MOLECULAR AND CELLULAR MECHANISMS CONTRIBUTING TO THE ACCELERATED AGING PHENOTYPE IN HUTCHINSON-GILFORD PROGERIA SYNDROME

Hutchinson-Gilford Progeria Syndrome (HGPS) is an autosomal dominant disorder caused by de novo mutations in the gene (LMNA) encoding lamin A that results in "premature aging" and early death. HGPS belongs to a group of disorders collectively referred to as, segmental progeroid syndromes, because multiple organs and tissues exhibit premature degenerative phenotypes consistent with physiological aging. The results presented here indicate that HGPS cells exhibit an elevated steady-state level of DNA double-stranded breaks (DSBs) and impaired repair of ionizing radiation (IR)-induced DSBs, both of which correlate strongly with the nuclear structural irregularities observed in a fraction of HGPS cells. These DNA damage-associated defects are due to the presence of Progerin in a dominant gain of function manner. Accordingly, reduction of Progerin levels by treatment with a farnesyl transferase inhibitor (FTI) improves repair of IR-induced DSBs. Interestingly, MRN (MRE11/Rad50/Nbs1) repair complex factors, involved in DSB repair, exhibit delayed localization to IR-induced DSBs, which may contribute to the repair defect. Furthermore, many segmental progeroid syndromes including HGPS exhibit phenotypes consistent with pre-natal developmental defects. These developmental defects are significant in their own right, but also because they may contribute to the post-natal progressive degeneration observed in HGPS. To begin investigating whether the HGPS mutation has adverse effects during pre-natal development, the expression of nuclear lamins was characterized in mouse (mESCs) and human (hESCs) embryonic stem cells which have been shown to be representative of pluripotent inner mass cells of blastocyst-stage embryos. This study shows that lamin A is not expressed in undifferentiated mESCs and hESCs, but becomes expressed quickly during in vitro hESC differentiation. Theses results suggest that Progerin is absent during early pre-natal development, but becomes expressed at later stages when the LMNA gene is activated, perhaps adversely affecting the developmental process. Altogether, theses studies identify impaired DNA DSB repair as a molecular mechanism that may contribute to the progressive degeneration phenotype and suggest that at least some of the pathophysiology may begin before birth