The effect of 9p21.3 coronary artery disease locus neighboring genes on atherosclerosis in mice: Kim, Effect of 9p21.3 genes on atherosclerosis

The 9p21.3 region of the genome has been identified as the locus with strongest association to coronary artery disease (CAD) and myocardial infarction (MI) in multiple independent large scale genome-wide association studies (GWAS).1-3 The locus is within a 58kb region that is devoid of protein coding genes, suggestive of a regulatory function (Figure 1). Interestingly, the neighboring genes in the region include well-known tumor suppressor genes, including CDKN2A and CDKN2B.4-6 The CDKN2A locus encodes a cyclin-dependent protein kinase (CDK) inhibitory protein (CKI), p16INK4A, and a p53-regulatory protein, p19ARF. The CDKN2B gene encodes another CKI, p15INK4B. Another gene in the region is methylthioadenosine phosphorylase (MTAP), which encodes a ubiquitously expressed metabolic enzyme S-methyl-5′-thioadenosine phosphorylase7 that processes the polyamine biosynthesis byproduct in the methionine salvage pathway. Loss or inactivation of MTAP has frequently been observed in a number of different human tumors, and it has been shown to have a tumor suppressive role in a mice model.8 Figure 1 The landscape of the 9p21.3 region Multiple studies demonstrated a potential role for cell cycle regulatory mechanisms in atherosclerosis progression. Previously, the master tumor suppressor gene p53 has been implicated in the development of atherosclerosis in apolipoprotein E (ApoE)-null mice9, 10, affecting both cell proliferation and apoptosis within the atheroma. Another tumor suppressor gene, p21Waf1, was also shown to increase the atheroma size in ApoE-null mice11, whereas the tumor suppressor p27Kip1 was shown to protect against atherosclerosis.12 Correlations of the 9p21 locus SNP genotype to differential expression of the neighboring genes have been observed in several studies with inconsistent findings.13-15 A knockout (KO) mouse model involving the entire region orthologous to the 9p21.3 CAD locus showed significant decreases in the expressed levels of Cdkn2a and Cdkn2b, and increased proliferation of primary smooth muscle cells (SMC) and mouse embryonic fibroblasts (MEF), although an effect on atherosclerosis in vivo was not demonstrated.16 Mice deficient in the p19Arf gene were found to have increased atherosclerotic lesions in an ApoE null background with significant attenuation of apoptosis in lesions as well as in cultured primary macrophages and vascular smooth muscle cells.17 However, to date no observation regarding atherosclerotic phenotype has been made involving the other neighboring genes. We set out to survey the 9p21.3 orthologous region using knockout mice models to systematically address the role of the neighboring protein-coding genes in atherosclerosis. We chose the APOE*3 Leiden sensitizing model because it is dominant, simplifying the construction of the models, and also because it exhibits relatively modest elevations of cholesterol, more realistically modeling the human disease than other widely used models.