Inactivation of the human retinoblastoma tumor-suppressor gene (RB1; MIM # 180200) is usually caused by mutations in the coding region, but mutations in the promoter region can also affect RB1 gene expression. A 103-bp deletion in the promoter region in a prostate cancer was reported to abrogate RB1 promoter activity . In addition, we found two distinct point mutations in the core promoter region of the RB1 gene at an ATF site and a retinoblastoma binding factor 1 (RBF-1) site from two low-penetrance retinoblastoma families, both of which caused a large reduction of the RB1 promoter activity . We have also reported that E4TF1/hGABP transcription factor preferentially binds to the RBF-1 site and can stimulate the RB1 promoter activity through the RBF-1 site . Cowell et al. reported a family with a novel mutation in a Sp1 site of the RB1 promoter at -150 to -155 relative to the initiating codon . Similar to the previously reported point mutations affecting other transcription factor binding sites , that family had members with only unilateral retinoblastoma and there were many unaffected carriers, indicating that the mutation was associated with reduced penetrance. We have speculated that the reason why point mutations in the promoter region cause fewer tumors compared to null mutations elsewhere in the translational unit is that they may be hypomorphic alleles with a minimal residual level of tumor suppressor function .
In this letter, we report our analysis of a total of 308 DNA samples (98 samples from primary retinoblastoma tumors and 210 samples from peripheral blood or fibroblasts of bilateral and/or hereditary retinoblastoma patients) that were expected to have abnormalities in the RB1 gene. These samples were analyzed by the SSCP technique to search for point mutations in the RB1 promoter from -327 to -89 (numbering is based on the first base of the initiation codon and is the