Deletion of RB exons 24 and 25 causes low-penetrance retinoblastoma.
A deletion in the tumor-suppressor gene, RB, 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 (delta24-25) is the largest ever observed in an LP retinoblastoma family. Unlike the usual RB mutations, which cause retinoblastoma in 95% of at-risk eyes and yield no detectable protein, the delta24-25 allele transcribed a message splicing exon 23 to exon 26, resulting in a detectable protein (pRBdelta24-25) 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 delta24-25-nuclear localization and repression of E2F-consistent with the idea that LP mutations generate "weak alleles" by reducing but not eliminating essential activities. However, delta24-25 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"). pRBdelta24-25 was also completely defective in suppressing growth of Saos-2 osteosarcoma cells. Targeting pRBdelta24-25 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 delta24-25 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.
Bremner
R
R
Eye Research Institute of Canada, Department of Ophthalmology, University of Toronto, Ontario. rbremner@playfair.utoronto.ca
Du
D C
DC
Connolly-Wilson
M J
MJ
Bridge
P
P
Ahmad
K F
KF
Mostachfi
H
H
Rushlow
D
D
Dunn
J M
JM
Gallie
B L
BL
D002170:Canada; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D002455:Cell Division; D002467:Cell Nucleus / Q000737:chemistry; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D005091:Exons / Q000235:genetics*; D005260:Female; D005786:Gene Expression Regulation / Q000235:genetics; D015972:Gene Expression Regulation, Neoplastic / Q000235:genetics; D016161:Genes, Retinoblastoma / Q000235:genetics*; D006801:Humans; D008297:Male; D008545:Melanoma / Q000235:genetics; D009687:Nuclear Proteins; D012516:Osteosarcoma; D010375:Pedigree; D011296:Prenatal Diagnosis; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051736:Proto-Oncogene Proteins c-mdm2; D011993:Recombinant Fusion Proteins; D012175:Retinoblastoma / Q000175:diagnosis / Q000235:genetics*; D016160:Retinoblastoma Protein / Q000032:analysis / Q000378:metabolism; D056465:Retinoblastoma-Binding Protein 1; D017384:Sequence Deletion / Q000235:genetics*; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000378:metabolism; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D011993:Recombinant Fusion Proteins; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; C497206:MDM2 protein, human; D051736:Proto-Oncogene Proteins c-mdm2