P roliferation of vascular smooth muscle cells (VSMCs) is a central axiom of most models of atherosclerosis, promoting atherogenesis as a response to injury 1 or inflammation. 2 However, most heart attacks are caused by rupture of a "vulnerable" plaque with a thin VSMC-poor fibrous cap overlying a relatively large necrotic core. 3,4 Plaque repair requires VSMC proliferation and is thus beneficial at this stage. However, VSMCs from advanced human plaques show poor proliferation and premature senescence in culture 5 and in vivo 6 ; furthermore, fibrous cap VSMCs show extensive DNA damage, marked telomere shortening, and markers of senescence. 7 Although these findings suggest that VSMC senescence may be important in atherogenesis, its mechanisms and direct consequences are unproven. Clinical Perspective on p 1919Replicative cell senescence is mediated in part by telomeres, which shorten during replication and ultimately trigger a DNA damage response (DDR) and growth arrest. Telomeres are composed of tandem DNA repeats that are maintained in a compact T-loop structure by the shelterin complex of telomere-associated proteins, including TRF1, TRF2, POT1, TIN2, RAP1, and TPP1. Shelterin proteins restrict access to telomerase and exonucleases/ligases, thus avoiding inappropriate telomere elongation and shortening, respectively, and prevent exposure of chromosome ends that are recognized as double-stranded DNA breaks (DSBs). Although each of the shelterin proteins is important for telomere maintenance, telomeric repeat-binding factor-2 (TRF2) has a particularly critical role. TRF2 regulates replicative senescence in part by reducing telomere length at senescence 8,9 and can also stop the ataxia telangiectasia kinase (ATM) from initiating a DDR from functional telomeres. 10 Loss of TRF2 induces multiple features of senescence, including irreversible growth arrest, expression of senescence-associated β-galactosidase, and telomere dysfunction with chromosomal fusions.11-13 TRF2 can also regulate cell longevity in a telomere-independent manner by direct association with multiple DDR proteins, including ATM, Nijmegen breakage syndrome-1, and checkpoint kinase-2.14-16 ATM phosphorylates TRF2 after DNA Background-Although vascular smooth muscle cell (VSMC) proliferation is implicated in atherogenesis, VSMCs in advanced plaques and cultured from plaques show evidence of VSMC senescence and DNA damage. In particular, plaque VSMCs show shortening of telomeres, which can directly induce senescence. Senescence can have multiple effects on plaque development and morphology; however, the consequences of VSMC senescence or the mechanisms underlying VSMC senescence in atherosclerosis are mostly unknown. Methods and Results-We examined the expression of proteins that protect telomeres in VSMCs derived from human plaques and normal vessels. Plaque VSMCs showed reduced expression and telomere binding of telomeric repeat-binding factor-2 (TRF2), associated with increased DNA damage. TRF2 expression was regulated by p53-dependent degradat...

Background— Mitochondrial DNA (mtDNA) damage occurs in both circulating cells and the vessel wall in human atherosclerosis. However, it is unclear whether mtDNA damage directly promotes atherogenesis or is a consequence of tissue damage, which cell types are involved, and whether its effects are mediated only through reactive oxygen species. Methods and Results— mtDNA damage occurred early in the vessel wall in apolipoprotein E–null (ApoE −/− ) mice, before significant atherosclerosis developed. mtDNA defects were also identified in circulating monocytes and liver and were associated with mitochondrial dysfunction. To determine whether mtDNA damage directly promotes atherosclerosis, we studied ApoE −/− mice deficient for mitochondrial polymerase-γ proofreading activity (polG −/− /ApoE −/− ). polG −/− /ApoE −/− mice showed extensive mtDNA damage and defects in oxidative phosphorylation but no increase in reactive oxygen species. polG −/− /ApoE −/− mice showed increased atherosclerosis, associated with impaired proliferation and apoptosis of vascular smooth muscle cells, and hyperlipidemia. Transplantation with polG −/− /ApoE −/− bone marrow increased the features of plaque vulnerability, and polG −/− /ApoE −/− monocytes showed increased apoptosis and inflammatory cytokine release. To examine mtDNA damage in human atherosclerosis, we assessed mtDNA adducts in plaques and in leukocytes from patients who had undergone virtual histology intravascular ultrasound characterization of coronary plaques. Human atherosclerotic plaques showed increased mtDNA damage compared with normal vessels; in contrast, leukocyte mtDNA damage was associated with higher-risk plaques but not plaque burden. Conclusions— We show that mtDNA damage in vessel wall and circulating cells is widespread and causative and indicates higher risk in atherosclerosis. Protection against mtDNA damage and improvement of mitochondrial function are potential areas for new therapeutics.

Objective-The goal of this study was to assess the role of B-cell activating factor (BAFF) receptor in B-cell regulation of atherosclerosis. Methods and Results-Male LDL receptor-deficient mice (Ldlr −/− ) were lethally irradiated and reconstituted with either wild type or BAFF receptor (BAFF-R)-deficient bone marrow. After 4 weeks of recovery, mice were put on a high-fat diet for 6 or 8 weeks. BAFF-R deficiency in bone marrow cells led to a marked reduction of conventional mature B2 cells but did not affect the B1a cell subtype. This was associated with a significant reduction of dendritic cell activation and Tcell proliferation along with a reduction of IgG antibodies against malondialdehyde-modified low-density lipoprotein. In contrast, serum IgM type antibodies were preserved. Interestingly, BAFF-R deficiency was associated with a significant reduction in atherosclerotic lesion development and reduced numbers of plaque T cells. Selective BAFF-R deficiency on B cells led to a similar reduction in lesion size and T-cell infiltration but in contrast did not affect dendritic cell activation. Conclusion-BAFF-