invention, it is possible to use one or more oligonucleotide primers which amplify Rh DNA. It is preferred, however, to employ primer(s) which selectively hybridize with and amplify Rh DNA. More preferably, it is desired that the primer or primers specifically hybridize with and amplify Rh DNA without substantial contribution from non-Rh DNA. The method of the invention can employ a set of primers which amplify different segments of Rh DNA. For example, the method of the invention can employ primers which differentially amplify RhD DNA and/or RhC Ee DNA. For example, the primer set may include a combination of primers, including at least one which either selectively or specifically amplifies RhD DNA, and at least one which either selectively or specifically amplifies RhCcEe DNA. In this way, information can be derived permitting the differential determination of the Did and the associated CcEe genotypes. The method of the invention typically further includes examining the Rh DNA fragments to derive information permitting discrimination of one or more aspects of Rh genotype. Such discrimination involves identifying particular fragments which are commonly, preferably consistently, representative of part or all of specific alleles or haplotypes. Numerous methods are known for deriving information sufficient to accomplish such differential discrimination. For example, the method of the invention may include separating Rh DNA fragments, such as on the basis of molecular weight, electrophoretic mobility, chromatographic mobility, etc. Because restriction enzymes cleave DNA into discrete fragments of particular sizes, a separation of the fragments will reveal a fragment pattern. Once separated, some or all of the fragments may be specifically or non-specifically identified according to known methods. For example, the Rh DNA fragments may be stained non-specifically, relying on the fragment pattern itself for a derivation of genotype information. Alternatively, one or more of the fragments may be identified by means of a hybridization probe composition including one or more oligonucleotide probes which hybridize with one or more fragments. Such hybridization may be induced to uniquely or selectively identify particular fragments. Identification of probe hybridization may be accomplished by means of probes to which have been attached detectable moieties. The method permits derivation of information characterizing, preferably specifically characterizing or differentiating, various aspects of Rh genotypes. The method advantageously enables the artisan to obtain information characterizing RhD genotype, RhCc genotype, RliEe genotype, RhCc.Ee genotype, RhDCαEe genotype, and combinations thereof. The method of the invention can include determining Rh genotype as described above, together with the determination of genotype of the subject with respect to other, non-Rh genes. Alternatively, the method may include a determination of specific phenotype information about the subject. In an especially preferred embodiment, for example, Rh phenotype is advantageously determined to provide confirmatory or complementary evidence of Rh genotype determination. Such phenotype determination may be performed by serological testing. In another embodiment, a diagnostic kit is provided for performing the Rh genotype determination method of the invention. In this embodiment, the diagnostic kit of the invention enables determining Rh blood group genotype(s) by detecting target nucleic acid sequences, such as sequences specific to the RhD and/or RhCαEe genes. The kit includes a restriction enzyme which can differentially cleave a DNA sample to generate Rh DNA fragments providing information concerning Rh genotype. The kit may include an amplification primer or primer set, i.e., oligonucleotides that may be used to bind to or cause elongation through sequences specific to D and/or CcEe DNA. Typically, the primer is provided in a first reagent container, and the restriction enzyme is provided in a second reagent container. The diagnostic kit may further include other container means, such as a microtiter plate having a plurality of wells. In such cases, the kit can include microtiter plates to which have been attached oligonucleotide capture probes having nucleic acid sequences which are substantially complementary to the D and/or CcEe target sequences. In addition, the kit may include additional components such as reagents, diluents, buffers, as well as control samples. The kits may also include detection means, such as nucleic acid stains or oligonucleotide probes. Kits may be constructed and sold in any configuration which enables performance of the specific embodiments of the method of the invention. Generally, for example, each of these is provided in a separate container. The kits may further include means for acquiring an erythroid tissue sample from a subject, means for extracting DNA from tissue samples, means for detecting Rh DNA, etc., without limitation. In a further embodiment, the invention provides a diagnostic composition for determining Rh genotype of a subject. In this embodiment, the composition includes DNA fragments characterizing all or part of the Rh genotypes of the subject. The characteristic DNA fragments result from selective cleavage of a DNA sample derived from a subject. Accordingly, as a result of the invention, there is now provided a safe and convenient diagnostic method for differentially determining Rh genotype of patients. in particular, RhDd and RhCαEe genotypes can be determined selectively or in conjunction with each other and with other genotype information. There is now provided a method for determining Rh genotypes in a fetus in utero which can be performed even without requiring the taking of a blood sample. A test based on determining Rh genotype(s), in particular Dd and/or CcEe genotypes, from DNA obtained from a tissue sample such as amniotic fluid or chorionic villus, now allows the clinician to reduce the degree of risk to the fetus being tested. Moreover, the new diagnostic method permits the accurate prediction of the potential of anti-Rh-associated hemolytic disease of the newborn. These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention have been chosen for purposes of illustration and description, but are not intended in any way to restrict the scope of the present invention. The preferred embodiments of certain aspects of the invention are shown in the accompanying drawings, wherein: Figure 1 is a diagram showing the relationships among a set of Rh gene probes used according to the method of the invention, which together span the 5' to 3' untranslated region of Rh cDNA. Figure 2 shows a Southern blot of genomic DNAs from eight unrelated individuals prepared according to the method of the invention. Figure 3 is a comprehensive Sphl restriction map for the Rh polypeptide genes, including schematic representation of the distribution of exons in the D and non-D genes as well as the relationship between the Sphl RFLPs and Rh haplotypes. Figure 4 shows a Southern blot of genomic DNAs from four families carrying different Rh haplotypes prepared according to the method of the invention, along with pedigrees of each family. Figure 5 is a diagram illustrating the transmission of Rh haplotypes from parents to children in a single family. Figure 6 shows a Southern blot of genomic DNAs from 15 unrelated random individuals prepared according to the method of the invention. Figure 7 shows a Southern blot of genomic DNAs from 15 unrelated individuals exhibiting variant Rh phenotypes prepared according to the method of the invention. Figure 8 shows a Southern blot of genomic DNAs from six nonhuman primates prepared according to the method of the invention. Figure 9 shows a Southern blot of genomic DNAs from amniotic fluid of fetuses at risk of alloimmunization prepared according to the method of the invention. DETAILED DESCRIPTION OF THE INVENTION As has been described in greater detail above, the determination Rh genotypes, especially with regard to Dd and CcEe genotypes, is of great clinical importance. However, until now the complexity of Rh genetics has been a significant burden which has never effectively been overcome. Despite such problems, it has now unexpectedly proven possible to characterize Rh genotypes with excellent precision and accuracy. The invention provides a method for the differential determination of Rh genotypes based on a set of RFLPs which are located in, and are highly descriptive of, both of the major Rh structural genes, i.e., D and CcEe . Exon-specific cDNA and oligonucleotide probes have been used to map the major Rh genes and to determine the position of endonuclease cleavage sites. A panel of restriction endonucleases has been screened in an effort to identify a set of restriction fragments which would provide the information necessary for determining Rh genotypes. It was unexpectedly found that it was possible to use a single endonuclease to generate a set of restriction fragments which permits direct and specific characterization of RhD and RhCc.Ee genotypes. The segregation pattern of restriction fragment length polymorphisms in informative families