An unexpected new role of mutant Ras : perturbation of human embryonic development The Ras signaling pathway controls important cellular responses to growth factors , and somatic mutations in RAS genes and other components of the Ras pathway , such as PTPN11 -LRB- encoding the protein-tyrosine phosphatase SHP-2 -RRB- and BRAF , are found in human malignancies . Ras proteins are guanosine nucleotide-binding proteins that cycle between active guanosine triphosphate -LRB- GTP -RRB- - bound and inactive guanosine diphosphate -LRB- GDP -RRB- - bound conformations . Neoplasia-associated Ras mutations frequently affect amino acids G12 , G13 , or Q61 and decrease the intrinsic guanosine triphosphatase -LRB- GTPase -RRB- activity by ten - to twentyfold . The GTPase activity is crucial for Ras inactivation by hydrolysis and release of a phosphate group from Ras · GTP to produce Ras · GDP . We and others have recently discovered germline mutations in the KRAS gene in individuals diagnosed with Noonan and cardio -- facio -- cutaneous -LRB- CFC -RRB- syndrome , two clinically overlapping disorders characterized by short stature , distinct facial anomalies , heart defects , and other abnormalities . Noonan syndrome-associated mutations V14I and T58I K-Ras activate Ras but have milder biochemical effects than somatic mutations encountered in cancers , offering an explanation why these K-Ras lesions are tolerated during embryonic development . Together with recent findings of BRAF , MEK1 , and MEK2 mutations in CFC syndrome and HRAS mutations in Costello syndrome , another clinically related disorder , it has now become clear that Noonan-like features -LRB- short stature , relative macrocephaly , facial anomalies , learning difficulties -RRB- that are found in these three related disorders are a result of constitutive activation of the Ras -- Raf -- extracellular signal-regulated and mitogen-activated protein kinase pathway . RAS genes The three human RAS genes , KRAS -LRB- isoforms A and B -RRB- , NRAS , and HRAS -LRB- isoforms 1 and 2 -RRB- , encode small , highly conserved guanosine triphosphatases -LRB- GTPases -RRB- that relay growth signals to a number of effector proteins to control fundamental cellular pathways -LRB- reviewed in -LSB- 1 -RSB- -RRB- . Ras proteins act as molecular switches by cycling between an active guanosine triphosphate -LRB- GTP -RRB- - bound and an inactive guanosine diphosphate -LRB- GDP -RRB- - bound state . Stimulated growth factor receptors recruit a number of adaptor proteins that activate guanosine nucleotide exchange factors -LRB- GEFs -RRB- to remove guanine nucleotides from Ras . Ras is then activated by binding to GTP , which is present at a tenfold higher concentration than GDP . In the GTP-bound state , the two switch regions of Ras -LRB- switch I and II -RRB- change their conformation . This conformational switch allows Ras to bind and activate Ras effector proteins such as Raf-1 . The `` on '' position is turned `` off '' by an intrinsic GTPase activity , which hydrolyses and releases a phosphate group from Ras · GTP to produce Ras · GDP . The conformational transition of the switch I and II regions that is associated with this reaction disrupts the interaction between Ras and its effectors . The intrinsic GTPase activity of Ras is slow and accelerated about 105-fold by GTPase activating proteins -LRB- GAPs -RRB- , such as neurofibromin or p120 GAP . These GAPs mediate Ras · GTP hydrolysis by inserting an arginine residue -LRB- arginine finger -RRB- into the phosphate-binding pocket of Ras -LRB- reviewed in -LSB- 2 , 3 -RSB- ; Fig. 1 -RRB- . Fig. 1Ras cycles between an active GTP-bound and an inactive GDP-bound conformation . In the active state , the two switch regions , switch I and II , change their conformation allowing Ras to activate effector proteins . The intrinsic GTPase hydrolyzes a phosphate group to produce Ras · GDP . This reaction is accelerated by GTPase activating proteins -LRB- GAPs -RRB- . A similar version of this figure has been previously