Attenuation of disease phenotype through alternative translation initiation in low-penetrance retinoblastoma.

Hereditary predisposition to retinoblastoma (RB) is caused by germline mutations in the retinoblastoma 1 (RB1) gene and transmits as an autosomal dominant trait. In the majority of cases disease develops in greater than 90% of carriers. However, reduced penetrance with a large portion of disease-free carrier is seen in some families. Unambiguous identification of the predisposing mutation in these families is important for accurate risk prediction in relatives and their genetic counseling but also provides conceptual information regarding the relationship between the RB1 genotype and the disease phenotype. In this study we report a novel mutation detected in 10 individuals of an extended family, only three of whom are affected by RB disease. The mutation comprises a 23-basepair (bp) duplication in the first exon of RB1 (c.43_65dup) producing a frameshift in exon 1 and premature chain termination in exon 2. Mutations resulting in premature chain termination classically are associated with high penetrance disease, as message translation may not generate functional product and nonsense mediated RNA decay (NMD) frequently eliminates the mutant transcript. However, appreciable NMD does not follow from the mutation described here and transcript expression in tissue culture cells and translation in vitro reveals that alternative in-frame translation start sites involving Met113 and possibly Met233 are used to generate truncated RB1 products (pRB94 and pRB80), known and suspected to exhibit tumor suppressor activity. These results strongly suggest that modulation of disease penetrance in this family is achieved by internal translation initiation. Our observations provide the first example for rescue of a chain-terminating mutation in RB1 through alternative translation initiation.

Selective ablation of retinoblastoma protein function by the RET finger protein.

The retinoblastoma tumor suppressor protein (Rb) affects gene transcription both negatively and positively and through this regulates distinct cellular responses. Although cell cycle regulation requires gene repression, Rb's ability to promote differentiation and part of its antiproliferative activity appears to rely on the activation of gene transcription. We present evidence here that the RET finger protein (RFP)/tripartite motif protein 27 (TRIM 27) inhibits gene transcription activation by Rb but does not affect gene repression. RFP binds to Rb and prevents the degradation of the EID-1 inhibitor of histone acetylation and differentiation. Furthermore, ablation of RFP in U2OS osteosarcoma cells augments a transcriptional program indicative of lineage-specific differentiation in response to Rb. These findings provide precedent for a regulatory pathway that uncouples different Rb-dependent activities and thus silences specific cellular responses to Rb in a selective way.