Familial liability to psychosis is a risk factor for multimorbidity in people with psychotic disorders and their unaffected siblings

BACKGROUND

Multimorbidity may impose an overwhelming burden on patients with psychosis and is affected by gender and age. Our aim is to study the independent role of familial liability to psychosis as a risk factor for multimorbidity.

METHODS

We performed the study within the framework of the Genetic Risk and Outcome of Psychosis (GROUP) project. Overall, we compared 1024 psychotic patients, 994 unaffected siblings and 566 controls on the prevalence of 125 lifetime diseases, and 19 self-reported somatic complaints. Multimorbidity was defined as the presence of two or more complaints/diseases in the same individual. Generalized linear mixed model (GLMM) were used to investigate the effects of gender, age (adolescent, young, older) and familial liability (patients, siblings, controls) and their interactions on multimorbidity.

RESULTS

Familial liability had a significant effect on multimorbidity of either complaints or diseases. Patients had a higher prevalence of multimorbidity of complaints compared to siblings (OR 2.20, 95% CI 1.79-2.69, P<0.001) and to controls (3.05, 2.35-3.96, P<0.001). In physical health multimorbidity, patients (OR 1.36, 95% CI 1.05-1.75, P=0.018), but not siblings, had significantly higher prevalence than controls. Similar finding were observed for multimorbidity of lifetime diseases, including psychiatric diseases. Significant results were observed for complaints and disease multimorbidity across gender and age groups.

CONCLUSION

Multimorbidity is a common burden, significantly more prevalent in patients and their unaffected siblings. Familial liability to psychosis showed an independent effect on multimorbidity; gender and age are also important factors determining multimorbidity.

Genome-wide association study of lifetime cannabis use based on a large meta-analytic sample of 32 330 subjects from the International Cannabis Consortium

Cannabis is the most widely produced and consumed illicit psychoactive substance worldwide. Occasional cannabis use can progress to frequent use, abuse and dependence with all known adverse physical, psychological and social consequences. Individual differences in cannabis initiation are heritable (40-48%). The International Cannabis Consortium was established with the aim to identify genetic risk variants of cannabis use. We conducted a meta-analysis of genome-wide association data of 13 cohorts (N=32 330) and four replication samples (N=5627). In addition, we performed a gene-based test of association, estimated single-nucleotide polymorphism (SNP)-based heritability and explored the genetic correlation between lifetime cannabis use and cigarette use using LD score regression. No individual SNPs reached genome-wide significance. Nonetheless, gene-based tests identified four genes significantly associated with lifetime cannabis use: NCAM1, CADM2, SCOC and KCNT2. Previous studies reported associations of NCAM1 with cigarette smoking and other substance use, and those of CADM2 with body mass index, processing speed and autism disorders, which are phenotypes previously reported to be associated with cannabis use. Furthermore, we showed that, combined across the genome, all common SNPs explained 13-20% (P<0.001) of the liability of lifetime cannabis use. Finally, there was a strong genetic correlation (rg=0.83; P=1.85 × 10(-8)) between lifetime cannabis use and lifetime cigarette smoking implying that the SNP effect sizes of the two traits are highly correlated. This is the largest meta-analysis of cannabis GWA studies to date, revealing important new insights into the genetic pathways of lifetime cannabis use. Future functional studies should explore the impact of the identified genes on the biological mechanisms of cannabis use.

Obesity is associated with more activated neutrophils in African American male youth

BACKGROUND

There is emerging evidence suggesting the role of peripheral blood leukocytes in the pathogenesis of obesity and related diseases. However, few studies have taken a genome-wide approach to investigating gene expression profiles in peripheral leukocytes between obese and lean individuals with the consideration of obesity-related shifts in leukocyte types.

METHOD

We conducted this study in 95 African Americans (AAs) of both genders (age 14-20 years, 46 lean and 49 obese). Complete blood count with differential test (CBC) was performed in whole blood. Genome-wide gene expression analysis was obtained using the Illumina HumanHT-12 V4 Beadchip with RNA extracted from peripheral leukocytes. Out of the 95 participants, 64 had neutrophils stored. The validation study was based on real-time PCR with RNA extracted from purified neutrophils.

RESULTS

CBC test suggested that, in males, obesity was associated with increased neutrophil percentage (P=0.03). Genome-wide gene expression analysis showed that, in males, the majority of the most differentially expressed genes were related to neutrophil activation. Validation of the gene expression levels of ELANE (neutrophil elastase) and MPO (myeloperoxidase) in purified neutrophils demonstrated that the expression of these two genes--important biomarkers of neutrophils activation--were significantly elevated in obese males (P=0.01 and P=0.02, respectively).

CONCLUSION

The identification of increased neutrophil percentage and activation in obese AA males suggests that neutrophils have an essential role in the pathogenesis of obesity-related disease. Further functional and mechanistic studies on neutrophils may contribute to the development of novel intervention strategies reducing the burden associated with obesity-related health problems.

Pleiotropic effects of obesity-susceptibility loci on metabolic traits: a meta-analysis of up to 37,874 individuals

AIMS/HYPOTHESIS

Genetic pleiotropy may contribute to the clustering of obesity and metabolic conditions. We assessed whether genetic variants that are robustly associated with BMI and waist-to-hip ratio (WHR) also influence metabolic and cardiovascular traits, independently of obesity-related traits, in meta-analyses of up to 37,874 individuals from six European population-based studies.

METHODS

We examined associations of 32 BMI and 14 WHR loci, individually and combined in two genetic predisposition scores (GPSs), with glycaemic traits, blood lipids and BP, with and without adjusting for BMI and/or WHR.

RESULTS

We observed significant associations of BMI-increasing alleles at five BMI loci with lower levels of 2 h glucose (RBJ [also known as DNAJC27], QPTCL: effect sizes -0.068 and -0.107 SD, respectively), HDL-cholesterol (SLC39A8: -0.065 SD, MTCH2: -0.039 SD), and diastolic BP (SLC39A8: -0.069 SD), and higher and lower levels of LDL- and total cholesterol (QPTCL: 0.041 and 0.042 SDs, respectively, FLJ35779 [also known as POC5]: -0.042 and -0.041 SDs, respectively) (all p < 2.4 × 10(-4)), independent of BMI. The WHR-increasing alleles at two WHR loci were significantly associated with higher proinsulin (GRB14: 0.069 SD) and lower fasting glucose levels (CPEB4: -0.049 SD), independent of BMI and WHR. A higher GPS-BMI was associated with lower systolic BP (-0.005 SD), diastolic BP (-0.006 SD) and 2 h glucose (-0.013 SD), while a higher GPS-WHR was associated with lower HDL-cholesterol (-0.015 SD) and higher triacylglycerol levels (0.014 SD) (all p < 2.9 × 10(-3)), independent of BMI and/or WHR.

CONCLUSIONS/INTERPRETATION

These pleiotropic effects of obesity-susceptibility loci provide novel insights into mechanisms that link obesity with metabolic abnormalities.

Determinants of HbA1c in nondiabetic Dutch adults: genetic loci and clinical and lifestyle parameters, and their interactions in the Lifelines Cohort Study

OBJECTIVES

Glycated haemoglobin (HbA1c) is associated with cardiovascular disease risk in individuals without diabetes, and its use has been recommended for diagnosing diabetes. Therefore, it is important to gain further understanding of the determinants of HbA1c. The aim of this study was to investigate the effects of genetic loci and clinical and lifestyle parameters, and their interactions, on HbA1c in nondiabetic adults.

DESIGN

Population-based cohort study.

SETTING

Three northern provinces of the Netherlands.

SUBJECTS

A total of 2921 nondiabetic adults participating in the population-based LifeLines Cohort Study.

