R. Bremner et al., DELETION OF RB EXON-24 AND EXON-25 CAUSES LOW-PENETRANCE RETINOBLASTOMA, American journal of human genetics, 61(3), 1997, pp. 556-570
A deletion in the tumor-suppressor gene, RE, discovered by quantitativ
e 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 r
etinoblastoma family, Unlike the usual RE mutations, which cause retin
oblastoma 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 ami
no acids from the C-terminal domain, proving that this domain is essen
tial for suppression of retinoblastoma. Two functions were partially i
mpaired by Delta 24-25-nuclear localization and repression of E2F-cons
istent with the idea that LP mutations generate ''weak alleles'' by re
ducing 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 non
essential for suppressing retinoblastoma. Alternatively, some homozygo
us 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 d
id not improve Saos-2 growth suppression, suggesting that C-terminal d
omain functions other than nuclear localization are essential for bloc
king proliferation in these cells. Since Delta 24-25 behaves like a nu
ll allele in these cells but Like an LP allele in the retina, pRB may
use different mechanisms to control growth in different cell types.