ABCC8 R1420H Loss-of-Function Variant in a Southwest American Indian Community: Association With Increased Birth Weight and Doubled Risk of Type 2 Diabetes

Abstract

Missense variants in KCNJ11 and ABCC8, which encode the KIR6.2 and SUR1 subunits of the β-cell KATP channel, have previously been implicated in type 2 diabetes, neonatal diabetes, and hyperinsulinemic hypoglycemia of infancy (HHI). To determine whether variation in these genes affects risk for type 2 diabetes or increased birth weight as a consequence of fetal hyperinsulinemia in Pima Indians, missense and common noncoding variants were analyzed in individuals living in the Gila River Indian Community. A R1420H variant in SUR1 (ABCC8) was identified in 3.3% of the population (N = 7,710). R1420H carriers had higher mean birth weights and a twofold increased risk for type 2 diabetes with a 7-year earlier onset age despite being leaner than noncarriers. One individual homozygous for R1420H was identified; retrospective review of his medical records was consistent with HHI and a diagnosis of diabetes at age 3.5 years. In vitro studies showed that the R1420H substitution decreases KATP channel activity. Identification of this loss-of-function variant in ABCC8 with a carrier frequency of 3.3% affects clinical care as homozygous inheritance and potential HHI will occur in 1/3,600 births in this American Indian population.

Figure 1

Type 2 diabetes status for R1420H carriers and noncarriers. A: Prevalence of type 2 diabetes shown in individuals grouped by their age at last exam. Numbers in parentheses indicate the number of subjects with diabetes (numerator) and total number of subjects (denominator) for each age-group. B: Type 2 diabetes cumulative incidence adjusted for sex, birth year, and ancestry (American Indian/European admixture based on genetic markers and self-reported fraction Pima Indian heritage). Mean onset age was calculated from the parameters of the Weibull model for an individual born in 1940, and the mean age for all other covariates was 45.0 years for R1420H and 52.1 years for R1420R.

Figure 2

The prevalence of type 2 diabetes for R1420H carriers and noncarriers in relation to BMI. Diabetes prevalence is shown in individuals stratified by their maximum BMI measured at age ≥15 years at a nondiabetic exam. Numbers in parentheses indicate the number of subjects with diabetes (numerator) and total number of subjects (denominator) for the BMI group. As a comparison of prevalence between R1420H and R1420R at each level of BMI, ORs (95% CI) are as follows: BMI <25 kg/m², 2.32 (0.73–7.32); BMI 25–30 kg/m², 2.45 (1.30–4.62); BMI >30–35 kg/m², 1.09 (0.55–2.14); BMI >35–40 kg/m², 1.88 (1.01–3.49); and BMI >40 kg/m², 1.04 (0.45–2.46).

Figure 3

Comparison of mean BMI for R1420H carriers and noncarriers in relation to diabetes status and age. A: Analysis of BMI restricted to subjects who developed diabetes. Mean BMI at all exams before and after diagnosis of diabetes in 2,458 individuals who eventually developed diabetes (117 R1420H carriers [the one H1420H was included with the heterozygote carriers] and 2,341 noncarriers). Mean BMI was calculated using data from all longitudinal exams (total exams = 9,086). Diabetes diagnosis is at 0 years; negative years represent time before diagnosis and positive years represent time following diagnosis. BMI is significantly lower in all exams from R1420H carriers (P = 7 × 10⁻⁴ adjusted for age, sex, birth year, time category, and ancestry in a mixed model that accounts for sibship and repeated examinations within individuals). B: Analysis of BMI restricted to subjects not known to have diabetes. The maximum BMI by age among 4,210 subjects who did not have diabetes as of their last exam (106 R1420H carriers and 4,104 noncarriers). The number of R1420H carriers/noncarriers at each age category are 15–24 years (50/1,922), 25–34 years (26/1,042), 35–44 years (19/747), 45–54 years (8/259), and ≥55 years (3/134). R1420H carriers had a lower BMI compared with noncarriers (P = 0.01 adjusted for age, sex, birth year, and ancestry in a model that accounts for sibship).

Figure 4

Mean birth weight for siblings discordant for the R1420H variant. Birth weight is adjusted for gestational age, birth year, sex, genetically derived estimate of American Indian/European ancestry, and maternal diabetes status during pregnancy (see Research Design and Methods). Diagonal line is line of identity. The mean birth weight of siblings with R1420H genotype was higher than that of siblings with the R1420R genotype in 18 of 23 sibships. P = 0.01 for within-sibship difference in birth weight.

Figure 5

CIR for R1420H carriers and noncarriers grouped by their age at last nondiabetic OGTT. CIR is standardized for insulin assay (three different assays were used in this longitudinal study) after logarithmic transformation. Means are adjusted for sex, BMI, HOMA-IR, and ancestry (American Indian/European admixture based on genetic markers and self-reported fraction Pima Indian heritage).

Figure 6

Decreased K_ATP activity for SUR1 (ABCC8) channels harboring R1420H or R1420C mutations. Relative ⁸⁶Rb⁺ efflux is shown as a function of time under basal conditions, in the presence of metabolic inhibition (MI), in the presence of K_ATP channel opener diazoxide, and for both control cells expressing green fluorescent protein and cells expressing R1420R, R1420H, or R1420C channels. Data are shown as mean ± SEM from four independent transfections. *P < 0.05, **P < 0.01 vs. R1420R for each case.