Early onset of diabetes in the proband is the major determinant of risk in HLA DR3-DQ2/DR4-DQ8 siblings.

Islet autoimmunity is initiated in infancy, and primary prevention trials require children at high genetic risk to be identified before autoantibodies appear. To inform screening strategies, we evaluated risks of autoimmunity and diabetes associated with HLA DR3-DQ2/DR4-DQ8 in U.K. families. Extended HLA haplotypes were determined in 2,134 siblings from the Bart's-Oxford Study followed to a median age of 22 years. Risks of diabetes and islet autoimmunity (more than two antibodies) were estimated by survival analysis. Of 138 informative DR3-DQ2/DR4-DQ8 siblings, 63% shared both haplotypes with their diabetic proband, 29% shared one, and 8% shared neither. In HLA-identical DR3-DQ2/DR4-DQ8 siblings, the cumulative risk of diabetes by age 15 was 17% (vs. 6% in those sharing one haplotype or none; P = 0.095). Risk varied, however, with the age at the onset of diabetes in the proband; the cumulative risk of autoimmunity and/or diabetes by age 15 was 61% in siblings of probands diagnosed when younger than 10 years old compared with only 4.7% in those diagnosed after age 10 years (P < 0.001). The age of the proband at diagnosis, but not HLA haplotype sharing, was an independent determinant of sibling risk. This suggests that non-HLA genes or epigenetic/environmental factors that accelerate the progression of type 1 diabetes in the proband strongly affect risk in siblings.