In a girl with variable features of cardiofaciocutaneous syndrome (CFC2; 615278), Stark et al. (2012) identified a de novo heterozygous 439A-G transition in exon 4 of the KRAS gene, resulting in a lys147-to-glu (K147E) substitution in a highly conserved residue close to known mutations. Lys147 is part of a motif involved in the binding network for guanine nucleotides, which determine the activation state of RAS proteins. In vitro studies demonstrated that the K147E mutant protein predominates in the active GTP-bound form, probably due to facilitated uncatalyzed GDP/GTP exchange. The patient was 1 of a female dizygotic twin pair; the other twin was unaffected. The patient had a high birth weight, macrocephaly, and abnormal craniofacial features, including proptosis, hypertelorism, downslanting palpebral fissures, low-set ears, and short neck, suggestive of Noonan syndrome. Reexamination at age 3.5 years showed coarser facial features more consistent with CFC. She also had hypertrophy of the interventricular myocardial septum, myocardial hypertrophy, and pulmonic stenosis. She had mildly delayed development.
K147 is a conserved amino acid within a motif required for guanine base binding by KRAS. K147 is also ubiquitinated, leading to increased KRAS activation by GEF proteins. Using in vitro assays and transfected COS-7 cells, Cirstea et al. (2013) found that the K147E mutation significantly increased nucleotide dissociation in KRAS, generating a self-activating protein that acted independently of upstream signaling. However, overactivity of K147E mutant KRAS was subject to normal downregulation by RasGAP (see 139150) and had 2-fold lower affinity for RAF1 kinase (164760).
