Effects of DNA damage in smooth muscle cells in atherosclerosis

Rationale: DNA damage and the DNA damage response (DDR) have been identified in human atherosclerosis, including in vascular smooth muscle cells (VSMCs). However, although double strand breaks (DSBs) are hypothesized to promote plaque progression and instability, in part by promoting cell senescence, apoptosis and inflammation, the direct effects of DSBs in VSMCs seen in atherogenesis are unknown. Objective: To determine the presence and effect of endogenous levels of DSBs in VSMCs on atherosclerosis. Methods and Results: Human atherosclerotic plaque VSMCs showed increased expression of multiple DDR proteins in vitro and in vivo, particularly the MRN complex (MRE11, RAD50, NBS1) that senses DSB repair. Oxidative stress-induced DSBs were increased in plaque VSMCs, but DSB repair was maintained. To determine the effect of DSBs on atherosclerosis, we generated two novel transgenic mice lines expressing NBS1 or C-terminal deleted NBS1 only in VSMCs, and crossed them with ApoE-/- mice. SM22α-NBS1/ApoE-/- VSMCs showed enhanced DSB repair and decreased growth arrest and apoptosis, whereas SM22α-(ΔC)NBS1/ApoE-/- VSMCs showed reduced DSB repair and increased growth arrest and apoptosis. Accelerating or retarding DSB repair did not affect atherosclerosis extent or composition. However, VSMC DNA damage reduced relative fibrous cap areas, whereas accelerating DSB repair increased cap area and VSMC content. Conclusions: Human atherosclerotic plaque VSMCs show increased DNA damage including DSBs and DDR activation. VSMC DNA damage has minimal effects on atherogenesis, but alters plaque phenotype inhibiting fibrous cap areas in advanced lesions. Inhibiting DNA damage in atherosclerosis may be a novel target to promote plaque stability