Deletion of RB exons 24 and 25 causes low-penetrance retinoblastoma

A deletion in the tumor-suppressor gene, RE, discovered by quantitative multiplex PCR, shows low penetrance (LP), since only 39% of eyes at risk in this family develop retinoblastoma. The 4-kb deletion spanning exons 24 and 25 (Delta 24-25) is the largest ever observed in an LP retinoblastoma family, Unlike the usual RE mutations, which cause retinoblastoma in 95% of at-risk eyes and yield no detectable protein, the Delta 24-25 allele transcribed a message splicing exon 23 to exon 26, resulting in a detectable protein (pRB(Delta 24-25)) that lacks 58 amino acids from the C-terminal domain, proving that this domain is essential for suppression of retinoblastoma. Two functions were partially impaired by Delta 24-25-nuclear localization and repression of E2F-consistent with the idea that LP mutations generate ''weak alleles'' by reducing but not eliminating essential activities. However, Delta 24-25 ablated interaction of pRB with MDM2. Since a homozygous LP allele is considered nontumorigenic, the pRB/MDM2 interaction may be semi-or nonessential for suppressing retinoblastoma. Alternatively, some homozygous LP alleles may not cause tumorigenesis because an additional event is required (the ''three-hit hypothesis''), or the resulting imbalance in pRB function may cause apoptosis (the ''death allele hypothesis''), pRB(Delta 24-25) also completely defective in suppressing growth of Saos-2 osteosarcoma cells. Targeting pRB(Delta 24-25) to the nucleus did not improve Saos-2 growth suppression, suggesting that C-terminal domain functions other than nuclear localization are essential for blocking proliferation in these cells. Since Delta 24-25 behaves like a null allele in these cells but Like an LP allele in the retina, pRB may use different mechanisms to control growth in different cell types.