Contribution of RIT1 Mutations to the Pathogenesis of Noonan Syndrome: Four New Cases and Further Evidence of Heterogeneity

Noonan syndrome (NS) is a common developmental disorder presenting with dysmorphic craniofacial features, heart defects, and short stature. It belongs to the group of RASopathies caused by germline mutations in genes encoding proteins involved in the RAS/MAPK signaling pathway. Although mutations in nine genes are known to cause NS, approximately 30% of the cases still have unexplained etiology. To identify the new causative genes, 42 patients with a clinical diagnosis of NS, who had negative results on Sanger sequencing of PTPN11, SOS1, and RAF1 (the most common NS genes), were selected for whole exome sequencing. In two patients, mutations in recently described new NS gene-RIT1 were found (c.244T>G [p.Phe82Val] and c.270G>C [p.Met90Ile]). Further analysis of a larger cohort (n = 64) of NS patients with classic Sanger sequencing revealed the presence of RIT1 mutation c.284G>C (p.Gly95Ala) in two additional patients. All the detected mutations were localized in switch II domain responsible for GTPase activity. The modeling of RIT1 protein structure revealed that the mutated amino acids and their interacting residues are evolutionary conserved and any residue replacement might change the structural stability and/or protein internal dynamics influencing catalytic activity of the protein. It seems that the identified mutations might alter protein function and therefore, the activity of ERK and P38 MAPK pathways, thus underlying the specific phenotype observed in NS patients. Our study independently confirms the role of RIT1 in the pathogenesis of Noonan syndrome. (C) 2014 Wiley Periodicals, Inc.