DEVELOPMENT OF RETINOBLASTOMA IN THE ABSENCE OF TELOMERASE ACTIVITY

Background: The length and stability of telomeres (essential functiona l structures at the end of eukaryotic chromosomes) have been implicate d in the control of cell lifespan. Most somatic cells lack telomerase, the enzyme that synthesizes telomeric DNA, and their telomeres shorte n with cell division. Cells immortalized in vitro, on the other hand, express telomerase and maintain their telomeres. Telomerase activity h as also been detected in the large majority of tumors from a variety o f cancers. These observations have suggested that telomere maintenance is required for unlimited cell proliferation and that telomerase is a marker for cell immortality in vitro and in vivo. Purpose: We investi gated whether telomerase is activated during the development of retino blastoma. This is a childhood eye cancer associated with a limited num ber of mutations in an embryonic tissue and thus likely to develop in cells that have long telomeres. The ease of defection of retinoblastom a makes it possible to screen relatively small tumors before extensive proliferation of the malignant cells. Methods: We measured telomerase activity in 34 samples of retinoblastoma, four retinoblastoma-derived cell lines, and six cell lines derived from other cancers. Only three of the cell lines from other cancers expressed the retinoblastoma pro tein. Telomerase activity was assayed by a polymerase chain reaction p rotocol in extracts prepared from tumors or cell lines. The level of e nzyme activity in cell extracts was quantified at several protein conc entrations and expressed relative to that in a positive control, after normalization for the amount of protein. Telomere length was measured by Southern blot hybridization of genomic DNA with a telomere-specifi c probe. Average values of telomere length in telomerase-positive and telomerase-negative tumors and in cell lines were compared by two-side d, two-sample Student's t test. Results: No telomerase activity was de tected in 17 (50%) of 34 retinoblastomas. Assays of cell lines derived from other cancers revealed no association between the presence or th e level of the enzyme activity and the expression of the retinoblastom a protein. Telomeres were significantly longer in telomerase-negative tumors than in telomerase-positive tumors (P = .0008). Conclusions: Ou r results indicate that retinoblastoma can develop when telomeres are still relatively long and in the absence of telomerase, Telomerase act ivity associated with short telomeres is, however, observed in 50% of the retinoblastomas and in retinoblastoma-derived cell lines. Implicat ions: Telomerase may not be a marker for acquisition of the malignant phenotype in the case of tumors that are derived from cells with long telomeres and that are associated with a low number of mutations.