Models for the longitudinal genetic analysis of same-age twins: application to HDL cholesterol

Genetic contribution to lipid levels in early life based on 158 loci validated in adults: the FAMILY study

The contribution of polymorphisms associated with adult lipids in early life is unknown. We studied 158 adult lipid polymorphisms in 1440 participants (544 children, 544 mothers and 324 fathers) of the Family Atherosclerosis Monitoring In early life (FAMILY) birth cohort. Total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C) and triglycerides (TG) measurements were collected at birth, 3 and 5 years of age. Polymorphisms were genotyped using the Illumina Cardio-Metabochip array. Genotype scores (GS) were calculated for TC, HDL-C, LDL-C and TG. Linear and mixed-effects regressions adjusted for sex, age and population stratification were performed. The GS was associated with LDL-C level at 3 and 5 years (β = 0.017 ± 0.003, P = 2.9 × 10⁻⁸; β = 0.020 ± 0.003, P = 5.7 × 10⁻⁹) and from birth to 5 years (β = 0.013 ± 0.003, P = 2.6 × 10⁻⁷). The GS was associated with TC level at 3 and 5 years (β = 0.009 ± 0.002, P = 9.1 × 10⁻⁷; β = 0.009 ± 0.002, P = 7.7 × 10⁻⁶). CETP rs3764261 was associated with the HDL-C level from birth to 5 years (β = 0.064 ± 0.014, P =  7.4 × 10⁻⁶). AMPD3 rs2923084 was associated with the HDL-C level at 5 years (β = 0.096 ± 0.024, P = 9.7 × 10⁻⁵). Known loci associated with blood lipids in adults are associated with TC, LDL-C and HDL-C, but not TG in early life. Genetically predisposed children may benefit from early lipid lowering preventative strategies.

Moderating the covariance between family member's substance use behavior

Twin and family studies implicitly assume that the covariation between family members remains constant across differences in age between the members of the family. However, age-specificity in gene expression for shared environmental factors could generate higher correlations between family members who are more similar in age. Cohort effects (cohort × genotype or cohort × common environment) could have the same effects, and both potentially reduce effect sizes estimated in genome-wide association studies where the subjects are heterogeneous in age. In this paper we describe a model in which the covariance between twins and non-twin siblings is moderated as a function of age difference. We describe the details of the model and simulate data using a variety of different parameter values to demonstrate that model fitting returns unbiased parameter estimates. Power analyses are then conducted to estimate the sample sizes required to detect the effects of moderation in a design of twins and siblings. Finally, the model is applied to data on cigarette smoking. We find that (1) the model effectively recovers the simulated parameters, (2) the power is relatively low and therefore requires large sample sizes before small to moderate effect sizes can be found reliably, and (3) the genetic covariance between siblings for smoking behavior decays very rapidly. Result 3 implies that, e.g., genome-wide studies of smoking behavior that use individuals assessed at different ages, or belonging to different birth-year cohorts may have had substantially reduced power to detect effects of genotype on cigarette use. It also implies that significant special twin environmental effects can be explained by age-moderation in some cases. This effect likely contributes to the missing heritability paradox.

Evidence of differential allelic effects between adolescents and adults for plasma high-density lipoprotein

A recent meta-analysis of genome-wide association (GWA) studies identified 95 loci that influence lipid traits in the adult population and found that collectively these explained about 25–30% of heritability for each trait. Little is known about how these loci affect lipid levels in early life, but there is evidence that genetic effects on HDL- and LDL-cholesterol (HDL-C, LDL-C) and triglycerides vary with age. We studied Australian adults (N = 10,151) and adolescents (N = 2,363) who participated in twin and family studies and for whom we have lipid phenotypes and genotype information for 91 of the 95 genetic variants. Heterogeneity tests between effect sizes in adult and adolescent cohorts showed an excess of heterogeneity for HDL-C (p_Het<0.05 at 5 out of 37 loci), but no more than expected by chance for LDL-C (1 out of 14 loci), or trigycerides (0 out 24). There were 2 (out of 5) with opposite direction of effect in adolescents compared to adults for HDL-C, but none for LDL-C. The biggest difference in effect size was for LDL-C at rs6511720 near LDLR, adolescents (0.021±0.033 mmol/L) and adults (0.157±0.023 mmol/L), p_Het = 0.013; followed by ZNF664 (p_Het = 0.018) and PABPC4 (p_Het = 0.034) for HDL-C. Our findings suggest that some of the previously identified variants associate differently with lipid traits in adolescents compared to adults, either because of developmental changes or because of greater interactions with environmental differences in adults.

Sex Differences in Genetic and Environmental Factors Contributing to Body-Height

Sex differences in the heritability of self-reported body-height in two Finnish twin cohorts were studied by using sex-limitation models. The first cohort was born in 1938–1949 (N = 4873 twin pairs) and the second in 1975–1979 (N = 2374 twin pairs). Body-height was greater in the younger cohort (difference of 3.1 cm for men and 2.9 cm for women). The heritability estimates were higher among men (h2 = 0.87 in the older cohort and h2 = 0.82 in the younger cohort) than women (h2 = 0.78 and h2 = 0.67, respectively). Sex-specific genetic factors were not statistically significant in either cohort, suggesting that the same genes contribute to variation in body height for both men and women. The stronger contribution of environmental factors to body-height among women questions the hypothesis that women are better buffered against environmental stress, at least for this phenotype.

