Permanent Neonatal/Infancy-Onset Diabetes Mellitus Caused by Seven Novel Mutations of the Insulin Gene

Results: Permanent Neonatal/Infancy-Onset Diabetes Mellitus is a rare disease, which occurs in about 1 in 140.000 live births in Italy. In a group of 37 patients with this condition we found 19 heterozygous activating mutations of Kir6.2 (KCNJ11) gene and 1 homozygous mutation of the glucokinase. In one of the remaining patients we observed a surprisingly high C-peptide level (8.5 ng/ml) at diabetes onset, a finding similar to that reported in patients with familial hyper(pro)insulinemia caused by mutations in the insulin (INS) gene. We thus screened the entire coding region of the insulin gene in KCNJ11-negative subjects. We detected 7 different heterozygous mutations in 9 Italian probands: the mutation R65C (at the dibasic doublet between the C-peptide and the A-chain) in 3 probands (2 familial cases), and the mutations LB6P, LB6V, LB11P, LB15YB16delinsH, CA6Y, and YA19X. In addition, we found the CA6Y mutation in a proband from Denmark. Diabetes onset spanned from 1 to 6 months from birth, but the affected parents of the 2 familial cases showed diabetes onset at 1 and 4 years of age, respectively. In patients carrying an INS gene mutation diabetes onset occurred later than individuals with KCNJ11 gene mutation by a median of 5 weeks. By transfecting HEK293 cells with wild type, neonatal diabetes (ND) mutations and the R65L familial hyper(pro)insulinemia (control) mutation (pro)insulin cDNAs, we found that wt (pro)insulin and the R65L mutation but not ND mutations are secreted by 293 cells. XBP1 splicing, a marker of endoplasmic reticulum stress, was induced in 293 cells transfected with ND mutations, but not by wt or R65L (pro)insulin expression. Annexin V (early apoptosis) and propidium iodide (late apoptosis) analyzed by FACS were found increased in 203 cells transfectd with the LB15YB16delinsH and CA6Y mutations. Accordingly, after transfection of INS-1E cells with wild type, the R65L or ND mutations only cells expressing wild type or R65L (pro)insulin were viable after 96 hours. We conclude that INS gene mutations can cause permanent neonatal/infancy onset diabetes by ER stress-induced apoptosis of the beta cells. Category: Genetics - Type 2 Diabetes