Molecular Pathogenesis
The four genes currently known to be associated with cardiofaciocutaneous (CFC) syndrome are in the Ras/mitogen-activated protein kinase (MAPK) signaling cascade. The MAPK signaling cascade of dual-specificity kinases [Rauen et al 2011] (see Figure 1) is highly conserved among eukaryotic organisms and is critically involved in cell proliferation, differentiation, motility, apoptosis, and senescence. The Ras/Raf/MEK/ERK signal transduction pathway is activated by extracellular stimuli. Activated Ras recruits Raf, the first kinase of the cascade, to the cell membrane. Activated Raf phosphorylates MEK1 (encoded by MAP2K1) and/or MEK2 (encoded by MAP2K2), which then phosphorylates ERK1 and/or ERK2 (aka MAPK). Noonan syndrome has been associated with pathogenic variants in PTPN11 (protein product SHP2), SOS1, SOS2, RAF1 (protein product CRAF), NRAS, RIT1, LZTR1, RRAS and KRAS. Pathogenic variants in HRAS are causative for Costello syndrome. CFC syndrome is associated with pathogenic variants in BRAF, MAP2K1, and MAP2K2. Because KRAS pathogenic variants were identified in individuals clinically diagnosed with CFC syndrome or with Noonan syndrome [Niihori et al 2006, Schubbert et al 2006], the role of its protein product GTPase KRas (KRAS) in CFC syndrome has yet to be clarified.
BRAF
Gene structure.
BRAF encodes BRAF, a member of the Raf family, which also includes CRAF and ARAF encoded by the X-linked gene ARAF. BRAF spans approximately 190 kb and contains 18 exons. For a detailed summary of gene and protein information, see Table A, Gene.
Pathogenic variants. The spectrum of BRAF pathogenic variants in individuals with CFC syndrome is similar to the spectrum of somatic pathogenic variants observed in cancer. However, pathogenic variants associated with CFC syndrome are more widely distributed within the gene and many are novel, never having been identified in cancer. Pathogenic variants are heterogeneous and cluster mainly in two regions, the cysteine-rich domain of the CR1 and the protein kinase domain. Nearly all pathogenic variants published to date have been de novo missense variants. However, rare in-frame deletions have been identified in BRAF exon 11 [Yoon et al 2007].
Table 2.
Selected BRAF Pathogenic Variants
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DNA Nucleotide Change | Predicted Protein Change | Reference Sequences |
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c.770A>G | p.Gln257Arg 1 |
NM_004333.4
NP_004324.2
|
c.1399T>G | p.Ser467Ala 1 |
c.1408_1410del | p.Thr470del 1 |
c.1455G>C | p.Leu485Phe 2 |
c.1600G>C | p.Gly534Arg 2 |
c.1787G>T | p.Gly596Val |
c.1799T>A | p.Val600Glu 3 |
c.1799T>G | p.Val600Gly |
Variants listed in the table have been provided by the author. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
- 1.
- 2.
Associated with Costello syndrome; see .
- 3.
Normal gene product. The protein product of BRAF is BRAF, a serine/threonine protein kinase that is one of the many direct downstream effectors of Ras. The Raf/MEK/ERK module of kinases is critically involved in cell proliferation, differentiation, motility, apoptosis, and senescence. BRAF has only two known downstream effectors, mitogen-activated protein kinase 1 and 2 (also known as MEK1 and MEK2). There are three conserved regions in BRAF. Conserved region 1 (CR1) contains the Ras binding domain and the cysteine-rich domain, both of which are required for recruitment of BRAF to the cell membrane. CR2 is the smallest of the conserved regions and CR3 is the kinase domain containing the glycine-rich loop (exon 11) and the activation segment (exon 15) of the catalytic domain.
Abnormal gene product. The type of BRAF pathogenic variants found in CFC syndrome is similar to the types of somatic pathogenic variants found in cancers with high kinase and kinase-impaired activities [Niihori et al 2006, Rodriguez-Viciana et al 2006]. In addition, CFC syndrome BRAF mutated proteins activate downstream effectors in vitro, as determined by measuring phosphorylated species of MEK and ERK. Both cancer and CFC syndrome-associated BRAF mutated proteins with elevated kinase activity induce higher levels of MEK and ERK phosphorylation compared with wild-type BRAF, whereas kinase-impaired BRAF mutated proteins are impaired in their ability to induce phosphorylation of MEK and ERK [Rodriguez-Viciana et al 2006]. The most common BRAF pathogenic variant identified in cancer, p.Val600Glu, has not been identified in CFC syndrome. Presumably, such a gain-of-function variant would be incompatible with life. However, a germline p.Val600Gly pathogenic variant has recently been reported in CFC [Champion et al 2011] and, like the BRAF p.Val600Glu pathogenic variant, has also been reported in cancer.
Cancer and benign tumors – solid tumors. Somatic pathogenic variants in BRAF have been reported in approximately 8% of tumors with BRAF most frequently mutated in melanoma, thyroid, colorectal, and ovarian. The vast majority of BRAF pathogenic variants are missense substitutions found in (but not limited to) exon 11 (the glycine-rich loop) and exon 15 (the activation segment) in the BRAF kinase domain [Wellbrock et al 2004]. One pathogenic variant, p.Val600Glu, which results in increased kinase activity, accounts for more than 90% of BRAF pathogenic variants identified in human cancer. Somatic BRAF p.Val600Glu pathogenic variants are also found in benign nevi and premalignant colon polyps. The common p.Val600Glu cancer pathogenic variant has never been identified in CFC syndrome. However, a BRAF p.Val600Gly pathogenic variant has been reported recently in an individual with CFC syndrome [Champion et al 2011].