published -LSB- 61 -RSB- , republication with permission For several decades , it has been well recognized that RAS genes are frequently mutated in human cancers -LRB- reviewed in -LSB- 4 -RSB- -RRB- . These mutations predominantly lead to amino acid substitutions at residues G12 , G13 , or Q61 and lock Ras in the active GTP-bound state by diminishing the intrinsic Ras GTPase activity and/or by causing resistance to GAPs . These mutations were long believed to occur only as somatic events , and in a mouse model system , widespread expression of endogenous K-RasG12D leads to embryonic death -LSB- 5 -RSB- . The highest incidence of RAS mutations are found in adenocarcinomas of the pancreas -LRB- 90 % -RRB- , the colon -LRB- 50 % -RRB- , and the lung -LRB- 30 % -RRB- , in thyroid tumors -LRB- 50 % -RRB- , and in myeloid leukemia -LRB- 30 % -RRB- -LSB- 4 -RSB- . Noonan syndrome and related disorders Noonan syndrome -LRB- NS ; MIM 163950 -RRB- is an autosomal dominant disorder characterized by short stature , distinct facial anomalies -LRB- Fig. 2 -RRB- , a typical spectrum of congenital heart defects , including pulmonic stenosis , hypertrophic cardiomyopathy , and septal defects , and developmental delays -LRB- reviewed -LSB- 6 -RSB- -RRB- . The disorder occurs in approximately 1 out of 2,000 individuals and shares many features with the much less common disorders of Costello syndrome -LRB- CS -RRB- , cardio -- facio -- cutaneous -LRB- CFC -RRB- syndrome , and lentigines , electrocardiographic conduction defects , ocular hypertelorism , pulmonary stenosis , abnormalities of the genitals ; retarded growth resulting in short stature , and deafness -LRB- LEOPARD -RRB- syndrome -LRB- LS -RRB- -LSB- 6 -RSB- . Patients with these disorders have in common Noonan-like facial features , a similar spectrum of cardiac anomalies , delayed growth , and -- to a variable degree -- developmental retardation , which is stronger in patients with CS or CFC . In addition , each of these disorders is characterized by unique phenotypic patterns : -LRB- 1 -RRB- CS patients have nasal papillomata , loose skin , and a strong predisposition to tumors -LRB- mainly rhabdomyosarcoma -RRB- -LSB- 7 -RSB- ; -LRB- 2 -RRB- CFC patients have ectodermal abnormalities with sparse curly hair , sparse , or absent eyelashes -LSB- 8 -RSB- ; and -LRB- 3 -RRB- LS patients have multiple lentigines typically emerging during adolescence -LSB- 6 -RSB- . NS and LS may be familial with an autosomal dominant inheritance pattern -LRB- except for rare instances of NS where rare recessive inheritance has been suspected -LSB- 9 -RSB- -RRB- , but most cases of these disorders and virtually all of the more severe conditions , CFC and CS , occur sporadically , suggesting dominant new mutations . Until recently , the genetic basis for these disorders was unknown . It was not clear whether they represented genetically distinct entities or if they were different -LRB- allelic -RRB- variants of a common disorder . By linkage analysis in families with NS , the disorder was mapped to chromosome 12q24 -LSB- 10 -RSB- , and subsequently , it has been shown that approximately 50 % of patients with NS carry germline mutations in PTPN11 -LSB- 11 -RSB- . The PTPN11 gene encodes for tyrosine-protein phosphatase -LRB- SHP-2 -RRB- , a phosphatase that relays growth signals from activated growth factor receptors to other signaling molecules , including Ras -LRB- reviewed in -LSB- 12 -RSB- -RRB- . Most PTPN11 mutations are predicted to disrupt the auto-inhibition of the catalytic phosphatase domain -LRB- PTPase -RRB- by the N-terminal src-homology 2 -LRB- N-SH2 -RRB- domain thereby promoting the active conformation of the protein -LSB- 12 -RSB- . PTPN11 mutations were not found in patients with CS or CFC syndromes -LRB- reviewed in -LSB- 6 -RSB- -RRB- . However , specific mutations in the same gene were uncovered in patients with LS -LSB- 13 -RSB- . Surprisingly , in contrast to NS , LS mutants are catalytically defective and may act as dominant negative mutations -LSB- 