was traced and their distribution among unrelated persons was examined. It was found that particular RFLP markers are co-transmitted with the various Rh haplotypes. This remarkable finding was unexpected, in view of previous efforts which have yet to completely define the molecular structure of the major Rh genes. The co-transmission of the new RFLP markers with the various Rh haplotypes not only allows correlation of genotypes with the corresponding blood group phenotypes, but also provides a direct approach to the determination of Rh zygosity related to the Rh-positive or negative status (DD , Dd or dd ) , as well as to Rh CcEe status (CCEE, CCEe, CcEE, CcEe, CCee , Ccee , ccEe, ccEE or cσee). These features clearly render the inventive method useful in determining Rh genotypes, and are expected to play a growing role in genetic counseling and prenatal assessment of Rh alloimmunization. For purposes of more clearly and accurately describing the invention herein, certain terminological conventions have been adopted in the following discussion. These conventions are intended to provide a practical means for enhancing description of the invention, but are not intended to be limiting, and the skilled artisan will appreciate that other and additional interpretations may be implied. In this description, for example, the term "D gene" is intended to refer to genes which encode Rh D proteins. The term "D protein" is intended to refer to protein which carries the D specificity or antigenic determinant. D/d genotype is said to be a product of the presence or absence (or silence) of a D gene in the genome, while the presence or absence of D protein expression determines D/d phenotype. It is noted that the term "d" or "d" is not generally understood to imply the existence of an allele, but refers to an absence of detectable D protein or D gene. Indeed, as has been noted elsewhere herein, no serologically detectable d antigen has been found, and no gene encoding a d protein has been identified. By contrast, the term "CcEe gene" is intended to refer to genes which encode CcEe proteins. In turn, the term "CcEe protein" is intended to refer to protein which carries C/c and/or E/e antigenic specificities. Accordingly, the CcEe genotype of an individual is believed to be determined by the specific pair of CcEe alleles in his/her genome, while the individual's CcEe phenotype is believed to be determined by the expression of CcEe proteins and the combinations of expressed C/c and E/e determinants. It should also be noted that the designations "D" and "CcEe" may be employed to identify proteins, peptides, antigens, nucleic acids, oligonucleotides, etc., which are traceable to or associated with the D and CcEe genes and proteins. In general, however, it is understood that Rh-related or Rh-based chromosomal DNA (including exons, introns, and parts thereof), nucleic acid templates, nucleic acid transcripts, as well as cDNA, may be designated by the use of italics, e.g., "D" and "CcEe" . This usage is consistent with usage in the art. "Polymorphic" or "DNA polymorphism" refers to the condition in which two or more variations of a specific DNA sequence coexist in the same interbreeding population. Polymorphic differences are caused by differences in amino acid sequence which may be due to events such as point mutations, gene rearrangements or alternative splicing. While a difference may exist between the nucleotide sequences of two versions of the same gene, such differences do not give rise to a phenotypic polymorphism unless a change in amino acid sequence is caused. Even so, if differences in nucleotide sequence can be detected even without a corresponding amino acid change, then genetic polymorphism still exists. In particular, if a change in nucleotide sequence causes the creation or deletion of a restriction enzyme cleavage site, then analytical use can be made of restriction fragment length polymorphisms. Typically, it is difficult to identify such RFLPs even in a single gene. Thus, it is quite unexpected that RFLPs have now been found which permit the determination of genotype with respect to two genes based on the use of a single restriction enzyme. The method of the invention takes advantage of restriction fragment polymorphisms to permit the determination of RhD and RhCcEe genotypes, either separately or in conjunction with one another. As a result of the present invention, it is now possible to design probes and/or primers comprising nucleic acid oligomers or oligonucleotides which can be employed to detect polymorphisms in the Rh genes. The probes or primers suitable for this purpose preferentially hybridize with or are specific for particular restriction fragments or regions incorporating restriction sites of Rh genes. Preferably, the primer permits the amplification of DNA which includes a restriction polymorphism site. The amplified DNA can be differentially digested depending on the presence or absence of the restriction cleavage site, thereby enabling the determination of Rh genotype. Typically, but not necessarily, such a restriction polymorphism involves a site or region in which a point change in nucleotide sequence causes a change in the Rh gene product. Nonetheless, even if the restriction site change does not cause a phenotypic polymorphism, such a change is consistently associated with another mutation which does cause such a phenotypic polymorphism. Accordingly, the probes or primers include those which hybridize with higher frequency alleles as well as those which hybridize with lower frequency alleles. Accordingly, a probe of the invention may be said to bind to or hybridize with Rh DNA if it recognizes a defined region of Rh DNA, such as a region which encodes a D or CcEe polymorphism locus. A primer of the invention may be said to amplify Rh DNA if it binds to or causes elongation through a defined region of Rh DNA. Thus, a primer amplifies D DNA or amplifies CcEe DNA if it preferentially amplifies all or part of the D locus or all or part of the CcEe locus, respectively. Moreover, the invention contemplates the use of "Rh primers" or primers which non-selectively amplify both D DNA and CcEe DNA. Such primers are possible due to the large measure of homology present in the D and CcEe genes. On the other hand, a primer is specific for a particular gene or allele, such as D ("D primer"), if it specifically amplifies only DNA associated with the D gene, while a primer specific for CcEe ( "CcEe primer") specifically amplifies only CcEe DNA. It is generally preferred that probes or primers be capable of differentiating two alleles which differ by no more than a single nucleotide modification. It is known in the art that it is possible to control the specificity of hybridization by selectively constructing probes and primers and by adjusting hybridization or other experimental conditions. There may be situations, however, in which it would be desirable to employ probes which hybridize somewhat less selectively. Accordingly, it is within a particular context that a probe or primer according to the invention is said to be "substantially complementary" to a specific Rh sequence. That is, if the situation demands high precision, a probe or primer is substantially complementary to a target sequence- if there exists only a very small probability that the oligomer will bind with a sequence other than the specific target sequence. In other situations, a probe or primer may be deemed to be substantially complementary to a target sequence if the probability of a unique hybridization is substantially less than 1.0. Thus, a probe or primer of the invention may be "substantially complementary" to a target region if it is either exactly complementary, or even only partially complementary, to the target region, depending on the required stringency of the method parameters. Thus, the invention employs probes and primers which hybridize with fragments of either or both of the Rh genes . Such probes are useful when both of the Rh genes or transcripts thereof are desired to be characterized. Moreover, the invention provides probes and primers which include part or all of one or more introns in the Rh locus in chromosomal DNA. Such probes are useful when the Rh genes themselves are desired to be characterized and additional information is desired to be obtained about the structure of the gene in a particular individual. This is true of a highly preferred embodiment of the invention. For example, the use of the Sphl enzyme requires the amplification of Rh intronic segments since no Sphl cleavage sites are known in the Rh cDNAs. The probes or primers of the invention may also include, as part of their nucleotide sequences, regions which are not substantially complementary to any region adjacent to or near a target sequence. Thus, in any probe or primer of the invention, at least a part of the probe or primer is substantially complementary to a target segment. The nucleic acid oligomers of the invention may be used as probes or primers to identify the presence of target nucleic acid fragments, such as D and/or CcEe fragments, through binding to or hybridizing with such target sequences. Accordingly, the nucleic