Molecular basis for incomplete penetrance of familial retinoblastoma

AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA 2777 We previously showed that kindreds with low penetrant (lp) retinoblastoma carry unique mutant RB alleles which are null for E2F1 binding but retain partial tumor suppressor activity. To define the molecular basis for incomplete penetrance, we now show that three common lp alleles representing single codon alterations (R661W, C712R, and del480) exhibit >50% wildtype RB activity as measured by i) site-specific cyclin-mediated phosphorylation at key regulatory serine residues, ii) by transcriptional activation in mammalian cells, and iii) by ‘flat cell’ differentiation in mammalian cells in vivo. These data demonstrate that these lp alleles, present in 30 different lp kindreds, retain substantially greater functional activity than previously suggested using in vitro GST-based binding assays. We, therefore, hypothesize that the molecular basis for incomplete penetrance associated with partially active RB alleles may result from the need for an additional genetic/epigenetic event for tumorigenesis that is not required for null RB alleles. Retinoblastoma and small-cell carcinoma are the only tumors that are commonly associated (>90%) with mutational inactivation of RB. Although retinoblastoma tumors from lp families were not available for this analysis, we have studied a small-cell tumor that is homozygous for a common lp RB allele. Genetic analysis of this tumor has allowed us to test several predictions about the molecular basis of incomplete penetrance and has identified candidate pathways for this phenotype.