Corruption of DNA End-Joining in Mammalian Chromosomes by Progerin Expression
Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare genetic condition characterized by features of accelerated aging and a life expectancy of about 14 years. HGPS is commonly caused by a point mutation in the LMNA gene which codes for lamin A, an essential component of the nuclear lamina. The HGPS mutation alters splicing of the LMNA transcript, leading to a truncated, farnesylated form of lamin A termed "progerin." HGPS is associated with accumulation of genomic DNA double-strand breaks (DSBs), suggesting altered DNA repair. DSB repair normally occurs by either homologous recombination (HR), an accurate, templated form of repair, or by non-homologous end joining (NHEJ), an error-prone non-templated rejoining of DNA ends. Some NHEJ events occur via high-fidelity joining of DNA ends and we refer to such events as precise ligation (PL). Previously, we reported that expression of progerin correlated with increased NHEJ relative to HR. We now report on progerins impact on the nature of DNA end-joining. We used a model system involving a DNA end-joining reporter substrate integrated into the genome of cultured thymidine kinase-deficient mouse fibroblasts. Some cells were engineered to express progerin. DSBs were induced in the substrate through expression of endonuclease I-SceI, and DSB repair events were recovered through selection for thymidine kinase function. Progerin expression correlated with a significant shift away from PL and toward error-prone NHEJ. Our work suggests that progerin suppresses interactions between complementary sequences at DNA termini, shifting DSB repair toward low-fidelity DNA end-joining and perhaps contributing to aging through compromised genome stability.