Elevated Proinsulin in Healthy Siblings of IDDM Patients Independent of HLA Identity

Based on the recent demonstration of elevated serum proinsulin levels in cystic fibrosis patients with impaired glucose tolerance, it was hypothesized that proinsulin could be an indicator of altered β-cell function. We therefore analyzed fasting proinsulin levels in 99 siblings of insulin-dependent diabetes mellitus (IDDM) patients, most of them discordant for diabetes for >6 yr. The results from this group were compared with the results from 41 healthy age- and sex-matched control subjects with no family history of diabetes. Median (range) fasting proinsulin in siblings was 8.9 pM (1.7–58 pM) vs. 3.8 pM (<1.2–28 pM) in control subjects (P < .00001). There was no difference between the groups in fasting blood glucose concentrations. Both groups had fasting insulin concentrations within the normal range with a tendency toward lower values in the siblings: 108 pM (60–237 pM) vs. 118 pM (71–175 pM) (P = .07). The 99 siblings were subdivided into groups according to HLA sharing with their diabetic proband. The concentration of proinsulin, insulin, and blood glucose among the groups of 33 HLA-identical, 40 HLA-haploidentical, and 26 nonidentical siblings did not differ significantly. The fasting proinsulin level did not correlate with fasting levels of insulin, blood glucose, age, or body weight. We conclude that fasting proinsulin is elevated in healthy siblings of IDDM patients, whereas fasting insulin is normal or slightly decreased independent of HLA identity with their diabetic sibling. Elevated proinsulin levels could represent a family trait, perhaps mirroring a β-cell more vulnerable to destruction, or it could reflect previous β-cell damage that does not lead to IDDM.