Germline gain-of-function mutations in SOS1 cause Noonan syndrome

A mouse model for Costello syndrome reveals an Ang II-mediated hypertensive condition

Germline activation of H-RAS oncogenes is the primary cause of Costello syndrome (CS), a neuro-cardio-facio-cutaneous developmental syndrome. Here we describe the generation of a mouse model of CS by introduction of an oncogenic Gly12Val mutation in the mouse H-Ras locus using homologous recombination in ES cells. Germline expression of the endogenous H-RasG12V oncogene, even in homozygosis, resulted in hyperplasia of the mammary gland. However, development of tumors in these mice was rare. H-RasG12V mutant mice closely phenocopied some of the abnormalities observed in patients with CS, including facial dysmorphia and cardiomyopathies. These mice also displayed alterations in the homeostasis of the cardiovascular system, including development of systemic hypertension, extensive vascular remodeling, and fibrosis in both the heart and the kidneys. This phenotype was age dependent and was a consequence of the abnormal upregulation of the renin-Ang II system. Treatment with captopril, an inhibitor of Ang II biosynthesis, prevented development of the hypertension condition, vascular remodeling, and heart and kidney fibrosis. In addition, it partially alleviated the observed cardiomyopathies. These mice should help in elucidating the etiology of CS symptoms, identifying additional defects, and evaluating potential therapeutic strategies.

Adults with genetic syndromes and cardiovascular abnormalities: clinical history and management

Cardiovascular abnormalities, especially structural congenital heart defects, commonly occur in malformation syndromes and genetic disorders. Individuals with syndromes comprise a significant proportion of those affected with selected congenital heart defects such as complete atrioventricular canal, interrupted arch type B, supravalvar aortic stenosis, and pulmonary stenosis. As these individuals age, they contribute to the growing population of adults with special health care needs. Although most will require longterm cardiology follow-up, primary care providers, geneticists, and other specialists should be aware of (1) the type and frequency of cardiovascular abnormalities, (2) the range of clinical outcomes, and (3) guidelines for prospective management and treatment of potential complications. This article reviews fundamental genetic, cardiac, medical, and reproductive issues associated with common genetic syndromes that are frequently associated with a cardiovascular abnormality. New data are also provided about the cardiac status of adults with a 22q11.2 deletion and with Down syndrome.

Ras oncogenes: split personalities

Extensive research on the Ras proteins and their functions in cell physiology over the past 30 years has led to numerous insights that have revealed the involvement of Ras not only in tumorigenesis but also in many developmental disorders. Despite great strides in our understanding of the molecular and cellular mechanisms of action of the Ras proteins, the expanding roster of their downstream effectors and the complexity of the signalling cascades that they regulate indicate that much remains to be learnt.

Genetic approaches for changing the heart and dissecting complex syndromes

The genetic, biochemical and molecular bases of human cardiac disease have been the focus of extensive research efforts for many years. Early animal models of cardiovascular disease used pharmacologic or surgical interventions, or took advantage of naturally occurring genetic abnormalities and the data obtained were largely correlative. The inability to directly alter an organism's genetic makeup and cellular protein content and accurately measure the results of that manipulation precluded rigorous examination of true cause-effect and structure-function relationships. Directed genetic manipulation in the mouse gave researchers the ability to modify and control the mammalian heart's protein content, resulting in the rational design of models that could provide critical links between the mutated or absent protein and disease. Two techniques that have proven particularly useful are transgenesis, which involves the random insertion of ectopic genetic material of interest into a "host" genome, and gene targeting, which utilizes homologous recombination at a pre-selected locus. Initially, transgenesis and gene targeting were used to examine systemic loss-of-function and gain-of-function, respectively, but further refinements in both techniques have allowed for investigations of organ-specific, cell type-specific, developmental stage-sensitive and dose-dependent effects. Genetically engineered animal models of pediatric and adult cardiac disease have proven that, when used appropriately, these tools have the power to extend mere observation to the establishment of true causative proof. We illustrate the power of the general approach by showing how genetically engineered mouse models can define the precise signaling pathways that are affected by the gain-of-function mutation that underlies Noonan syndrome. Increasingly precise and modifiable animal models of human cardiac disease will allow researchers to determine not only pathogenesis, but also guide treatment and the development of novel therapies.

Electrocardiography in Noonan syndrome PTPN11 gene mutation--phenotype characterization

Noonan syndrome is a developmental disorder with distinctive facial features, short stature and cardiac abnormalities. In this cross-sectional study, we evaluated characteristic electrocardiographic (ECG) findings and cardiac abnormalities in 84 patients with Noonan syndrome, 56 (67%) of who were positive for a PTPN11 mutation. As reported previously, pulmonary stenosis was the most common cardiac abnormality, followed by atrial septal defect and hypertrophic cardiomyopathy. The ECG showed at least one characteristic finding in 50% of cases including left axis deviation in 38 (45%), small R waves in the left precordial leads in 20 (24%) and an abnormal Q wave in 5 (6%) patients with Noonan syndrome. A wide QRS complex was not detected in any of these patients. The characteristic ECG findings of Noonan syndrome patients were not associated with a PTPN11 gene mutation, or with a (specific) cardiac anomaly. We conclude that there are characteristic ECG findings in Noonan syndrome, but the ECG pattern is neither a useful tool for the phenotype characterization of a PTPN11 mutation, nor for the presence or type of cardiac abnormality.

