We are studying the role of the RB/p16 tumor suppressor pathway in human cancer focusing primarily on the model of lung cancer. We have continued to identify novel subtle mutations within the RB gene in lung cancer samples to study their functional properties. We have also examined novel mutant RB alleles from families with low penetrance of familal retinoblastoma to study the molecular basis for the phenotype of incomplete penetrance. We have recently demonstrated that many families with low penetrance carry unstable mutant alleles with temperature-sensitive pocket binding activity which suggest a model for the variable frequency of tumor progression in these patients. A major goal of this work is to define more precisely tumor suppressor domains within the RB product. For example, several low-penetrant RB alleles retain 'cell differentiation properties', but lack E2F binding. We have also characterized the roles of several other candidate 'lung cancer' genes, including the RB-related gene family, SV40 large T, and FHIT. In addition, we had previously established that hypermethylation and gene silencing of p16 occurs in 40% of human non-small cell lung cancers and can be reversed by decitabine therapy. At the Surgery Branch with Dr. Schrump we have begun a phase 1 clinical trial with decitabine to determine clinical response in lung cancer and to test the ability to hypomethylate specific genetic loci in vivo. We also have generated a series of mouse models to 1) to determine the effect of RB versus p16 inactivation as the initial genetic hit in a murine lung cancer model, 2) to study the genetic basis of neuroendocrine murine tumors, 3) to develop a mouse model for incomplete pentrance of RB, and 4) to detemine the biological relevance of a conserved caspase cleavage site near the C-terminus of the RB product.
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