MEASUREMENTS

Body mass index (BMI), waist circumference, HbA1c, fasting plasma glucose (FPG) and erythrocyte indices were measured. Data on current smoking and alcohol consumption were collected through questionnaires. Genome-wide genotyping was performed, and 12 previously identified single-nucleotide polymorphisms (SNPs) were selected for replication and categorized as 'glycaemic' and 'nonglycaemic' SNPs according to their presumed mechanism(s) of action on HbA1c. Genetic risk scores (GRSs) were calculated as the sum of the weighted effect of HbA1c-increasing alleles.

RESULTS

Age, gender, BMI, FPG, mean corpuscular haemoglobin, mean corpuscular haemoglobin concentration, current smoking and alcohol consumption were independent predictors of HbA1c, together explaining 26.2% of the variance in HbA1c, with FPG contributing 10.9%. We replicated three of the previously identified SNPs and the GRSs were also found to be independently associated with HbA1c. We found a smaller effect of the 'nonglycaemic GRS' in females compared with males and an attenuation of the effect of the GRS of all 12 SNPs with increasing BMI.

CONCLUSIONS

Our results suggest that a substantial portion of HbA1c is determined by nonglycaemic factors. This should be taken into account when considering the use of HbA1c as a diagnostic test for diabetes.

Genome-wide association analysis of coffee drinking suggests association with CYP1A1/CYP1A2 and NRCAM

Coffee consumption is a model for addictive behavior. We performed a meta-analysis of genome-wide association studies (GWASs) on coffee intake from 8 Caucasian cohorts (N=18 176) and sought replication of our top findings in a further 7929 individuals. We also performed a gene expression analysis treating different cell lines with caffeine. Genome-wide significant association was observed for two single-nucleotide polymorphisms (SNPs) in the 15q24 region. The two SNPs rs2470893 and rs2472297 (P-values=1.6 × 10(-11) and 2.7 × 10(-11)), which were also in strong linkage disequilibrium (r(2)=0.7) with each other, lie in the 23-kb long commonly shared 5' flanking region between CYP1A1 and CYP1A2 genes. CYP1A1 was found to be downregulated in lymphoblastoid cell lines treated with caffeine. CYP1A1 is known to metabolize polycyclic aromatic hydrocarbons, which are important constituents of coffee, whereas CYP1A2 is involved in the primary metabolism of caffeine. Significant evidence of association was also detected at rs382140 (P-value=3.9 × 10(-09)) near NRCAM-a gene implicated in vulnerability to addiction, and at another independent hit rs6495122 (P-value=7.1 × 10(-09))-an SNP associated with blood pressure-in the 15q24 region near the gene ULK3, in the meta-analysis of discovery and replication cohorts. Our results from GWASs and expression analysis also strongly implicate CAB39L in coffee drinking. Pathway analysis of differentially expressed genes revealed significantly enriched ubiquitin proteasome (P-value=2.2 × 10(-05)) and Parkinson's disease pathways (P-value=3.6 × 10(-05)).

ABCB1 gene variants influence tolerance to selective serotonin reuptake inhibitors in a large sample of Dutch cases with major depressive disorder

P-glycoprotein (P-gp), an ATP-driven efflux pump in the blood-brain barrier, has a major impact on the delivery of antidepressant drugs in the brain. Genetic variants in the gene ABCB1 encoding for P-gp have inconsistently been associated with adverse effects. In order to resolve these inconsistencies, we conducted a study in a large cohort of patients with major depressive disorder with the aim to unravel the association of ABCB1 variants with adverse effects of antidepressants and in particular with selective serotonin reuptake inhibitors (SSRIs), which display affinity as substrate for P-gp. The Netherlands Study of Depression and Anxiety (NESDA) study was used as a clinical sample. For 424 patients data were available on drug use, side effects. We selected six ABCB1 gene variants (1236T>C, 2677G>T/A, 3435T>C, rs2032583, rs2235040 and rs2235015) and analyzed them for association with adverse drug effects using multinomial regression analysis for both single variants and haplotypes. We found a significant association between the number of SSRI-related adverse drug effects and rs2032583 (P=0.001), rs2235040 (P=0.002) and a haplotype (P=0.002). Moreover, serotonergic effects (sleeplessness, gastrointestinal complaints and sexual effects) were significantly predicted by these variants and haplotype (P=0.002/0.003). We conclude that adverse drug effects with SSRI treatment, in particular serotonergic effects, are predicted by two common polymorphisms of the ABCB1 gene.

Association of complement C3 gene variants with renal transplant outcome of deceased cardiac dead donor kidneys

Local renal complement activation by the donor kidney plays an important role in the pathogenesis of renal injury inherent to kidney transplantation. Contradictory results were reported about the protective effects of the donor C3F allotype on renal allograft outcome. We investigated the influence of the donor C3F allotype on renal transplant outcome, taking all different donor types into account. C3 allotypes of 1265 donor-recipient pairs were determined and divided into four genotypic groups according to the C3F allotype of the donor and the recipient. The four genotypic groups were analyzed for association with primary nonfunction (PNF), delayed graft function, acute rejection, death-censored graft survival and patient survival. Considering all donor types, multivariable analysis found no association of the donor C3F allotype with renal allograft outcome. Also, for living and deceased brain-dead donors, no association with allograft outcome was found. Post hoc subgroup analysis within deceased cardiac dead (DCD) donors revealed an independent protective association of donor C3F allotype with PNF. This study shows that the donor C3F allotype is not associated with renal allograft outcome after kidney transplantation. Subgroup analysis within DCD donors revealed an independent protective association of the donor C3F allotype with PNF, which is preliminary and warrants further validation.

Genetics of testosterone and the aggression-hostility-anger (AHA) syndrome: a study of middle-aged male twins

The aim of this study was to determine the genetic contribution to the variation in testosterone and the aggression-hostility-anger (AHA) syndrome in middle-aged twins. Moreover, the relation between testosterone and this syndrome, and possible common genetic mechanisms were investigated. Towards this end, blood samples were collected at two time points; the AHA syndrome was measured using three questionnaires: the Buss-Durkee Hostility Inventory with seven subscales, the Jenkins Activity Survey and the Spielberger State-Trait Anger Scale. The results showed substantial heritabilities for testosterone (approximately 60%) and moderate to fair heritabilities for the nine measures of the AHA syndrome (23–53%). The best fitting model for testosterone at two time points included a small age component and additive genetic and unique environmental factors, while a multivariate analysis of the nine AHA subscales resulted in an independent pathway model with two common additive genetic and two common unique environmental factors. No correlation between the common genetic factor influencing testosterone and the AHA subscales was found. We did, however, detect a negative correlation between the common environmental factor underlying testosterone and both common environmental factors influencing the nine AHA subscales, which may reflect a tendency for testosterone levels to rise and hostility to drop (or vice versa) after repeatedly experiencing success (or failure). Twin Research (2000) 3, 266–276.

Heritability of blood pressure increases during mental stress

We studied the influence of mental stress on the contributions of genes and environment to individual variation in systolic (SBP) and diastolic (DBP) blood pressure by structural equation modelling in 320 adolescent male and female twins. Blood pressure data were collected during rest and during a reaction time and a mental arithmetic task. Univariate analyses of SBP and DBP showed familial aggregation for blood pressure. A genetic explanation for this resemblance was most likely, although during rest conditions a model that attributed familial resemblance to shared environmental factors, also fitted the data. There was no evidence for sex differences in heritabilities. Multivariate analyses showed significant heterogeneity between sexes for the intercorrelations of the blood pressure data measured under different rest and task conditions. Multivariate genetic analyses were therefore carried out separately in males and females. For SBP and DBP in females and for SBP in males an increase in heritabilities was seen for blood pressure measured during stress, as compared to rest measurements. The influence of shared environ-mental factors decreased during stress. For DBP in males no significant contributions of shared environment were found. The multivariate analyses indicated that the same genetic and environmental influences are expressed during rest and stress conditions.

