This report describes Noonan syndrome with multiple lentigines 2 (NSML2, previously LEOPARD syndrome 2), which is a subtype of Noonan syndrome with multiple lentigines; NSML2 exhibits autosomal dominant inheritance. The human gene implicated in this disease is RAF1, which encodes Raf-1 proto-oncogene, serine/threonine kinase, a kinase that acts within the RAS-RAF-MEK-ERK pathway. RAF1 is also associated with the diseases Noonan syndrome 5 (OMIM:611553, FBhh0000131) and dilated cardiomyopathy-1NN (OMIM:615916, FBhh0000156). See, in addition, FBhh0000558, a fly model of RAS-RAF-MEK-ERK-pathway-related cancer using RAF1. There is one high-scoring fly ortholog, Raf, for which RNAi targeting constructs, alleles caused by insertional mutagenesis, and classical amorphic alleles have been generated. Dmel\Raf is orthologous to two additional human genes, ARAF and BRAF. BRAF is also associated with a form of Noonan syndrome (see FBhh0001344).
Multiple transgenic constructs of the human Hsap\RAF1 gene have been introduced into flies, including wild-type, genes carrying disease-implicated variants, gain-of-function RAF1, and genes carrying an N-terminal deletion. Functional conservation between the human and fly genes has been demonstrated in fly several systems; for example, an activated form of the human gene has been shown to recapitulate phenotypes observed for an activated form of, or overexpression of, the fly gene. Variant(s) implicated in human disease tested (as transgenic human gene, RAF1): the S275L and and L613V variant forms of the human gene have been introduced into flies; these variants are also implicated in Noonan syndrome 5 (FBhh0000131). Experiments with the human variants include assessments of pharmaceutical candidates.
Hypertrophic cardiomyopathy, which is frequently observed in NSML2 patients, has been investigated in fly models using the human Hsap\RAF1 gene and the fly Raf gene; see 'cardiomyopathy, hypertrophic (postulated), RAF-related' (FBhh0000737).
Animals homozygous for amorphic mutations of Dmel\Raf exhibit lethality in the late larval stage; imaginal discs are undeveloped. Embryos lacking all Dmel\Raf activity (derived from homozygous null germline clones in the mother and not rescued by paternal contribution) die in early embryogenesis. Many physical and genetic interactions have been described for Dmel\Raf; see below and in the gene report for Raf.
[updated May 2021 by FlyBase; FBrf0222196]
The phenotypes typically observed include multiple lentigines (small pigmented skin spots), electrocardiographic conduction abnormalities, ocular hypertelorism, pulmonic stenosis, abnormal genitalia, retardation of growth, and sensorineural deafness (Gorlin et al., 1969; pubmed:5771505). [from OMIM:151100; 2020.08.11]
[LEOPARD SYNDROME 2; LPRD2](https://omim.org/entry/611554)
[RAF1 PROTOONCOGENE, SERINE/THREONINE KINASE ; RAF1](https://omim.org/entry/164760)
Of 17 Noonan syndrome patients with a RAF1 mutation in either of two hotspots (clustering around ser259 or ser612), 16 (94%) had hypertrophic cardiomyopathy (CMH; see OMIM:192600), compared with an 18% prevalence of CMH among Noonan syndrome patients in general (Pandit et al., 2007, pubmed:17603483). Pandit et al. (2007) also scanned RAF1 exons mutated in Noonan and LEOPARD syndrome patients in 241 unrelated individuals with nonsyndromic CMH who did not have mutations in 8 myofilament genes known to cause CMH, and the authors identified a thr260-to-ile mutation in the RAF1 gene in one patient (Pandit et al., 2007, pubmed:17603483). [from OMIM:164760, 2016.01.19]
A large percentage (70%-90%) of persons with Noonan syndrome and a RAF1 pathogenic variant have hypertrophic cardiomyopathy (HCM), which is significantly higher than the 20% with HCM observed in the general NS population. Moreover, this genotype-phenotype correlation seems to be domain-specific, as HCM appears to be associated with pathogenic variants affecting the region encoding the N-terminal 14-3-3 consensus site or the C-terminus. [from GeneReviews, Noonan Syndrome with Multiple Lentigines, pubmed:20301557 2016.01.20]
RAF1 is a serine-threonine kinase that activates MEK1 (OMIM:176872) and MEK2 (OMIM:601263). Ectopically expressed RAF1 mutants from the two hypertrophic cardiomyopathy (CMH; see OMIM:192600) hotspots (around ser259 or ser612) linked to LEOPARD syndrome 2 and/or Noonan syndrome 5 had increased kinase activity and enhanced ERK (see 176948) activation, whereas non-CMH-associated mutants were kinase impaired. (Pandit et al., 2007, pubmed:17603483). Mutations in the CR2 domain, but not the CR3 domain, of RAF1 are associated with hypertrophic cardiomyopathy (Razzaque et al., 2007, pubmed:17603482). [From OMIM:611553 and OMIM:164760, 2016.01.19]
RAF1 is ubiquitously expressed and encodes a protein of 648 amino acids with three conserved regions (CR). CR1 contains a Ras-binding domain; CR2 is a site of regulatory phosphorylation and association with the 14-3-3 protein. CR1 and CR2 both have negative regulatory function, removal of which results in oncogenic activity. The kinase domain, CR3, also associates with 14-3-3. The protein is highly regulated with numerous serine and threonine residues that can be phosphorylated, resulting in activation or inactivation. The serine at residue 259, which is in CR2, is particularly important. In the inactive state, the N-terminus of RAF1 interacts with and inactivates the kinase domain at the C-terminus. This conformation is stabilized by 14-3-3 protein dimers that bind to phosphorylated Ser259 and Ser261. Dephosphorylation of Ser259 facilitates binding of RAF1 to RAS-GTP and propagation of the signal through the RAS-MAPK cascade via RAF1 MEK kinase activity. [from GeneReviews, Noonan Syndrome with Multiple Lentigines, pubmed:20301557 2016.01.20]
Many to one: 3 human to 1 Drosophila.
Ortholog of human BRAF, ARAF, and RAF1 (1 Drosophila to 3 human).
Dmel\Raf shares 43% identity and 54% similarity with human BRAF, 44% identity and 58% similarity with human ARAF, and 47% identity and 60% similarity with human RAF1.