DBA/2J Haplotype on Distal Chromosome 2 Reduces Mertk Expression, Restricts Efferocytosis, and Increases Susceptibility to Atherosclerosis.

OBJECTIVE Arch atherosclerosis 4 (Aath4) is a quantitative trait locus for atherosclerotic plaque formation in the inner curve of the aortic arch previously identified in an F2 cross of Apoe-/- mice on DBA/2J and 129S6 backgrounds. C-mer proto-oncogene tyrosine kinase (Mertk), coding for a ligand-activated transmembrane tyrosine kinase, is a candidate gene within the same chromosomal region. Our objective was to determine whether strain differences in Mertk influence plaque formation. APPROACH AND RESULTS To dissect the strain effects of Mertk on atherosclerosis, we first established a congenic mouse line (Aath4aDBA/DBA ) in which a 5' region of Aath4 of DBA/2J, including Mertk, was backcrossed onto a 129S6-Apoe-/- background. The resulting Aath4aDBA/DBA male mice developed significantly larger plaques compared with control mice (Aath4a129/129 ), proving that the DBA/2J allele of Aath4a is proatherogenic. Thioglycollate-elicited peritoneal macrophages from Aath4aDBA/DBA mice express less than 50% of Mertk mRNA and cell-surface MERTK protein compared with those from the control mice. Moreover, both large and small peritoneal Aath4aDBA/DBA macrophages showed reduced phagocytosis of apoptotic cells. When Mertk cDNAs from 129S6 and DBA/2J mice were overexpressed in HEK293T (human embryonic kidney 293T) cells, phagocytosis of apoptotic cells was equally enhanced in direct proportion to Mertk levels, indicating that phagocytosis is modulated by the amount of MERTK, but that it is not affected by MERTK amino acid differences between 129S6 and DBA/2J. CONCLUSIONS Reduced transcription of Mertk, rather than differences in MERTK protein structure, determines the reduced efficiency of apoptotic cell clearance in the Aath4aDBA/DBA mice, which, in turn, contributes to their increased susceptibility to atherosclerosis.