Costello syndrome (CS) is a rare congenital disorder characterized by failure to 
thrive, craniofacial dysmorphisms, cardiac and skin abnormalities, mental 
retardation, and predisposition to malignancies. CS is caused by heterozygous 
gain-of-function mutations in HRAS that also occur as somatic alterations in 
human tumors. HRAS is one of the three classical RAS proteins and cycles between 
an active, GTP- and an inactive, GDP-bound conformation. We used primary human 
skin fibroblasts from patients with CS as a model system to study the functional 
consequences of HRAS mutations on endogenous signaling pathways. The GTP-bound 
form of HRAS was significantly enriched in CS compared with normal fibroblasts. 
Active HRAS is known to stimulate both the RAF-MEK-ERK and the PI3K-AKT 
signaling cascade. Phosphorylation of MEK and ERK was normal in CS fibroblasts 
under basal conditions and slightly prolonged after epidermal growth factor 
(EGF) stimulation. Interestingly, basal phosphorylation of AKT was increased yet 
more in CS fibroblasts. Moreover, AKT phosphorylation was diminished in the 
early and enhanced in the late phase of EGF stimulation. Taken together, these 
results document that CS-associated HRAS mutations result in prolonged signal 
flux in a ligand-dependent manner. Our data suggest that altered cellular 
response to growth factors rather than constitutive activation of HRAS 
downstream signaling molecules may contribute to some of the clinical features 
in patients with CS.