Shp2 knockdown and Noonan/LEOPARD mutant Shp2-induced gastrulation defects

Shp2 is a cytoplasmic protein-tyrosine phosphatase that is essential for normal development. Activating and inactivating mutations have been identified in humans to cause the related Noonan and LEOPARD syndromes, respectively. The cell biological cause of these syndromes remains to be determined. We have used the zebrafish to assess the role of Shp2 in early development. Here, we report that morpholino-mediated knockdown of Shp2 in zebrafish resulted in defects during gastrulation. Cell tracing experiments demonstrated that Shp2 knockdown induced defects in convergence and extension cell movements. In situ hybridization using a panel of markers indicated that cell fate was not affected by Shp2 knock down. The Shp2 knockdown-induced defects were rescued by active Fyn and Yes and by active RhoA. We generated mutants of Shp2 with mutations that were identified in human patients with Noonan or LEOPARD Syndrome and established that Noonan Shp2 was activated and LEOPARD Shp2 lacked catalytic protein-tyrosine phosphatase activity. Expression of Noonan or LEOPARD mutant Shp2 in zebrafish embryos induced convergence and extension cell movement defects without affecting cell fate. Moreover, these embryos displayed craniofacial and cardiac defects, reminiscent of human symptoms. Noonan and LEOPARD mutant Shp2s were not additive nor synergistic, consistent with the mutant Shp2s having activating and inactivating roles in the same signaling pathway. Our results demonstrate that Shp2 is required for normal convergence and extension cell movements during gastrulation and that Src family kinases and RhoA were downstream of Shp2. Expression of Noonan or LEOPARD Shp2 phenocopied the craniofacial and cardiac defects of human patients. The finding that defective Shp2 signaling induced cell movement defects as early as gastrulation may have implications for the monitoring and diagnosis of Noonan and LEOPARD syndrome.

Mutation detection using automated fluorescence-based sequencing

The development of high-throughput DNA sequencing techniques has made direct DNA sequencing of PCR-amplified genomic DNA a rapid and economical approach to the identification of polymorphisms that may play a role in disease. Point mutations as well as small insertions or deletions are readily identified by DNA sequencing. The mutations may be heterozygous (occurring in one allele while the other allele retains the normal sequence) or homozygous (occurring in both alleles). Sequencing alone cannot discriminate between true homozygosity and apparent homozygosity due to the loss of one allele due to a large deletion. In this unit, strategies are presented for using PCR amplification and automated fluorescence-based sequencing to identify sequence variation. The size of the project and laboratory preference and experience will dictate how the data is managed and which software tools are used for analysis. A high-throughput protocol is given that has been used to search for mutations in over 200 different genes at the Harvard Medical School - Partners Center for Genetics and Genomics (HPCGG, http://www.hpcgg.org/).

Many faces of Ras activation

Ras proteins were originally identified as the products of oncogenes capable of inducing cell transformation. Over the last twenty-five years they have been studied in great detail because mutant Ras proteins are associated with many types of human cancer. Wild type Ras proteins play a central role in the regulation of proliferation and differentiation of various cell types. They alternate between an active GTP-bound state and an inactive GDP-bound state. Their activation is catalysed by a specialized group of enzymes known as guanine nucleotide exchange factors (GEFs). To date, four subfamilies of GEF molecules have been identified. Although all of them are able to activate Ras, their structure, tissue expression and regulation are significantly diverse. In this review we will summarize the various mechanisms by which these exchange factors activate Ras.

SOS1 mutations are rare in human malignancies: implications for Noonan Syndrome patients

Germ line gain-of-function mutations in several members of the RAS/ERK pathway, including PTPN11, KRAS, and RAF1, cause the autosomal dominant genetic disorder Noonan Syndrome (NS). NS patients are at increased risk of leukemia/myeloproliferative disease and possibly some solid tumors, such as neuroblastoma. Recently, SOS1 gain of function mutations have also been shown to cause NS. Somatic PTPN11, KRAS, and RAF1 mutations occur (although at different frequencies) in a variety of sporadic neoplasms, but whether SOS1 mutations are associated with human cancer has not been evaluated. We sequenced DNA from a total of 810 primary malignancies, including pancreatic, lung, breast, and colon carcinomas, and acute myelogenous leukemia, as well as several neuroblastoma cell lines. From this large, diverse series, missense SOS1 mutations were identified in a single pancreatic tumor, one lung adenocarcinoma, and a T-cell acute lymphoblastic leukemia cell line. Our findings suggest that SOS1 is not a significant human oncogene in most cancers. Furthermore, NS patients with SOS1 mutations may not be at increased risk of developing cancer.

Membrane-dependent signal integration by the Ras activator Son of sevenless

The kinetics of Ras activation by Son of sevenless (SOS) changes profoundly when Ras is tethered to membranes, instead of being in solution. SOS has two binding sites for Ras, one of which is an allosteric site that is distal to the active site. The activity of the SOS catalytic unit (SOS(cat)) is up to 500-fold higher when Ras is on membranes compared to rates in solution, because the allosteric Ras site anchors SOS(cat) to the membrane. This effect is blocked by the N-terminal segment of SOS, which occludes the allosteric site. We show that SOS responds to the membrane density of Ras molecules, to their state of GTP loading and to the membrane concentration of phosphatidylinositol-4,5-bisphosphate (PIP2), and that the integration of these signals potentiates the release of autoinhibition.

Genetic determinants of cardiac hypertrophy

PURPOSE OF REVIEW

Cardiac hypertrophy is a common phenotypic response of the heart to stimulants. It is associated with increased morbidity and mortality in various cardiovascular disorders. Genetic factors are important determinants of phenotypic expression of cardiac hypertrophy, whether in single-gene disorders or in complex traits. We focus on the molecular genetics of cardiac hypertrophy in various conditions with an emphasis on hypertrophic cardiomyopathy, a genetic paradigm of cardiac hypertrophic response.

RECENT FINDINGS

The molecular genetic basis of cardiac hypertrophy in single-gene disorders has been partially elucidated. Likewise, the impact of genetics on the expression of cardiac hypertrophy in the general population has been demonstrated. Identification of mutations in the Z disk proteins has expanded the spectrum of causal mutations beyond the thin and thick filaments of the sarcomeres. In addition, modifier loci have been mapped and shown to impart considerable effects on the expression of cardiac hypertrophy in hypertrophic cardiomyopathy. Elucidation of the molecular genetics of sarcomeric hypertrophic cardiomyopathy and many of the phenocopies has highlighted the limitations of clinical diagnosis as a determinant of management and prognostic advice. The findings have raised the importance of diagnosis and treatment algorithms, which are based on both genotype and phenotype information.

