Evaluation of recurrence risk in siblings of diabetic children: importance of age and birth order in relation to HLA genotypes.

In order to identify factors other than the HLA-linked susceptibility involved in the risk to the siblings of diabetic children, we compared the age at onset, birth order and HLA genotypes in 23 IDD multiplex (Mx) families and 140 simplex (Sx) families. The results showed a relationship between age and recurrence risk in sibs. Probands (= 1st affected sibs) of Mx families were significantly younger at onset (5.8 +/- 0.8 yr) than probands of Sx families (9.0 +/- 0.4 yr, p less than 0.01). The 2nd affected sibs of Mx families, although affected at an older age (12.4 +/- 1.6 yr), had been significantly younger at the time of the proband's onset, than the siblings who have remained unaffected (6.2 +/- 1.2 vs 11.5 +/- 0.5 yr, p less than 0.01). Probands of Mx families were more often the first born and probands of Sx families, more often came later in the birth order (p less than 0.02). Prevalence of IDD was higher among siblings born after than among those born before the proband (12% vs 4%, p less than 0.002). The HLA haplotype distribution in unaffected siblings deviated from the random assortment in relation to birth order, showing an excess of HLA-identical sibs born before the proband and, inversely, an excess of non-identical sibs born after the proband. The results suggest that age-dependent factors are likely to increase the penetrance of HLA-linked IDD susceptibility. These factors could be related to the environmental insult. However, a genetically determined form of type I diabetes characterized by higher familial penetrance and earlier onset cannot be ruled out.