INCOMPLETE PENETRANCE OF FAMILIAL RETINOBLASTOMA LINKED TO GERM-LINE MUTATIONS THAT RESULT IN PARTIAL LOSS OF RB FUNCTION

To study the molecular basis for the clinical phenotype of incomplete penetrance of familial retinoblastoma, we have examined the functional properties of three RB mutations identified in the germ line of five different families with low penetrance, RB mutants isolated from commo n adult cancers and from classic familial retinoblastoma (designated a s classic RB mutations) are unstable and generally do not localize to the nucleus, do not undergo cyclin-dependent kinase (cdk)-mediated hyp erphosphorylation, show absent protein ''pocket'' binding activity, an d do not suppress colony growth of RB(-) cells, In contrast, two low-p enetrant alleles (661W and ''deletion of codon 480'') retained the abi lity to localize to the nucleus, showed normal cdk-mediated hyperphosp horylation in vivo, exhibited a binding pattern to simian virus 40 lar ge T antigen using a quantitative yeast two-hybrid assay that was inte rmediate between classic mutants (null) and wild-type RE, and had abse nt E2F1 binding in vitro, A third, low-penetrant allele, ''deletion of RB exon 4,'' showed minimal hyperphosphorylation in vivo but demonstr ated detectable E2F1 binding in vitro, In addition, each low-penetrant RE mutant retained the ability to suppress colony growth of RB(-) tum or cells. These? findings suggest two categories Of mutant, low-penetr ant RE alleles, Class 1 alleles correspond to promotor mutations, whic h are believed to result in reduced or deregulated levels of wild-type RE: protein, whereas class 2 alleles result in mutant proteins that r etain partial activity, Characterization of the different subtypes of class 2 low-penetrant genes may help to define more precisely function al domains within the RE product required for tumor suppression.