SUMMARY

Cardiac hypertrophy, regardless of the cause, is the phenotypic consequence of complex interactions between genetic and nongenetic factors.

Duplication of chromosome band 12q24.11q24.23 results in apparent Noonan syndrome

Noonan syndrome is an autosomal dominant disorder with an estimated incidence of 1 in 1,000 to 1 in 2,500 live births. It is characterized by postnatal-onset short stature, characteristic facial changes, webbed neck, pectus carinatum, or excavatum, congenital heart defects, and bleeding abnormalities. Gain-of-function mutations in the PTPN11, KRAS, SOS1, and RAF1 genes that are components of the RAS/MEPK signaling pathway are identified in about 70-85% of individuals with Noonan syndrome. We report here a case of duplication of chromosome region 12q24.11q24.23 identified by array comparative genomic hybridization (aCGH) that includes the PTPN11 gene in a 3-year-old girl with apparent Noonan syndrome. The patient presented with postnatal-onset failure-to-thrive, developmental delay, microcephaly, velopalatal incompetence, pectus excavatum, coarctation of aorta, atrial and ventricular septal defects, decreased muscle tone, and minor facial anomalies consistent with Noonan syndrome. At 3 years of age her speech, gross and fine motor development were at the level of a 12-18 month old child. This degree of developmental delay was atypical for an individual with Noonan syndrome, raising concerns for a chromosomal abnormality. Array-CGH showed an interstitial duplication of 10 Mb including the PTPN11 gene. Sequencing of PTPN11, KRAS, SOS1 and the coding region of RAF1 did not identify mutations. The increased gene dosage of the PTPN11 gene in the form of duplication is expected to have the same consequence as gain-of-function mutations seen in Noonan syndrome. We propose that at least some of the 15-30% of individuals with Noonan syndrome who do not have a mutation by sequencing may have a gain in copy number of PTPN11 and recommend that comprehensive testing for Noonan syndrome should include analysis for copy number changes of PTPN11.

Familial myelodysplasia and acute myeloid leukaemia--a review

Familial occurrence of myelodysplasia (MDS) and/or acute myeloid leukaemia (AML) is rare but can provide a useful resource for the investigation of predisposing mutations in these myeloid malignancies. To date, examination of families with MDS/AML has lead to the detection of two culprit genes, RUNX1 and CEBPA. Germline mutations in RUNX1 result in familial platelet disorder with propensity to myeloid malignancy and inherited mutations of CEBPA predispose to AML. Unfortunately, the genetic cause remains obscure in most other reported pedigrees. Further insight into the molecular mechanisms of familial MDS/AML will require awareness by clinicians of new patients with relevant family histories.

The developmental genetics of congenital heart disease

Congenital heart disease is the leading cause of infant morbidity in the Western world, but only in the past ten years has its aetiology been understood. Recent studies have uncovered the genetic basis for some common forms of the disease and provide new insight into how the heart develops and how dysregulation of heart development leads to disease.

De novo HRAS and KRAS mutations in two siblings with short stature and neuro-cardio-facio-cutaneous features

Mutations in genes involved in Ras signalling cause Noonan syndrome and other disorders characterised by growth disturbances and variable neuro-cardio-facio-cutaneous features. We describe two sisters, 46 and 31 years old, who presented with dysmorphic features, hypotonia, feeding difficulties, retarded growth and psychomotor retardation early in life. The patients were initially diagnosed with Costello syndrome, and autosomal recessive inheritance was assumed. Remarkably, however, we identified a germline HRAS mutation (G12A) in one sister and a germline KRAS mutation (F156L) in her sibling. Both mutations had arisen de novo. The F156L mutant K-Ras protein accumulated in the active, guanosine triphosphate-bound conformation and affected downstream signalling. The patient harbouring this mutation was followed for three decades, and her cardiac hypertrophy gradually normalised. However, she developed severe epilepsy with hippocampal sclerosis and atrophy. The occurrence of distinct de novo mutations adds to variable expressivity and gonadal mosaicism as possible explanations of how an autosomal dominant disease may manifest as an apparently recessive condition.

Perinatal autopsy findings in three cases of jugular lymphatic obstruction sequence and cardiac polyvalvular dysplasia

Three infants with a prenatal diagnosis of Noonan's syndrome suffered fetal hydrops and immediate neonatal death. The infants all had the external appearance of jugular lymphatic obstruction sequence with wide-spaced nipples, redundant posterior nuchal skin, and edema of the dorsa of the feet and hands. All 3 demonstrated thick, redundant leaflets of all 4 cardiac valves, and 2 had a membranous ventricular septal defect. One female infant had a mutation of the PTPN11 gene. Two males had no common mutation of PTPN11. The males demonstrated other abnormalities in common, including small penis, testicular malformation, rosette-like appearance of the pituicytes, and an eosinophil infiltration of the pancreatic islets with islet cell hypertrophy. Detailed anatomy of cases of lymphatic obstruction sequence fetuses can be correlated with an increasing number of genetic mutations associated with Noonan's syndrome and related syndromes in mice and humans.