14 , 15 -RSB- . Molecular modeling and biochemical studies suggest that LS mutations disrupt the SHP-2 catalytic domain to result in open , inactive forms of SHP-2 . Thus , the pathogenesis of LS and NS is distinct . It is unknown why the clinical phenotypes of LS and NS are similar although the underlying mutations have opposite biochemical effects . There are several possible explanations for this paradox . These are discussed in detail by Kontaridis et al. -LSB- 14 -RSB- : -LRB- 1 -RRB- The NS and LS phenotypes potentially result from differential effects of mutant SHP-2 on different receptor tyrosine kinase pathways at distinct developmental times . For example , recent work by others suggests that the main effect of NS mutants is to enhance epithelial -- mesenchymal transformation/mesenchymal cell proliferation by increasing ErbB2/3 -LRB- and/or ErbB3/4 -RRB- signaling . By contrast , LS mutants antagonize HB-EGF/ErbB1 signaling at later times -LSB- 14 -RSB- ; -LRB- 2 -RRB- Other phenotypes common to NS and LS such as facial abnormalities and short stature might involve defective migration and/or differentiation , which might result from increased or decreased signaling involving the same pathway ; -LRB- 3 -RRB- Yet , undetermined or poorly understood functions of SHP-2 might underlie pathogenesis of one or both of these disorders -LSB- 14 -RSB- . Fig. 2a Craniofacial phenotype of young children with NS , CFC , and CS aged between 10 and 18 months . Their genotypes are PTPN11 G503R , BRAF K499E , and HRAS G13C , respectively . Note the similarities of facial features , including hypertelorism , broad forehead , and low-set ears . Coarse facial features are particularly typical of CS . b The same disorders in older children and adolescents with the mutations PTPN11 N308D , MEK2 F57I , and HRAS G12S , respectively . Facial features become more distinct with age , although the similarities of the three syndromes are still evident . Courtesy of Prof. Rainer König , Frankfurt -LRB- images of CFC patients -RRB- and Prof. Kerstin Kutsche , Hamburg -LRB- images of CS patients -RRB- The Noonan gene PTPN11 also acts as oncogene One of the myeloid malignancies that has been found to be particularly related to perturbed Ras signaling is juvenile myelomonocytic leukemia -LRB- JMML -RRB- , a myeloproliferative disorder -LRB- MPD -RRB- of early childhood -LSB- 16 -RSB- , in which mutations occur in NRAS or KRAS -LRB- ∼ 25 % -RRB- or in NF1 -LRB- clinical diagnosis of neurofibromatosis type 1 in ∼ 11 % -RRB- -LSB- 17 -- 19 -RSB- . In a murine model , somatic activation of K-Ras in hematopoietic cells initiates a rapidly fatal MPD modeling JMML -LSB- 20 , 21 -RSB- . Despite a low annual incidence estimated at 1 -- 2 per million , JMML has attracted many clinical and basic researchers because of the severe and often lethal clinical course and the association with NS -LSB- 22 -RSB- and neurofibromatosis type 1 -LRB- NF1 ; MIM 162200 ; reviewed in -LSB- 19 -RSB- -RRB- . Shortly after the discovery of PTPN11 mutations in individuals with NS , specific germline PTPN11 alterations were identified in young children with NS who developed a JMML-like disorder within the first few weeks of birth -LRB- NS/JMML -RRB- -LSB- 23 , 24 -RSB- , and somatic PTPN11 mutations were subsequently uncovered in JMML cells from 35 % of children with non-syndromic JMML -LSB- 23 -- 25 -RSB- . Somatic PTPN11 mutations also occur in B-cell precursor acute lymphoblastic leukemia -LSB- 26 -RSB- and rarely other malignancies -LSB- 27 -RSB- . Somatic mutations observed in patients with JMML differ from those mutations found in patients with NS/JMML and from those detected in patients with NS alone . In sporadic JMML , the most common mutation predicts an E76K substitution . This mutation has never been found in patients with NS -LRB- N308D most common -RRB- or NS/JMML -LRB- T73I most common -RRB- -LSB- 6 , 25 -RSB- . In elegant functional experiments , several groups have shown that somatic PTPN11 mutations associated with sporadic JMML