acid oligomers may be detectably labeled by being linked to a detectable marker moiety such as a fluorescent label, an electron dense substance, a reporter moiety, a specific or nonspecific binding moiety, or other detectable moiety such as is known in the art. Optionally, the oligomers of the invention may further include a reactive moiety permitting cross-linking with a target nucleic acid sequence. Furthermore, the oligomers of the invention may be linked to a substrate, for example, to a gel or resin to immobilize the oligomers. The molecular genetic method of the invention generally involves obtaining DNA from a biological sample, preferably a sample of erythroid tissue, of a mammalian subject, typically a human patient. Commonly, the method is used to analyze a blood sample, but other types of samples which contain erythroid tissue are useful. In a particularly preferred embodiment, the invention provides a method for determining Rh blood group genotype in a fetus. In this embodiment, the preferred tissue sample includes a sample of amniotic fluid or chorionic villus. It is understood that such samples can also be obtained from sample libraries or tissue archives such as blood banks, or from forensic evidence, etc. Accordingly, the method may be used on a unique or irreplaceable sample or may be used to screen large numbers of samples. The molecular genetic method described herein can easily be applied to DNA samples obtained from amniotic fetal cells and/or chorionic villus cells and is useful in determining the Dd and/or CcEe genotype of the fetus. This test can assist in the identification of those pregnancies at risk for hemolytic disease of the newborn resulting from Rh alloimmunization. Once the tissue sample has been obtained, it is desired that the nucleic acid be extracted from the cells in the sample. This can be achieved by any method known in the art. See, for example, the method of Sambrook et al. (Ref. 24). The purified nucleic acid fraction is then employed directly for the Rh genotype determination, or may be stored until needed. In the method of the invention, the nucleic acid sample is selectively amplified by means known in the art to generate an amplified nucleic acid product, in which a disproportionate fraction is Rh-related nucleic acid. "Amplification" according to the method of the invention refers to any molecular biology technique for detection of trace levels of a specific nucleic acid sequence by exponentially amplifying a template nucleic acid sequence. In particular, suitable amplification techniques include such techniques as the PCR and the ligase chain reaction (LCR) . The PCR is known to be a highly sensitive technique, and is in wide use. The LCR is known to be highly specific, and is capable of detecting point mutations. In certain circumstances it is desirable to couple the two techniques to improve precision of detection. The PCR is a well-known technique and is described, for example, in Innis et al. (Ref. 25). The LCR is more recently developed and is described in Landegren et al. (Ref. 25) and Barany et al. (Ref. 27). An LCR kit is available from Stratagene. Other amplification techniques may be expected to be employed according to the method of the invention. Regardless of the precise amplification technique employed for the method of the invention, one or more primers is employed. "Primer" as used herein refers to an oligonucleotide, whether natural, synthetic or semisynthetic, which is capable of initiating DNA synthesis for exponential amplification of a target or template nucleic acid sequence by an amplification technique. For PCR the primer acts as a point of initiation of DNA synthesis under conditions in which synthesis of a primer extension product complementary to a nucleic acid strand is induced, i.e., in the presence of four different- nucleoside triphosphates and an agent for polymerization (i.e., DNA polymerase or reverse transcriptase) in an appropriate buffer and at a suitable temperature. For LCR the primer is capable of annealing to a target nucleic acid and of being ligated to an adjacent primer to serve as a template for amplification. Also for purposes of the LCR, the primer generally includes paired sets of adjacent, complementary oligonucleotides which can anneal to single stranded target molecules and ligate together. For LCR ampli ication of DNA, the primers include two sets of adjacent, complementary oligonucleotides. A primer is preferably an oligodeoxyribonucleotide and is single stranded for maximum efficiency in amplification, but may also be double stranded. If double stranded, the primer is generally first treated to separate its strands before being used to prepare extension products. Typically, LCR primers are double stranded. The exact length of a primer will depend on many factors, but typically ranges from 15 to 25 nucleotides. Short primer molecules generally require cooler temperatures to form sufficiently stable hybrid complexes with the template. A primer need not reflect the exact sequence of the template but must be sufficiently complementary to hybridize with a template. "Primer," as used herein, may refer to more than one primer, particularly in the case where there is some ambiguity in the information regarding one or both ends of the target region to be amplified. For instance, if a nucleic acid sequence is inferred from a protein sequence, a "primer" is actually a collection of primer oligonucleotides containing sequences representing all possible codon variations based on the degeneracy of the genetic code. One of the primer oligonucleotides in this collection will be homologous with the end of the target sequence. Likewise, if a "conserved" region shows significant levels of polymorphism in a population, mixtures of primers can be prepared that will amplify adjacent sequences. A primer or probe can be labeled, if desired, by incorporating a label detectable by spectroscopic, photochemical, biochemical, immunochemical, or chemical means. For example, useful labels include 3Ϊ P, fluorescent dyes, electron-dense reagents, reporter molecules such as enzymes (as commonly used in ELISAs), biotin, or haptens or proteins for which antisera or monoclonal antibodies are available. Other labels may be used by the skilled artisan according to the invention. A label can also be used to "capture" the primer, so as to facilitate the immobilization of either the primer or amplified DNA on a solid support. "Oligonucleotide" or "nucleic acid oligomer" refers to primers, probes, nucleic acid fragments to be detected, nucleic acid controls, and unlabeling blocking oligomers and is defined as a molecule comprised of two or more deoxyribonucleotides or ribonucleotides. The nucleic acid oligomers of the invention may be single-stranded oligomers of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The exact size of an oligonucleotide will depend upon many factors and the ultimate function or use of the oligonucleotide. The oligodeoxyribonucleotides and oligoribonucleotides may be obtained or derived by known methods from natural sources. Alternatively, the oligonucleotides may be produced synthetically according to methods known in the art. Such methods include, for example, cloning and restriction of appropriate sequences and direct chemical synthesis by a method such as the phosphotriester method of Narang et al. (Ref. 28); the phosphodiester method of Brown et al. (Ref. 29); the diethylphosphoramidite method of Beaucage et al. (Ref. 30); and the solid support method in U.S. Patent No. 4,458,066. It is preferred that the oligomers be at least substantially purified to avoid introduction of artifacts into the genotype determination method. The method of the invention also involves the digestion of Rh nucleic acid by means of a restriction endonuclease, that is, an enzyme, usually of bacterial origin, which cuts or cleaves double stranded DNA only at defined sites. Because these enzymes exhibit such highly precise cleavage of DNA, a given restriction enzyme will produce a characteristic pattern of fragments any time it is employed to cleave the same region of DNA. In the event that a change in DNA causes a cleavage site or "restriction site" to be deleted or a new site to be created, the total amount of digestion and the distribution of "restriction fragments" may shift. Thus, the term "restriction fragment length polymorphism" or "RFLP" refers to the differences in DNA nucleotide sequences that are randomly distributed throughout the entire genome and that produce different restriction fragment patterns for different individuals upon digestion of genomic DNA with restriction endonucleases. The restriction enzyme digestion process may be accomplished according to known methods. Preferably, the digestion process employs a single restriction enzyme. Combinations of such enzymes may be employed if they are effective in generating informative and/or practical restriction fragments. A particularly preferred restriction enzyme is Sphl , which, as will be seen below, has shown itself to be well suited for performing the method of the invention. The restriction enzyme digestion is analyzed according to the invention to