Noonan syndrome: psychological and psychiatric aspects

Although Noonan syndrome (NS) is a disorder with a relatively high prevalence, virtually no information in adult patients is available about the psychological and psychopathological profile. In the present clinical report the first series of 10 NS patients from an ongoing project is presented. The purpose of the study is to investigate the psychopathology, social cognition and adaptation as well as the quality of life in NS patients aged 16 years or more. PTPN11 mutations were present in six patients and KRAS and SOS1 in one patient, respectively. In two patients no known mutation was found. The results demonstrate a variable level of intelligence and suggest moderately impaired social cognition in terms of emotion recognition and alexithymia. In some patients mild signs of anxiety and lowered mood are found that, however, do not meet the criteria for a specific psychiatric disorder. It is concluded that NS in adults is associated with a behavioral phenotype in which deficiencies in social and emotional recognition and expression may be key elements.

Mutation analysis in Costello syndrome: functional and structural characterization of the HRAS p.Lys117Arg mutation

Costello syndrome is a mental retardation syndrome characterized by high birth weight, postnatal growth retardation, coarse face, loose skin, cardiovascular problems, and tumor predisposition. De novo heterozygous missense mutations in HRAS codon 12 and 13 disturbing the intrinsic GTP hydrolysis cause Costello syndrome. We report a patient with typical Costello syndrome and a novel heterozygous missense mutation in codon 117 (c.350A>G, p.Lys117Arg) of the HRAS gene, resulting in constitutive activation of the RAS/MAPK pathway similar to the typical p.Gly12Ser and p.Gly12Ala mutations. Recombinant HRAS p.Lys117Arg demonstrates normal intrinsic GTP hydrolysis and responsiveness to GTPase-activating proteins, but the nucleotide dissociation rate is increased 80-fold. Consistent with the biochemical data, the crystal structure of the p.Lys117Arg mutant indicates an altered interaction pattern of the side chain that is associated with unfavorable nucleotide binding properties. Together, these data show that a RAS mutation that only perturbs guanine nucleotide binding has similar functional consequences as mutations that impair GTP hydrolysis and causes human disease.

Isolation of a distinct class of gain-of-function SHP-2 mutants with oncogenic RAS-like transforming activity from solid tumors

SHP-2 protein tyrosine phosphatase plays an important role in activation of the RAS-dependent signaling. Gain-of-function mutations in the PTPN11 gene, which encodes SHP-2, have been found in the leukemia-prone developmental disorder Noonan syndrome as well as sporadic childhood leukemias, indicating that SHP-2 is a bona fide human oncoprotein. However, the role of SHP-2 mutations in non-hematological malignancies remains obscure. Here, we screened for PTPN11 mutations in primary solid tumors and identified a 1520C>A mutation that causes threonine-507 to lysine (T507K) substitution in the phosphatase domain of SHP-2 in a case of hepatocellular carcinoma. T507K SHP-2 exhibited altered substrate specificity with slightly elevated basal phosphatase activity. Upon expression in NIH3T3 cells, T507K SHP-2 induced transformed foci, which was not observed with wild type, Noonan-specific or leukemia-specific SHP-2. Furthermore, NIH3T3 cells transformed by T507K SHP-2 showed anchorage-independent growth and developed tumors in nude mice. These results indicate that quantitative and/or qualitative alteration in phosphatase activity determines the transforming potential as well as target cell/tissue spectrum of individual SHP-2 mutants as oncoproteins. Although rare in solid tumors, the identified T507K SHP-2 represents a distinct class of SHP-2 mutants with oncogenic RAS-like transforming activity, which could contribute to the development of solid tumors.

In vitro assays to characterize inhibitors of the activation of small G proteins by their guanine nucleotide exchange factors

Guanine nucleotide exchange factors (GEFs) are essential regulators of the spatiotemporal conditions of small GTP-binding protein (SMG) activation. Their cellular activities combine the biochemical stimulation of GDP/GTP exchange, which leads to the active conformation of the SMG, to the detection of upstream signals and, in some cases, interaction with downstream effectors. Inhibition of GEF activities by small molecules has become recently a very active field, both for understanding biology with the tools of chemistry and because GEFs are emerging as therapeutic targets. The natural compound brefeldin A (BFA) was the first inhibitor of a GEF to be characterized, and several inhibitors of SMG activation have since been discovered using a variety of screening methods. An essential step toward their use in basic research or as leads in therapeutics is the characterization of their mechanism of inhibition. GEFs function according to a multistep mechanism, involving transient ternary (nucleotide-bound) and binary (nucleotide-free) intermediates. This mechanism thereby offers many opportunities for blockage, but a thorough analysis is necessary to define the inhibition mechanism and the steps of the reaction that are affected by the inhibitor. Here, based on the case study of how BFA inhibits the activation of Arf activation by Sec7 domains, we describe a flowchart of assays to decipher the mechanism of inhibitors of the activation of SMGs by their GEFs.

Genetics of autosomal recessive non-syndromic mental retardation: recent advances

The identification of the genes mutated in autosomal recessive non-syndromic mental retardation (ARNSMR) has been very active recently. This report presents an overview of the current knowledge on clinical data in ARNSMR and progress in research. To date, 12 ARNSMR loci have been mapped, and three genes identified. Mutations in known ARNSMR genes have been detected so far in only a small number of families; their contribution to mental retardation in the general population might be limited. The ARNSMR-causing genes belong to different protein families, including serine proteases, Adenosine 5'-triphosphate-dependent Lon proteases and calcium-regulated transcriptional repressors. All of the mutations in the ARNSMR-causing genes are protein truncating, indicating a putative severe loss-of-function effect. The future objective will be the development of diagnostic kits for molecular diagnosis in mentally retarded individuals in order to offer at-risk families pre-natal diagnosis to detect affected offspring.