exhibit stronger biochemical and biological effects than germline PTPN11 mutations , leading to the concept that only milder SHP-2 activation may be tolerated during embryonic development -LSB- 28 -- 30 -RSB- . The situation is too complex however to be explained by a simple model of strong activating somatic vs less activating germline mutations alone . Enzymatic , structural , and mathematical modeling analyses show that these mutants can affect basal activation , SH2 domain-phosphopeptide affinity , and/or substrate specificity to varying degrees , and there is no absolute correlation between the mutants ' extents of basal activation and the diseases they induce -LSB- 31 -RSB- . A murine knock-in Ptpn11D61G / + model of NS has been constructed , revealing that endocardial cushions of these mice have increased activation of extracellular signal-regulated kinase -LSB- 32 -RSB- . This finding suggests that the phenotype exhibited by PTPN11 mutations is mediated through hyperactive Ras signaling . Neurofibromatosis type 1 JMML is also associated with NF1 -LRB- MIM 162200 -RRB- , an autosomal dominant disorder that occurs in 1 of 4,000 births and is characterized by pigmentary anomalies -LRB- multiple café-au-lait spots -RRB- and a predisposition to benign and malignant tumors of mainly neurogenic origin . The disorder is caused by mutations in the NF1 tumor suppressor gene , which encodes the RasGAP neurofibromin . The incidence of JMML is increased approximately 200-fold in children with NF1 -LRB- reviewed in -LSB- 33 -RSB- -RRB- . Loss of the normal NF1 allele -LRB- LOH , loss of heterozygosity -RRB- is common in JMML cells from children with NF1 -LSB- 34 , 35 -RSB- , and this results in severely deregulated Ras signaling and causes aberrant growth of hematopoietic progenitor colonies in vitro -LSB- 36 -RSB- . In addition , adoptive transfer of homozygous Nf1 mutant fetal liver cells or somatic inactivation of a conditional mutant Nf1 allele in hematopoietic cells induces a JMML-like MPD in mice -LSB- 37 , 38 -RSB- . Remarkably , many NF1 patients have mild features reminiscent of NS -LSB- 39 -RSB- , and some may even develop a mixed phenotype , which has prompted the definition of neurofibromatosis -- NS -LRB- NFNS ; MIM 601321 -RRB- as a separate entity . Most patients who are diagnosed with this condition harbor NF1 mutations without an obvious genotype -- phenotype correlation -LSB- 40 -RSB- . Additional genes mutated in Noonan syndrome To date , the identification of additional NS genes was hampered by the fact that the vast majority of PTPN11 mutation-negative NS-patients represent sporadic cases -LSB- 41 -RSB- , rendering genetic linkage studies impossible . However , recent findings by our group and others identified germline mutations in components of the Ras signaling pathway in individuals with NS or related syndromes -LSB- 42 -- 46 -RSB- . We systematically screened patients with NS and CFC for RAS mutations after having discovered a novel de novo germline KRAS mutation in a patient with NS/JMML -LSB- 42 -RSB- . The mutation c. 173C > T -LRB- p.T58I -RRB- detected in the index patient affects a highly conserved amino acid residue of K-Ras flanking the switch II region -LRB- amino acids 59 -- 67 -RRB- of the protein . De novo KRAS mutations were identified in 4 of 174 sporadic cases with NS previously excluded for PTPN11 mutations . All germline KRAS mutations were novel and not known to occur in human cancer . NS-associated KRAS alleles included recurrent mutations , V14I and D153V . The latter was confirmed in patients with more severe phenotypes -LSB- 44 , 46 -RSB- . Analogous to the dual role of PTPN11 as both oncogene and developmental gene , this discovery led to the hypothesis that activating KRAS mutations do not only act as oncogenes ; aberrant K-Ras can also cause developmental disorders when mutations -- probably conferring relatively mild effects -- emerge in the germline . To prove our hypothesis , we studied the functional properties of V14I and T58I K-Ras -LSB- 42 -RSB- and found both NS-associated mutants V14I and T58I to have intermediate biochemical -LRB- e.g. , ability to hydrolyze Ras · GTP -RRB- and biological properties -LRB- e.g. , growth behavior in response to growth factors after retroviral transfection -RRB- when compared with wild-type K-Ras and the oncogenic mutant K-Ras G12D -LSB- 42 -RSB- . The two aforementioned mutations , V14I and T58I , are located adjacent to amino acid residues that are typically altered in cancer -LRB- G12 , G13 , or Q61 -RRB- . They locate to regions of Ras that are known to be involved in GTP binding . By contrast , in a subset of NS patients , KRAS mutations lead to substitutions in protein regions not obviously involved in GTP binding -LRB- e.g. , D153V K-Ras -RRB- , indicating the existence of previously unappreciated mechanisms of Ras activation . Carta et al. -LSB- 46 -RSB- performed a structural analysis on the two K-Ras mutants , V152G and D153V , that these investigators identified in two patients with severe NS . Their computer-based analysis indicated that both mutations disturb the conformation of the guanine ring-binding pocket favoring the active GTP-bound conformation by increasing the guanine nucleotide dissociation rate . Additionally , these residues are predicted to be important for binding the RasGEF son of sevenless -LRB- SOS -RRB- -LSB- 47 -RSB- . In the meantime , we have detected a number of additional KRAS mutations associated with NS -LRB- Q22R , P34L , P34Q , I36M -RRB- , all of which affect highly conserved amino acid residues and are assumed to confer -LRB- mild -RRB- gain of function -LSB- 48 -RSB- . In this study on additional 236 PTPN11-negative NS patients , we detected KRAS mutations in seven individuals -LSB- 48 -RSB- , thus confirming that KRAS accounts for less than 5 % of NS cases . Genes mutated in cardio -- facio -- cutaneous syndrome In our initial study , we also discovered a novel K-Ras mutation , K-Ras P34R , in 1 of 12 patients with CFC syndrome -LSB- 42 -RSB- . This mutation is now known to be located in another mutational hotspot associated with NS -LSB- 48 -RSB- . A P34R H-Ras lesion has been previously characterized by Stone et al. -LSB- 49 -RSB- . These investigators employed a mutagenesis strategy during which they found that H-Ras P34R binds to GTP in vivo . In vitro , H-Ras P34R is not stimulated by GAPs -LSB- 49 -RSB- . Considering the highly conserved G-domain structure of Ras proteins , the P34R K-Ras mutation presumably has very similar -LRB- if not identical -RRB- biochemical properties . The presence of K-Ras mutations in CFC syndrome was confirmed by Niihori et al. -LSB- 44 -RSB- who analyzed DNA specimens from 43 individuals with CFC syndrome and found two KRAS germline mutations -LRB- G60R and D153V -RRB- in three patients . Notably , the D153V K-Ras mutation also has been described in patients with NS -LSB- 42 , 46 , 48 -RSB- . In a more recent study , we identified additional K-Ras mutations in individuals with CFC or NS/CFC including Q22E and F156I . The observations that mutations , such as K-Ras F156I , may be associated with an overlap of NS and CFC suggest that there is not a very strict genotype -- phenotype correlation -LSB- 48 -RSB- . Simultaneous to the finding of mutated K-Ras in CFC , Niihori et al. identified BRAF germline mutations in 16 of 43 CFC patients -LSB- 36 -RSB- . At the same time , another group found BRAF germline mutations in 18 of 23 CFC patients and MEK1 or MEK2 germline mutations in 3 of the remaining 5 CFC patients studied -LSB- 45 -RSB- . Like PTPN11 and RAS , BRAF is a known oncogene , and somatic mutations are frequently found in cancer -LSB- 50 -RSB- . The molecular basis of Costello syndrome Shortly before germline KRAS mutations were described in patients with NS and CFC , HRAS germline mutations were reported to cause CS -LSB- 43 -RSB- . Later , other investigators also detected these mutations in the majority -LRB- ∼ 90 % -RRB- of CS patients studied -LSB- 51 -- 53 -RSB- . Surprisingly , these germline lesions