Mammalian responses to dysfunctional telomeres

Telomeres are specialized structure composed of tandem repeats bound by proteins at the end of linear chromosomes. All eukaryotes have evolved a system that monitors and preserves the functionality of the telomeres. The tandem repeats are elongated de novo during every cell cycle by telomerase, a ribonuclear protein with both RNA and catalytic component. Telomeres prevent genomic instability that can lead to mutations and tumorgenesis.; The present work examines the cellular and organismal responses to dysfunctional telomeres resulted from the absence of telomerase. First, telomere dysfunction mediated responses in fibroblasts were studied. In fast growing fibroblasts with short telomeres, there was an increase in genomic instability accompanying growth retardation. The retardation in growth rate vanished after crisis, suggesting either an adaptation to the presence of the short telomeres or inactivation of checkpoints. When exogenous DNA damage was introduced into the post-crisis fibroblasts, they responded similarly as the cells without telomere dysfunction. The protein level of DNA damage checkpoints p53, gamma-H2AX and Chk2 did not correlate with cellular growth. This suggests that one cannot determine if cells have dysfunctional telomeres by looking at the globally DNA damage checkpoint protein level. Next, the mechanism of the dysfunctional telomeres was investigated. Individual telomeres in T cells and fibroblasts were examined directly and it was found that short telomeres are recognized directly by DNA damage proteins in the absence of telomerase. Lastly, in vivo mechanism and responses to dysfunctional telomere were examined. Telomerase RNA mutations have been reported in autosomal dominant dyskeratosis congenita patients and aplastic anemic patients. By using a mouse strain that has short telomeres, we demonstrated that haploinsufficiency of telomerase RNA can lead to telomere dysfunction in gastrointestinal track, bone marrow and germ cells. This finding provides a plausible mechanism for how mutations in telomerase leading to diseases dyskeratosis congenita and aplastic anemia. We also uncovered a previously unknown aspect of telomere biology, the parental effect of telomere length regulation in which wildtype mice that inherit short telomeres show cell disease phenotypes. This phenomenon could potentially cause stem cell failure in genetically normal people.