Neurological complications of cardio-facio-cutaneous syndrome

Cardio-facio-cutaneous syndrome (CFC) is a multiple congenital anomaly disorder characterized by craniofacial dysmorphia, ectodermal abnormalities, congenital heart defects, and developmental and growth delay. Neurological complications associated with CFC remain to be clearly defined. Recent discovery of causative mutations in genes of the MAPK pathway (BRAF, MEK1, and MEK2) now permit accurate molecular diagnosis of CFC. The aim of the study was to characterize neurological features of participants with molecularly-confirmed CFC. Medical records, and laboratory and imaging data were reviewed for 39 mutation-positive individuals with CFC. Participants with a clinical diagnosis of CFC but a negative result on mutation screening of the BRAF, MEK1, and MEK2 genes were excluded from the study. Mean age of participants was 9 years 4 months (range 18 mo-24 y); there were 24 females and 15 males. Mutations in B RA F were present in 32 participants, MEK1 in five, and MEK2 in two participants. Hypotonia, motor delay, speech delay, and learning disability were universally present in this cohort. Macrocephaly was present in 13 participants, ptosis in 11, strabismus in 14, and nystagmus in 11 of the 22 participants who underwent a neurological exam. Corticospinal tract findings were present in seven participants. Ventriculomegaly or hydrocephalus was present in 14 of 32 participants who underwent brain imaging. Other findings on magnetic resonance imaging included prominent Virchow-Robin spaces (n=6), abnormal myelination (n=4), and structural anomalies (n=5). Seizures were present in 15 participants. No specific genotype-phenotype correlation was observed.

Costello syndrome and related disorders

PURPOSE OF REVIEW

Costello syndrome is a rare congenital disorder affecting multiple organ systems, encompassing severe failure to thrive, cardiac anomalies including hypertrophic cardiomyopathy and atrial tachycardia, tumor predisposition, and cognitive impairment. Costello syndrome shares findings with cardio-facio-cutaneous syndrome and the diagnosis can be challenging. The discovery of gene mutations underlying these and other closely related disorders allows for molecular confirmation of a clinical diagnosis.

RECENT FINDINGS

The identification of germline HRAS mutations in Costello syndrome, and mutations in BRAF, MEK1 and MEK2 in cardio-facio-cutaneous syndrome, uncovered the biologic mechanism for the shared phenotypic findings based on the close interaction of the gene products within the Ras-mitogen-activated protein kinase pathway. Changes in other genes encoding mitogen-activated protein kinase pathway proteins are responsible for Noonan syndrome and the KRAS mutation phenotype.

SUMMARY

Costello syndrome is caused by heterozygous de-novo point mutations in HRAS, resulting in increased activation of the mitogen-activated protein kinase pathway. Despite their overlapping presentation, Costello syndrome and its related disorders are distinct, and the phenotypes become more distinctive with age. Molecular testing is available and a clinical diagnosis should be reconsidered if it is inconsistent with the molecular result.

New insights into the mechanisms of SOS activation

The activation of the small guanosine triphosphatase Ras is critical for many biological events. It is therefore not surprising that the ubiquitously expressed Ras guanine nucleotide exchange factor (GEF) SOS (Son of Sevenless), which couples protein tyrosine kinases to Ras activation, is under tight autoinhibitory control. Several studies have revealed how multiple regulatory domains might affect SOS activity. Most notably, a second Ras-binding site on SOS allosterically regulates the duration and amplitude of Ras activation. This allosteric Ras-GTP is produced by another GEF, Ras guanine nucleotide-releasing protein 1 (RasGRP1). SOS and RasGRP1 are both activated downstream of phospholipase D(2), and gain-of-function mutants of SOS contribute to inherited diseases. These studies not only enable us to better appreciate the complexity of the regulation of GEFs but also prompt us to reevaluate our current understanding of pathways that lead to Ras activation.

Mutation and phenotypic spectrum in patients with cardio-facio-cutaneous and Costello syndrome

Cardio-facio-cutaneous (CFC) and Costello syndrome (CS) are congenital disorders with a significant clinical overlap. The recent discovery of heterozygous mutations in genes encoding components of the RAS-RAF-MAPK pathway in both CFC and CS suggested a similar underlying pathogenesis of these two disorders. While CFC is heterogeneous with mutations in BRAF, MAP2K1, MAP2K2 and KRAS, HRAS alterations are almost exclusively associated with CS. We carried out a comprehensive mutation analysis in 51 CFC-affected patients and 31 individuals with CS. Twelve different BRAF alterations were found in twenty-four patients with CFC (47.0%), two MAP2K1 mutations in five (9.8%) and two MAP2K2 sequence variations in three CFC-affected individuals (5.9%), whereas three patients had a KRAS alteration (5.9%). We identified four different missense mutations of HRAS in twenty-eight cases with CS (90.3%), while KRAS mutations were detected in two infants with a phenotype meeting criteria for CS (6.5%). In 14 informative families, we traced the parental origin of HRAS alterations and demonstrated inheritance of the mutated allele exclusively from the father, further confirming a paternal bias in the parental origin of HRAS mutations in CS. Careful clinical evaluation of patients with BRAF and MAP2K1/2 alterations revealed the presence of slight phenotypic differences regarding craniofacial features in MAP2K1- and MAP2K2-mutation positive individuals, suggesting possible genotype-phenotype correlations.

Mutation analysis of the genes involved in the Ras-mitogen-activated protein kinase (MAPK) pathway in Korean patients with Noonan syndrome

Noonan syndrome (NS) is a congenital abnormality that affects multiple parts of the body. Approximately 50% of cases are caused by mutations in the PTPN11 gene. NS shares many clinical features with a group of developmental disorders including Costello syndrome and cardio-facio-cutaneous (CFC) syndrome. Recently, KRAS and SOS1 were identified as causative genes for NS. Moreover, mutations in several genes associated with the Ras-mitogen-activated protein kinase (MAPK) pathway, including HRAS, BRAF, MEK1, and MEK2 were identified in patients with Costello syndrome and CFC syndrome. Accordingly, this study carried out mutation analysis of nine genes including PTPN11, SOS1, GRB2, KRAS, HRAS, NRAS, BRAF, MEK1, and MEK2 associated with the Ras-MAPK pathway in 14 Korean patients with NS. Seven patients were found to have mutations in the PTPN11 gene. Mutation analyses of the other genes did not reveal any disease causing mutations except for one unclassified variation in the 3'-untranslated region of the HRAS gene (c.*1C>T). The patient's father also had the same substitution with the normal phenotype. Therefore, this variation is believed to be either a rare polymorphism or a disease-related variation with variable penetrance. The Ras-MAPK pathway has now emerged as a key cascade in a group of similar developmental disorders as well as in many types of cancers. This study found that, with the exception of PTPN11, mutations in genes related to the Ras-MAPK pathway appear to be uncommon, at least in Korean patients with NS.

