Recent evidence indicates the existence of a genetic locus in chromosome region 13q14 that confers susceptibility to retinoblastoma, a cancer of the eye in children. A gene encoding a messenger RNA (mRNA) of 4.6 kilobases (kb), located in the proximity of esterase D, was identified as the retinoblastoma susceptibility (RB) gene on the basis of chromosomal location, homozygous deletion, and tumor-specific alterations in expression. Transcription of this gene was abnormal in six of six retinoblastomas examined: in two tumors, RB mRNA was not detectable, while four others expressed variable quantities of RB mRNA with decreased molecular size of about 4.0 kb. In contrast, full-length RB mRNA was present in human fetal retina and placenta, and in other tumors such as neuroblastoma and medulloblastoma. DNA from retinoblastoma cells had a homozygous gene deletion in one case and hemizygous deletion in another case, while the remainder were not grossly different from normal human control DNA. The gene contains at least 12 exons distributed in a region of over 100 kb. Sequence analysis of complementary DNA clones yielded a single long open reading frame that could encode a hypothetical protein of 816 amino acids. A computer-assisted search of a protein sequence database revealed no closely related proteins. Features of the predicted amino acid sequence include potential metal-binding domains similar to those found in nucleic acid-binding proteins. These results provide a framework for further study of recessive genetic mechanisms in human cancers.

Mutational inactivation of the retinoblastoma susceptibility (RB) gene has been proposed as a crucial step in the formation of retinoblastoma and other types of human cancer. This hypothesis was tested by introducing, via retroviral-mediated gene transfer, a cloned RB gene into retinoblastoma or osteosarcoma cells that had inactivated endogenous RB genes. Expression of the exogenous RB gene affected cell morphology, growth rate, soft agar colony formation, and tumorigenicity in nude mice. This demonstration of suppression of the neoplastic phenotype by a single gene provides direct evidence for an essential role of the RB gene in tumorigenesis.

Introduction of a normal retinoblastoma gene (RB) into retinoblastoma cells was previously shown to suppress several aspects of their neoplastic phenotype, including tumorigenicity in nude mice, thereby directly demonstrating a cancer suppression function of RB. To explore the possibility of a similar activity in a common adult tumor, RB expression was examined in three human prostate carcinoma cell lines. One of these, DU145, contained an abnormally small protein translated from an RB messenger RNA transcript that lacked 105 nucleotides encoded by exon 21. To assess the functional consequences of this mutation, normal RB expression was restored in DU145 cells by retrovirus-mediated gene transfer. Cells that maintained stable exogenous RB expression lost their ability to form tumors in nude mice, although their growth rate in culture was apparently unaltered. These results suggest that RB inactivation can play a significant role in the genesis of a common adult neoplasm and that restoration of normal RB-encoded protein in tumors could have clinical utility.

The human gene (RB) that determines susceptibility to hereditary retinoblastoma has been identified recently by molecular genetic techniques. Previous results indicate that complete inactivation of the RB gene is required for tumour formation. As a 'cancer suppressor' gene, RB thus functions in a manner opposite to that of most other oncogenes. Sequence analysis of RB complementary DNA clones demonstrated a long open reading frame encoding a hypothetical protein with features suggestive of a DNA-binding function. To further substantiate and identify the RB protein, we have prepared rabbit antisera against a trypE-RB fusion protein. The purified anti-RB IgG immunoprecipitates a protein doublet with apparent relative molecular mass (Mr) of 110,000-114,000. The specific protein(s) are present in all cell lines expressing normal RB mRNA, but are not detected in five retinoblastoma cell lines examined. The RB protein can be metabolically labelled with 32P-phosphoric acid, indicating that it is a phosphoprotein. Biochemical fractionation and immunofluorescence studies demonstrate that the majority of the protein is located within the nucleus. Furthermore, the protein can be retained by and eluted from DNA-cellulose columns, suggesting that it is associated with DNA binding activity. Taken together, these results imply that the RB gene product may function in regulating other genes within the cell.

Mutations of the gene encoding p53, a 53-kilodalton cellular protein, are found frequently in human tumor cells, suggesting a crucial role for this gene in human oncogenesis. To model the stepwise mutation or loss of both p53 alleles during tumorigenesis, a human osteosarcoma cell line, Saos-2, was used that completely lacked endogenous p53. Single copies of exogenous p53 genes were then introduced by infecting cells with recombinant retroviruses containing either point-mutated or wild-type versions of the p53 cDNA sequence. Expression of wild-type p53 suppressed the neoplastic phenotype of Saos-2 cells, whereas expression of mutated p53 conferred a limited growth advantage to cells in the absence of wild-type p53. Wild-type p53 was phenotypically dominant to mutated p53 in a two-allele configuration. These results suggest that, as with the retinoblastoma gene, mutation of both alleles of the p53 gene is essential for its role in oncogenesis.