MAP2K1, MAP2K2
Gene structure. MEK, like Raf, exists as a multigene family. MAP2K1 spans approximately 104 kb. MAP2K2 spans approximately 34 kb. Each gene contains 11 exons. For a detailed summary of gene and protein information, see Table A, Gene.
Pathogenic variants. Missense variants in MAP2K1 and MAP2K2 cause CFC syndrome in approximately 25% of clinically diagnosed individuals. Pathogenic variants are heterogeneous missense substitutions, with the majority identified in exons 2 and 3 of both MAP2K1 and MAP2K2. The amino acid substitutions in MEK1 and MEK2 are similar, suggesting that the functional consequences in the two family isoforms may be similar.
Table 3.
Selected MAP2K1 Pathogenic Variants
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DNA Nucleotide Change | Predicted Protein Change | Reference Sequences |
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c.389A>G | p.Tyr130Cys |
NM_002755.3
NP_002746.1
|
c.199G>A | p.Asp67Asn |
c.171G>T | p.Lys57Asn |
Variants listed in the table have been provided by the author. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
Table 4.
Selected MAP2K2 Pathogenic Variants
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DNA Nucleotide Change | Predicted Protein Change | Reference Sequences |
---|
c.170T>G | p.Phe57Cys 1 |
NM_030662.3
NP_109587.1
|
c.383C>A | p.Pro128Gln |
c.401A>G | p.Tyr134Cys 1 |
Variants listed in the table have been provided by the author. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
- 1.
Normal gene product.
MAP2K1 and MAP2K2 encode threonine/tyrosine kinases with both isoforms having the ability to activate ERK1 and ERK2. MAP2K1 encodes the mitogen activated protein kinase 1 (MEK1). MAP2K2 encodes MEK2. The proteins have approximately 85% amino acid identity. MEK1 and MEK2 proteins do not serve redundant purposes as determined in mouse development.
Abnormal gene product. In vitro functional studies of MEK proteins encoded by CFC syndrome-associated MAP2K1 and MAP2K2 pathogenic variants determine that they overstimulate ERK phosphorylation compared to wild-type MEK; however, they are less stimulating than artificially generated constitutively active MEK pathogenic variants [Rodriguez-Viciana et al 2006].
Cancer and benign tumors. The first functional MAP2K1 pathogenic variant, p.Asp67Asn, was identified in an ovarian cancer cell line with functional studies determining that this mutated protein has increased activity as measured by an increase in ERK phosphorylation [Estep et al 2007]. Subsequently, MAP2K1 p.Lys57Asn pathogenic variants were identified in non-small-cell lung carcinoma [Marks et al 2008]. Since these initial publications, somatic pathogenic variants in MAP2K1 and MAP2K2 have been reported in various tumor types.
KRAS
Gene structure.
KRAS has four coding exons with intervening sequences and spans approximately 45 kb. Two alternative splice variants exist, with KRAS4b being ubiquitously expressed. For a detailed summary of gene and protein information, see Table A, Gene.
Pathogenic variants. Germline missense KRAS variants, which are distinct from those identified in cancer, have been identified in coding exons 1, 2, and 4b [Carta et al 2006, Niihori et al 2006, Schubbert et al 2006, Zenker et al 2007].
Normal gene product. The GTPase KRAS belongs to a large superfamily of small GTPases; it and its major counterparts H-Ras and N-Ras are the most extensively studied of the Ras proteins. Ras proteins regulate cell growth, proliferation, and differentiation. Ras activates several downstream cascades, some of which include the mitogen-activated protein kinase (MAPK), phosphotidylinositol 3-kinase (PI3K), RAL guanine nucleotide dissociation stimulator (RALGDS), and phospholipase Cε (PLCε).
Abnormal gene product. Abnormal protein products deregulate single transduction and cause growth factor hypersensitivity of hematopoietic cells. Functional studies of NS/CFC syndrome-associated KRAS pathogenic variants revealed reduced intrinsic GTPase activity compared to the wild-type protein, although not to the level of mutated K-Ras protein typically found in cancer [Schubbert et al 2006]. Such a gain-of-function pathogenic variant would presumably be incompatible with life.
Cancer and benign tumors. Aberrant activation of Ras is frequently found in cancer, occurring in approximately 20% of all tumors. The vast majority of oncogenic pathogenic variants occur in hotspots in codons 12, 13, or 61. These are not the same pathogenic variants found in Noonan syndrome or CFC syndrome. Single-nucleotide variants in KRAS account for approximately 85% of pathogenic variants in the Ras gene family. NRAS (~15% of total) and HRAS (~1% of total) pathogenic variants are found less frequently. Amino acid substitutions caused by pathogenic missense variants in KRAS affect guanine nucleotide binding and cause a reduction of GTP hydrolysis, resulting in a gain of function of the protein.