What's new in the neuro-cardio-facial-cutaneous syndromes?

The RAS-MAPKinase pathway is a signal transduction cascade which has been studied extensively during the last decades for its role in human oncogenesis. Activation of this cascade is controlled by cycling of the RAS protein between an inactive and an active state and by phosphorylation of downstream proteins. The signalling cascade regulates cell proliferation, differentiation and survival. Disturbed RAS signalling in malignancies is caused by acquired somatic mutations in RAS genes or other components of this pathway. Recently, germline mutations in genes coding for different components of the RAS signalling cascade have been recognized as the cause of several phenotypically overlapping disorders, recently referred to as the neuro-cardio-facial-cutaneous syndromes. Neurofibromatosis type 1, Noonan, LEOPARD, Costello and cardiofaciocutaneous syndromes all present with variable degrees of psychomotor delay, congenital heart defects, facial dysmorphism, short stature, skin abnormalities and a predisposition for malignancy. These findings point to important roles for this evolutionary conserved pathway in oncogenesis, development, cognition and growth.

CONCLUSION

it has become obvious in recent years that the neuro-cardio-facial-cutaneous syndromes all share a common genetic and pathophysiologic basis. Dysregulation of the RAS-MAPKinase pathway is caused by germline mutations in genes involved in this pathway. Undoubtedly more genes causing related syndromes will be discovered in the near future since there are still a substantial number of genes in the pathway that are not yet associated with a known syndrome.

Noonan syndrome

Noonan syndrome is a common autosomal dominant condition caused by multiple genes in the RasMAPK pathway. The adult phenotype can be extremely subtle, and many adults are diagnosed only after the birth of a more obviously affected child. Whether diagnosis is made in childhood or adulthood, initial and ongoing evaluation of many systems can have considerable health benefits.

Mitogen-activated protein kinases in heart development and diseases

Mitogen-activated protein (MAP) kinases belong to a highly conserved family of Ser-Thr protein kinases in the human kinome and have diverse roles in broad physiological functions. The 4 best-characterized MAP kinase pathways, ERK1/2, JNK, p38, and ERK5, have been implicated in different aspects of cardiac regulation, from development to pathological remodeling. Recent advancements in the development of kinase-specific inhibitors and genetically engineered animal models have revealed significant new insights about MAP kinase pathways in the heart. However, this explosive body of new information also has yielded many controversies about the functional role of specific MAP kinases as either detrimental promoters or critical protectors of the heart during cardiac pathological processes. These uncertainties have raised questions on whether/how MAP kinases can be targeted to develop effective therapies against heart diseases. In this review, recent studies examining the role of MAP kinase subfamilies in cardiac development, hypertrophy, and survival are summarized.

Mediating ERK 1/2 signaling rescues congenital heart defects in a mouse model of Noonan syndrome

Noonan syndrome (NS) is an autosomal dominant disorder characterized by a wide spectrum of defects, which most frequently include proportionate short stature, craniofacial anomalies, and congenital heart disease (CHD). NS is the most common nonchromosomal cause of CHD, and 80%-90% of NS patients have cardiac involvement. Mutations within the protein tyrosine phosphatase Src homology region 2, phosphatase 2 (SHP2) are responsible for approximately 50% of the cases of NS with cardiac involvement. To understand the developmental stage- and cell type-specific consequences of the NS SHP2 gain-of-function mutation, Q79R, we generated transgenic mice in which the mutated protein was expressed during gestation or following birth in cardiomyocytes. Q79R SHP2 embryonic hearts showed altered cardiomyocyte cell cycling, ventricular noncompaction, and ventricular septal defects, while, in the postnatal cardiomyocyte, Q79R SHP2 expression was completely benign. Fetal expression of Q79R led to the specific activation of the ERK1/2 pathway, and breeding of the Q79R transgenics into ERK1/2-null backgrounds confirmed the pathway's necessity and sufficiency in mediating mutant SHP2's effects. Our data establish the developmental stage-specific effects of Q79R cardiac expression in NS; show that ablation of subsequent ERK1/2 activation prevents the development of cardiac abnormalities; and suggest that ERK1/2 modulation could have important implications for developing therapeutic strategies in CHD.

Ras proteins: paradigms for compartmentalised and isoform-specific signalling

Ras GTPases mediate a wide variety of cellular processes by converting a multitude of extracellular stimuli into specific biological responses including proliferation, differentiation and survival. In mammalian cells, three ras genes encode four Ras isoforms (H-Ras, K-Ras4A, K-Ras4B and N-Ras) that are highly homologous but functionally distinct. Differences between the isoforms, including their post-translational modifications and intracellular sorting, mean that Ras has emerged as an important model system of compartmentalised signalling and membrane biology. Ras isoforms in different subcellular locations are proposed to recruit distinct upstream and downstream accessory proteins and activate multiple signalling pathways. Here, we summarise data relating to isoform-specific signalling, its role in disease and the mechanisms promoting compartmentalised signalling. Further understanding of this field will reveal the role of Ras signalling in development, cellular homeostasis and cancer and may suggest new therapeutic approaches.

