Susanne M. Clee

Common Genetic Variation in ABCA1 Is Associated With Altered Lipoprotein Levels and a Modified Risk for Coronary Artery Disease

Background — Low plasma HDL cholesterol (HDL-C) is associated with an increased risk of coronary artery disease (CAD). We recently identified the ATP-binding cassette transporter 1 (ABCA1) as the major gene underlying the HDL deficiency associated with reduced cholesterol efflux. Mutations within the ABCA1 gene are associated with decreased HDL-C, increased triglycerides, and an increased risk of CAD. However, the extent to which common variation within this gene influences plasma lipid levels and CAD in the general population is unknown. Methods and Results — We examined the phenotypic effects of single nucleotide polymorphisms in the coding region of ABCA1. The R219K variant has a carrier frequency of 46% in Europeans. Carriers have a reduced severity of CAD, decreased focal (minimum obstruction diameter 1.81±0.35 versus 1.73±0.35 mm in noncarriers, P =0.001) and diffuse atherosclerosis (mean segment diameter 2.77±0.37 versus 2.70±0.37 mm, P =0.005), and fewer coronary events (50% versus 59%, P =0.02). Atherosclerosis progresses more slowly in carriers of R219K than in noncarriers. Carriers have decreased triglyceride levels (1.42±0.49 versus 1.84±0.77 mmol/L, P =0.001) and a trend toward increased HDL-C (0.91±0.22 versus 0.88±0.20 mmol/L, P =0.12). Other single nucleotide polymorphisms in the coding region had milder effects on plasma lipids and atherosclerosis. Conclusions — These data suggest that common variation in ABCA1 significantly influences plasma lipid levels and the severity of CAD.

Increased ABCA1 activity protects against atherosclerosis.

The ABC transporter ABCA1 plays a key role in the first steps of the reverse cholesterol transport pathway by mediating lipid efflux from macrophages. Previously, it was demonstrated that human ABCA1 overexpression in vivo in transgenic mice results in a mild elevation of plasma HDL levels and increased efflux of cholesterol from macrophages. In this study, we determined the effect of overexpression of ABCA1 on atherosclerosis development. Human ABCA1 transgenic mice (BAC(+)) were crossed with ApoE(-/-) mice, a strain that spontaneously develop atherosclerotic lesions. BAC(+)ApoE(-/-) mice developed dramatically smaller, less-complex lesions as compared with their ApoE(-/-) counterparts. In addition, there was increased efflux of cholesterol from macrophages isolated from the BAC(+)ApoE(-/-) mice. Although the increase in plasma HDL cholesterol levels was small, HDL particles from BAC(+)ApoE(-/-) mice were significantly better acceptors of cholesterol. Lipid analysis of HDL particles from BAC(+)ApoE(-/-) mice revealed an increase in phospholipid levels, which was correlated significantly with their ability to enhance cholesterol efflux.

Positional cloning of Sorcs1, a type 2 diabetes quantitative trait locus.

We previously mapped the type 2 diabetes mellitus-2 locus (T2dm2), which affects fasting insulin levels, to distal chromosome 19 in a leptin-deficient obese F2 intercross derived from C57BL/6 (B6) and BTBR T+ tf/J (BTBR) mice. Introgression of a 7-Mb segment of the B6 chromosome 19 into the BTBR background (strain 1339A) replicated the reduced insulin linked to T2dm2. The 1339A mice have markedly impaired insulin secretion in vivo and disrupted islet morphology. We used subcongenic strains derived from 1339A to localize the T2dm2 quantitative trait locus (QTL) to a 242-kb segment comprising the promoter, first exon and most of the first intron of the Sorcs1 gene. This was the only gene in the 1339A strain for which we detected amino acid substitutions and expression level differences between mice carrying B6 and BTBR alleles of this insert, thereby identifying variation within the Sorcs1 gene as underlying the phenotype associated with the T2dm2 locus. SorCS1 binds platelet-derived growth factor, a growth factor crucial for pericyte recruitment to the microvasculature, and may thus have a role in expanding or maintaining the islet vasculature. Our identification of the Sorcs1 gene provides insight into the pathway underlying the pathophysiology of obesity-induced type 2 diabetes mellitus.