derive or produce information concerning Rh genotypes. The type and extent of the desired information, will affect the choice of restriction enzyme. For example, if RhD and RhCαEe genotype information is desired to be obtained, the Sphl enzyme has been found to be unusually useful. Other enzymes might be chosen when such complete information is not required. The skilled artisan will appreciate that the method of the invention can be modified in other ways to suit the practical requirements of particular assays of Rh genotypes. The effectiveness of a restriction enzyme or combination of such enzymes can be determined by employing means for the separation of the resultant restriction fragments. Various such means are known to provide useful results . A preferred method in the art is fragments separation by means of polyacrylamide gel electrophoresis (PAGE). This method was employed in the Examples described hereinbelow. The restriction fragments can be identified by means of specific probes. Suitable assay techniques for purposes of the present invention are known in the art. For example, such detection can be accomplished using a dot blot format in which unlabeled amplified sample is bound to a membrane and the membrane is incubated with labeled probe(s) under suitable hybridization conditions. Unhybridized probe is removed by washing and the membrane is then examined for the presence of labeled probe. Another suitable approach is accomplished in tandem with PAGE in which the separated fragments are stained non-specifically with a nucleic acid-staining substance such as ethidium bromide. Such methods are described, for example, in Sambrook et al., Molecular Cloning: A Laboratory Manual, 2d. ed., Cold Spring Harbor Laboratory, NY (1989) (Ref. 24). Other suitable methods will suggest themselves to the skilled artisan. The method of the invention may further include steps related to the determination of Rh genotypes through inferences based on Rh serological phenotypes. For example, to avoid misleading or false results, it may be desirable to supplement or confirm the information gained from genomic testing with conventional serological testing. In addition, certain restriction enzymes may be used according to the invention which produce somewhat ambiguous results inasmuch as the resultant set of restriction fragments might fail to uniquely describe every genotype. In such cases, it may be possible to generate unique information for each and every genotype by employing serological testing in conjunction with genomic analysis using a restriction enzyme. Such serological testing steps may be performed either prior to, in parallel with, or after the prescribed genomic testing. Moreover, the planning of testing procedures may be based on preliminary partial results which rule out or imply the possibility of, for example, certain rare genotypes. It may also be desirable to perform genotype testing for other markers or blood groups together with the Rh genotyping method of the invention. For example, the genotyping method described in U.S. application Serial No. 08/337,268 filed November 10, 1994, or application Serial No. 08/484,570 filed June 7, 1995, relating to the determination of Kell genotypes, may be performed in conjunction with the method of the invention to provide counsel as to a broader spectrum of possible difficulties in pregnancy. The Rh genotype determination methods of the invention are of particular utility in the determination of the genotype of fetuses to avoid hemolytic disease of the newborn. The methods of the invention are also useful, however, in a variety of other situations in which molecular genetic information about a person is desired. For example, the methods of the invention are useful in ' situations in which it is desired to obtain information concerning the identity of an individual from forensic samples. Alternatively, the methods of the invention are useful for obtaining genetic information enabling the determination of paternity in those situations in which paternity is in doubt or dispute. In addition, the methods of the invention are useful for the determining the Rh genotype of a recipient of a blood transfusion, as well as for the screening of stored blood for Rh genotypes, to avoid transfusion incompatibility. Other applications of the methods of the invention will suggest themselves to the skilled artisan. The following examples are intended to assist in a further understanding of the invention. The particular materials and conditions employed are intended to be further illustrative of the invention and are not limiting upon the reasonable scope thereof. EXAMPLES For each of the Examples described herein, the molecular biology techniques employed were performed generally in accordance with methods accepted in the art. See, for example, Sambrook et al. (Ref. 24), and Innis et al. (Ref. 25 ) , the disclosures of which are incorporated herein by reference. Human Subjects As described in greater detail below, 86 human subjects were studied. Of these, 71 were normal, whereas 15 carried low incidence Rh antigenic variants or Rh deficiency syndromes. The group of normal subjects included 38 Caucasians, 16 Blacks, 12 Asians (Chinese, Japanese and Indian), three native Americans and two Black-Asians. Subjects were either randomly chosen or selected on the basis of known Rh phenotypes (such as the selection of Rh-negative individuals). In addition, the subjects were either unrelated individuals or related members of families. The Rh antigenic specificities of blood samples (phenotypes) were determined by standard hemagglutination tests. Of normal subjects, 48 were determined to be Rh-positive, and 23 were Rh-negative. The D, C, c, E, and e blood group phenotypes are summarized as follows : Rh-positive: 17 D+C+c+E-e+, 10 D+C-c+E-e+, 8 D+C+c-E-e+, 7 D+C+c+E+e+, 3 D+C+c-E+e, 2 D+C-c+E+e-, and 1 D+C-c+E+e+; and Rh-negative: 11 D-C-c+E-e+, 5 D-C-c+E+e+, 2 D-C-c+E+e-, 2 D-C+c+E-e+, 2 D-C+c-E-e+, and 1 D-C+c+E+e+. With regard to the Rh genetic variants, all 15 subjects were unrelated. Five of these showed absence of non-D antigens (3 D—, 1 DC"-, and 1 Dc-), three exhibited Rh deficiency (2 Rhnull and 1 Rhmod), and the remainder carried low incidence antigens. Preparation of Genomic DNAs In the following Examples, high molecular weight genomic DNAs were isolated from human and nonhuman primate peripheral leukocytes by either a total cell lysis procedure (Ref. 31) or a sequential cell lysis method (Ref. 32). Preparation of Genomic Probes Nucleic acid probes used in the following analyses of Rh genes included cDNA, genomic DNA, and oligonucleotide probes. Rh cDNA probes encompassing the 5' region (exons 1 to 4) and the 3' region (exons 4 to 9) were synthesized by reverse transcriptase-polymerase chain reaction (RT-PCR) . The RT-PCR was performed using primers selected from published sequences (Refs. 8-9), according to a previously described method (Ref. 33). Exon-specific probes were generated by PCR amplification, using a thermostable DNA polymerase (Ref. 34). The genomic probe containing the 5' region and a portion of exon 1 (designated Ex 1) was amplified from a purified Rh phage clone (Ref. 33). Probes directed to other exons, including exon 2 to exon 10, were selectively amplified from Rh cDNAs. All DNA probes were purified by native polyacrylamide gel electrophoresis (PAGE) according to conventional methods. Figure 1 shows the sizes and nucleotide positions of the exonic probes employed for these studies . The sequence of full-length Rh cDNA spanning the 5' to 3' untranslated (UT) region is divided into 10 exons, as drawn. The locations of initiation and termination codons are illustrated. The cDNA probes specific for individual exons are shown by vertical lines, and their nucleotide positions relative to the first residue of codon ATG are indicated. All of the probes used in these examples cross-hybridize with both D and non-D Rh polypeptide genes. Oligonucleotide probes (25 to 27 mers) were synthesized on a 380A automated DNA synthesizer and purified by 15% PAGE containing 7.0 mol/L urea. These oligomers were mainly used to confirm the results of exon mapping (see below). E∑on Mapping by Southern Blot and Direct Gel Hybridizations Genomic DNAs were digested to completion with restriction endonucleases, electrophoresed on 0.8% agarose gel and subjected to either Southern blot (Ref. 35) or direct gel hybridization (Ref. 36) using labeled DNA probes. The DNA probes were labeled with α- 3 P-dCTP by the random primer extension method (Ref. 37). Synthetic oligomer probes were labeled with γ- 32 P-ATP by forward reaction of the T 4 polynucleotide kinase (Ref. 24) Washing conditions were adjusted as necessary to assure specific hybridizations with the probes as described previously (Refs. 31, 36). EXAMPLE 1 Identification of RFLPs Associated with the D and Non-D Genes Using the Rh cDNAs as probes, genomic DNAs from unrelated individuals were analyzed in an effort to identify RFLPs that might be linked with the Rh genes. A panel of restriction endonucleases (BamHI, .EcoRI, Hindlll, Pstl, and Sphl ) was tested according to conventional methods (Refs. 31, 33). The enzyme Sphl was