Evidence for Independent Heritability of the Glycation Gap (Glycosylation Gap) Fraction of HbA1c in Nondiabetic Twins

ROBERT M. COHEN 2 HAROLD SNIEDER PHD 0 1 CHRISTOPHER J. LINDSELL PHD 2 HURIYA BEYAN PHD 4 MOHAMMED I. HAWA BSC 4 STUART BLINKO PHD 5 RAYMOND EDWARDS PHD 3 TIMOTHY D. SPECTOR MSC 0 R. DAVID G. LESLIE 4 0 Twin Research & Genetic Epidemiology Unit, St. Thomas' Hospital , London , U.K. 1 Department of Pediatrics, Georgia Prevention Institute, Medical College of Georgia , Augusta , Georgia; the 2 Division of Endocrinology, Medicine, General Clinical Research Center, Emergency Medicine, University of Cincinnati, Medical Service, Cincinnati Veterans Affairs Medical Center , Cincinnati, Ohio; the 3 The Royal London Medical School and NETRIA, St. Bar- tholomew's Hospital, London, U.K. Institute of Cell and Molecular Science , London E1 2AT, U.K 4 Centre for Diabetes and Metabolic Medicine, Institute of Cell and Molecular Science, St. Bartholomew's Hospital , London , U.K. 5 Abbott Murex Biotech , Dartford , U.K.; the OBJECTIVE - HbA1c (A1C) is substantially determined by genetic factors not shared in common with glucose. Fractions of the variance in A1C, the glycation gap (GG; previously called the glycosylation gap) and the hemoglobin glycosylation index, correlate with diabetes complications. We therefore tested whether GG (measured A1C A1C predicted from glycated serum proteins [GSPs]) was genetically determined and whether it accounted for the heritability of A1C. RESEARCH DESIGN AND METHODS - We conducted a classic twin study on A1C and GSP collected in 40 and 46 pairs of monozygotic and dizygotic healthy female twins, respectively. The predicted A1C was based on the regression line between A1C and GSP in a separate population spanning the pathophysiologic range. RESULTS - GG was more strongly correlated between monozygotic (r 0.65) than dizygotic (r 0.48) twins, adjusted for age and BMI. The best-fitting quantitative genetic model adjusted for age and BMI showed that 69% of population variance in GG is heritable, while the remaining 31% is due to unique environmental influences. In contrast, GSP was similarly correlated between monozygotic (r 0.55) and dizygotic (r 0.49) twins, hence not genetically determined. GG was strongly correlated to A1C (r 0.48), attributable mostly to genetic factors. About one-third of the heritability of A1C is shared with GG; the remainder is specific to A1C. CONCLUSIONS - Heritability of the GG accounts for about one-third of the heritability of A1C. By implication, there are gene(s) that preferentially affect erythrocyte lifespan or glucose and/or nonenzymatic glycation or deglycation in the intracellular, rather than extracellular, compartment. - V of glycemic control within subjects ariation between different measures with diabetes is a common clinical finding (1 4). Yudkin et al. (5) and Gould et al. (6) described persistent differences between HbA1c (A1C) and blood glucose in nondiabetic subjects and categorized these differences as high glycator and low glycator subsets. This observation has recently led to efforts to fractionate the variance in A1C to determine whether there are components that are more closely related to glycemic control and components that seem to remain constant despite variations in glycemic control. The strategy taken to fractionate the vari ance in A1C has been to examine the relationship between A1C and other measures of glycemic control, including, in one instance, glycated serum proteins (GSPs) using the measure fructosamine (i.e., resulting in a measure identified as the glycation gap [GG; previously called the glycosylation gap]) and in the other instance, by the mean of capillary blood glucose measured throughout the day (yielding a measure referred to as the hemoglobin glycation index [HGI]). Cohen et al. (7) reported that the GG is reproducible over time, despite variation in glycemic control reflected in A1C and GSPs. GG correlated with the development of diabetic nephropathy in a retrospective study. McCarter et al. (8) found that HGI was likewise reproducible over time and that retinopathy and nephropathy risk were predicted by the HGI determined on numerous extended capillary glucose profiles throughout the duration of the Diabetes Control and Complications Trial. Evidence from both healthy and diabetic twins indicates that A1C levels are genetically determined, which provides an independent line of evidence that A1C is in part determined by factors other than glycemic control (9). Given the two independent lines of evidence about A1C variance and heritability (79), a logical question that arises is whether the heritable components of A1C are associated preferentially with the GG fraction or the GSPs fraction of the A1C variance, as this would narrow the range of mechanisms involved and inform candidate gene studies. We therefore studied a cohort of healthy nondiabetic female monozygotic and dizygotic twins to determine the contributions of genetic and environmental factors to the GG and GSPs. RESEARCH DESIGN AND METHODS We studied 86 healthy female nondiabetic monozygotic (n 40) and dizygotic (n 46) twin pairs (age range 2176 years) from the St. Thomas U.K. Adult Twin Registry. We used only female subjects to avoid a sex effect. The current twin sample and criteria for selection are the same as previously described except for the exclusion of two twin pairs Figure 1Reference line for predicting A1C from GSP. The sample consisted of 56 subjects with mean (SD) age 44 18.7 years; 24 (43%) were female, and 19 (34%) were individuals with diabetes. r2 0.456 in this reference population. in whom GSP was determined on only one twin of the pair and one twin pair in whom the GSP was a clear outlier in one of the twins (6 SDs above twin group mean) (9). Samples were not available for GSP measurements on the diabetic twin subjects included in that study. Exclusion of frank diabetes at the time of sampling was made by a random whole blood glucose 10.0 mmol/l (180 mg/dl) or a fasting glucose 6.1 mmol/l (110 mg/dl). The separate reference population (Fig. 1) was selected consecutively from patients or their partners attending a hospital clinic because they 1) were either normal or had type 1 diabetes, 2) were aged 2176 years, 3) had normal serum creatinine without proteinuria, and 4) had no current condition other than diabetes. All subjects gave informed consent, and the St. Thomas Hospital and St. Bartholomews Hospital ethics committees approved the study. Both diabetic and nondiabetic subjects were included in the reference population to better describe the relationship between A1C and GSP, which spans the normal and hyperglycemic range. In previous studies, there had been no sex difference in the A1C-GSP relationship (7). Biochemical analyses and confirmation of zygosity Zygosity was determined by standardized questionnaire and confirmed by DNA fingerprinting. A1C was measured as previously reported (9) with interassay coefficient of variation (CV) 2.5% (...truncated)