Poor replication of candidate genes for major depressive disorder using genome-wide association data

Data from the Genetic Association Information Network (GAIN) genome-wide association study (GWAS) in major depressive disorder (MDD) were used to explore previously reported candidate gene and single-nucleotide polymorphism (SNP) associations in MDD. A systematic literature search of candidate genes associated with MDD in case-control studies was performed before the results of the GAIN MDD study became available. Measured and imputed candidate SNPs and genes were tested in the GAIN MDD study encompassing 1738 cases and 1802 controls. Imputation was used to increase the number of SNPs from the GWAS and to improve coverage of SNPs in the candidate genes selected. Tests were carried out for individual SNPs and the entire gene using different statistical approaches, with permutation analysis as the final arbiter. In all, 78 papers reporting on 57 genes were identified, from which 92 SNPs could be mapped. In the GAIN MDD study, two SNPs were associated with MDD: C5orf20 (rs12520799; P=0.038; odds ratio (OR) AT=1.10, 95% CI 0.95-1.29; OR TT=1.21, 95% confidence interval (CI) 1.01-1.47) and NPY (rs16139; P=0.034; OR C allele=0.73, 95% CI 0.55-0.97), constituting a direct replication of previously identified SNPs. At the gene level, TNF (rs76917; OR T=1.35, 95% CI 1.13-1.63; P=0.0034) was identified as the only gene for which the association with MDD remained significant after correction for multiple testing. For SLC6A2 (norepinephrine transporter (NET)) significantly more SNPs (19 out of 100; P=0.039) than expected were associated while accounting for the linkage disequilibrium (LD) structure. Thus, we found support for involvement in MDD for only four genes. However, given the number of candidate SNPs and genes that were tested, even these significant may well be false positives. The poor replication may point to publication bias and false-positive findings in previous candidate gene studies, and may also be related to heterogeneity of the MDD phenotype as well as contextual genetic or environmental factors.

High-intensity interval training in an adolescent with cystic fibrosis: a physiological perspective

Nutritional, musculoskeletal, and/or ventilatory status can lead to a decreased exercise capacity in children with cystic fibrosis (CF). Exercise training is already part of the usual care; however, the "optimal" intensity and volume of exercise training to improve exercise capacity is still unknown. Six weeks of high-intensity interval training (HIT) for a patient with CF with a ventilatory limitation was evaluated by a cardiopulmonary exercise test (CPET). Peak oxygen uptake and peak workload increased 19% and 16%, respectively, and there was a rise in peak ventilation from 50 L/min to 75 L/min, with an increase in both breathing depth and respiratory rate. A relative short period of HIT resulted in a significant increase in exercise capacity. In patients with CF, HIT might be an effective and efficient training regimen, especially in CF patients with a ventilatory limitation. Further research is necessary to investigate whether HIT is a better alternative than traditional aerobic training programs especially in ventilatory limited patients with CF.

Genetic and environmental influences on objective intermediate asthma phenotypes in Dutch twins

It is unclear to what extent the same set of environmental or genetic factors regulate objective intermediate asthma phenotypes. We examined heritabilities of these phenotypes and estimated their environmental and genetic overlap. We studied baseline lung function (forced expiratory volume in 1 s (FEV(1)), forced vital capacity (FVC) and FEV(1)/FVC), bronchial hyperresponsiveness, number of positive skin prick tests (SPT) to 11 allergens, serum total immunoglobulin (Ig)E, number of positive specific IgE tests to four allergens and eosinophil counts. 103 twin pairs were studied (46 monozygotic and 57 dizygotic; mean age: 22.5 yrs, range: 17.0-27.0 yrs). Univariate and bivariate genetic analyses were performed after adjustment for significant covariates. All intermediate asthma phenotypes showed significant heritabilities (47-83%). Most phenotypes were substantially correlated, which was mainly due to shared genetic factors. Pairs of phenotypes with the largest genetic correlations were specific IgE and SPT (0.98), and total IgE with specific IgE (0.87), with SPT (0.72), and with eosinophils (0.62). SPT showed significant environmental correlations with total IgE (0.65), specific IgE (0.70) and bronchial hyperresponsiveness (0.44). Genetic effects explain the majority of the variation in objective intermediate asthma phenotypes. Additionally, correlations between pairs of these traits are also mainly explained by genetic rather than environmental factors.

Heritability of insulin sensitivity and lipid profile depend on BMI: evidence for gene-obesity interaction

AIMS/HYPOTHESIS

Evidence from candidate gene studies suggests that obesity may modify genetic susceptibility to type 2 diabetes and dyslipidaemia. On an aggregate level, gene-obesity interactions are expected to result in different heritability estimates at different obesity levels. However, this hypothesis has never been tested.

METHOD

The present study included 2,180 British female twins. BMI was used as an index of general obesity. Outcome measures were insulin sensitivity (indexed by quantitative insulin-sensitivity check index [QUICKI]) and fasting plasma lipid profile. Structural equation modelling was used to test whether BMI interacted with latent genetic and environmental effects to impact on the outcome measures.

RESULTS

Genetic influences on triacylglycerol increased with BMI (p < 0.001) whereas the unique environmental influence on QUICKI decreased with BMI (p < 0.001), resulting in a higher heritability estimate for both measures at higher BMI levels. This was further illustrated by stratified analysis in twin pairs concordant for normal weight and twin pairs concordant for overweight. Heritability was 19 percentage points higher for triacylglycerol (p < 0.001) and 31 percentage points higher for QUICKI (p < 0.01) among twins concordant for overweight than among twins concordant for normal weight. BMI had no moderator effect on the latent genetic and environmental factors for total cholesterol and HDL-cholesterol.

CONCLUSIONS/INTERPRETATION

Our results suggest that the expression of genes influencing triacylglycerol and insulin sensitivity can vary as a function of obesity status. The substantial increases in the genetic contribution to the total variance in insulin sensitivity and triacylglycerols at higher BMIs may prove extremely valuable in the search for candidate genes.

Bivariate genetic modelling of the response to an oral glucose tolerance challenge: a gene x environment interaction approach

AIMS/HYPOTHESIS

Twin and family studies have shown the importance of genetic factors influencing fasting and 2 h glucose and insulin levels. However, the genetics of the physiological response to a glucose load has not been thoroughly investigated.

METHODS

We studied 580 monozygotic and 1,937 dizygotic British female twins from the Twins UK Registry. The effects of genetic and environmental factors on fasting and 2 h glucose and insulin levels were estimated using univariate genetic modelling. Bivariate model fitting was used to investigate the glucose and insulin responses to a glucose load, i.e. an OGTT.

RESULTS

The genetic effect on fasting and 2 h glucose and insulin levels ranged between 40% and 56% after adjustment for age and BMI. Exposure to a glucose load resulted in the emergence of novel genetic effects on 2 h glucose independent of the fasting level, accounting for about 55% of its heritability. For 2 h insulin, the effect of the same genes that already influenced fasting insulin was amplified by about 30%.

CONCLUSIONS/INTERPRETATION

Exposure to a glucose challenge uncovers new genetic variance for glucose and amplifies the effects of genes that already influence the fasting insulin level. Finding the genes acting on 2 h glucose independently of fasting glucose may offer new aetiological insight into the risk of cardiovascular events and death from all causes.

Neuroticism, recall bias and attention bias for valenced probes: a twin study

BACKGROUND

Prior research on the nature of the vulnerability of neuroticism to psychopathology suggests biases in information processing towards emotional rather than neutral information. It is unclear to what extent this relationship can be explained by genetic or environmental factors.

METHOD

The genetic relationship between a neuroticism composite score and free recall of pleasant and unpleasant words and the reaction time on negative probes (dot-probe task) was investigated in 125 female twin pairs. Interaction effects were modelled to test whether the correlation between neuroticism and cognitive measures depended on the level of the neuroticism score.

