Background: Glycated hemoglobin (HbA1c) is used to diagnose type 2 diabetes (T2D) and assess glycemic control in 233 patients with diabetes. Previous genome-wide association studies (GWAS) have identified 18 HbA1c-associated genetic 234 variants. These variants proved to be classifiable by their likely biological action, into erythrocytic (also associated with 235 erythrocyte traits) or glycemic (associated with other glucose-related traits). In this study we tested the hypotheses that in a 236 very large scale GWAS, we would identify more genetic variants associated with HbA1c and that HbA1c variants 237
implicated in erythrocytic biology would affect the diagnostic accuracy of HbA1c. We therefore expanded the number of 238 HbA1c-associated loci, and tested the effect of genetic risk-scores comprised of erythrocytic or glycemic variants on 239 incident diabetes prediction, and on prevalent diabetes screening performance. Throughout this multi-ancestry study, we 240 kept a focus on inter-ancestry differences in HbA1c genetics performance that might influence race-ancestry differences in 241 health outcomes. 242
Methods & Findings: Using genome-wide association meta-analyses in up to 159,940 individuals from 82 cohorts of 244 European, African, East and South Asian ancestry, we identified 60 common genetic variants associated with HbA1c. We 245 classified variants as implicated in glycemic, erythrocytic or unclassified biology and tested whether additive genetic scores 246 of erythrocytic variants (GS-E) or glycemic variants (GS-G) were associated with higher T2D incidence in multi-ethnic 247 longitudinal cohorts (N=33,241). Nineteen glycemic and 22 erythrocytic variants were associated with HbA1c at genome-248 wide significance. GS-G was associated with higher T2D risk (incidence OR=1.05, 95% CI 1.04 -1.06, per HbA1c-raising 249 allele, P=3×10-29); whereas GS-E was not (OR=1.00, 95% CI 0.99 - 1.01, P=0.60). In Europeans and Asians, erythrocytic 250 variants in aggregate had only modest effects on the diagnostic accuracy of HbA1c. Yet, in African Americans, the X-linked 251 G6PD G202A variant (T-allele frequency 11%) was associated with an absolute decrease in HbA1c of 0.81%-units (95% CI 252 0.66-0.96) per allele in hemizygous men, and 0.68%-units (95% CI 0.38-0.97) in homozygous women. The G6PD variant 253 may cause ~2% (N=0.65 million, 95% CI 0.55-0.74) of African American adults with T2D to remain undiagnosed when 254 screened with HbA1c. Limitations include the smaller sample sizes for non-European ancestries and the inability to classify 255 approximately one third of the variants. Further studies in large multi-ethnic cohorts with HbA1c, glycemic and erythrocytic 256 traits are required to better determine the biological action of the unclassified variants. 257
Conclusions: As G6PD deficiency can be clinically silent until illness strikes we recommend investigation of the possible 259 benefits of screening for the G6PD genotype along with using HbA1c to diagnose T2D in populations of African ancestry 260 or groups where G6PD deficiency is common. Screening with direct glucose measurements, or genetically-informed HbA1c 261 diagnostic thresholds in people with G6PD deficiency, may be required to avoid missed or delayed diagnoses.