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Mutational inactivation of the retinoblastoma gene (RB) is found in all retinoblastomas and in a subset of other human neoplasms, including sarcomas of bone or soft tissue and carcinomas of lung or breast. Exogenous copies of wild-type RB have been shown to suppress the tumorigenicity of several types of tumor cells with endogenous RB mutations, including a previously described human prostatic carcinoma cell line. To further support a role for RB inactivation in the genesis ofprostate cancer, seven primary or metastatic prostate carcinoma specimens were examined for evidence of RB mutation. By the use of immunoblot analysis and immunostaining of histologic sections, RB-encoded protein was readily detected in tumor cells offive specimens, was equivocally detected in one specimen, and was apparently absent from tumor cells of one specimen. RB mutations in the latter case were precisely characterized as (I) a deletion of 103 nucleotides containing transcriptional start sites and (it) loss of the second RB allele.The 103-base-pair deletion was sufficient to abolish the promoter activity of upstream DNA sequences in a heterologous expression system. These results (i) demonstrate thatRB can be inactivated in vivo by mutation of its promoter, (ii) confirm the existence ofRB mutations in some human prostate carcinomas, and (iii) suggest the use of immunohistochemical methods to screen for RB mutations in clinical samples of common adult neoplasms. mutations, as predicted by Knudson (9). RB mutations are also found in a subset of osteosarcomas, soft-tissue sarcomas, and carcinomas of breast and lung, suggesting a broad role for RB inactivation in the genesis of human tumors (8).Finally, restoration of the normal RB gene product, ppllORB, into pp11ORB-deficient (RB-) tumor cells alters several aspects of their neoplastic phenotype, including suppression of tumorigenicity in nude mice (10, 11). Because of these properties, RB is a model for studying other candidate tumor suppressor genes.In a previous study of three human prostate carcinoma cell lines, one was found to express a mutated RB mRNA and an abnormally small RB protein that was functionally inactive in tumor suppression (11). To establish the existence of RB mutations in native prostate carcinomas, primary or metastatic tumor specimens were screened for loss of RB protein expression. Two tumors were found to have severely reduced or undetectable amounts of RB protein by immunoblot analysis and by immunostaining of histologic sections. Direct detection of mutated RB alleles is hindered by the large size (';200 kilobases) and structural complexity (27 exons) of this gene (12). Nevertheless, a detailed genetic analysis of one tumor revealed both mutations leading to RB inactivation: (i) a 103-base-pair (bp) deletion that abolished activity of the RB promoter and (ii) loss of the second normal RB allele. Prostate carcinoma is the most common cancer in men (1).

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