RESULTS

The only significant correlation was between neuroticism and the proportion of recalled unpleasant words (heritability is 30%), and was only detectable at the higher end of the neuroticism distribution. This interaction effect seems to be due to environmental effects that make people in the same family more similar (e.g. parental discipline style), rather than genetic factors. An interesting sub-finding was that faster reaction times for left versus right visual field probes in the dot-probe task suggest that cognitive processing in the right hemisphere is more sensitive to subliminal (biologically relevant) cues and that this characteristic is under substantial genetic control (49%). Individual differences in reaction times on right visual field probes were due to environmental effects only.

CONCLUSIONS

There is no evidence that the predisposition of individuals to focus on negative (emotional) stimuli is a possible underlying genetic mechanism of neuroticism.

Prostasin: a possible candidate gene for human hypertension

BACKGROUND

Prostasin, a serine protease, is suggested to be a novel mechanism regulating the epithelial sodium channel (ENaC) expressed in the distal nephron. This study aimed to evaluate whether the human prostasin gene is a novel candidate gene underlying blood pressure (BP) elevation.

METHODS

In a sample of healthy African-American (AA) and European-American (EA) twin subjects aged 17.6 +/- 3.3 years (n = 920, 45% AAs), race-specific tagging single-nucleotide polymorphisms (tSNPs) were identified to tag all the available SNPs +/- 2 kb up- and downstream of the prostasin gene from HapMap at r2 of 0.8-1.0. Selection yielded four tSNPs in AAs and one in EAs, with one tSNP (rs12597511: C to T) present in both AAs and EAs.

RESULTS

For rs12597511, CT and TT genotypes exhibited higher systolic BP (SBP) than CC genotype (115.9 +/- 1.1 mm Hg vs. 113.7 +/- 0.6 mm Hg, P = 0.025 (AAs); and 110.7 +/- 0.5 mm Hg vs. 109.6 +/- 0.6 mm Hg, P = 0.115 (EAs)). CT and TT genotypes compared with CC genotype showed a significant increase in diastolic BP (DBP) in both racial groups (62.5 +/- 0.7 mm Hg vs. 60.4 +/- 0.4 mm Hg, P = 0.003 (AAs); and 58.2 +/- 0.3 mm Hg vs. 56.7 +/- 0.4 mm Hg, P = 0.007 (EAs)). Furthermore, there was an increase in radial pulse wave velocity (PWV) in subjects with CT and TT genotype as compared with those with CC genotype (6.5 +/- 0.1 vs. 6.1 +/- 0.1 m/s, P < 0.0001) (EAs); and 6.7 +/- 0.1 vs. 6.6 +/- 0.1 m/s, P = 0.354 (AAs)). Analyses combining AAs and EAs consistently demonstrated a statistical significance of rs12597511 on all the phenotypes including SBP/DBP and PWV.

CONCLUSION

Genetic variation of the prostasin gene may be implicated in the development of hypertension in youths..

Change of genetic determinants of left ventricular structure in adolescence: longitudinal evidence from the Georgia cardiovascular twin study

BACKGROUND

Genetic contribution to left ventricular (LV) structure is generally recognized, but whether and how this influence varies by ethnicity or with age is unknown.

METHODS

Participants were 517 European-American (EA) and African-American (AA) twin pairs (mean age: 14.6 +/- 3.0) at visit 1 and 422 EA and AA twin pairs at follow-up 4.1 years later. Echocardiograms were obtained on both visits. Data were analyzed using the structural equation modeling software Mx.

RESULTS

Body mass index (BMI) was a strong predictor for all LV measures at both visits 1 and 2, accounting for 3.5-24.2% of the total variance. Hemodynamics explained up to 4.5% additional LV measures variance. After adjusting for BMI, LV measures showed substantial heritability (range: 21-71%). Best-fitting longitudinal models revealed considerable novel genetic effects on the interventricular septum, posterior wall-, and relative wall thickness (RWT) (but not LV internal diameter), accounting for 32-41% of the phenotypic variance at visit 2, with no significant gender and ethnic effects. There was a gender difference for LV mass index in AAs (P < 0.01), with a significant influence of novel genetic effects in males (47%), but not in females. No gender difference was seen in EAs, with 34% of the phenotypic variance at visit 2 attributable to novel genetic effects.

CONCLUSIONS

The heritability of cardiac structure and geometry was equally substantial in both AAs and EAs. Significant novel genetic influences were detected for all measures but LV inner diameter and LV mass index in AA females. Further developmental genetic studies are warranted to elucidate the nature of the emerging gene effects during the transition from adolescence to adulthood.

TUB is a candidate gene for late-onset obesity in women

AIMS/HYPOTHESES

We recently reported significant associations between BMI and three TUB single nucleotide polymorphisms (SNPs) in two Dutch cohorts enriched for type 2 diabetes. Here, we attempted a replication of these associations in a large population-based cohort of female twins comprehensively phenotyped for measures of general and central obesity.

METHODS

Two TUB SNPs (rs2272382, rs2272383) and a third (rs1528133), 22 kb distal to RIC3, were genotyped in 2694 Europid women from the St Thomas' UK Adult Twin Registry (Twins UK) (mean age +/- SD: 47.6 +/- 12.7 years; 42.8% postmenopausal). We explored the hypothesis that TUB is a candidate gene for late-onset obesity in humans through testing the interaction of the SNPs by menopausal status.

RESULTS

In the whole cohort, none of the three SNPs showed a significant main effect on measures of general or central obesity. However, for central obesity the rs2272382 SNP showed a significant interaction with menopausal status (p = 0.036). Postmenopausal women homozygous for the minor allele of rs2272382 showed significantly more general obesity (p = 0.022) and central obesity (p = 0.009) than carriers of the major allele. Differences (beta [95% CI]) between the two genotype groups were 0.92 kg/m2 (0.03-1.81) for BMI (p = 0.036), 2.73 cm (0.62-4.84) for waist circumference (p = 0.013) and 2.43% (0.27-4.60) for per cent central fat (p = 0.027). These associations were confirmed by a sibling transmission disequilibrium test for central obesity, waist circumference and per cent central fat.

CONCLUSIONS/INTERPRETATION

We have replicated associations of TUB SNP rs2272382 with measures of general and central obesity in normal postmenopausal women. These findings confirm TUB as a candidate gene for late-onset obesity in humans.

Heritability of flow-mediated dilation: a twin study

BACKGROUND

Endothelial dysfunction assessed by brachial artery flow-mediated dilation (FMD) is a marker for early atherosclerotic vascular disease and future cardiovascular events.

OBJECTIVE

To estimate the heritability of brachial artery FMD using a twin design.

METHODS

We estimated the heritability of FMD using 94 middle-aged male twin pairs. FMD was measured by ultrasound, and traditional coronary heart disease risk factors were measured. Genetic modeling techniques were used to determine the relative contributions of genes and environment to the variation in FMD.

RESULTS

The mean age of the twin participants was 54.9 +/- 2.8 years. The mean FMD was 0.047 +/- 0.030. The intraclass correlation coefficient was higher in MZ twins [0.38, 95% confidence interval (CI) 0.32-0.43] than in DZ twins (0.19, 95% CI 0.11-0.26), suggesting a role of genetic influence in FMD variation. Structural equation modeling showed that both genetic and unique environmental factors contributed significantly to the variation in FMD. The crude FMD heritability was 0.37 (95% CI 0.15-0.54). After adjustment for traditional cardiovascular risk factors, including age, total cholesterol, blood pressure, and body mass index, the heritability of FMD was 39% (95% CI 0.18-0.56). The remaining variation in FMD could be explained by individual-specific environment.

CONCLUSION

This is the first study using twins to estimate the relative contributions of genetics and environment to the variation in FMD in a US population. Our results demonstrate a moderate genetic effect on brachial artery FMD, independent of traditional coronary risk factors. Our data also highlight the importance of unique environment on the variability in FMD.

