The catalytic and the RNA subunits of human telomerase are required to immortalize equid primary fibroblasts.

Many human primary somatic cells can be im- mortalized by inducing telomerase activity through the ex- ogenous expression of the human telomerase catalytic subunit (hTERT). This approach has been extended to the immortalization of cell lines from several mammals. Here, we show that hTERT expression is not sufficient to immor- talize primary fibroblasts from three equid species, namely donkey, Burchelli's zebra and Grevy's zebra. In vitro anal- ysis of a reconstituted telomerase composed by hTERT and an equid RNA component of telomerase (TERC) revealed a low activity of this enzyme compared to human telomerase, suggesting a low compatibility of equid and human telomer- ase subunits. This conclusion was also strengthened by comparison of human and equid TERC sequences, which revealed nucleotide differences in key regions for TERC and TERT interaction. We then succeeded in immortalizing equid fibroblasts by expressing hTERT and hTERC con- comitantly. Expression of both human telomerase subunits led to telomerase activity and telomere elongation, indicat- ing that human telomerase is compatible with the other equid telomerase subunits and proteins involved in telomere metabolism. The immortalization procedure described here- in could be extended to primary cells from other mammals. The availability of immortal cells from endangered species could be particularly useful for obtaining new information on the organization and function of their genomes, which is relevant for their preservation.