Reprogramming of telomerase activity and rebuilding of telomere length in cloned cattle

Nuclear reprogramming requires the removal of epigenetic modifications impo sed on the chromatin during cellular differentiation and division. The mamm alian oocyte can reverse these alterations to a state of totipotency, allow ing the production of viable cloned offspring from somatic cell nuclei. To determine whether nuclear reprogramming is complete in cloned animals, we a ssessed the telomerase activity and telomere length status in cloned embryo s, fetuses, and newborn offspring derived from somatic cell nuclear transfe r. In this report, we show that telomerase activity was significantly (P < 0.05) diminished in bovine fibroblast donor cells compared with embryonic s tem-like cells, and surprisingly was 16-fold higher in fetal fibroblasts co mpared with adult fibroblasts (P < 0.05). Cell passaging and culture period s under serum starvation conditions significantly decreased telomerase acti vity by approximately 30-50% compared with nontreated early passage cells ( P < 0.05). Telomere shortening was observed during in vitro culture of bovi ne fetal fibroblasts and in very late passages of embryonic stem-like cells . Reprogramming of telomerase activity was apparent by the blastocyst stage of postcloning embryonic development, and telomere lengths were longer (15 -23 kb) in cloned fetuses and offspring than the relatively short mean term inal restriction fragment lengths (14-18 kb) observed in adult donor cells. Overall, telomere lengths of cloned fetuses and newborn calves (<approxima te to>20 kb) were not significantly different from those of age-matched con trol animals (P > 0.05), These results demonstrate that cloned embryos inhe rit genomic modifications acquired during the donor nuclei's in vivo and in vitro period but are subsequently reversed during development of the clone d animal.