SHP-2 and PI3-kinase genes PTPN11 and PIK3R1 may influence serum apoB and LDL cholesterol levels in normal women

Insulin regulates apoB metabolism via activation of PI3K or regulation of MTP via MAPK/ERK signalling. SHP-2 enhances both pathways through increased IRS-1 phosphorylation. We hypothesized that variants in the SHP-2 gene PTPN11 and PI3K p85alpha subunit gene PIK3R1 may influence fasting levels of plasma apoB and/or LDL cholesterol. We tested association of tagging SNPs (tSNPs) in each gene with serum lipids in a large sample of unselected population-based Caucasian female twins (n=2771, mean age 47.4+/-12.5 years) and then tested interaction between tSNPs in determining apoB and LDL levels. PTPN11 tSNP rs11066322 was associated with apoB (P=0.007) and rs11066320 was associated with LDL cholesterol (P=0.016). PIK3R1 tSNP rs251406 was associated with apoB (P=0.0003) and rs706713 was associated with LDL cholesterol (P=0.009). PTPN11 tSNP rs11066322 interacted with PIK3R1 tSNP rs251406 in determining serum apoB levels (P=0.012) and with PIK3R1 tSNP rs40318 in determining LDL cholesterol levels (P=0.009). Association of single tSNPs with both apoB and LDL cholesterol as well as interactions between the two genes suggest that variants influencing SHP-2 activity may modulate the acute pathway by which insulin regulates these lipids.

Candidate genes and growth curves for adiposity in African- and European-American youth

OBJECTIVE

Obesity is associated with multiple health problems and often originates in childhood. This study investigated the association of genes with the development of general and central obesity from childhood into adulthood.

DESIGN

Individual growth curves for measures of general adiposity were examined in an 11-year (1987-1998) cohort study. Single-nucleotide polymorphisms (SNPs) in 11 candidate genes were genotyped.

SUBJECTS

Five hundred and twenty-six subjects classified by race (49% African American (AA)), sex (47% male) and socio-economic status (SES).

RESULTS

AA female carriers of the 27Glu allele in the ADRB2 gene had a larger waist circumference (P<0.05). Subjects of high SES with the ApoB 4145Lys allele had a larger mean waist circumference than those without this allele (P<0.05). Only in the presence of an adverse environment (low SES) did carriers of the NOS3 298Asp allele have a larger mean body mass index, waist circumference and sum of skinfolds (P<0.05).

CONCLUSION

These results suggest that several polymorphisms are associated with the mean level of adiposity, with the effects depending on other factors such as race, sex and/or SES.

Phosphatidylinositol 3-kinase p85alpha regulatory subunit gene PIK3R1 haplotype is associated with body fat and serum leptin in a female twin population

AIMS/HYPOTHESIS

Phosphatidylinositol 3-kinase (PI3K) couples the leptin and insulin signalling pathways via the insulin receptor substrates IRS1 and IRS2. Hence, defective activation of PI3K could be a novel mechanism of peripheral leptin or insulin resistance. We investigated associations of tagging single-nucleotide polymorphisms (tSNPs) in the PI3K p85alpha regulatory subunit gene PIK3R1 with anthropometry, leptin, body fat and insulin sensitivity in a female twin population of European extraction.

MATERIALS AND METHODS

Eight tSNPs were genotyped in 2,778 women (mean age 47.4+/-12.5 years) from the St Thomas' UK Adult Twin Registry (Twins UK).

RESULTS

SNP rs1550805 was associated with serum leptin (p=0.028), BMI (p=0.025), weight (p=0.019), total fat (p=0.004), total fat percentage (p=0.002), waist circumference (p=0.025), central fat (p=0.005) and central fat percentage (p=0.005). SNPs rs7713645 and rs7709243 were associated with BMI (p=0.020 and p=0.029, respectively), rs7709243 with weight, total and central fat (p=0.026, p=0.031 and p=0.023, respectively) and both SNPs with fasting glucose (p=0.003 and p=0.001, respectively) and glucose 2-h post OGTT (p=0.023 and p=0.007, respectively). Subjects with haplotype 222 (frequency 7.2%) showed higher serum leptin concentration (p=0.007) and body fat measures (p< or =0.001 for all), and those with haplotype 221 (frequency 38.7%) showed higher fasting and 2-h glucose (p=0.035 and p=0.021, respectively) compared with subjects with the most common haplotype, 111 (frequency 45.5%).

CONCLUSIONS/INTERPRETATION

Association of the PIK3R1 SNP rs1550805 with serum leptin and body fat may reflect a diminished ability of PI3K to signal via IRS1 or IRS2 in response to leptin.

AMP-kinase alpha2 subunit gene PRKAA2 variants are associated with total cholesterol, low-density lipoprotein-cholesterol and high-density lipoprotein-cholesterol in normal women

BACKGROUND

5'-AMP-activated protein kinase (AMPK) inactivates critial ensymes in fatty acid and cholesterol synthesis. We hypothesised that the serum lipid profile may be influenced by genetic variation in the AMPK catalytic alpha2 subunit.

METHOD

We examined association of 5 tagging SNPs (tSNPs) in the PRKAA2 gene with serum lipids in 2777 normal Caucasian females (mean age 47.4+/-12.5 years).

RESULTS

All tSNPs were associated with total- and LDL-cholesterol, (p<0.001 to 0.034), explaining variances of 0.13-0.59% and 0.11-0.55% respectively. One haplotype (frequency 34.7%) showed lower total- and LDL-cholesterol compared with the most common haplotype (frequency 45.7%) (p< or =0.001), explaining 0.78% of total- and 0.75% of LDL-cholesterol. Another haplotype (frequency 10.5%) was significantly associated with lower HDL-cholesterol (p = 0.005), explaining 0.59% of variance.

CONCLUSIONS

PRKAA2 gene variants are significantly associated with serum lipoproteins in a large sample of normal female Caucasians.

Sleep disordered breathing in pediatric patients with tetralogy of Fallot

Adverse effects on the pulmonary circulation in obstructive sleep disordered breathing (SDB) may place children with heart lesions affecting the right ventricle at increased risk for morbidity and mortality. We examined the distribution and effects of SDB in pediatric patients with tetralogy of Fallot (TOF). Families of 37 pediatric patients with TOF completed a survey of cardiac symptoms and school performance as well as a Pediatric Sleep Questionnaire (PSQ), a validated questionnaire for the screening of SDB in children 2-18 years of age. Medical records were reviewed for growth parameters, medical history, and most recent electrocardiogram (ECG) findings. Data from patients with SDB (PSQ score > or = 8, n = 14) were compared to data from patients without SDB (PSQ score < 8; n = 23). The prevalence of SDB in this population (38%) was significantly higher than the published prevalence of 5% in a healthy general pediatric population (p < 0.001). No significant difference was found in age, gender, or age and sex standardized body mass index between patients with or without SDB. No difference was seen in medication use or timing of surgical repair, whether primary or palliative. Patients with SDB had a significantly higher cardiac symptom score (p = 0.01) and increasing PSQ scores correlated with worsening cardiac symptom scores (p = 0.006). Increasing PSQ scores also correlated with worsening school performance (p = 0.001). No differences were seen in ECG data. The screened prevalence of SDB in the pediatric population with TOF is higher than in the general population; patients with TOF and SDB are more likely to have worse cardiac symptoms and poor school performance.

Evidence of altered bone turnover, vitamin D and calcium regulation with knee osteoarthritis in female twins

BACKGROUND

Osteoarthritis (OA) is a disorder of the whole synovial joint organ. There is growing evidence of the importance of bone turnover in OA, and human studies have demonstrated that the subchondral bone is metabolically active in OA. The aim of this study was to assess the relationships of radiographic knee OA with altered bone turnover and calcium regulation.

