Variants in the Ca V 2.3 (alpha 1E) subunit of voltage-activated Ca2+ channels are associated with insulin resistance and type 2 diabetes in Pima Indians.

OBJECTIVE: Linkage to type 2 diabetes has been reported on chromosome 1q21-25 in Pima Indians. Fine mapping identified single nucleotide polymorphisms (SNPs) near the CACNA1E gene associated with this disease. CACNA1E encodes the voltage-dependent calcium channel Ca(v)2.3 Ca(2+), and mice lacking this channel exhibit impaired glucose tolerance and insulin secretion. Therefore, CACNA1E was investigated as a positional candidate gene. RESEARCH DESIGN AND METHODS: CACNA1E was sequenced, and 28 SNPs were genotyped in the same group of Pima subjects who had been analyzed in the linkage study. Allele-specific expression was used to functionally evaluate a variant in the 3' untranslated region (UTR). RESULTS: A novel G/A variant in the 3'-UTR was associated with young-onset type 2 diabetes (odds ratio 2.09 per copy of the G-allele [95% CI 1.31-3.33], adjusted P = 0.001) and had an effect on the evidence for linkage at chromosome 1q21-25 (P = 0.004). Among 372 nondiabetic Pima subjects who had undergone metabolic testing, the risk allele was associated with reduced insulin action including increased fasting, 30, 60, and 120 min plasma glucose concentrations and increased fasting plasma insulin during an oral glucose tolerance test (all P < 0.01), as well as a decreased rate of insulin-stimulated glucose disposal at both physiologically and maximally stimulated insulin concentrations (both P < 0.002). Functional analysis of this variant showed that the nonrisk allele had a 2.3-fold higher expression compared with the risk allele. CONCLUSIONS: A functional variant in CACNA1E contributes to type 2 diabetes susceptibility by affecting insulin action. This variant partially explains the linkage to type 2 diabetes on chromosome 1q21-25 in Pima Indians.

CHRM3 gene variation is associated with decreased acute insulin secretion and increased risk for early-onset type 2 diabetes in Pima Indians.

The muscarinic acetylcholine receptor subtype M3 (CHRM3) gene is expressed in islet beta-cells and has a role in stimulating insulin secretion; therefore, CHRM3 was analyzed as a candidate gene for type 2 diabetes in Pima Indians. Ten variants were genotyped in a family-based sample (n = 1,037), and 1 variant (rs3738435) located in the 5' untranslated region of an alternative transcript was found to be modestly associated with both early-onset type 2 diabetes and the acute insulin response in a small subset of these subjects. To better assess whether this variant has a role in acute insulin secretion, which could affect risk for early-onset type 2 diabetes, rs3738435 was genotyped in a larger group of normal glucose-tolerant Pima Indians who had measures of acute insulin secretion (n = 282) and a larger case-control group of Pima Indians selected for early-onset type 2 diabetes (n = 348 case subjects with age of onset <25 years; n = 392 nondiabetic control subjects aged >45 years). Genotyping in these larger sets of subjects confirmed that the C allele of rs3738435 was associated with a reduced acute insulin response (adjusted P = 0.00006) and was also modestly associated with increased risk of early-onset type 2 diabetes (adjusted P = 0.02).

Habitual physical activity in children: the role of genes and the environment.

BACKGROUND: Understanding the factors that contribute to physical inactivity in children is important because sedentary behavior strongly relates to metabolic disorders such as obesity and diabetes. OBJECTIVE: We aimed to quantify the genetic and environmental influences on physical activity energy expenditure (PAEE) in 100 sex-concordant dizygotic (n = 38) and monozygotic (n = 62) twin pairs aged 4-10 y. DESIGN: Resting metabolic rate (RMR) was assessed by using respiratory gas exchange, total energy expenditure (TEE) by using doubly labeled water, and body composition by using dual-energy X-ray absorptiometry. Structural equation modeling was used to partition the phenotypic variance into additive genetic (a2) and common (c2) and unshared (e2) environmental components. RESULTS: Because PAEE [TEE - (RMR + 0.1 x TEE)] depends on body weight, which is highly heritable, we tested several models: 1) after adjustment for age, sex, ethnicity, study date, season, and weight, a2 explained none of the phenotypic variance in PAEE (95% CI: 0%, 38%), whereas c2 and e2 accounted for 69% (33%, 77%; P = 0.001) and 31% (23%, 39%; P < 0.001) of the variance, respectively; 2) after adjustment for the cofactors in model 1, a2 explained 19% of the phenotypic variance in TEE (0%, 60%; P = 0.13), whereas c2 and e2 accounted for 59% (16%, 79%; P = 0.007) and 23% (17%, 31%; P < 0.0001) of the variance, respectively; 3) in models adjusted as above (excluding weight), a2 explained no variance in physical activity level (TEE/RMR) (0%, 32%; P = 0.50), whereas c2 and e2 explained 65% (34%, 60%; P = 0.001) and 35% (28%, 45%; P < 0.0001) of the variance, respectively. CONCLUSIONS: Our data suggest that the familial resemblance in physical activity in these children is explained predominantly by shared environmental factors and not by genetic variability.

