Gene Variance Research Articles

Hyper- and hypo-responsiveness to dietary fat and cholesterol among inbred mice: searching for level and variability genes

A concept proposed by Berg (Berg, K. 1989. Arteriosclerosis. 9: I-50-I-58) is that a combination of level and variability genes determine an individual's overall plasma lipid levels and atherosclerotic risk. Our goal was to determine which inbred mouse strains could be used to identify candidate level and variability genes controlling lipid levels and atherosclerosis susceptibility. Nine common inbred mouse strains were examined for responsiveness with respect to plasma lipoprotein and tissue lipid levels upon feeding diets rich in cholesterol and fat. Marked quantitative variations were observed in plasma cholesterol and triglyceride levels among mice fed rodent chow and the high fat test diets. Mice of strains DBA/2 and AKR appeared to be hyporesponsive to diets containing high levels of fat and cholesterol as compared to rodent chow. In contrast, several strains were primarily hyperresponsive to either dietary fat or cholesterol, or both ingredients. Determination of cholesterol absorption for selected strains fed test diets suggested that decreased cholesterol absorption, in part, contributes to hyporesponsiveness as seen in DBA/2 mice. Levels of mRNA for cholesterol 7 alpha-hydroxylase were estimated and shown to vary markedly among strains. An inverse correlation was seen among strains between cholesterol 7 alpha-hydroxylase mRNA, and plasma and hepatic cholesterol levels for some diets. Thus, genes controlling cholesterol absorption and bile acid synthesis are candidates for further study as level and variability genes affecting plasma cholesterol levels. Overall, inbred mouse strains will prove useful for identifying genes controlling level and variability traits.

Power and validity of methods to identify variability genes

A variability gene model [Magnus et al., Clin Genet 19:67-70, 1981] hypothesizes that environmental influences on the expression of additive genes for a quantitative trait such as cholesterol are under the control of alleles at a separate nonadditive locus. They suggest identifying such genes using an analysis of variance to compare absolute intrapair monozygotic twin trait differences between the genotypes of the postulated variability locus. However, quantitative traits such as cholesterol often have skewed distributions with a long right tail; what are the effects of such non-normality on the procedure suggested by Magnus et al. [1981]? We show that their method is a special case of the Levene tests, robust tests for variability differences. We introduce a statistical model representing sources of variability in twin pair differences and demonstrate with simulation studies that although the Levene tests have robust Type I error, power is enhanced when nonnormal data are transformed before analysis, and the apparent presence and degree of variability differences are dependent on the scale of analysis. These findings indicate the importance of appropriate transformation of the trait before analysis. Analysis of a well-characterized twin data set illustrates these conclusions.

Risk Factor Variability and Coronary Heart Disease

Present attempts to identify genes contributing to coronary heart disease (CHD) risk focus on "candidate genes". With respect to CHD this could be any gene whose protein product is directly or indirectly involved in atherogenesis, thrombogenesis or thrombolysis/fibrinolysis. Genes that exhibit associations with absolute risk factor levels may be referred to as "level genes" to distinguish them from "variability genes", which are genes involved in establishing the framework within which environmental influences may cause risk factor variation. In a series of persons recruited from the Norwegian Twin Panel, confirmatory evidence for level gene effect with respect to apolipoprotein B (apoB) concentration was found with an XbaI polymorphism in DNA at the apoB locus corresponding to residue 2,488 in the mature protein. Evidence for variability gene effect with respect to apoB as well as body mass index emerged with DNA variants in the 3' part of the apoB gene. Level gene effect with respect to apolipoprotein A-I (apoA-I) and high density lipoprotein (HDL) cholesterol as well apparent variability gene effect with respect to total and LDL cholesterol were detected with a DNA polymorphism at the cholesteryl ester transfer protein (CETP) locus. The first example of interaction between normal genes in determining risk factor level was uncovered in analysis of the apolipoprotein E (apoE) polymorphism and a restriction fragment length polymorphism (RFLP) at the low density lipoprotein receptor (LDLR) locus. An LDLR gene identified by presence of a PvuII restriction site eliminated completely the well known effect of the apoE4 allele on cholesterol level. Finally, in families where high Lp(a) lipoprotein level (a well established risk factor for CHD) segregated as a Mendelian trait, very close linkage with an RFLP at the plasminogen locus was established and DNA variation at the LPA locus reflecting varying numbers of a structure homologous to the "kringle IV" region of plasminogen was uncovered.

“Variability gene” effect of cholesteryl ester transfer protein (CETP) genes

Cholesteryl ester transfer protein (CETP) may have important roles in transfer of lipids from cells to serum lipoproteins or between circulating lipoprotein particles. Restriction fragment length polymorphisms (RFLPs) in DNA at the CETP locus have been detected. In the present study we have used RFLPs detectable with the restriction enzyme TaqI to examine if CETP influences serum lipid variability (as opposed to absolute lipid levels). We have compared within-pair difference in serum lipid and apolipoprotein levels in monozygotic twin pairs of various genotypes in the B polymorphism at the CETP locus and uncovered significant differences between genotypes. We conclude that the CETP locus has "variability genes" (as opposed to "level genes") with respect to total and LDL cholesterol variability. A person's total genetic risk for coronary heart disease may depend on his or her combination of "level genes" and "variability genes". The method of analysis applied may be the best available for the study of gene - environment interaction.

Variability gene effect on cholesterol at the Kidd blood group locus.

The within-pair difference in lipid levels was examined in 142 monozygotic (MZ) twin pairs drawn from the population-based Norwegian Twin Panel. Mean within-pair difference in serum total cholesterol was lower in MZ pairs who were heterozygous for blood group Kidd genes or homozygous for the Jkb gene than in pairs who were homozygous for the Jka gene. The difference between the two categories of homozygotes was significant at the 2% level. The analyses suggest that the main reason for the difference observed is a restrictive effect of the Jkb gene on total cholesterol variability. No effect on triglycerides or HDL cholesterol variability was detected and there was no association between Kidd blood groups and sex and age-adjusted levels of cholesterol, triglycerides or HDL cholesterol. The present data confirm findings we reported when we introduced the study of within-pair variability in MZ twins as a method to analyze gene-environment interactions and validate the "variability gene" concept. A person's net risk for coronary heart disease may depend on his or her combination of "level genes" and "variability genes" as well as on environmental or life-style factors.