METHODS

We performed a matched and unmatched case-control study using twins assessed for OA. The subjects were 1644 female Caucasian twin pairs (266 monozygotic and 556 dizygotic) aged 24-79 yr from the St Thomas' UK Adult Twin Registry. Assays for measures of bone turnover [bone-specific alkaline phosphatase, osteocalcin and urinary deoxypyridinoline (DPD)] and calcium regulation [serum parathyroid hormone (PTH), 25-hydroxyvitamin D, serum calcium, serum magnesium and serum phosphate] were performed. The radiological features of knee OA were graded on a four-point scale (0-3) for osteophytes and a five-point scale (0-4) for Kellgren and Lawrence classification. Adjustment for age, body mass index (BMI) and relatedness was made. Conditional logistic regression analysis was also used to estimate the odds ratio (OR) and 95% confidence intervals (CI) for having radiological features of knee OA per standardized unit difference of serum variable between twins.

RESULTS

Of the 1644 women studied, 474 (28.8%) had radiological evidence of knee osteophytes. There was evidence of increased bone turnover, increased PTH levels and decreased vitamin D levels in this group compared with those without osteophytes. No association was seen with joint space loss. After adjusting for age, BMI and relatedness, all of the differences disappeared except for a significant increase 10% in urinary DPD (P = 0.04). Discordant twin pair analysis (performed on a subgroup of 229 pairs) confirmed modest increases in bone resorption indicated by urinary DPD (OR 1.67, 95% CI 0.88-3.16) and a significant decrease in serum magnesium (OR 0.65, 95% CI 0.46-0.92) in the co-twins with OA.

CONCLUSION

Bone resorption is increased in women with knee OA, consistent with metabolically active subchondral bone. However, bone formation, vitamin D and calcium regulation were not different after adjusting for age and BMI. The results suggest that bone resorption is increased in the presence of OA. Although we cannot clearly differentiate a cause or effect relationship, these results suggest that this is related to disease mechanisms and point to potential diagnostic or therapeutic avenues for bone resorption in OA.

Genetic and environmental influences on psychological distress in the population: General Health Questionnaire analyses in UK twins

BACKGROUND

The General Health Questionnaire (GHQ) is the most popular screening instrument for detecting psychiatric disorders in community samples. Using longitudinal data of a large sample of UK twin pairs, we explored (i) heritabilities of the four scales and the total score; (ii) the genetic stability over time; and (iii) the existence of differential heritable influences at the high (ill) and low (healthy) tail of the distribution.

METHOD

At baseline we assessed the GHQ in 627 MZ and 1323 DZ female pairs and at a second occasion (3.5 years later) for a small subsample (90 MZ and 270 DZ pairs). Liability threshold models and raw ordinal maximum likelihood were used to estimate twin correlations and to fit longitudinal genetic models. We estimated extreme group heritabilities of the GHQ distribution by using a model-fitting implementation of the DeFries-Fulker regression method for selected twin data.

RESULTS

Heritabilities for Somatic Symptoms, Anxiety, Social Dysfunction, Depression and total score were 0.37, 0.40, 0.20, 0.42 and 0.44, respectively. The contribution of shared genetic factors to the correlations between time points is substantial for the total score (73%). Group heritabilities of 0.48 and 0.43 were estimated for the top and bottom 10% of the total GHQ score distribution, respectively.

CONCLUSION

The overall heritability of the GHQ as a measure of psychosocial distress was substantial (44%), with all scales having significant additive genetic influences that persisted across time periods. Extreme group analyses suggest that the genetic control of resilience is as important as the genetic control of vulnerability.

The influence of genetics and environmental factors in the pathogenesis of acne: a twin study of acne in women

Acne is common and often leads to significant psychologic and physical morbidity. From clinical experience, acne appears to run in families; however, very few studies have investigated the genetic basis of this very common skin disease. A large twin study based on 458 pairs of monozygotic and 1099 pairs of dizygotic twins, all women with a mean age of 46 y was performed to investigate the relative contribution of genetic and environmental factors on the liability to acne. In addition, potential risk factors were assessed in twins with and without acne in a nested cross-sectional design. Fourteen percent of the twins reported a history of acne. Genetic modeling using acne scores showed that 81% (95% confidence interval 73-87%) of the variance of the disease was attributable to additive genetic effects. The remaining 19% was attributed to unique (i.e., unshared) environmental factors. Of the potential risk factors tested in 400 acne twins and 2414 unaffected twins, only apolipoprotein A1 serum levels were significantly lower in acne twins even after adjusting for age and weight. Family history of acne was also significantly associated with an increased risk. No significant differences were found between acne twins and nonacne twins for weight, body mass index, height, birth weight, hair thinning, reproductive factors as well as cholesterol, triglycerides, high-density lipoprotein, and glucose levels. The lower serum levels of apolipoprotein A1 in acne twins were also confirmed when analyzing acne discordant twin pairs. The evidence of a major genetic influence on acne should stimulate the search for potential genes that may lead to new therapeutic approaches.

Are twins and singletons comparable? A study of disease-related and lifestyle characteristics in adult women

The classic twin study is sometimes described as "the perfect natural experiment" for the investigation of the aetiology of complex disease, but assumptions of the twin design need to be empirically tested if their results are to be considered unbiased and representative of singleton populations. In this study comparisons of disease and prevalence of lifestyle characteristics have been made between twin participants in the St Thomas' Hospital UK adult twin registry, the largest twin volunteer register in the UK for the study of diseases of ageing, and a parallel population-based study of singleton women. The only differences found were for weight, where monozygotic (MZ) twins were lighter and had a smaller variance than dizygotic (DZ) twins and singletons. For the other variables studied, volunteer twins were not found to differ from age-matched singleton women in distribution or prevalence of: bone mineral density, osteoarthritis, blood pressure, hypertensive drug use, height, history of hysterectomy and ovariectomy, menopausal status and current alcohol and overall tobacco consumption. We conclude that the results of twin studies can be generalised to singleton populations for these measures and disease outcomes.

HbA(1c) levels are genetically determined even in type 1 diabetes: evidence from healthy and diabetic twins

HbA(1c), a measure of blood glucose regulation, reflects glucose levels in the preceding months. In diabetes, HbA(1c) levels predict the risk of microvascular complications. The aim of this study was to determine whether genetic factors could influence HbA(1c) levels in normal subjects and type 1 diabetic patients. We performed a classical twin study of HbA(1c) in healthy nondiabetic female twins and 42 monozygotic (MZ) and 47 dizygotic (DZ) pairs. Interclass correlations (r) were higher in MZ (r = 0.77) compared with DZ (r = 0.53) twin pairs, suggesting a substantial genetic effect; this was confirmed by quantitative genetic model fitting. Additive genetic effects (heritability) explained 62% (95% CI 47-75) of population variance in HbA(1c); the remainder was attributable to the influence of unique environment (23% [15-36]) and age (14% [5-28]). Multivariate modeling showed that genetic factors also have a substantial influence on fasting glucose levels (51%). However, HbA(1c) heritability could not be explained by genes in common with fasting glucose. In the patients with type 1 diabetes, HbA(1c) levels were correlated in 33 MZ twins concordant for diabetes (r = 0.68; P < 0.001) but also in 45 MZ twins discordant for the disease (r = 0.52; P < 0.001). These significant correlations for HbA(1c) in both concordant and discordant pairs indicate a diabetes-independent familial effect. Thus, HbA(1c) levels are largely genetically determined and independent of the genes influencing fasting glucose. Even in type 1 diabetes, familial (i.e., diabetes-independent) factors influence protein glycation, implying that familial factors may explain, in part, the risk for microvascular complications, as indicated by high HbA(1c) levels.

Lack of heritability of circulating leptin concentration in humans after adjustment for body size and adiposity using a physiological approach

OBJECTIVE

To construct a simple physiological model of leptin kinetics, based on measures of body size and composition, which is suitable for investigating the influence of genetic and other influences on circulating leptin levels in humans.