A novel missense substitution (Val1483Ile) in the fatty acid synthase gene (FAS) is associated with percentage of body fat and substrate oxidation rates in nondiabetic Pima Indians.

Inhibition of fatty acid synthase (FAS) induces a rapid decline in fat stores in mice, suggesting a role for this enzyme in energy homeostasis. The human FAS gene (FAS) maps to chromosome 17q25, a region previously shown to have suggestive linkage to adiposity in a genome-wide linkage scan for genetic determinants of obesity in Pima Indians. To investigate the potential role of FAS in the pathophysiology of human obesity, the FAS gene was sequenced and 13 single nucleotide polymorphisms (SNPs) were identified. Five representative SNPs were genotyped in 216 full-blooded, nondiabetic Pima Indians for association analyses. A Val1483Ile polymorphism (GTC to ATC; allele frequency of A = 0.10) was associated with percentage of body fat and 24-h substrate oxidation rates measured in a respiratory chamber. Compared with homozygotes for the Val variant, subjects with Ile/x had a lower mean percentage of body fat (30 +/- 1 vs. 33 +/- 1%, P = 0.002; adjusted for age, sex, and family membership) and a lower mean carbohydrate oxidation rate (983 +/- 41 vs. 1,094 +/- 19 kcal/day, P = 0.03), which resulted in a lower mean 24-h respiratory quotient (0.845 +/- 0.01 vs. 0.850 +/- 0.01 kcal/day, P = 0.04; both adjusted for age, sex, family membership, percentage of body fat, and energy balance). Our findings indicate that the Val1483Ile substitution in FAS is protective against obesity in Pima Indians, an effect possibly explained by the role of this gene in the regulation of substrate oxidation.

Weight, adiposity, and physical activity as determinants of an insulin sensitivity index in pima Indian children.

OBJECTIVE: To determine whether measures of physical activity are related to an insulin sensitivity index ([ISI] 10(4)/fasting insulin x glucose) independent of weight or adiposity in children. RESEARCH DESIGN AND METHODS: We conducted a longitudinal study of 90 Pima Indian children (39 boys and 51 girls) at 5 and 10 years of age measuring adiposity (dual-energy X-ray absorptiometry), physical activity behavior (questionnaire: number of activities per week [ACT], average hours per week [TIME]), and energy expenditure (doubly labeled water: physical activity level [PAL]). RESULTS: In cross-sectional analyses, ACT was correlated with ISI at 5 years of age (r = 0.24, P = 0.02) and at 10 years of age (r = 0.21, P = 0.05), but these relationships were not independent of weight or adiposity. PAL was correlated with ISI at 10 years of age (r = 0.39, P = 0.03) but was not independent of weight or adiposity. Longitudinally, ISI decreased from 5 to 10 years of age, and increases in weight and adiposity were associated with decreases in ISI (r = -0.51 and -0.41, respectively; both P < 0.0001). ACT decreased from 5 to 10 years of age, but children who had smaller decreases in ACT had smaller decreases in ISI, independent of increases in weight or adiposity (partial r = 0.22, P = 0.04 adjusted for either weight or adiposity). CONCLUSIONS: These data suggest that early establishment and maintenance of an active lifestyle can have a beneficial effect on ISI that is partially independent of changes in weight or adiposity. This is particularly relevant considering the current epidemics of both obesity and type 2 diabetes in children.

Assessing risk factors for obesity between childhood and adolescence: II. Energy metabolism and physical activity.

OBJECTIVE: To assess the effect of energy expenditure, including resting metabolic rate (RMR), total energy expenditure (TEE), and activity energy expenditure (AEE), as well as substrate oxidation (respiratory quotient [RQ]), on the development of obesity in a large cohort of Native American children with a high propensity for obesity. METHODS: During the summer months of 1992 to 1995 and again 5 years later, 138 (65 boys and 73 girls) 5-year-old Pima Indian children were studied. At baseline and follow-up, height and weight were measured; body composition was assessed with the use of 18O dilution; RMR and RQ were assessed with the use of indirect calorimetry; TEE was measured with the use of the doubly-labeled water method; and AEE was calculated (TEE - [RMR + 0.1 x TEE]). In addition, an activity questionnaire was used to assess participation in sporting activities as well as television viewing during the previous year. Linear regression models were used to assess the effects of the baseline variables on the development of obesity. RESULTS: Pima Indian children were markedly overweight at both 5 and 10 years of age. Cross-sectionally, percentage of body fat and body weight at 5 and 10 years of age were negatively correlated with sports participation and positively correlated with television viewing. Most important, there was a marked change in the correlation between body size and activity between 5 and 10 years of age: at age 5 years, weight was positively correlated with AEE and PAL, but at age 10 years, the correlation with AEE was lost and that with PAL was negative. However, prospectively, none of the variables measured at baseline was a predictor of percentage of body fat at age 10 years after adjustment for percentage of body fat at age 5 years. CONCLUSIONS: At age 5 years, obesity is associated with decreased participation in sports and increased television viewing but not with a decreased PAL. At age 10 years, obesity is associated with decreased participation in sports, increased television viewing, and a decreased PAL, suggesting that a decrease in PAL in free-living conditions seems to follow, not precede, the development of obesity.