affect the same amino acid residues of H-Ras that are also mutated in cancer . The observation that activating oncogenic H-Ras mutations affecting codons G12 or G13 are tolerated in the germline , whereas oncogenic K-Ras mutations affecting these residues are not , underscores the notion that although H-Ras and K-Ras are structurally highly similar , they play different roles during embryonic development . Knockout studies have shown that only K-Ras is essential for embryonic development , whereas N-Ras and H-Ras are not -LSB- 54 -RSB- . Different phenotypes exhibited by mutations in various Ras isoforms may be due to heterogenous expression patterns of these proteins . Additionally , various Ras isoforms undergo different processing , e.g. , de/repalmitoylation kinetics that regulate subcellular localization and activity of Ras isoforms -LSB- 55 , 56 -RSB- . Although the vast majority of patients with CS harbor HRAS mutations , very recent studies have indicated that a CS phenotype may occasionally be associated with specific mutations of BRAF -LSB- 57 -RSB- or KRAS -LSB- 48 -RSB- . It remains to be determined if these patients also have the increased risk of neoplasia , which is typical of CS . Therefore , we doubt it is useful to classify these patients with mutations in Ras pathway genes other that HRAS as CS . Neuro -- cardio -- facial -- cutaneous syndrome The molecular explanation of how different germline K-Ras or H-Ras mutations cause a wide spectrum of different phenotypes is still largely unknown . Activating germline K-Ras mutations may be associated with the broadest spectrum of clinical manifestations -LSB- 42 , 44 , 46 -RSB- . However , these syndromes are not just a less or more severe expression of essentially the same disorder . Therefore , it is conceivable that the different RAS mutations associated with divergent phenotypes do not only vary quantitatively in their degree of constitutive activation . Unique lesions may also have different qualitative effects on downstream signaling pathways . Additionally , modifier loci may play a role . Recently , the term ` neuro -- cardio -- facial -- cutaneous ' -LRB- NCFC -RRB- syndrome -LSB- 58 -RSB- was coined to illustrate that clinically overlapping disorders of the NS spectrum , including NF1 , NS , CS , LS , and CFC syndrome , are caused by mutations in components of the Ras signaling pathway -LRB- Fig. 3 -RRB- . We speculate that the phenotypic variability between these disorder results from -LRB- 1 -RRB- different expression patterns of affected genes/isoforms and -LRB- 2 -RRB- variable mechanisms by which certain mutants interact with downstream effectors or regulatory proteins ; these mutants therefore perturb Ras signaling in varying degrees . Further research is required to determine the basis of genotype -- phenotype correlations in NS and related disorders . Fig. 3The Ras signaling pathway relays growth signals from activated growth factor receptors to the nucleus . Somatic mutations in several molecules of the pathway have been implicated in cancer . It is now recognized that germline mutations of identical molecules may cause disorders of the Noonan spectrum . A similar version of this figure has been previously published -LSB- 61 -RSB- , republication with permission Neurofibromatosis Noonan syndrome and NS-like features in neurofibromatosis The rapidly increasing knowledge of the Ras signaling pathway and its relation to developmental disorders of the NS spectrum suggests that Noonan-like features are the clinical correlate of any genetic abnormality occurring in the germline and leading to a generalized mild deregulation of the Ras signaling cascade -LRB- s -RRB- during embryonic development . The new insights gained by recent research in this field may explain the phenotypic similarities between individuals with NF1 and NS . In addition to the well-known symptoms defining NF1 , namely multiple café-au-lait spots and cutaneous neurofibromas , patients with NF1 are known to exhibit NS features , including relative