The Cincinnati lipid research clinic family study: Analysis of commingling and family resemblance for fasting blood glucose

Exercise training, genetics and type 2 diabetes-related phenotypes

Type 2 diabetes mellitus (T2DM) is at virtually pandemic levels world-wide. Diabetes has been referred to as 'a geneticist's nightmare'. However, dramatic advances in our understanding of the genetics of T2DM have occurred in the past 5 years. While endurance exercise training and increased habitual physical activity levels have consistently been shown to improve or be associated with improved T2DM-related phenotypes, there is substantial interindividual variation in these responses. There is some evidence that T2DM-related phenotype responses to exercise training are heritable, indicating that they might have a genetic basis. Genome-wide linkage studies have not identified specific chromosomal loci that could account for these differences, and no genome-wide association studies have been performed relative to T2DM-related phenotype responses to exercise training. From candidate gene studies, there are relatively strong and replicated data supporting a role for the PPARγ Pro12Ala variant in the interindividual differences in T2DM-related phenotype responses to training. This is a potentially important candidate locus because it affects T2DM susceptibility, has high biological plausibility and is the target for the primary pharmaceutical method for treating T2DM. Is it time to conduct a hypothesis-driven large-scale exercise training intervention trial based on PPARγ Pro12Ala genotype with T2DM-related phenotypes as the primary outcome measures, while also assessing potential mechanistic changes in skeletal muscle and adipose tissue? Or would it be more appropriate to propose a smaller trial to address the specific skeletal muscle and adipose tissue mechanisms affected by the interaction between the PPARγ Pro12Ala genotype and exercise training?

Clustering of insulin resistance, total and central abdominal fat: same genes or same environment?

Obesity, insulin resistance and disturbed glucose metabolism cluster within the Insulin Resistance Syndrome (IRS). Whether this reflects shared genetic or environmental factors detectable in ‘normal’ populations (not selected for IRS features) is unknown. This study estimated (i) genetic influences on IRS traits and (ii) shared and specific genetic and environmental factors on the relationships between these traits in healthy female twins. Fasting insulin, glucose, total and central fat were measured in 59 monozygotic (MZ) and 51 dizygotic (DZ) female twin pairs aged ( ± SD) 52 ± 13 years. Body fat was measured by dual-energy X-ray absorptiometry, insulin resistance and secretion by a modified homeostasis model assessment. Using intraclass correlation coefficients and univariate model-fitting analyses, genetic influences were found in total fat, central fat, insulin resistance, fasting glucose and insulin secretion, with genetic factors explaining 64, 57, 59, 75 and 68% of their variance, respectively, using the latter technique. In matched analysis intra-pair differences in total and central fat related to intra-pair differences in insulin resistance (r2 = 0.19, P < 0.001). Multivariate model-fitting showed a close genetic relationship between total and central fat (r = 0.88). The genetic correlation between IR and central fat (0.41) was significantly greater than that for total fat (0.24), suggesting that central fat is not only a predictor of, but shares considerable genetic influence with, insulin resistance. In Cholesky analysis, these genetic influences were separate from those shared between central and total fat. In conclusion, both shared and specific genetic factors regulate components of the IRS in healthy females. However, there were discrete genetic influences on -cell insulin secretion, not shared with other IRS components, suggesting that a separate genetic propensity exists for Type2 diabetes. These findings suggest we may understand the genetic and environmental influences on IRS from the study of the normal population.

Nutrient and food intakes of middle-aged adults at low risk of cardiovascular disease: the international study of macro-/micronutrients and blood pressure (INTERMAP)

PURPOSE

Individuals with favorable levels of readily measured cardiovascular disease (CVD) risk factors (low risk, LR) experience low long-term rates of CVD mortality and greater longevity. The purpose of the current study was to compare nutrient/food intakes of LR participants with participants not LR in the INTERMAP study.