Cholesterol efflux regulatory protein, Tangier disease and familial high-density lipoprotein deficiency.

Cellular cholesterol efflux, by which cholesterol is transported from peripheral cells to HDL acceptor molecules for transport to the liver, is the first step of reverse cholesterol transport. Two genetic disorders, Tangier disease and some cases of familial HDL deficiency, have defects of cellular cholesterol efflux. The recent discovery of mutations in the ABC1 gene, which encodes the cholesterol efflux regulatory protein, in both these disorders establishes cholesterol efflux regulatory protein as a rate-limiting factor in reverse cholesterol transport.

ABCA1 Regulatory Variants Influence Coronary Artery Disease Independent of Effects on Plasma Lipid Levels

The authors have previously shown that individuals heterozygous for ABCA1 mutations have decreased high density lipoprotein cholesterol, increased triglycerides and an increased frequency of coronary artery disease (CAD), and that single nucleotide polymorphisms (SNPs) in the coding region of the ABCA1 gene significantly impact plasma lipid levels and the severity of CAD in the general population. They have now identified several SNPs in non‐coding regions of ABCA1 which may be important for the appropriate regulation of ABCA1 expression (i.e. in the promoter, intron 1 and the 5′ untranslated region), and have examined the phenotypic effects of these SNPs in the REGRESS population. Out of 12 SNPs, four were associated with a clinical outcome. A threefold increase in coronary events with an increased family history of CAD was evident for the G‐191C variant. Similarly, the C69T SNP was associated with a twofold increase in events. In contrast, the C‐17G was associated with a decrease in coronary events and the InsG319 was associated with less atherosclerosis. For all these SNPs, the changes in atherosclerosis and CAD occurred without detectable changes in plasma lipid levels. These data suggest that common variation in non‐coding regions of ABCA1 may significantly alter the severity of atherosclerosis, without necessarily influencing plasma lipid levels.

SORCS1: A Novel Human Type 2 Diabetes Susceptibility Gene Suggested by the Mouse

OBJECTIVE—A small number of susceptibility genes for human type 2 diabetes have been identified by candidate gene analysis or positional cloning. Genes found to influence diabetes or related traits in mice are likely to be susceptibility genes in humans. SorCS1 is the gene identified as responsible for the mouse chromosome 19 T2dm2 quantitative trait locus for fasting insulin levels, acting via impaired insulin secretion and increased islet disruption in obese females. Genes that impair compensatory insulin secretion in response to obesity-induced insulin resistance may be particularly relevant to human diabetes. Thus, we sought to determine whether variation in the human SORCS1 gene was associated with diabetes-related traits. RESEARCH DESIGN AND METHODS—We assessed the contribution of variation in SORCS1 to human insulin–related traits in two distinct Mexican-American cohorts. One cohort (the Mexican-American Coronary Artery Disease [MACAD] cohort) consisted of nondiabetic individuals, allowing assessment of genetic association with subclinical intermediate insulin-related traits; the second cohort (the San Antonio Family Diabetes Study [SAFADS]) contained individuals with diabetes, allowing association analyses with overt disease. RESULTS—We first found association of SORCS1 single nucleotide polymorphisms and haplotypes with fasting insulin levels and insulin secretion in the MACAD cohort. Similar to our results in the mice, the genetic association was strongest in overweight women. We then observed association with diabetes risk and age at diagnosis in women of the SAFADS cohort. CONCLUSIONS—Identification of SORCS1 as a novel gene affecting insulin secretion and diabetes risk is likely to provide important insight into the biology of obesity-induced type 2 diabetes.