found to be uniquely informative, in that this was the only restriction endonuclease in the panel which permitted D and non-D gene fragments to be readily distinguished. Figure 2 shows the pattern of Sphl RFLPs in eight unrelated individuals bearing the common haplotypes (4 Rh-positive and 4 Rh-negative). Genomic DNAs were digested with Sphl , blotted onto filters, and hybridized with probes as indicated. Lanes 1 to 7 represent individuals who were informative homozygotes for given haplotypes. Lane 8 shows a dCE/dce heterozygote, as dCE/dCE homozygotes are very rare among human populations. It was also noted that the dCe/dCe homozygote carried a partial deletion in the D gene. The DNA size marker is indicated at the left margin of Figure 2, and the origin of restriction fragments from D and non-D genes is at the right margin. Note that there are three separate exon 10-containing fragments; the band that is missing in Rh-negative (i.e., Rh cf) is assigned as exon 10 of the D gene (D 10 ). Based on extensive analysis, four Sphl RFLP frameworks in the genomic region encompassing exon 4 through exon 7 were recognized which correlated with the occurrence of distinct Rh haplotypes (Table 1). Of the five bands detected by Ex4-7, the 8.9 and 1.2 kb bands were found only in individuals carrying the Rh-positive haplotypes. It was inferred that these fragments were most likely derived from the D gene. The 7.3, 5.4 and 1.9 kb fragments were observed in both Rh-positive and Rh-negative individuals, indicating that these fragments originate from the non-D gene(s) . Compared with the D-specific bands in Rh-positive haplotypes, the allelic fragments from ce, cE, Ce and CE varied in size. Moreover, the frequency of their occurrence was apparently related to the linkage status of D or d (Table 1). Specifically, the ce gene was found to produce either one or two bands when it occurred as Dee, but only one band in the dee haplotype. The cΞ and CE genes were associated with the presence of the 7.3 kb fragment regardless of their linkage with D or d. The fourth alternative, i.e., the Ce gene, always gave rise to two bands when cis to D. However, when d occurred cis to Ce , the Ce gene resulted in either one or two bands (Table 1). All other probes, including Ex 1 and Ex 10 (Figure 2, panels 1 and 5), were less discriminatory due to the co-migration of non-D gene fragments. EXAMPLE 2 A Comprehensive Sphl Map for the D Gene and Non-D Alleles A sequence search showed that there is no SpΛI cutting site in Rh cDNA. That is, Sphl cuts Rh-related DNA only in intronic regions, if at all. Accordingly, it was inferred that each of the restriction fragments generated using Sphl should contain at least one exon. Taking advantage of this feature, the exon content of various Sphl bands was determined, and a comprehensive restriction map for the major Rh genes was derived, as shown in Figure 3A. The locations and approximate distances (in kb) of SphΙ(S) cleavage sites that flank each exonic fragment are indicated. The assignment of exons was based on the parallel analysis of Rh-positive and negative individuals and on the fact that the D gene was completely absent from the dee, dcE and dCE haplotypes (Figure 2). These results, obtained by use of exon-specific probes (Figure 1), were further confirmed by direct gel mapping with synthetic oligonucleotides (data not shown). Figure 3A shows that the structural organization of D is quite similar to that of the non-D gene (Ref. 38). Nevertheless, the majority of D exons (8 out of 10 except exons 3 and 8) were distinguishable from those of the non-D gene because of the asymmetric distribution of Sphl cleavage sites. Although some fragments originating from different regions of the two genes co-migrated, they could be readily differentiated on the basis of their exonic content. It was apparent for all Rh genes that while their introns 4 and 6 lacked the Sphl recognition sequence, introns 1, 2, 3, 7, 8 and 9 each retained at least one cutting site, in the case of intron 5, it harbored a unique Sphl site in the D gene, but was polymorphic for the non-D alleles (Figure 3A) . This distribution of Sphl cleavage sites permits a full explanation of the banding patterns observed among the eight Rh haplotypes (Figure 2), and thus establishes the relationship between the Sphl RFLP frameworks and Rh haplotypes (Figure 3B). EXAMPLE 3 Transmission of Sphl RFLPs with Rh Haplotypes in Families To explore whether the Sphl RFLPs could be used to determine the Rh genotypes, their transmission was traced in several families. This tracing enabled the establishment of a correlation of the genotypes with the phenotypes (Table 2). Figure 4 shows a Southern blot of genomic DNAs of four families (A-D). Each individual in each family is assigned a number. Certain family members were not examined (designated NT in the family trees) due to unavailability of blood samples. The genomic DNAs were digested with Sphl , and hybridized with Ex4-7. Gel lanes are matched with family pedigree, and the deduced genotypes are shown. The size of each Sphl band is noted, and the D-specific bands are marked by two arrows at the right margin of panel 4. In family A, all members were phenotypically D+C+c-E-e+. Such uniformity of phenotype among related individuals could result from two genotypes: DCe/DCe or DCe/dCe . Using the method of the invention, genomic blots indicated that family members Al, A3 and A4 were DCe/DCe homozygotes, whereas A2 and A5 were DCe/dCe heterozygotes (Figure 4, panel 1). This conclusion was supported by a decreased intensity of D-specific bands in the latter two subjects. In family B, the inference of two genotypes (Dee I ee and dee/ dee ) was more straightforward. As shown above, Dee and dee haplotypes are specified by different RFLP frameworks (Table 1). Accordingly, individuals with the Dce/dce genotype should display a Sphl banding pattern different from those carrying the DcelDce or dee I dee genotype (Figure 2). This was indeed the case for family B in which members Bl and B4 could be unequivocally defined as Dce/dce heterozygotes (Figure 4, panel 2 and Table 2). These results illustrated a direct determination of Rh zygosity related to the Rh-positive or negative status (D/D, Did or did) . The genotyping of family C was complicated by the expression of multiple Rh phenotypes which could be specified by three or six haplotype combinations (Table 2). Nevertheless, the finding of no decrease of intensity in the D gene bands (Figure 4, panel 3) excluded the involvement of all d-containing haplotypes. Therefore, Cl should bear the DCe/Dce genotype, and the others should bear either the DCe/DcE or the DCE/Dce genotype. If C2 carried DCe/DcE (which would be the most probable case), both C3 and C4 should have inherited DCe from Cl and DcE from C2 (Table 2). Figure 5 shows the inheritance of Rh haplotypes in family D as deduced from the Sphl banding patterns shown in Figure 4, panel 4. Compared with Dl, D2 showed reduced intensity in D gene bands. Therefore, Dl was a DID homozygote carrying the DCelDce genotype, whereas D2 was a Did heterozygote carrying the Dceldce genotype. As the Dee haplotype in Dl and D2 was specified by different frameworks, the random assortment of parent Rh haplotypes resulted in three different genotypes in the children that matched with the Sphl banding patterns (Figure 4, panel 4) as well as the typed blood group specificities (Table 2). EXAMPLE 4 Distribution of Sphl RFLPs Among Unrelated Individuals To assess whether Rh genotypes could be determined when family data are not available, we examined a number of unrelated individuals whose Rh phenotypes were serologically characterized. Figure 6 shows a gel autoradiogram which is representative of this analysis, including seven Rh-negative and eight Rh-positive individuals. Genomic DNAs were digested with Sphl , and probed with Ex4-7. Lanes 1-6 and 10 are the Rh-negative subjects, with lanes 1, 6, 10 being Caucasians, 2 being Indian, 3,5 being blacks, and 4 being Japanese. The other lanes (7-9, 11-15) are Rh-positive Caucasians, all of whom are D/d heterozygotes, except for lane 7 who is DCelDCe homozygote. Of the total of 23 Rh-negative subjects examined, the 7.3 kb band was detectable in all except the two dCeldCe homozygotes, and its occurrence appeared to be independent of the ethnic origin of individuals (Figure 6). Furthermore, the Did heterozygosity as to the DCeldce and DcEldce genotypes could be directly determined. This ability to directly differentiate among these genotypes was particularly significant because DCe, dee and DcE are the three most frequent haplotypes encountered in Caucasians (Ref. 6). EXAMPLE 5 Association and Dissociation of Sphl RFLPs with Rh Genetic Variants As has been mentioned above, the Rh system is highly polymorphic, containing many low incidence antigens and variant phenotypes (Refs. 6, 39). To delineate the relationship of these low incidence antigens with Sphl RFLPs, we examined 15 unrelated individuals, each carrying a distinct Rh genetic variant. This analysis revealed a complex pattern of Sphl fragments indicating that the Rh phenotypic diversification has taken place on different genetic