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Costello syndrome (CS) is a developmental disorder characterized by postnatal reduced growth, facial dysmorphism, cardiac defects, mental retardation and skin and musculo-skeletal defects. CS is caused by  HRAS  germline mutations. In the majority of cases, mutations affect Gly 12  and Gly 13  and are associated with a relatively homogeneous phenotype. The same amino acid substitutions are well known as somatic mutations in human tumors and promote constitutive HRAS activation by impairing its GTPase activity. In a small number of cases with mild phenotype, a second class of substitutions involving codons 117 and 146 and affecting GTP/GDP binding has been described. Here, we report on the identification and functional characterization of two different three-nucleotide duplications resulting in a duplication of glutamate 37 (p.E37dup) associated with a homogeneous phenotype reminiscent of CS. Ectopic expression of HRAS E37dup  in COS-7 cells resulted in enhanced growth factor-dependent stimulation of the MEK-ERK and phosphoinositide 3-kinase (PI3K)-AKT signaling pathways. Recombinant HRAS E37dup  was characterized by slightly increased GTP/GDP dissociation, lower intrinsic GTPase activity and complete resistance to neurofibromin 1 GTPase-activating protein (GAP) stimulation due to dramatically reduced binding. Co-precipitation of GTP-bound HRAS E37dup  by various effector proteins, however, was inefficient because of drastically diminished binding affinities. Thus, although HRAS E37dup  is predominantly present in the active, GTP-bound state, it promotes only a weak hyperactivation of downstream signaling pathways. These findings provide evidence that the mildly enhanced signal flux through the MAPK and PI3K-AKT cascades promoted by these disease-causing germline  HRAS  alleles results from a balancing effect between a profound GAP insensitivity and inefficient binding to effector proteins.

human molecular genetics

Duplication of Glu 37  in the switch I region of HRAS impairs effector/GAP binding and underlies Costello syndrome by promoting enhanced growth factor-dependent MAPK and AKT activation