Heterozygous mutation of ataxia-telangiectasia mutated gene aggravates hypercholesterolemia in apoE-deficient mice

Janice M. LaPlante,DongFang Wu,ZhongMao Guo,LiChun Zhou,MingJian Shi,Billy R. Ballard,Hong Yang,George Julies,Wei Xiang

doi:10.1194/jlr.m400430-jlr200

Janice M. LaPlante, DongFang Wu + Show 7 more

Open Access

https://doi.org/10.1194/jlr.m400430-jlr200

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Abstract

Individuals with a heterozygous mutation at the ataxia-telangiectasia mutated gene (ATM) have been reported to be predisposed to ischemic heart disease. This report examined for the first time the effect of a heterozygous ATM mutation (ATM(+)(/-)) on plasma lipid levels and atherosclerosis intensity using ATM(+/-), ATM(+)(/+) (wild type), ATM(+)(/+)/LDLR(-)(/-) (low density lipoprotein receptor knockout), ATM(+)(/-)/LDLR(-)(/-), ATM(+)(/+)/ApoE(-)(/-) (apolipoprotein E knockout), and ATM(+)(/-)/ApoE(-)(/-) mice. Our data demonstrated that the plasma cholesterol and triglyceride levels in ATM(+)(/-) and ATM(+)(/-)/LDLR(-)(/-) mice were approximately the same as those in ATM(+)(/+) and ATM(+)(/+)/LDLR(-)(/-) control mice, respectively. In contrast, the plasma cholesterol level was significantly higher in ATM(+)(/-)/ApoE(-)(/-) mice than in ATM(+)(/+)/ApoE(-)(/-) control mice. In addition, the ATM(+)(/-)/ApoE(-)(/-) mice showed higher plasma apoB-48 levels, slower clearance for plasma apoB-48-carrying lipoproteins, and more advanced atherosclerotic lesions in the aorta compared with the ATM(+)(/+)/ApoE(-)(/-) mice. These novel results suggest that the product of ATM is involved in an apoE-independent pathway for catabolism of apoB-48-carrying remnants; therefore, superimposition of a heterozygous ATM mutation onto an ApoE deficiency background reduces the clearance of apoB-48-carrying lipoproteins from the blood circulation and promotes the formation of atherosclerosis.

Highlights

ATMϩ/Ϫ/ApoEϪ/Ϫ mice showed increased plasma apoB-48 and reduced clearance for apoB-48-carrying lipoproteins. These results indicate that, in the background of an ApoE deficiency, a heterozygous mutation at ataxia-telangiectasia mutated gene (ATM) reduces the removal of plasma apoB-48-carrying lipoproteins, resulting in severe hypercholesterolemia and atherosclerosis
This report clearly demonstrates that the ATMϩ/Ϫ/ ApoEϪ/Ϫ mice suffered severe hypercholesterolemia compared with their ATMϩ/ϩ/ApoEϪ/Ϫ littermates, whereas the plasma cholesterol levels in the ATMϩ/Ϫ and ATMϩ/Ϫ/ LDLRϪ/Ϫ mice were only slightly higher than those of their controls under normal chow or a high-fat diet
fast-performance liquid chromatography (FPLC) analysis revealed that the increased cholesterol levels observed among the ATMϩ/Ϫ/ApoEϪ/Ϫ mice were distributed primarily in the VLDL and LDL fractions

Summary

MATERIALS AND METHODS

The ATM mutation mice were kindly provided by Dr Anthony Wynshaw-Boris (University of California, San Diego, CA). A 100 ␮l plasma sample obtained from individual mice was fractionated using fast-performance liquid chromatography (FPLC) (Äkta FPLC 900; Amersham Biosciences, Piscataway, NJ) in a buffer containing 0.15 M NaCl, 0.01 M Na2HPO4, and 0.1 mM EDTA, pH 7.5, at a flow rate of 0.5 ml/ min. The levels of triglycerides, phospholipids, total cholesterol, and free cholesterol in the plasma and FPLC fractions were measured by spectrophotometric quantification using reagents obtained from Sigma Chemical Co. Aliquoted plasma and FPLC fractions were mixed with triglyceride, phophospholipid, or cholesterol reaction reagents on a glass microplate These mixtures were incubated at 37°C for 10 min. ApoB-48-carrying lipoproteins were prepared from the plasma of ApoB48/48/ApoEϪ/Ϫ mice. The radiolabeled apoB-48-carrying lipoproteins (1.5 ␮g protein/4 ␮l/g body weight) were injected into the tail vein of the ATMϩ/Ϫ/ ApoEϪ/Ϫ and ATMϩ/ϩ/ApoEϪ/Ϫ mice. All statistical analyses were performed with STASTIX software (Analytical Software, Tallahassee, FL)

RESULTS

31 Ϯ 11 14 Ϯ 7

57 Ϯ 5 63 Ϯ 5 342 Ϯ 61a 355 Ϯ 30a

DISCUSSION