backgrounds. Figure 7 shows Southern blots of genomic DNAs of 15 unrelated subjects exhibiting variant Rh phenotypes, which were digested with Sphl and probed with Ex4-7. Lanes 1 and 2 are DCCee and dccee controls. Note that lanes 1-5 and 6-17 were run on separate agarose gels. The mobility of bands, therefore, differs between the two panels, but the correspondence of bands is indicated between lanes 5 and 6. The sizes of the various bands is noted at the right margin of the figure. Rhnull (A) and (R) denote Rhnull amorph and regulator types, respectively. Of the five variants, the non-D antigen(s) (lanes 3-7), D— was either similar to DC"-, either missing the non-D fragments or associated with a silent Ce gene. The Dc- subject apparently carried a Dceldce genotype, yet the e antigen was not expressed. For the Rh deficiency syndromes (lanes 8-10), the amorph and regulator types of Rhnull were associated with the Rh-negative and DCe haplotypes, respectively, whereas Rhmod was accompanied by an increase of the D gene. Similar to Rhmod, Rh40 produced all five Sphl bands on the genomic blot, but it was associated with an increase in Ce rather than in D (lane 11). In the case of Rh33 and Rh-32,-46, both displayed the 7.3 kb band only, suggesting their occurrence on the Rh-negative background (lanes 12 and 13). On the other hand, Rh32,-46 occurred on the DCe background with decreased intensity in the 8.9 kb band (lane 14). The r α r variant lacked the D-gene fragments but contained both ce and Ce fragments (lane 15), suggesting that the marker antigen G arose from the Ce background. As to the variants of E/e series (lanes 16 and 17), the E" carrier was apparently a Dc∑fldce heterozygote, most likely bearing a subtle change in the cE gene. By contrast, hr s -hr B + was probably a variant of e associated with the Dee haplotype. Taken together, the results suggested that diverse molecular mechanisms have been operative in shaping the allelic diversity of Rh antigen genes (Table 3). EXAMPLE 6 Comparison of Sphl Banding Patterns between Humans and Nonhuman Primates To understand the origin of Sphl frameworks in evolutionary perspective, genomic DNAs from six species of nonhuman primates were analyzed, and their Sphl banding patterns were determined. As shown in Figure 8, Rh-like genes were identified in all nonhuman primates by cross-hybridization with the human Ex4-7 probe under very stringent washing conditions, suggesting a high sequence conservation throughout primate evolution. Nevertheless, each species displayed a distinct Sphl pattern, ranging from one band in baboon to five bands in gorilla. Among higher primates, the chimpanzee and gorilla appeared to be most similar to humans bearing framework I for the Dee or DCe haplotype (Table 1). This observation reconciled the expression of D- and C-analogous antigens on red cells from the two species (Ref. 40) and the presence of two or three Rh-like genes in their genomes (Ref. 41). The other four species each appeared to carry a single Rh-like gene, as they showed only one or two Sphl bands corresponding to the human 1.9, 5.4 and 8.9 kb counterparts. None of the six primate species examined, however, displayed the human 7.3 kb band that defines the Rh-negative haplotypes ( dee , dcE and dCE) . EXAMPLE 7 Prenatal assessment of fetuses at risk of alloimmunization To show the usefulness of Sphl frameworks in prenatal assessment of fetuses at risk of Rh alloimmunization, genomic DNAs isolated from leukocytes and amniocytes (cells in amniotic fluid of pregnant women) were analyzed in parallel, and the genetic makeup of Rh genes in the fetuses were determined. Figure 9 shows a genomic Southern blot prepared according to the method of the invention following techniques described herein. This analysis demonstrates that the DID, Did, or did status of the fetus can be unequivocally determined without prior determination on serologic typing. In this molecular test, two mothers were found to be homozygous for dce/dce, yet they each carried a D-positive fetus heterozygous for Dce/dce (Figure 9, middle and right panels). This finding indicated a possibility for the occurrence of fetomaternal Rh alloimmunization. In the third case, both the mother and the fetus were DCe/DCe homozygotes, thus excluding the occurrence of hemolytic disease in the newborn (Figure 9, left panel). Without wishing to be bound by theory, we now describe the mapping and inheritance of the human Rh polypeptide genes that determine the expression of D, C, c, E, and e blood group antigens. The location of Rh at the short arm of chromosome 1 has been previously established by cytogenetic analysis (Ref. 42) and recently confirmed by in situ hybridization with the Rh cDNA probe (Refs. 43-44). The construction of the comprehensive restriction map described here, however, took advantage of the features intrinsic to the Rh genes but previously not recognized: the absence of Sphl cleavage sites in their exons and the asymmetric distribution of those sites around their non-coding regions including the intronic sequences. This direct genomic mapping reveals a similar, if not identical, structural organization between the D and non-D genes and complements, respectively, the partial EcoRI map of D and the BamΑl/XhoI map of CE derived from analysis of Rh genomic clones (Refs. 12, 38). It became clear in the course of developing the invention that a set of Sphl RFLPs is tightly linked with the Rh structural genes. Using exon-specific probes, we have localized the Sphl polymorphic sites to the genomic region encompassing exon 4 through exon 7. Of the five Sphl restriction fragments covering these four exons, the 1.2 and 8.9 kb bands were assigned to the D gene and others to the non-D genes including ce, cE, Ce and CE alleles. Analysis of informative individuals homozygous for the common Rh haplotypes revealed that these Sphl fragments occur in several different banding patterns that constitute four major organizational frameworks. Family studies at the DNA level demonstrated further that such Rh frameworks are inherited en bloc in Mendelian codominant fashion, each representing a distinct combination of D (or d) with one of the alleles of the non-D gene. These results provided a molecular definition of the Rh haplotypes and established a genetic correlation of the Rh genotypes with the blood group phenotypes. The Rh system contains nearly 50 different antigens and many variant phenotypes and, thus, is the most complex of all known red cell blood group polymorphisms (Refs. 6, 39). The present studies on the linkage of Sphl RFLPs with naturally occurring Rh variants attest to the underlying genetic heterogeneity, suggesting alterations in gene structure and/or regulatory mechanisms. As shown herein, some variants (i.e., D—, DC"-, and r a r) are associated with new Sphl banding patterns and others occur on existing haplotype backgrounds. Although our analysis has until now been limited to just 15 variant individuals, the results indicate strongly that diversity of the Rh locus is primarily based on allelisms at the population level. This is similar to the case of other, well-characterized, human gene families, such as the major histocompatibility complex genes (Ref. 45) and the MNSs blood group system (Ref. 46). The clinical importance of the Rh system stems from the extreme immunogenicity of its antigens and the high frequency of Rh-negative phenotypes in the population (Ref. 1). The potent antigenicity of D, C, c, E, and e epitopes remains unexplained although the hydropathy model predicts that the D and non-D Rh polypeptides are similarly organized into multiple membrane-spanning domains (Refs. 8-10). Regarding the Rh-negative haplotypes, dee occurs with a highest frequency and is most prevalent among Caucasians and Blacks (see Table 1) . The extensive mapping of Rh-negative genomic DNAs shown here pinpoints d as the consequence of either a deletion or an alteration of the D gene, consistent with the results of the previous studies (Refs. 13, 47). In addition, the present studies show that the d haplotypes are specified by two different frameworks. Specifically, while dCe is associated with either the 7.3 kb or the 1.9 and 5.4 kb bands, dee, dcE and dCE are always associated with the 7.3 kb fragment. Whether this difference in linkage disequilibrium reflects two separate events in the evolution of Rh-negative haplotypes and whether the high incidence and ethnic confinement of dee arose from a founder effect are some questions that remain to be addressed. It is the method of the invention which now permits such detailed analysis. Fetomaternal alloimmunization resulting from incompatible Rh blood types is still the principal cause for hemolytic disease of the newborn. Our observation that each of the four Sphl frameworks is associated with a distinct Rh haplotype is immediately useful as a molecular procedure for genotyping fetuses at risk. As already demonstrated here, the Sphl banding patterns in conjunction with serologic typing allow actual determination of all of the common Rh genotypes. Although changes of the Sphl banding patterns have been observed (see above), they represent very rare genetic