title: Evidence for independent heritability of the glycation gap (glycosylation gap) fraction of HbA1c in nondiabetic twins.
abstract: OBJECTIVE
  HbA(1c) (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.
  
  
  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.
authors.affiliation: Division of Endocrinology, Medicine, General Clinical Research Center, Emergency Medicine, University of Cincinnati, Ohio, USA.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Cohen
authors.forename: Harold
authors.initials: H
authors.lastname: Snieder
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Lindsell
authors.forename: Huriya
authors.initials: H
authors.lastname: Beyan
authors.forename: Mohammed I
authors.initials: MI
authors.lastname: Hawa
authors.forename: Stuart
authors.initials: S
authors.lastname: Blinko
authors.forename: Raymond
authors.initials: R
authors.lastname: Edwards
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Spector
authors.forename: R David G
authors.initials: RD
authors.lastname: Leslie
mesh_terms: D000328:Adult; D000368:Aged; D003920:Diabetes Mellitus; D005260:Female; D006442:Glycated Hemoglobin / Q000378:metabolism*; D006031:Glycosylation; D006801:Humans; D008875:Middle Aged; D014430:Twins, Monozygotic
chemical_list: D006442:Glycated Hemoglobin