METHODS

Men and women (40-59 years) from 17 population samples in four countries (China, Japan, UK, US) provided four 24-h dietary recalls and two timed 24-h urine collections. LR was defined as meeting all of the following CVD risk criteria: systolic/diastolic blood pressure (BP) ≤ 120/ ≤ 80 mmHg; no drug treatment for high BP, hyperlipidemia, or CVD; non-smoking; BMI <25.0 kg/m(2) (US, UK) or <23.0 kg/m(2) (China, Japan); alcohol consumption <26.0 g/day (men)/<13.0 g/day (women); and no history of diabetes or CVD. Multivariate logistic regression was used to examine associations of nutrient/food intakes with LR.

RESULTS

LR individuals reported higher intake of vegetable protein, fiber, magnesium, non-heme iron, potassium; lower energy intake; lower intake of cholesterol, saturated fatty acids, animal protein; and lower 24-h urinary sodium compared with individuals not LR. With regard to foods, LR individuals reported higher intake of fruits, vegetables, grains, pasta/rice, fish; lower intakes of meats, processed meats, high-fat dairy, and sugar-sweetened beverages than individuals not LR.

CONCLUSIONS

Lower energy intake and differential intake of multiple specific nutrients and foods are characteristic of individuals at low risk for developing CVD. Identification of dietary habits associated with LR is important for further development of public health efforts aimed at reduction/prevention of CVD.

Twin study of genetic and environmental influences on glucose tolerance and indices of insulin sensitivity and secretion

AIMS/HYPOTHESIS

Family and twin studies have reported different estimates of the relative contribution of genetic and environmental factors to the quantitative traits glucose tolerance, insulin secretion, and insulin sensitivity. Our aims were to estimate these relative influences in a large sample of twins from the population and to assess the effect of age.

METHODS

In this population-based, cross-sectional study we gave an oral glucose tolerance test to 317 women and 290 men who were same-sex healthy twin pairs between 18 to 67 years of age. The genetic, common environmental and individual environmental variance components for fasting and 120-min glucose and for fasting and 30-min insulin as well as the linear effects of age on these components were estimated by multivariate analysis (using the software FISHER).

RESULTS

In women and men the heritability for fasting glucose was 12 and 38%, for 120-min glucose it was 38 and 43%, for fasting insulin it was 54 and 37%, and for 30-min insulin it was 57 and 47%, respectively. Under the assumption of no non-additive genetic effects (no intra- or inter-gene interaction) there was no strong evidence for common environmental effects, barring significant effects for fasting glucose in women. Heritability decreased with age for 120-min glucose in women and fasting insulin in men, whereas it increased for 120-min glucose in men.

CONCLUSION/INTERPRETATION

This study indicates a limited additive genetic influence on the result of an OGTT, possibly with sex-specific age effects, and generally little or no influence of the common environment. Accordingly, there is a considerable individual environmental variation.

Heritability of subclinical atherosclerosis in Latino families ascertained through a hypertensive parent

Although clinical coronary heart disease and many cardiovascular risk factors are well known to aggregate within families, the heritability of carotid artery intima-media thickness (IMT) is less well documented. We report IMT heritability estimates in Mexican American, Salvadoran American, or Guatemalan American (all referred to as Latino) families ascertained through a hypertensive proband. IMT and cardiovascular risk factors (age, sex, blood pressure, body mass index, lipids, fasting glucose, and insulin sensitivity) were measured in 204 adult offspring of 69 hypertensive probands, along with 82 parents (54 probands and 28 spouses). In the offspring, variance component analysis revealed a heritability for IMT of 64% (P< 0.0001) after adjustment for significant cardiovascular risk factors. Genetic factors accounted for 50% of the total variation in IMT, whereas significant cardiovascular risk factors explained 22% (14% were due to age). For offspring and parents combined, adjusted IMT heritability was less, 34% (P=0.0005), with genetic factors accounting for 18% of the total IMT variation, whereas significant cardiovascular risk factors explained 46% (38% were due to age). We conclude that variation in common carotid artery IMT is heritable in Latino families with a hypertensive proband. Heritability is particularly evident in younger family members, suggesting that acquired factors contribute progressively to IMT variability with aging.