variations. Therefore, these changes should not be a major limiting factor in Rh genotype determination. Currently, the fetal D typing method depends entirely on the detection of D-specific genomic sequences by PCR amplification (Refs. 19-20), which can result in misdiagnosis for a significant number of cases (Refs. 21-23). Compared with these methods, the application of Sphl frameworks to Rh genotyping will be advantageous in that: (1) the distinction of Rh zygosity related to the D/D, D/d and di status can be made directly and (2) the genotypes composed of non-D alleles can be deduced simultaneously. This new direct genotyping procedure will greatly facilitate genetic counseling and prenatal assessment of Rh alloimmunization. The invention further provides diagnostic and experimental kits which include means to enable the performance of the method of the invention in a precise, accurate, and reproducible manner. The kits of the invention may include a primer or set of primers which can be used to amplify Rh DNA. The kits may include a restriction enzyme or set of restriction enzymes suitable for generating information about Rh genotype. The kits may further include positive and/or negative control reagents as well as other reagents for adapting the use of the method of the invention to particular experimental and/or diagnostic techniques as desired. The kits include one or more containers for containing the primer and/or restriction enzyme, as well as for containing any addition reagents in the kit. Thus, while there have been described what are presently believed to be the preferred embodiments of the present invention, those skilled in the art will realize that other and further embodiments can be made without departing from the spirit of the invention, and it is intended to include all such further modifications and changes as come within the true scope of the claims set forth herein. REFERENCES 1. Mo-llison PL, Engelfriet CP, and Contreras M, Blood Transfusion in Clinical Medicine, 9th ed., Blackwell, Oxford (1992). 2. Luban NLC, "The new and the old — Molecular diagnostics and hemolytic disease of the newborn", New England J Med 329(9):658-60 (1993). 3. Agre P, and Cartron J-P, "Molecular biology of the Rh antigens", Blood 78:551-63 (1991). 4. Cartron J-P, and Agre P, "Rh blood group antigens: protein and gene structure", Semin Hematol 30:193-208 (1993). 5. Anstee DJ, and Tanner MJ, "Biochemical aspects of the blood group Rh (rhesus) antigens", Baillieres Clin Haematol 6:402-22 (1993). 6. Race RR, and Sanger R, Blood Groups in Man, 6th ed., Blackwell, Oxford (1975) pp. 178-260. 7. Tippett PA, "Speculative model for the Rh blood groups", Ann Hum Genet 50:241-47 (1986). 8. Chέrif-Zahar B, Bloy C, Le van Kim C, Blanchard D, Bailly P, Hermand P, Salmon C, et al., "Molecular cloning and protein structure of a human blood group Rh polypeptide", Proc Natl Acad Sci USA 87:6243-47 (1990). 9. Avent ND, Ridgwell K, Tanner MJA, and Anstee DJ, "cDNA cloning of a 30 kDa erythrocyte membrane protein associated with Rh (Rhesus)-blood-group-antigen expression", Biochem J 271:821-25 (1990). 10. Le Van Kim C, Mouro I, Chέrif-Zahar B, Raynal V, Cherrier C, Cartron J-P, and Colin Y, "Molecular cloning and primary structure of the human blood group RhD polypeptide", Proc Natl Acad Sci USA 89:10925-29 (1992). 11. Kajii E, Umenishi F, Iwamoto S, and Ikemoto S, "Isolation of a new cDNA clone encoding an Rh polypeptide associated with the Rh blood group system", Hum Genet 91:157-62 (1993) . 12. Arce MA, Thompson ES, Wagner S, Coyne KE, Ferdman BA, and Lublin DM, "Molecular cloning of RhD cDNA derived from a gene present in RhD-positive, but not RhD-negative individuals", Blood 82:651-55 (1993). 13. Colin Y, Chέrif-Zahar B, Le Van Kim C, Raynal V, Van Huffel V, and Cartron J-P, "Genetic basis of the RhD-positive and RhD-negative blood group polymorphism as determined by Southern analysis", Blood 78:2747-52 (1991). 14. Mouro I, Colin Y, Chέrif-Zahar B, Cartron J-P, and Le Van Kim C, "Molecular genetic basis of the human Rhesus blood group system", Nature Genet 5:62-65 (1993). 15. Walker RH et al., eds.. Technical Manual, 11th ed., American Association of Blood Banks, Arlington, Virginia (1993) 16. Zelinski T, "The use of DNA restriction fragment length polymorphisms in conjunction with blood group serology", Transfusion. 31:762-70 (1991). 17. Hopkinson DA, "The long [E/e} and the short [C/c] of the rhesus polymorphism", Nature Genetics 5:6-7 (1993). 18. Le Van Kim C, Chέrif-Zahar B, Raynal V, Mouro I, Lopez M, Cartron J-P, and Colin Y, "Multiple Rh messenger RNA isoforms are produced by alternative splicingj Blood 80:1074-78 (1992). 19. Bennett PR, Le Van Kim C, Colin Y, Warwick RM, Cherif-Zahar B, Fisk NM, and Cartron J-P, "Prenatal determination of fetal RhD type of DNA amplification", New England J Med 329:607-10 (1993). 20. Fisk NM, Bennett P, Warwick RM, Letsky EA, Welch R, Vaughn Jl, and Moore G, "Clinical utility of fetal RhD typing in alloimmunized pregnancies by means of polymerase chain reaction on amniocytes or chorionic villi". Am J Obstet Gvnecol 171:50-54 (1994). 21. Carritt B, Steers FJ, and Avent ND, "Prenatal determination of fetal RhD type", Lancet 344:205-06 (1994) (Letter) . 22. Simsek S, Bleeker PM, and von dem Borne AE, "Prenatal determination of fetal RhD type", New England J Med 339:795-96 (1994). 23. Bennett P, Warwick R, and Cartron J-P, New England J Med 330(11) :795-96 (1994) (Response to letter by Simsek et al. ) . 24. Sambrook J, Fritsch EF, and Maniatis T, Molecular Cloning: A Laboratory Manual, 2d. ed., Cold Spring Harbor Laboratory, NY (1989). 25. Innis MA, Gelfand DH, Sninsky JJ, and White TJ, PCR Protocols: A Guide to Methods and Applications, Academic Press, Inc., San Diego (1990). 26. Landegren U, et al., Science 241:1077 (1988). 27. Barany F, PCR Methods and Applications 1:5 (1991). 28. Narang et al.. Methods Enzymol 68:90 (1979). 29. Brown et al., Methods Enzv ol 68:109 (1979). 30. Beaucage et al., Tetrahedron Lett 22:1859 (1981). 31. Huang C-H, Guizzo ML, McCreary J, Leigh EM, and Blumenfeld 00, "Typing of MNSs blood group specific sequences in the human genome and characterization of a restriction fragment tightly linked to S-s- alleles", Blood 77:381-86 (1991). 32. Goossens M, and Kan YW, "DNA analysis in the diagnosis of hemoglobin disorders", Methods Enzymol 76:805-17 (1981). 33. Huang C-H, Reid ME, and Chen Y, "Identification of a partial internal deletion in the RH locus causing the erythrocyte D— phenotype", Blood (In Press). 34. Saiki RK, Gelfand DH, Stoffel S, Scharf SJ, Higuchi R, Horns GT, Mullis KB, et al., "Primer-directed enzymatic amplification of DNA with a thermostable DNA polymerase", Science 239:487-91 (1988). 35. Southern EM, "Gel electrophoresis of restriction fragments", Methods Enzvmol 68:152-76 (1979). 36. Huang C-H, and Blumenfeld 00, "Characterization of a genomic hybrid specifying the human erythrocyte antigen Dantu: Dantu gene is duplicated and linked to a delta glycophorin gene deletion", Proc Natl Acad Sci USA 85:9640-44 (1988). 37. Feinberg AP, and Vogelstein B, "A technique for radiolabeling DNA restriction endonuclease fragments to high specific activity" Anal Biochem 137:266-67 (1984). 38. Chέrif-Zahar B, Le Van Kim C, Rouillac C, Raynal V, Cartron J-P, and Colin Y, "Organization of the gene (RHCE) encoding the human blood group RhCcEe antigens and characterization of the promoter region", Genomics 19:68-74 (1994). 39. Issitt PD, "The Rh blood group system (1988: Eight new antigens in nine years and some observations on the biochemistry and genetics of the system". Transfusion Med Rev 3:1-12 (1989). 40. Socha WW, and Ruffie J, Blood Groups of Primates: Theory. Practice and Evolutionary Meaning, pp. 75-90, Alan Liss, New York (1983) . 41. Salvignol I, Blancher A, Calvas P, Socha WW, Colin Y, Cartron J-P, and Ruffie J, "Relationship between chimpanzee Rh-like genes and the R-C-E-F blood group system", J Med Primatol 22:19-28 (1993). 42. Marsh WL, Chaganti RS, Gardner FH, Mayer K, Nowell PC, and German J, "Mapping human autosomes: evidence supporting assignment of rhesus to the short arm of chromosome No. 1", Science 183:966-68 (1974). 43. Chέrif-Zahar B, Mattei MG, Le Van Kim C, Bailly P, Cartron J-P, and Colin Y, "Localization of the human Rh blood group gene structure to chromosome region Ip34.3-lp36.1 by in situ hybridization", Hum Genet 86:398-400 (1991). 44. MacGeoch C, Mitchell CJ, Carritt B, Avent ND, Ridgwell K, Tanner MJ, and Spurr NK, "Assignment of the chromosomal locus of the human 30-kDal Rh (rhesus) blood group-antigen-related protein (Rh30A) to chromosome region lp36.13 p34" Cytogenet Cell Genet 59:261-63 (1992). 45. Rapes D, and Stro inger JL, "Human class II major histocompatibility complex genes and proteins', Ann Rev Biochem 57:991-1028 (1988). 46. Huang C-H, and Blumenfeld 00, "MNSs blood groups and major glycophorins: Molecular basis for allelic variation", in Cartron J-P, and Rouger P, (eds.), Blood Cell Biochemistry 6:153-88, Plenum Press, New York (1995). 47. Hyland CA, Wolter LC, and Saul A, "Three unrelated Rh D gene polymorphisms identified among blood donors with Rhesus CCee (r'r') phenotypes". Blood 84:321-24 (1994).

