Genetic Targeting at Apolipoprotein E Locus to Knockdown Scavenger Receptor Class B Type I in Mice: Characterization of a Novel Mouse Model of Coronary Heart Disease

Background: Scavenger Receptor Class B Type I (SRBI) is the key player mediating HDL-dependent reverse cholesterol transport (RCT) in hepatocyte, and SRBI mutation causes coronary heart disease (CHD) in humans. Mice lacking SRBI and apolipoprotein E (apoE), fed on chow diet, develop cardinal features of CHD and succumb to premature death. However, the application of this model is limited due to the short lifespan, early onset of CHD symptom, the difficulty to maintain genetic mutations at two different loci, and infertility. The latter may be related to adrenal SRBI deficiency. Methods and Results: A genetic mouse model, named as SRBI KD ApoE -/- was produced by using CRISPR/Cas9 technique, in which an SRBI knock-down cassette (at both mRNA and proteins levels) is inserted downstream of the indogenous apoE promoter, thus abolishing apoE expression. Real-time PCR and Western blot characterization showed that SRBI expression in liver was significantly reduced (33% vs. 100% by RT-PCR, 0.5% vs. 100% by WB, P<0.001), but no significant difference was found in other examined tissues. SRBI KD ApoE -/- mice are fertile and do not exhibit pre-mature cardiovascular death on chow diet up to 10 months. Compared to ApoE -/- mice, SRBI KD ApoE -/- mice showed significant increase in serum HDL-C (3.11 vs 1.70 mmol/L, P<0.01), whereas serum triglyceride or LDL-C was similar (triglyceride: 0.87 vs. 0.9 mmol/L; LDL-C: 8.35 vs. 7.74 mmol/L). When treated with high-fat diet (HFD: 21% fat, 0.2% cholesterol in weight, n=6/group) for 8 weeks (starting at 8-week old), SRBI KD ApoE -/- mice gained significantly more weight than ApoE -/- mice(P<0.05 for females), and exhibited higher weight ratio of heart to body than that of ApoE -/- mice (females: 6.51% vs. 5.16%, P=0.051; males: 6.54% vs. 4.64%, P<0.01). Splenomegaly was observed more frequently in these mice than in ApoE -/- mice (females: 17.7% vs. 5.99%, P<0.001; males: 6.12% vs. 3.99%, P<0.01).