Evidence for age as a modifier of genetic associations for lipid levels

In order to identify novel genetic variants that influence plasma lipid concentrations, we performed a genome-wide association study (GWAS) comprised of 411 children under 18 years of age, ascertained at St. Jude Children's Research Hospital, all of whom were of European, African, or Mexican descent. Promising associations (p < 10(-5)) were subsequently examined in 1040 additional youths and 3508 adults from the Third National Health and Nutrition Examination Survey (NHANES III), a diverse population-based study. Three genotype-phenotype associations replicated in NHANES III youths and three associated in NHANES III adults at p < 0.05; however, no single association was significant in both youths and adults. The most significant association (p= 0.009) in NHANES III youths was between low-density lipoprotein cholesterol (LDL-C) and intronic rs2429917 among participants of African descent. Given the known age dependency of lipid levels, we also tested for gene-age interactions in NHANES III participants across all ages. We identified a significant (p= 0.024) age-dependent association between SGSM2 rs2429917 and LDL-C. This finding illustrates the utility of using children to discover novel variants associated with complex phenotypes and the importance of considering age-dependent genetic effects in association studies of lipid levels.

Metabolic syndrome in Argentine women: ATPIII, IDF, IDF/AHA/NHLBI; what criterion should be used?

INTRODUCTION

The menopause and the metabolic syndrome (MS) are related with the increase of the risk of cardiovascular diseases.

OBJECTIVES

To evaluate the existence of metabolic risk factors and the prevalence of MS in pre- and post-menopausal women.

SUBJECTS

253 women: 120 pre-menopausal and 133 post-menopausal. Anthropometric, arterial pressure and waist circumference measurements were carried out. Glycaemia, lipids, creatinine, hepatogram, uric acid and thyroid-stimulating hormone were dosed.

RESULTS

Statistical higher registries of arterial pressure, levels of glycaemia, total cholesterol, low-density lipoprotein, triglycerides and TG/high-density lipoprotein relation were observed among post-menopausal patients. As regards this group, the MS diagnosis was considerably higher, being the same by all the criteria.

CONCLUSIONS

We found more prevalence of cardiometabolic and MS risk factors among the group of post-menopausal patients. The responsible mechanisms would respond to the secondary hypoestrogenaemia at the cease of the ovarian function. Because of this fact, menopausal women should be considered a risk group for the development of MS.

Genetic and environmental influences on serum lipid tracking: a population-based, longitudinal Chinese twin study

We conducted cross-sectional and longitudinal twin analysis to explore genetic and environmental contribution to serum lipid tracking during childhood and adolescence. The study sample was part of a population-based twin cohort that was recruited in the rural areas of the Anhui Province of China. The baseline recruitment of twins was carried out from 1998 through 2000 and the follow-up from 2005 through 2007. Serum lipids showed significant tracking during childhood and adolescence. Participants with lipids at the highest tertile at the baseline tended to remain high at follow-up across ages and Tanner stages, whereas subjects with lipids at the lowest tertile at the baseline tended to remain low at follow-up. Using twin modeling, we showed that genetic and environmental factors contributed to individual variations in lipid levels and tracking from the baseline to the follow-up visit. The estimated tracking correlations for total cholesterol, triglyceride, and LDL cholesterol were in the range of 0.25-0.53 and were predominantly influenced by genetic factors. In contrast, the phenotypic tracking of HDL cholesterol was influenced by both genetic and environmental factors. Our study underscores the importance of considering both environmental and genetic factors in studying the etiology of dyslipidemia.

Heritability of serum apolipoprotein concentrations in middle-aged Japanese twins

Background

Studies of the genetic and environmental influences on apolipoproteins have been conducted, but few have used data from Japanese twins. The aim of this study was to quantify and compare the genetic and environmental causes of individual differences in the serum concentrations of apolipoproteins in Japanese middle-aged twins.

Methods

Apo A-I, apo A-II, apo B, apo C-II, apo C-III, and apo E were studied. A total of 142 twin pairs, aged 45 through 65 years, were enrolled: 85 monozygotic pairs (59 male, 26 female) and 57 same-sexed dizygotic pairs (43 male, 14 female). The intraclass correlation coefficient and structural equation modeling were used to estimate the best-fitting model and heritability.

Results

Sixteen percent to 75% of the total variances of apo A-I, apo C-II, and apo C-III were attributable to genetic influence; apo A-I and apo C-II were influenced by dominant genetic factors. Twenty percent to 73% of the total variances of apo A-II, apo B, and apo E were attributable to additive genetic influence; apo B was clearly influenced by common environmental factors. Furthermore, the heritability of all apolipoproteins was higher among females than among males.

Conclusions

Genetic factors, including additive genetic effects (A) and dominant effects (D), influence apolipoprotein levels. However, a common environment does not influence the variances of these apolipoproteins, with the exception of apo B. Furthermore, the heritability of apolipoprotein phenotypes differs by sex.

Relative effect of genetic and environmental factors on body height: differences across birth cohorts among Finnish men and women

OBJECTIVES

This study examined the change in heritability of adult body height across birth cohorts in Finland.

METHODS

In 1981, cross-sectional questionnaires were completed by 10,968 twin pairs born before 1958. The effect of genetic factors was estimated via genetic modeling.

RESULTS

Heritability increased from the cohort born before 1929 (0.76, 95% confidence interval [CI] = 0.65, 0.88 in men; 0.66, 95% CI = 0.55, 0.77 in women) to that born in 1947 through 1957 (0.81, 95% CI = 0.73, 0.87 in men; 0.82, 95% CI = 0.75, 0.89 in women).

CONCLUSIONS

Heritability of height increased across Finnish birth cohorts born in the first half of this century and leveled off after World War II. Environmental factors, compared with genetic factors, appear to be more important among women than men.