Genetic and environmental contributions to cardiovascular risk factors in Mexican Americans. The San Antonio Family Heart Study

BACKGROUND

The familial aggregation of coronary heart disease can be in large part accounted for by a clustering of cardiovascular disease risk factors. To elucidate the determinants of cardiovascular disease, many epidemiological studies have focused on the behavioral and lifestyle determinants of these risk factors, whereas others have examined whether specific candidate genes influence quantitative variation in these phenotypes.

METHODS AND RESULTS

Among Mexican Americans from San Antonio (Tex), we quantified the relative contributions of both genetic and environmental influences to a large panel of cardiovascular risk factors, including serum levels of lipids, lipoproteins, glucose, hormones, adiposity, and blood pressure. Members of 42 extended families were studied, including 1236 first-, second-, and third-degree relatives of randomly ascertained probands and their spouses. In addition to the phenotypic assessments, information was obtained regarding usual dietary and physical activity patterns, medication use, smoking habits, alcohol consumption, and other lifestyle behaviors and medical factors. Maximum likelihood methods were used to partition the variance of each phenotype into components attributable to the measured covariates, additive genetic effects (heritability), household effects, and an unmeasured environmental residual. For the lipid and lipoprotein phenotypes, age, gender, and other environmental covariates accounted in general for < 15% of the total phenotypic variance, whereas genes accounted for 30% to 45% of the phenotypic variation. Similarly, genes accounted for 15% to 30% of the phenotypic variation in measures of glucose, hormones, adiposity, and blood pressure.

CONCLUSIONS

These results highlight the importance of considering genetic factors in studies of risk factors for cardiovascular disease.

Mixture distributions in human genetics research

The use of mixture distributions in genetics research dates back to at least the late 1800s when Karl Pearson applied them in an analysis of crab morphometry. Pearson's use of normal mixture distributions to model the mixing of different species of crab (or 'families' of crab as he referred to them) within a defined geographic area motivated further use of mixture distributions in genetics research settings, and ultimately led to their development and recognition as intuitive modelling devices for the effects of underlying genes on quantitative phenotypic (i.e. trait) expression. In addition, mixture distributions are now used routinely to model or accommodate the genetic heterogeneity thought to underlie many human diseases. Specific applications of mixture distribution models in contemporary human genetics research are, in fact, too numerous to count. Despite this long, consistent and arguably illustrious history of use, little mention of mixture distributions in genetics research is made in many recent reviews on mixture models. This review attempts to rectify this by providing insight into the role that mixture distributions play in contemporary human genetics research. Tables providing examples from the literature that describe applications of mixture models in human genetics research are offered as a way of acquainting the interested reader with relevant studies. In addition, some of the more problematic aspects of the use of mixture models in genetics research are outlined and addressed.

Family studies of non-insulin-dependent diabetes mellitus in South Indians

Though a genetic basis for non-insulin-dependent diabetes mellitus (NIDDM) is clear, the likely mode of inheritance is not known. The segregation of NIDDM was studied in 64 nuclear South Indian pedigrees (449 individuals) ascertained through an affected proband having both parents and more than 1 sibling alive and available for oral glucose tolerance testing. A high proportion of parents were found to be of abnormal glucose tolerance [89 of 128 (70%) diabetic and 11 of 128 (9%) impaired]. Complex segregation analysis was performed using (1) POINTER which implements the mixed model and distinguishes major gene, multifactorial and non-transmitted environmental contributions to affection and (2) COMDS which implements an oligogenic model with major gene, modifier gene and environmental contributions to a) affection and b) diathesis (an ordered polychotomy amongst non-affected family members, based on 2-h plasma glucose level). Using POINTER, there was no formal support for a major gene and the most parsimonious solutions were achieved with multifactorial models. Using COMDS, we found i) significant improvements in models when information on glucose levels in nondiabetic family members (diathesis) was included, ii) support for segregation of a diallelic gene as well as background familial resemblance, and iii) under the best-supported model, this diallelic locus featured incomplete dominance (d = 0.8) and a disease-predisposing allele frequency of 14%. In South Indians, segregation of NIDDM is inadequately described by simple major gene models: more complex models provide more satisfactory descriptions. This finding, if applicable in other populations, has important implications for the search for diabetes-susceptibility genes.

