Development of retinoblastoma in the absence of telomerase activity

The length and stability of telomeres (essential functional structures at the end of eukaryotic chromosomes) have been implicated in the control of cell lifespan. Most somatic cells lack telomerase, the enzyme that synthesizes telomeric DNA, and their telomeres shorten with cell division. Cells immortalized in vitro, on the other hand, express telomerase and maintain their telomeres. Telomerase activity has also been detected in the large majority of tumors from a variety of 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.

We investigated whether telomerase is activated during the development of retinoblastoma. This is a childhood eye cancer associated with a limited number of mutations in an embryonic tissue and thus likely to develop in cells that have long telomeres. The ease of detection of retinoblastoma makes it possible to screen relatively small tumors before extensive proliferation of the malignant cells.

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 protein. Telomerase activity was assayed by a polymerase chain reaction protocol in extracts prepared from tumors or cell lines. The level of enzyme activity in cell extracts was quantified at several protein concentrations 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-specific probe. Average values of telomere length in telomerase-positive and telomerase-negative tumors and in cell lines were compared by two-sided, two-sample Student's t test.

No telomerase activity was detected in 17 (50%) of 34 retinoblastomas. Assays of cell lines derived from other cancers revealed no association between the presence or the level of the enzyme activity and the expression of the retinoblastoma protein. Telomeres were significantly longer in telomerase-negative tumors than in telomerase-positive tumors (P = .0008).

Our results indicate that retinoblastoma can develop when telomeres are still relatively long and in the absence of telomerase. Telomerase activity associated with short telomeres is, however, observed in 50% of the retinoblastomas and in retinoblastoma-derived cell lines.

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.