SOS1 is the second most common Noonan gene but plays no major role in cardio-facio-cutaneous syndrome

BACKGROUND

Heterozygous gain-of-function mutations in various genes encoding proteins of the Ras-MAPK signalling cascade have been identified as the genetic basis of Noonan syndrome (NS) and cardio-facio-cutaneous syndrome (CFCS). Mutations of SOS1, the gene encoding a guanine nucleotide exchange factor for Ras, have been the most recent discoveries in patients with NS, but this gene has not been studied in patients with CFCS.

METHODS AND RESULTS

We investigated SOS1 in a large cohort of patients with disorders of the NS-CFCS spectrum, who had previously tested negative for mutations in PTPN11, KRAS, BRAF, MEK1 and MEK2. Missense mutations of SOS1 were discovered in 28% of patients with NS. In contrast, none of the patients classified as having CFCS was found to carry a pathogenic sequence change in this gene.

CONCLUSION

We have confirmed SOS1 as the second major gene for NS. Patients carrying mutations in this gene have a distinctive phenotype with frequent ectodermal anomalies such as keratosis pilaris and curly hair. However, the clinical picture associated with SOS1 mutations is different from that of CFCS. These findings corroborate that, despite being caused by gain-of-function mutations in molecules belonging to the same pathway, NS and CFCS scarcely overlap genotypically.

GEFs in growth factor signaling

Evolutionary conserved members of the Ras superfamily of small GTP-binding proteins function as binary molecular switches to control diverse biological processes. In the context of cellular signaling, these include functions in exocytic and endocytic trafficking, as well as roles in signal relay downstream of various cell surface receptors. We previously reviewed roles played by the large family of GTPase, activating proteins in these processes. In this companion review, we highlight recent findings relating to the regulation of another major class of Ras superfamily regulatory proteins, the guanine nucleotide exchange factors.

Role of JAK2 in the pathogenesis and therapy of myeloproliferative disorders

The myeloproliferative disorders polycythaemia vera (PV), essential thombocythaemia (ET), and primary myelofibrosis (PMF) are clonal disorders of multipotent haematopoietic progenitors. The genetic cause of these diseases was not known until 2005, when several independent groups demonstrated that most patients with PV, ET and PMF acquire a single point mutation in the cytoplasmic tyrosine kinase JAK2 (JAK2V617F). These discoveries have changed the landscape for diagnosis and classification of PV, ET and PMF, and show the ability of genomic technologies to identify new molecular targets in human malignancies with pathogenetic, diagnostic and therapeutic significance.

Germline loss-of-function mutations in SPRED1 cause a neurofibromatosis 1-like phenotype

We report germline loss-of-function mutations in SPRED1 in a newly identified autosomal dominant human disorder. SPRED1 is a member of the SPROUTY/SPRED family of proteins that act as negative regulators of RAS->RAF interaction and mitogen-activated protein kinase (MAPK) signaling. The clinical features of the reported disorder resemble those of neurofibromatosis type 1 and consist of multiple café-au-lait spots, axillary freckling and macrocephaly. Melanocytes from a café-au-lait spot showed, in addition to the germline SPRED1 mutation, an acquired somatic mutation in the wild-type SPRED1 allele, indicating that complete SPRED1 inactivation is needed to generate a café-au-lait spot in this syndrome. This disorder is yet another member of the recently characterized group of phenotypically overlapping syndromes caused by mutations in the genes encoding key components of the RAS-MAPK pathway. To our knowledge, this is the first report of mutations in the SPRY (SPROUTY)/SPRED family of genes in human disease.

Cardio-facio-cutaneous and Noonan syndromes due to mutations in the RAS/MAPK signalling pathway: genotype-phenotype relationships and overlap with Costello syndrome

Cardio-facio-cutaneous (CFC) syndrome, Noonan syndrome (NS), and Costello syndrome (CS) are clinically related developmental disorders that have been recently linked to mutations in the RAS/MEK/ERK signalling pathway. This study was a mutation analysis of the KRAS, BRAF, MEK1 and MEK2 genes in a total of 130 patients (40 patients with a clinical diagnosis of CFC, 20 patients without HRAS mutations from the French Costello family support group, and 70 patients with NS without PTPN11 or SOS1 mutations). BRAF mutations were found in 14/40 (35%) patients with CFC and 8/20 (40%) HRAS-negative patients with CS. KRAS mutations were found in 1/40 (2.5%) patients with CFC, 2/20 (10%) HRAS-negative patients with CS and 4/70 patients with NS (5.7%). MEK1 mutations were found in 4/40 patients with CFC (10%), 4/20 (20%) HRAS-negative patients with CS and 3/70 (4.3%) patients with NS, and MEK2 mutations in 4/40 (10%) patients with CFC. Analysis of the major phenotypic features suggests significant clinical overlap between CS and CFC. The phenotype associated with MEK mutations seems less severe, and is compatible with normal mental development. Features considered distinctive for CS were also found to be associated with BRAF or MEK mutations. Because of its particular cancer risk, the term "Costello syndrome" should only be used for patients with proven HRAS mutation. These results confirm that KRAS is a minor contributor to NS and show that MEK is involved in some cases of NS, demonstrating a phenotypic continuum between the clinical entities. Although some associated features appear to be characteristic of a specific gene, no simple rule exists to distinguish NS from CFC easily.

New perspectives for the elucidation of genetic disorders

For almost 15 years, genome research has focused on the search for major risk factors in common diseases, with disappointing results. Only recently, whole-genome association studies have begun to deliver because of the introduction of high-density single-nucleotide-polymorphism arrays and massive enlargement of cohort sizes, but most of the risk factors detected account for only a small proportion of the total genetic risk, and their diagnostic value is negligible. There is reason to believe that the complexity of many "multifactorial" disorders is primarily due to genetic heterogeneity, with defects of different genes causing the same disease. Moreover, de novo copy-number variation has been identified as a major cause of mental retardation and other complex disorders, suggesting that new mutations are an important, previously overlooked factor in the etiology of complex diseases. These observations support the notion that research into the previously neglected monogenic disorders should become a priority of genome research. Because of the introduction of novel high-throughput, low-cost sequencing methods, sequencing and genotyping will soon converge, with far-reaching implications for the elucidation of genetic disease and health care.