Commingling and complex segregation analysis of fasting plasma glucose in the Lipid Research Clinics family study

Commingling and segregation patterns of fasting plasma glucose (GL) were examined in family data from 5 clinics (Cincinnati, Stanford, Iowa, Minnesota, and Oklahoma) of the Lipid Research Clinics (LRC) family study. In addition to the primary question of whether there was a major gene for GL, a secondary purpose was to investigate the possibility of genetic heterogeneity among the 5 clinics. No statistical support was found for heterogeneity among clinics, either in the commingling of distributions or in the segregation patterns. For the combined clinics sample, both a major effect and a multifactorial component were significant. However, the major effect (accounting for 73% of the variance) was not found to be consistent with a major gene, as the hypothesis of Mendelian transmission was rejected. The most parsimonious model involved equal transmission probabilities, which suggests that the major effect is not transmitted from parents to offspring. Possible sources of this major non-Mendelian effect were explored. The multifactorial component accounted for 10% of the variance in GL levels, and no generational differences were noted. Although our study was unable to provide evidence in favor of a major gene effect, it should be noted that a major gene cannot be firmly refuted. For example, a variety of interactions, such as genotype-dependent age effects, could have masked the transmission probabilities.

Commingling and segregation analyses: comparison of results from a simulation study of a quantitative trait

Commingling analysis is commonly used to provide preliminary evidence for a single genetic locus with a major effect on the quantitative trait of interest. In this paper, the effectiveness of commingling analysis as a screening technique to identify samples for segregation analysis is assessed by applying both commingling and segregation analyses to samples of simulated pedigree data in which a major locus is segregating in the presence of polygenes and an individual-specific environmental effect. Under the circumstances simulated here, there is evidence for a single locus from segregation analysis but not from commingling analysis in at least 20% of the samples. No more than 2% of the samples provided evidence for commingling but not for segregation of a single locus. Comparisons of the samples that give evidence for both commingling and segregation, evidence for one but not the other, and no evidence for either show that evidence for commingling depends on the distributional characteristics of the trait in the sample, while support for the single locus from segregation analysis depends on both the distributional characteristics as well as the transmission of the rarer allele from parents to offspring. Since lack of commingling does not rule out the existence of a single locus in the realistic situations considered here, commingling analysis has limited usefulness as a screening technique for the presence for a single locus. In contrast, evidence for commingling does suggest the possibility that a single locus has a major effect on the trait and commingling analysis can provide guidance in the choice of initial parameter estimates for segregation analysis.

Family resemblance for fasting blood glucose: the Jerusalem Lipid Research Clinic

Familial aggregation of fasting plasma glucose was studied in a sample of families examined at the Jerusalem Lipid Research Clinic. We first examined homogeneity of familial correlations across major origin groups in the Israeli sample. Correlations were generally homogeneous across origin groups, except for mother-son pairs. The pooled familial correlations were relatively low for unadjusted blood glucose values, and somewhat higher upon adjustment for sex, age, ethnicity, education, seasonality, body mass, cigarette smoking, alcohol consumption and dietary intake. Genetic and cultural determinants of blood glucose were estimated utilizing a path model with 10 parameters to be estimated from a total of 16 correlations. Under a reduced model genetic heritability (h2) was estimated to be 0.18 +/- 0.08 and cultural heritability (c2) was 0.10 +/- 0.02. However, within this population the additive variation could be explained by a cultural model of inheritance without introducing genetic parameters, and most of the variance is due to "random" unmeasured environmental factors. Commingling analysis was also performed, and our findings imply that there is no evidence for admixture in the distribution of fasting blood glucose in this Israeli population sample.