Telomere Dynamics during Bovine Preimplantation Development

Telomeres are dynamic structures that protect the ends of linear eukaryotic chromosomes from aberrant fusions or from being misidentified as DNA breaks, playing an important role in development, differentiation, senescence and health. Although linear DNA shortens at each cell division due to incomplete replication of the 5’-end, telomeres have two mechanisms to maintain their length: a specialized enzyme called telomerase and the Alternative Lengthening of Telomeres (ALT) mechanism, based on homologous recombination. In complex organisms like mammals, both mechanisms are turned off in somatic cells after the preimplantation development, leading to a programmed cellular senescence, whereas immortal cells such as stem cells and most cancer cells possess mechanisms to maintain long telomeres. Short telomere length (TL) in newborns has been related with the appearance of certain disorders such as Dyskeratosis congenital, and telomere length has been proposed to be reset during preimplantation development. Telomere lengthening dynamics and mechanisms during preimplantation development have been widely studied in the laboratory mouse. However, laboratory mice contain longer telomeres than other mammals, including wild mouse, bovine and humans, and thus there may be also differences in telomere lengthening dynamics during preimplantation development. The objectives of this Master Thesis have been to determine the dynamics of telomere length during bovine preimplantation development and to test the effect of the injection of a plasmid expressing mouse Tert in bovine zygotes on the telomere length of the blastocyst. Telomere length was analyzed by qPCR at different stages (oocytes, zygotes, 2-cell, morula and blastocyst) observing that a significant increase occurred after embryonic genome activation, doubling its length at the morula stage and ending at the blastocyst stage with a telomeres 10 times longer than those of the oocyte. However, the increase in telomere length observed at the 2-cell stage in mouse embryos was not noted in bovine embryos, suggesting species-specific differences in the mechanisms involved in telomere lengthening at these developmental stages. Telomere length of the blastocysts derived from bovine zygotes injected with a plasmid encoding for mouse Tert did not differ from that of blastocysts obtained after injection with a control plasmid expressing EGFP. This result suggest that mouse Tert was not able to further elongate telomeres at the blastocyst stage because either it is not be able to form the telomerase complex with bovine components, or telomere length is tightly regulated during preimplantation development to reach a maximum length that cannot be exceeded by exogenous expression of telomerase components.