GEFs and GAPs: critical elements in the control of small G proteins

Guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) regulate the activity of small guanine nucleotide-binding (G) proteins to control cellular functions. In general, GEFs turn on signaling by catalyzing the exchange from G-protein-bound GDP to GTP, whereas GAPs terminate signaling by inducing GTP hydrolysis. GEFs and GAPs are multidomain proteins that are regulated by extracellular signals and localized cues that control cellular events in time and space. Recent evidence suggests that these proteins may be potential therapeutic targets for developing drugs to treat various diseases, including cancer.

Gain-of-function RAF1 mutations cause Noonan and LEOPARD syndromes with hypertrophic cardiomyopathy

Noonan and LEOPARD syndromes are developmental disorders with overlapping features, including cardiac abnormalities, short stature and facial dysmorphia. Increased RAS signaling owing to PTPN11, SOS1 and KRAS mutations causes approximately 60% of Noonan syndrome cases, and PTPN11 mutations cause 90% of LEOPARD syndrome cases. Here, we report that 18 of 231 individuals with Noonan syndrome without known mutations (corresponding to 3% of all affected individuals) and two of six individuals with LEOPARD syndrome without PTPN11 mutations have missense mutations in RAF1, which encodes a serine-threonine kinase that activates MEK1 and MEK2. Most mutations altered a motif flanking Ser259, a residue critical for autoinhibition of RAF1 through 14-3-3 binding. Of 19 subjects with a RAF1 mutation in two hotspots, 18 (or 95%) showed hypertrophic cardiomyopathy (HCM), compared with the 18% prevalence of HCM among individuals with Noonan syndrome in general. Ectopically expressed RAF1 mutants from the two HCM hotspots had increased kinase activity and enhanced ERK activation, whereas non-HCM-associated mutants were kinase impaired. Our findings further implicate increased RAS signaling in pathological cardiomyocyte hypertrophy.

Germline gain-of-function mutations in RAF1 cause Noonan syndrome

Noonan syndrome is characterized by short stature, facial dysmorphia and a wide spectrum of congenital heart defects. Mutations of PTPN11, KRAS and SOS1 in the RAS-MAPK pathway cause approximately 60% of cases of Noonan syndrome. However, the gene(s) responsible for the remainder are unknown. We have identified five different mutations in RAF1 in ten individuals with Noonan syndrome; those with any of four mutations causing changes in the CR2 domain of RAF1 had hypertrophic cardiomyopathy (HCM), whereas affected individuals with mutations leading to changes in the CR3 domain did not. Cells transfected with constructs containing Noonan syndrome-associated RAF1 mutations showed increased in vitro kinase and ERK activation, and zebrafish embryos with morpholino knockdown of raf1 demonstrated the need for raf1 for the development of normal myocardial structure and function. Thus, our findings implicate RAF1 gain-of-function mutations as a causative agent of a human developmental disorder, representing a new genetic mechanism for the activation of the MAPK pathway.

Integrating signals from RTKs to ERK/MAPK

Signals received at the cell surface must be properly transmitted to critical targets within the cell to achieve the appropriate biological response. This process of signal transduction is often initiated by receptor tyrosine kinases (RTKs), which function as entry points for many extracellular cues and play a critical role in recruiting the intracellular signaling cascades that orchestrate a particular response. Essential for most RTK-mediated signaling is the engagement and activation of the mitogen-activated protein kinase (MAPK) cascade comprised of the Raf, MEK and extracellular signal-regulated kinase (ERK) kinases. For many years, it was thought that signaling from RTKs to ERK occurred only at the plasma membrane and was mediated by a simple, linear Ras-dependent pathway. However, the limitation of this model became apparent with the discovery that Ras and ERK can be activated at various intracellular compartments, and that RTKs can modulate Ras/ERK signaling from these sites. Moreover, ERK scaffolding proteins and signaling modulators have been identified that play critical roles in determining the strength, duration and location of RTK-mediated ERK signaling. Together, these factors contribute to the diversity of biological responses generated by RTK signaling.

The spectrum of cardiac anomalies in Noonan syndrome as a result of mutations in the PTPN11 gene

OBJECTIVE

Noonan syndrome is a clinically homogeneous but genetically heterogeneous condition. Type 1 Noonan syndrome is defined by the presence of a mutation in the PTPN11 gene, which is found in approximately 40% of the cases. Phenotype descriptions and cardiac defects from cohorts with Noonan syndrome were delineated in the "pregenomic era." We report the heart defects and links to gene dysfunction in cardiac development in a large cohort of patients with type 1 Noonan syndrome.

METHODS

This was a retrospective, multicenter study based on clinical history, pictures, and medical and cardiologic workup over time. Data were collected by referral geneticists. Mutation screening was performed by direct sequencing of exons 2, 3, 4, 7, 8, 12, and 13 and their intron-exon boundaries, which harbor 98% of identified mutations the PTPN11 gene.

RESULTS

A PTPN11 gene mutation was identified in 104 (38.25%) of 274 patients with Noonan syndrome. Heart defect was present in 85%. The most prevalent congenital heart defects were pulmonary valve stenosis (60%), atrial septal defect, ostium secundum type (25%), and stenosis of the peripheral pulmonary arteries (in at least 15%). Pulmonary valve stenosis and atrial septal defect, ostium secundum type, were significantly associated with the identification of a mutation in the PTPN11 gene. Ventricular septal defect and most left-sided heart defects showed a trend toward overrepresentation in the group without a mutation.

CONCLUSION

We compared our data with previous series and integrated the comprehension of molecular PTPN11 gene dysfunction in heart development.