METHODS

Consideration of the kinetics of the secretion and clearance of leptin led to a predicted linear relationship between ln(leptin), ln(fat mass), and a function of non-fat body compartments. Results obtained from this model were compared with those from two published empirical models based on adjustment for fat mass alone or for body mass index. Overnight fasted leptin levels, body composition data (dual-energy X-ray absorptiometry) and questionnaire responses were obtained from 527 twin pairs (127 monozygotic, 400 dizygotic; 37 male (age 18-68 y, BMI 18-32 kg/m2), 489 female (age 18-71, BMI 17-44) drawn from the St Thomas' UK Adult Twin Registry.

RESULTS

In a partial correlation analysis ln(fat mass) and ln(height) (r=0.80, P<0.0001) and r=-0.22, P<0.0001 respectively) were independent predictors of ln(leptin) in females but ln(lean mass) was not (r=-0.01). A regression model incorporating ln(fat mass), ln(height) and a second order polynomial in age provided an adequate fit of the ln(leptin) data in females (r2=71%). ln(Leptin) values adjusted for body size and composition using the model were not significantly heritable (P=0.11), were significantly related to gender (r2=2.3%) and to ln(insulin) (r2=5.7%), but not to menopausal status (r2=0.7%), hormone replacement therapy (r2=0.4%), past or current smoking (r2=1.1%), or percentage trunk fat (r2=0.5%). Both empirical models found significant heritability (h2=36-42%), overestimated the effect of gender in the data (r2=14-16%), and produced significant relationships between adjusted ln(leptin) and percentage trunk fat (r2=4-12%).

CONCLUSIONS

We conclude that our physiologically based model provides an adequate description of the relationship between leptin and body composition and provides a more reliable framework than current empirical approaches for the investigation of other influences on circulating leptin levels. Heritable variations in the control of leptin secretion are unlikely to contribute significantly to variations in leptin levels at the population level.

Genetic determinism in the relationship between human CD4+ and CD8+ T lymphocyte populations?

The adaptive immune system in mammals acts in a coordinated manner to eliminate environmentally derived pathogens. Humans, mice and rats show within species variation in the levels and ratios of their peripheral CD4+ and CD8+ T cells and to a significant degree this variation is under the control of polymorphic genes. Whether genes act separately to specify CD4+ and CD8+ subpopulation levels or whether CD8+ variation is controlled through gene and environmental action on CD4+ cells or vice versa, is not known. We use a quantitative modelling approach in identical and non-identical female human twins to delineate the lines of control which act upon and between CD4+ and CD8+ subsets. The major findings of the study are: (1) genetic variation controls CD8+ T cell levels through two major routes-the first is via an effect on CD4+ T cells which accounts for the observed co-variation between CD4+ and CD8+ T cells, the second is through direct action on CD8+ T cell levels. (2) No evidence of a gene effect from CD8+ T cells on CD4+ cells is observed. Our findings have implications for the evolution of the complex defence system of which CD4+ and CD8+ T cells are a crucial part and encourage further work towards locating common pleiotropic quantitative trait loci responsible for variation in numbers of T cells.

Genes and environment in refractive error: the twin eye study

PURPOSE

A classical twin study was performed to examine the relative importance of genes and environment in refractive error.

METHODS

Refractive error was examined in 226 monozygotic (MZ) and 280 dizygotic (DZ) twin pairs aged 49 to 79 years (mean age, 62.4 years). Using a Humphrey-670 automatic refractor, continuous measures of spherical equivalent, total astigmatism, and corneal astigmatism were recorded. Univariate and bivariate maximum likelihood model fitting was used to estimate genetic and environmental variance components using information from both eyes.

RESULTS

For the continuous spectrum of myopia/hyperopia, a model specifying additive genetic and unique environmental factors showed the best fit to the data, yielding a heritability of 84% to 86% (95% confidence interval [CI], 81%-89%). If myopia and hyperopia (< or = -0.5 D and > or = 0.5 D, respectively) were treated as binary traits, the heritability was 90% (95% CI, 81%-95%) for myopia and 89% (95% CI, 81%-94%) for hyperopia. For total and corneal astigmatism, modeling showed dominant genetic effects are important; dominant genetic effects accounted for 47% to 49% of the variance of total astigmatism (95% CI, 37%-55%) and 42% to 61% of corneal astigmatism variance (95% CI, 8%-71%), with additive genetic factors accounting for 1% to 4% and 4% to 18%, respectively (95% CIs, 0%-13% and 0%-60%, respectively).

CONCLUSIONS

Genetic effects are of major importance in myopia/hyperopia; astigmatism appears to be dominantly inherited.

Genetic correlates of musical pitch recognition in humans

We used a twin study to investigate the genetic and environmental contributions to differences in musical pitch perception abilities in humans. We administered a Distorted Tunes Test (DTT), which requires subjects to judge whether simple popular melodies contain notes with incorrect pitch, to 136 monozygotic twin pairs and 148 dizygotic twin pairs. The correlation of DTT scores between twins was estimated at 0.67 for monozygotic pairs and 0.44 for dizygotic pairs. Genetic model-fitting techniques supported an additive genetic model, with heritability estimated at 0.71 to 0.80, depending on how subjects were categorized, and with no effect of shared environment. DTT scores were only weakly correlated with measures of peripheral hearing. This suggests that variation in musical pitch recognition is primarily due to highly heritable differences in auditory functions not tested by conventional audiologic methods.

The heritability of age-related cortical cataract: the twin eye study

PURPOSE

A classical twin study was performed to establish the relative importance of genes and environment in cortical cataract.

METHODS

Five hundred six pairs of unselected female twin volunteers (226 monozygotic and 280 dizygotic) with a mean age of 62 years (range, 49-79 years) were examined. Cortical cataract was assessed using the slit-lamp-based Oxford Clinical Cataract Classification and Grading System (clinical grading) and the Wilmer Automated Grading System, which analyzed digital retroillumination images of subjects' lenses (digital grading). The worse eye categorized score for each individual was used in maximum likelihood path modeling of the correlations within twin pairs. These correlations were used to determine the underlying liability to cortical cataract.

RESULTS

Prevalence of significant cortical cataract (>/=5% of lens area) was similar in monozygotic and dizygotic twins, occurring in 19.4% and 20.6% with the clinical grading system and 24% and 23% using the digital grading system, respectively. Modeling suggested liability to cortical cataract is explained by additive and dominant genes, individual environment, and age. Estimates of the broad sense heritability of cortical cataract were 58% (95% confidence interval [CI], 51%-64%) for the clinical grading system and 53% (95% CI, 45%-60%) for the digital system. Dominant genes were estimated to contribute to 38% (95% CI, 1%-64%) of the genetic effect with the clinical grading and 53% (95% CI, 28%-60%) with the digital grading. Individual environment explained 26% and 37% and age 16% and 11% of cortical cataract variance in clinical and digital gradings, respectively.

CONCLUSIONS

Genetic effects are important in the development of cortical cataract and involve the action of dominant genes.

Genetic contribution to bone metabolism, calcium excretion, and vitamin D and parathyroid hormone regulation

A classical twin study was performed to assess the relative contribution of genetic and environmental factors to bone metabolism, calcium homeostasis, and the hormones regulating them. It was examined further whether the genetic effect is menopause dependent. The subjects were 2136 adult twins (98.3% female): 384 monozygotic (MZ) and 684 dizygotic (DZ) twin pairs. The intraclass correlations were calculated, and maximum likelihood model fitting was used to estimate genetic and environmental variance components. The intraclass correlations for all of the variables assessed were higher in MZ twin pairs. The heritabilities (95% CIs) obtained from model fitting for hormones regulating bone metabolism and calcium homeostasis were parathyroid hormone (PTH), 60% (54-65%); 25-hydroxyvitamin D [25(OH)D]; 43% (28-57%), 1,25-hydroxyvitamin D [1,25(OH)], 65% (53-74%); and vitamin D binding protein 62% (56-66%). The heritabilities (95% CIs) for markers of bone formation also were assessed; bone-specific alkaline phosphatase (BSAP), 74% (67-80%), and osteocalcin, 29% (14-44%); marker of bone resorption deoxypyridinoline (DPD), 58% (52-64%); and measure of calcium homeostasis 24 h urine calcium, creatinine (Cr), 52% (41-61%). The magnitude of genetic influence differed with menopause for most variables. This study provides evidence for the importance of genetic factors in determining bone resorption and formation, calcium excretion, and the hormones regulating these processes. It shows for the first time a clear genetic effect on bone resorption in premenopausal women and the regulation of PTH, vitamin D metabolism, and calcium excretion. The genes controlling bone hormones and markers are likely to be useful therapeutic and diagnostic targets.

