Characterization of telomeric defects and signal transduction pathways in Dyskeratosis Congenita cells

Telomere attrition is a natural process that occurs due to inadequate telomere maintenance. Once at a critically short threshold, telomeres signal the cell to cease division and enter a cell fate termed senescence. Telomeres can be elongated by the enzyme telomerase, which adds de novo telomere repeats to the ends of chromosomes. Mutations in the telomerase complex or telomere-related genes give rise to the premature aging disorder Dyskeratosis Congenita (DC). DC provides a unique model system to study human aging in relation to telomerase insufficiency and the subsequent accelerated telomere attrition. In this thesis, skin fibroblasts as well as keratinocytes and T-cells were analyzed from patients with Autosomal Dominant Dyskeratosis Congenita (AD DC) caused by a single allele mutation in the telomerase RNA component (TERC) that leads to telomerase haploinsufficiency. These cells were determined to have a severe proliferative defect and extremely short telomeres. It is demonstrated that the short telomeres in AD DC cells initiate a DNA damage response transduced by the p53/p21WAF/CIP pathway which mediate an elevation in steady-state levels of mitochondrially-derived superoxide and oxidative stress. Exogenous expression of the catalytic reverse transcriptase component of telomerase (TERT) activated telomerase in DC fibroblasts but resulted in reduced activity (~50% compared to control fibroblasts); however telomeres were successfully maintained, albeit at a short length. Simultaneous expression of both TERT and TERC led to robust telomerase activity and elongation of telomeres, indicating that TERC haploinsufficiency is a rate-limiting step in telomere maintenance in DC cells. Reconstitution of telomerase activity in AD DC cells ameliorated the proliferative defects, reduced the p53/p21WAF/CIP response and decreased oxidative stress. Increased superoxide and slow proliferation found in DC cells could also be mitigated by inhibiting p21WAF/CIP or by decreasing the oxygen tension to which the cells are exposed. Together, these results support the hypothesis that the insufficient telomerase leads to critically short telomeres which signal the activation of p21WAF/CIP, leading to increased steady-state levels of mitochondrial superoxide and metabolic oxidative stress as a means to engage senescence. These studies provide insight into mechanisms whereby shortened telomeres lead to premature aging in a humans and point to potential strategies to reduce the effects of tissue dysfunction in DC patients.