Temperature-sensitive RB mutations linked to incomplete penetrance of familial retinoblastoma in 12 families

The tumor-suppressor activity of the retinoblastoma protein (RB) is encoded within a protein-binding ("pocket") domain that is targeted for mutations in all cases of familial retinoblastoma and in many common adult cancers. A lthough familial retinoblastoma is a paradigm for a highly penetrant, reces sive model of tumorigenesis, the molecular basis for the phenotype of incom plete penetrance of familial retinoblastoma is undefined. We studied the RE pocket-binding properties of three independent, mutant RE alleles that are present in the germline of 12 kindreds with the phenotype of incomplete pe netrance of familial retinoblastoma. Each arises from alterations of single codons within the RE pocket domain (designated "Delta 480," "661W," or "71 2R"). Under the same conditions, we studied the properties of wild-type (WT ) RE, an RE point mutant isolated from a lung carcinoma sample (706F) and a n adjacent, in vitro-generated point mutant (707W). The Delta 480, 661W, an d 712R mutants lack pocket protein-binding activity in vitro but retain the WT ability to undergo cyclin-mediated phosphorylation in vivo. Each of the low-penetrant RE mutants exhibits marked enhancement of pocket protein bin ding when the cells are grown at reduced temperature. In contrast, in this temperature range, no change in binding activity is seen with WT RE, the 70 6F mutant, or the 707W mutant. We have demonstrated that many families with incomplete penetrance of familial retinoblastoma carry unstable, mutant RE alleles with temperature-sensitive pocket protein-binding activity. The va riable frequency for tumor development in these families may result from re versible fluctuations in a threshold level of RE pocket-binding activity.