Differences in the Phenotype Between Children With Familial Defective Apolipoprotein B-100 and Familial Hypercholesterolemia

Familial defective apolipoprotein B-100 (FDB) is a dominantly inherited genetic disorder resulting from a point mutation in the apolipoprotein (apo) B gene and is associated with significantly elevated plasma total and LDL cholesterol levels. Despite numerous descriptions outlining the phenotype of children with familial hypercholesterolemia (FH), no study has described the biochemical and clinical phenotype in a cohort of children with FDB. The phenotypes of FH and FDB, therefore, have not been compared in children. We have studied a cohort of 38 Dutch children (all <20 years old) with FDB from 21 different families. Lipid and lipoprotein levels and the clinical phenotype were compared with 97 age-matched FH heterozygotes, as defined by molecular analysis, and with age-matched non-FDB, non-FH control subjects. Female FDB carriers (n=23) had significantly lower total cholesterol (P<.001), LDL cholesterol (P=.001), total cholesterol:HDL ratio (P<.001), and apoB levels (P=.001) than age-matched female FH heterozygotes (n=50). Similar results were noted in male FDB carriers (n=15) compared with male FH heterozygotes (n=47; P=.005, P=.007, P=.014, and P=.074, respectively). Within the FDB group, female FDB heterozygotes had higher LDL cholesterol (P=.038) and a trend to higher total cholesterol levels (P=.165) than age-matched males. Both male and female FDB carriers had significantly higher total cholesterol, LDL cholesterol, and total cholesterol:HDL ratio than age- and sex-matched control subjects, which was evident even in children <10 years of age, providing additional evidence that this mutation is penetrant in early life. These results provide evidence for a milder biochemical phenotype in children with FDB than in children with FH. The phenotype observed is intermediate between that of control subjects and FH heterozygotes matched for age and sex. As the incidence of coronary artery disease is related to both the extent and duration of cholesterol elevation, our findings might explain in part the lower incidence of clinical atherosclerosis seen in adults with this condition than in adults with FH.

Expressing the nature of quantitative traits

One of the biggest challenges facing geneticists today is to identify the specific genetic variation contributing to complex diseases such as diabetes, obesity, cardiovascular disease, and arthritis. The high prevalence of these disorders suggests that the underlying nucleotide alterations are common in the general population, whilst the polygenic nature of these disorders suggests that changes in several genes, each with possibly only small phenotypic effects, interact to contribute to the development of these diseases. Furthermore, the environmental component of most complex diseases suggests that certain physiological stresses may exaggerate the phenotypic effects of the genetic pre-disposing factors. One type of genetic variation that may be especially important to the development of complex disease is that which influences the regulation of expression of key genes. One can easily envision that differences in the regulation of a gene may have functional consequences that depend on the genetic and environmental contexts in which they are found, and furthermore, that this variation in a regulator may have pleiotropic consequences on multiple downstream targets of that gene and thus might contribute to several aspects of a disease process. Novel genetic approaches are being utilized to identify such regulatory factors. The mRNA abundance of a given gene is a heritable quantitative trait, varying in the population as do factors such as plasma cholesterol or glucose levels. As such, mRNA levels can be treated as a phenotype for genetic mapping, just like any other quantitative trait, to identify the factors underlying their variation. These differences in mRNA levels might arise not only due to genetic variation in the regulatory elements (e.g. promoter, introns, or untranslated regions) of that gene, cis-acting variation, but also at other sites in the genome (e.g. in transcription factors that control their expression, RNA-binding proteins that stabilize their expression, factors involved in their processing, or genes regulating the abundance of any of these factors), trans-acting variation (Fig. 1). By expanding this concept and combining it with methods of genome-wide expression profile analysis, one does not need to know a priori what transcript is important to the development of disease, as all transcripts in a given tissue of interest are studied simultaneously and can be correlated with various aspects of the disease. One can examine the expression of all genes thought to be important in the disease process, identifying loci important for their regulation. Alternatively, by combining expression quantitative trait loci (QTL), or eQTL, data with that of traditional QTL for disease-associated phenotypes, it is possible to identify eQTL that are coincident with the physiological (or behavioral) QTL, identifying a list of genes that might contribute to the disease. Identification of the gene underlying these QTL could identify master regulators influencing the expression of these genes and determining disease susceptibility. Three recent articles the journal Nature Genetics illustrate the power of this approach in diverse complex diseases.

Sweet successes in diabetes genetics

Variant of transcription factor 7‐like 2 (TCF7L2) gene confers risk of type 2 diabetesu2028Grant et al. (2006)u2028Nature Genetics 38: 320–323