Correlation between retinoblastoma gene expression and differentiation in human testicular tumors

Inactivation of the retinoblastoma gene (RB gene) is associated with the development of several human malignancies including retinoblastomas, some osteo- and soft tissue sarcomas, small cell lung cancer, and possibly breast and bladder cancers. To our knowledge, this gene has not been evaluated in human germ-cell malignancies. In this study 67 primary testicular germ-cell tumors and 4 testicular non-germ-cell malignancies were examined to determine the prevalence and nature of RB gene alterations. Decreased expression of RB gene mRNA was found in all testicular germ-cell tumors (both seminomas and nonseminomas) examined. The RB protein could not be detected by immunohistochemical analysis in the undifferentiated cells of any germ-cell tumors whereas the differentiated malignant cells present in 14/15 teratocarcinomas expressed the protein. No gross alterations of the RB gene were found at DNA level in any of the examined specimens. This and the presence of the RB protein in the more differentiated tumor cells of teratocarcinomas suggest that changes in transcript levels rather than mutation(s) of the gene may be responsible for the absent or decreased RB expression in human germ-cell tumors. To date studies on the mechanism of RB regulation have demonstrated that it occurs at the protein level by phosphorylation of the p105 gene product. The findings presented here indicate that additional regulation might occur at the transcript level.