Oncosuppressor genes: mechanisms of inactivation for the retinoblastoma gene and p53.

Mutations that affect oncosuppressor genes contribute to transformed phenotype. The recessive characteristics of these genes require mutations on both alleles. For this reason alterations of the oncosuppressor genes can be transmitted with the germ line. In this review we focus on mechanisms of inactivation of the retinoblastoma gene and p53. The products of these genes are nuclear phosphoproteins involved in controlling cellular proliferation. Inactivated Rb genes have been found in several tumor types. The phosphorylation of the Rb gene product, p105Rb, is regulated by cyclin-dependent kinases in accordance with the cell cycle. Hypophosphorylated wild-type p105Rb is tightly bound to the nuclear matrix and seems to be critical in inhibition of cellular proliferation. Hyperphosphorylation is a physiological mechanism of inactivation of p105Rb. The p53 gene product is a transcriptional factor that blocks the progression of cell division. Mutations in the p53 gene are frequently found in many human cancers and are localized in the highly conserved region of the gene. The protein product of the mutated gene looses its function as a negative regulator of cellular proliferation. The wild-type protein can also be inactivated by forming stable complex with a mutated p53 protein or with the SV large T antigen.