Claims: WHAT IS CLAIMED IS; 1.. A diagnostic method for determining Rh genotype of a subject, comprising: selectively cleaving a DNA sample derived from a subject to provide Rh gene fragments in an amount sufficient to characterize an Rh genotype. 2. The diagnostic method of Claim 1, comprising selectively cleaving said DNA sample to provide Rh gene fragments in an amount sufficient to characterize RhDd genotype. 3. The diagnostic method of Claim 1, comprising selectively cleaving said DNA sample to provide Rh gene fragments in an amount sufficient to characterize RhCc genotype. 4. The diagnostic method of Claim 1, comprising selectively cleaving said DNA sample to provide Rh gene fragments in an amount sufficient to characterize RhEe genotype. 5. The diagnostic method of Claim 1 , comprising selectively cleaving said DNA sample to provide Rh gene fragments in an amount sufficient to characterize RhCαEe genotype. 6. The diagnostic method of Claim 1, comprising selectively cleaving said DNA sample to provide Rh gene fragments in an amount sufficient to characterize RhDd and RhCcEe genotype. 7. The diagnostic method of Claim 1, wherein said selectively cleaving step includes digesting said DNA sample with a restriction enzyme which cleaves Rh genes differentially based on Rh polymorphism. 8. The diagnostic method of Claim 7, wherein said selectively cleaving step includes digesting said DNA sample with a restriction enzyme which cleaves Rh genes differentially based on polymorphism in one or more Rh intronic regions. 9. The diagnostic method of Claim 8, wherein said selectively cleaving step includes digesting said DNA sample with a restriction enzyme which comprises Sphl . 10. The diagnostic method of Claim 1, further comprising selectively amplifying said DNA sample prior to said selectively cleaving step. 11. The diagnostic method of Claim 10, wherein said selectively amplifying step includes amplifying said DNA sample by polymerase chain reaction, ligase chain reaction, or a combination thereof. 12. The diagnostic method of Claim 10, wherein said selectively amplifying step comprises amplifying said DNA sample by at least one primer which amplifies RhD DNA. 13. The diagnostic method of Claim 10, wherein said selectively amplifying step comprises amplifying said DNA sample by at least one primer which amplifies RhCcEe DNA. 14. The diagnostic method of Claim 10, wherein said selectively amplifying step comprises amplifying said DNA sample by at least one primer which amplifies RhD DNA and at least one primer which amplifies RhCcEe DNA. 15. The diagnostic method of Claim 10, wherein said selectively amplifying step comprises amplifying said DNA sample by a primer which amplifies RhD DNA and RhCcEe DNA. 16. The diagnostic method of Claim 1, wherein said DNA sample comprises genomic DNA. 17. The diagnostic method of Claim 1, further comprising deriving information characterizing Rh genotype from said Rh DNA fragments. 18. The diagnostic method of Claim 17, wherein said deriving step includes deriving information characterizing Rh genotype, wherein said Rh genotype is selected from the group consisting of RhDd genotype, RhCc genotype, RtiEe genotype, RhCαEe genotype, RhDCc.Ee genotype, and combinations thereof. 19. The diagnostic method of Claim 17, wherein said deriving step further comprises: separating said Rh DNA fragments to obtain a Rh fragment pattern, wherein said Rh fragment pattern provides information specifically characterizing the Rh genotype of the subject. 20. The diagnostic method of Claim 19, wherein said deriving step further comprises detecting some or all of the Rh DNA fragments in said Rh fragment pattern. 21. The diagnostic method of Claim 20, wherein said detecting step includes marking said Rh DNA fragments with a non-specific probe composition. 22. The diagnostic method of Claim 20, wherein said detecting step includes marking one or more of said Rh DNA fragments with a hybridization probe composition. 23. The diagnostic method of Claim 22, wherein said marking step includes marking one or more of said Rh DNA fragments with a hybridization probe composition comprising a hybridization probe specific for an RhD DNA fragment. 24. The diagnostic method of Claim 22, wherein said marking step includes marking one or more of said Rh DNA fragments with a hybridization probe composition comprising a hybridization probe specific for an RhCcEe DNA fragment. 25. The diagnostic method of Claim l, further comprising determining a non-Rh genotype of said subject. 26. The diagnostic method of Claim 1, further comprising determining an Rh phenotype of said subject. 27. The diagnostic method of Claim 26, wherein said determining step comprises determining an Rh phenotype of said subject by serological testing. 28. The diagnostic method of Claim 1, further comprising obtaining a DNA sample from a subject. 29. The diagnostic method of Claim 28, wherein said obtaining step comprises obtaining a DNA sample from a biological sample containing erythroid tissue of the subject. 30. The diagnostic method of Claim 29, wherein said biological sample comprises amniotic fluid or chorionic villus and wherein said subject is a fetus in utero. 31. The diagnostic method of Claim 29, wherein said biological sample comprises a blood sample. 32. A diagnostic kit for the determination of Rh genotype, co prising: (a) a first reagent container including a primer which amplifies Rh DNA; and (b) a second reagent container including a restriction enzyme which cleaves Rh DNA to provide Rh DNA fragments characterizing an Rh genotype. 33. The diagnostic kit of Claim 32, wherein said primer comprises a primer which amplifies RhD DNA and RhCcEe DNA. 34. The diagnostic kit of Claim 32, wherein said primer comprises a primer which amplifies Rh DNA selected from the group consisting of RhD DNA, RhCc DNA, RhEe DNA, RhCcEe DNA, 'RhDCcEe DNA, and combinations thereof. 35. The diagnostic kit of Claim 32, wherein said restriction enzyme cleaves Rh DNA differentially based on Rh polymorphism. 36. The diagnostic kit of Claim 35, wherein said restriction enzyme cleaves Rh DNA differentially based on polymorphism in one or more Rh gene intronic regions. 37. The diagnostic kit of Claim 36, wherein said restriction enzyme is Sphl . 38. The diagnostic kit of Claim 32, wherein said kit further comprises means for separating Rh DNA fragments. 39. The diagnostic kit of Claim 38, wherein said separating means comprises means for separating Rh DNA fragments on the basis of molecular weight, electrophoretic mobility, or chromatographic mobility. 40. The diagnostic kit of Claim 32, wherein said kit further comprises means for detecting Rh DNA. 41. The diagnostic kit of Claim 32, wherein said kit further comprises means for detecting Rh phenotype. 42. The diagnostic kit of Claim 41, wherein said Rh phenotype detecting means comprises means for determining Rh phenotype serologically. 43. The diagnostic kit of Claim 32, wherein said kit further comprises means for acquiring a biological sample from a subject. 44. A diagnostic composition for determining Rh genotype, comprising: DNA fragments characterizing Rh genotype resulting from selective cleavage by a restriction enzyme of a DNA sample derived from a subject.

Inventors: HUANG CHENG-HAN, HUANG, CHENG-HAN
Applicants: NEW YORK BLOOD CENTER INC, THE NEW YORK BLOOD CENTER, INC.
Application year: 1996
Main classification: C12Q1
IPC codes: C12N15, C12Q1