Determination of twin zygosity: a comparison of DNA with various questionnaire indices

This study examined cross-validation and test-retest reliability of questions and questionnaire indices commonly used for twin zygosity classification. Mothers of 58 monozygotic (MZ) and 52 dizygotic (DZ) same sex twin pairs were interviewed by telephone to answer questions regarding the similarity of their twins (mean age = 14.6 +/- 2.8 years). A logistic regression equation correctly classified 91% of both MZ and DZ twin pairs in our sample using 7 of the 12 zygosity questions. The internal consistency for the total questionnaire (Cronbach's alpha) was 0.88. The median two month temporal stability estimate for the individual questions was r = .56 and r = .79 for the test total. For the cross-validation, zygosity classification indices taken from 9 previous studies were applied to our sample and compared to classification according to DNA microsatellite analyses (agreement range = 44 to 100%). The accuracy of the classification indices was significantly lower than the original studies for 62% of the comparisons. If zygosity determination with DNA markers or blood group typing for all subjects is not feasible, rather than using classification indices based on other studies, an optimal classification scheme can be achieved by using a zygosity questionnaire of which the reliability and validity of the questions is established in a random subsample of the same twin cohort.

The genetics of haemostasis: a twin study

BACKGROUND

The concentrations of fibrinogen, factor VII and VIII, von Willebrand factor, plasminogen activator inhibitor-1 (PAI-1), and tissue plasminogen activator have been associated with coronary-heart disease. In addition, polymorphisms in the genes coding for fibrinogen, factor VII, PAI-1, and factor XIII have been reported to affect both protein concentrations and cardiovascular disease risk.

METHODS

We did a classic twin study to assess heritabilities of these haemostatic factors. We enrolled 1002 female twins; 149 pairs of monozygotic and 352 pairs of dizygotic twins. 89 monozygotic and 196 dizygotic twin pairs were analysed for factor VII.

FINDINGS

Quantitative genetic model fitting showed that genetic factors contributed to about 41-75% of the variation in concentrations of fibrinogen, factor VII, factor VIII, PAI-1, tissue plasminogen activator, factor XIII A-subunit and B-subunit, and von Willebrand factor. Factor XIII activity showed higher (82%) and factor XIIa lower (38%) heritability.

INTERPRETATION

We have shown that genetic factors have a major effect on plasma concentrations of haemostatic proteins. Our results stress the importance of research into the genetic regulation of proteins involved in haemostasis and atherothrombotic disorders, including myocardial infarction and stroke.

Evidence of association and linkage disequilibrium between a novel polymorphism in the transforming growth factor beta 1 gene and hip bone mineral density: a study of female twins

OBJECTIVE

Bone mineral density (BMD) in later life is a major determinant of osteoporotic fracture risk and has been shown to be under strong genetic influence. Transforming growth factor beta 1 (TGF-beta 1) is an important regulatory cytokine, is found in high concentrations in the bone matrix, and is a plausible candidate for the genetic regulation of BMD.

METHODS

This study investigated whether a novel polymorphism within the TGF-beta 1 gene is associated with BMD in a large normal female population of 1706 dizygotic (DZ) twins (age range 18-76 yr).

RESULTS

A C--->T [corrected] polymorphism was identified in intron 5, the T [corrected] allele having a frequency of 0.25. Subjects homozygous for the presence of the TGF-beta 1 T [corrected] allele had a 4% reduction in femoral neck BMD compared with the other two genotype groups (P<0.025). No effect was seen at the lumbar spine or ultradistal radius, or with calcaneal ultrasound measurements. Results were unaffected after adjustment for potential confounders. These findings were predominantly seen in pre-menopausal subjects, suggesting that this locus has an effect on the attainment of peak BMD. In pre-menopausal women, subjects who were homozygous for the T [corrected] allele had a 5-fold excess risk of having osteoporosis at the femoral neck compared with the other genotype groups. A within-pair analysis using the sibling transmission disequilibrium test confirmed these findings in pre-menopausal women and supported the candidacy of the TGF-beta 1 locus in the genetic regulation of hip BMD.

CONCLUSIONS

These results indicate that allelic variation at the TGF-beta 1 gene contributes to the development of osteoporosis at the hip. The study also highlights the power of candidate gene analysis in twins, in whom loci having modest effects on disease risk can be identified.

Testing the fetal origins hypothesis in twins: the Birmingham twin study

AIMS/HYPOTHESIS

To test whether the link between birthsize and raised blood pressure or glucose tolerance is due to genetic or intrauterine factors, we studied whether differences in birthweight between pairs of monozygous and dizygous twins are associated with adult differences in blood pressure and glucose tolerance.

METHODS

A sample of 58 monozygous and 140 dizygous twins were identified from a register of births in Birmingham, United Kingdom, between 1950 and 1954. The twins had their blood pressure measured and underwent an oral glucose tolerance test.

RESULTS

There were no statistically significant associations between birthweight, length or ponderal index, and either blood pressure or glucose tolerance in the twins. Although there were substantial within-pair differences in birthweight between monozygous and dizygous twin pairs, these differences did not correlate with the adult outcomes. Monozygous correlations, however, for both blood pressure and glucose tolerance were statistically significantly higher than dizygous correlations and a quantitative genetic model suggested statistically significant heritability for these traits. In contrast correlations of birthsize were similar in monozygous and dizygous pairs suggesting only a small genetic component in determining fetal size.

CONCLUSION/INTERPRETATION

Our results show that birthsize in twins does not predict adult blood pressure or glucose tolerance. We also suggest that shared genetic determinants for fetal growth and adult outcomes are not likely to be prevalent or powerful.

Genetic variation in bone mineral density and calcaneal ultrasound: a study of the influence of menopause using female twins

The aim of the study was to determine whether the genetic variance in bone mineral density (BMD) and calcaneal ultrasound differs in pre- and postmenopausal women and to establish whether the genes operating before the menopause are the same as those after the menopause. Twins aged 18-75 years were recruited from the St Thomas' UK Adult Twin Registry. Quantitative model fitting techniques were used to test for differences in genetic influences in pre- and postmenopausal twins of several BMD sites and calcaneal ultrasound measures accounting for age. BMD and heel ultrasound variables were measured in 2490 female twins: 360 monozygotic pairs and 885 dizygotic pairs. The heritability in the group overall ranged from 19% to 76%. A significant increase in total variance was seen for most BMD sites after the menopause. The proportion of total variance explained by genetic influence was higher premenopausally at all sites except the femoral neck. For example, the genetic proportion of total variance for spine BMD was 88% premenopausally and 77% postmenopausally. In contrast there was no significant difference in total variance of ultrasound measures with menopause. There was no indication that traits are influenced by different genes before and after menopause. This study demonstrates that genetic and environmental influences differ significantly in pre- and postmenopausal groups for BMD, but not for calcaneal ultrasound. The total variance in BMD is greater postmenopausally, but there is evidence that the same genes are involved. These data stress the importance of accounting for menopause-gene interactions in the genetic analysis of data on osteoporosis.