short stature , relative macrocephaly , mild facial anomalies , thorax deformities , and learning difficulties . NFNS probably just represents the extreme end of a highly variable expression of these features -LSB- 59 -RSB- . Although the association of NF1 and NS was described in one family with independently segregating mutations in NF1 and PTPN11 -LSB- 60 -RSB- , this double chance event is not the causative mechanism in many other patients with NF1 and NS-like features . The majority of patients with the NFNS phenotype have been found to harbor NF1 mutations , some of which also occur in patients with `` pure '' NF1 -LSB- 40 , 59 -RSB- . We propose that the NS-like features that are often present in patients with NF1 are potentially due to NF1 haploinsufficiency leading to decreased inactivation of Ras · GTP . This might cause mild constitutional deregulation of Ras signaling , which may occasionally reach a level that leads to a NF1-NS phenotype . Frequently , patients with NF1 have mild features of NS . These may be explained by a mild activation of Ras , which is due to loss-of-function of one NF1 allele . Unknown genetic modifiers may play a role . Malignant cells arising in patients with NF1 somatically loose the wild-type NF1 allele . This second somatic hit ablates the GAP function of neurofibromin resulting in strongly enhanced Ras signaling -LRB- Fig. 4 -RRB- . Fig. 4Model illustrating the molecular basis of how Ras signaling may be increased in patients with neurofibromatosis type 1 leading to Noonan-like feature in these patients . Although only one NF1 allele is inactive in the germline , tumors of patients with neurofibromatosis type 1 somatically lose the second , wild-type NF1 allele Work in progress Although the vast majority of cases with CFC and CS are explained by mutations in KRAS , BRAF , MEK1 , MEK2 , and HRAS , the underlying genetic defect in 50 % of NS and 10 -- 30 % of CFC cases remains unknown . Based on the assumption that the NS phenotype results from hyperactive Ras signaling , we and others are currently screening DNA specimens from patients for mutations in genes encoding for other components , including negative regulators , of the Ras pathway . Analogous to PTPN11 and KRAS , currently unknown genes of the Ras pathway mutated in NS are presumably playing double roles in both development and oncogenesis . Mutations in BRAF and MEK1/2 have been excluded as major NS genes -LRB- M.Z. , unpublished data -RRB- . It will be of interest to study how the different expression patterns of various RAS genes and isoforms influence clinical phenotypes when these genes/isoforms are mutated in the germline . It will be crucial to elucidate the structural mechanisms to understand how these new lesions perturb signaling on a molecular level . Characterization of the biological consequences of these mutations will be largely improved by the construction of murine knock-in models . Note added in proof After this paper was accepted we uncovered mutations in SOS1 encoding for the homonymous RasGEF in patients with NS . Meanwhile , however , two other groups have published elegant reports on activating SOS1 mutations in approximately 10 % of cases with NS : Roberts AE , Araki T , Swanson KD , Montgomery KT , Schiripo TA , Joshi VA , Li L , Yassin Y , Tamburino AM , Neel BG , Kucherlapati RS . Germline gain-of-function mutations in SOS1 cause Noonan syndrome . Nat Genet . 2006 Dec 3 ; -LSB- Epub ahead of print -RSB- Tartaglia M , Pennacchio LA , Zhao C , Yadav KK , Fodale V , Sarkozy A , Pandit B , Oishi K , Martinelli S , Schackwitz W , Ustaszewska A , Martin J , Bristow J , Carta C , Lepri F , Neri C , Vasta I , Gibson K , Curry CJ , Siguero JP , Digilio MC , Zampino G , Dallapiccola B , Bar-Sagi D , Gelb BD . Gain-of-function SOS1 mutations cause a distinctive form of Noonan syndrome . Nat Genet . 2006 Dec 3 ; -LSB- Epub ahead of print -RSB- '' .

O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O

noonan syndrome
costello syndrome
neurofibromatosis type 1
ras signaling