Memorial Sloan Kettering Cancer Center, New York
Until recently, the only known germline oncogenic mutations known to cause human cancers involved RET, which confers predisposition to MEN2 and FmTC. We now know that distinct germline activating mutations of genes encoding effectors in the Ras signaling pathway are involved in the pathogenesis of a group of related human developmental disorders: PTPN11 in Noonan syndrome (NS), HRAS in Costello syndrome (CS) and either KRAS, BRAF, MEK1 or MEK2 in cardio-facio-cutaneous syndrome (CFC). CS is associated with a characteristic facies, severe feeding difficulty, and failure to thrive. CS is the only one of these disorders associated with an increased risk of neoplasia, primarily papillomas, rhabdomyosarcomas and bladder carcinomas. Although most individuals affected with CS have HRAS mutations that encode a mutant G protein with weak transforming activity, one child with a severe form of CS had a germline HRASG12V substitution, which is one of the most common somatic mutations of this gene in human cancers. Although individuals affected with CS do not develop endocrine neoplasms, potentially important insights on mechanisms of tumor initiation can be obtained by studying mouse models that recapitulate this disease. We developed mice with Cre-mediated germline activation of Hras to study CS pathogenesis and the mechanisms of tumor initiation by oncogenic Hras. mice with global endogenous expression of HrasG12V had high perinatal mortality, abnormal cranial dimensions, defective ameloblasts of incisors and molars, nasal septal deviation, epidermal and sebaceous gland hyperplasia of the auditory canal, squamous papillomas of skin and mucosae, and angiosarcomas. Endogenous expression of the mutant oncoprotein was not by itself sufficient to initiate tumor development in any cell type. Indeed, papillomas and angiosarcomas had higher HrasG12V gene copy number, augmented Hras signaling and activation of the DNA damage response as well as of markers of senescence, which were not present in adjacent epidermis. Endogenous expression of HrasG12V was also associated with a higher mutation rate in vivo, as determined by crossing mice with generalized Hras gene activation with a lacZ (“small blue”) mutation reporter mouse. Tumor initiation by endogenous HrasG12V expression likely requires augmentation of signal output, which in papillomas and angiosarcomas is achieved via increased Hras gene copy number, which may in turn be favored by a higher mutation frequency in cells expressing the oncoprotein. Hence, a high threshold of signaling output by the oncoprotein may be required for tumor initiation. This may also apply to other germline oncogenic syndromes, such as mEN2 itself, as an association of RET point mutation with copy number gains has been previously demonstrated.