Recessive mutations in the INS gene result in neonatal diabetes through reduced insulin biosynthesis

Abstract

Heterozygous coding mutations in the INS gene that encodes preproinsulin were recently shown to be an important cause of permanent neonatal diabetes. These dominantly acting mutations prevent normal folding of proinsulin, which leads to beta-cell death through endoplasmic reticulum stress and apoptosis. We now report 10 different recessive INS mutations in 15 probands with neonatal diabetes. Functional studies showed that recessive mutations resulted in diabetes because of decreased insulin biosynthesis through distinct mechanisms, including gene deletion, lack of the translation initiation signal, and altered mRNA stability because of the disruption of a polyadenylation signal. A subset of recessive mutations caused abnormal INS transcription, including the deletion of the C1 and E1 cis regulatory elements, or three different single base-pair substitutions in a CC dinucleotide sequence located between E1 and A1 elements. In keeping with an earlier and more severe beta-cell defect, patients with recessive INS mutations had a lower birth weight (−3.2 SD score vs. −2.0 SD score) and were diagnosed earlier (median 1 week vs. 10 weeks) compared to those with dominant INS mutations. Mutations in the insulin gene can therefore result in neonatal diabetes as a result of two contrasting pathogenic mechanisms. Moreover, the recessively inherited mutations provide a genetic demonstration of the essential role of multiple sequence elements that regulate the biosynthesis of insulin in man. (8-12). In contrast, abnormalities in chromosome 6q24 are the most common cause of TNDM (13), followed by mutations in the KCNJ11 and ABCC8 genes (14). Despite these advances, the etiology of neonatal diabetes is still not known in at least 30% of patients with PNDM, suggesting other genetic causes are still to be found (9).Insulin is secreted from islet beta cells of the pancreas. Insufficient secretion of insulin results in hyperglycemia and diabetes, whereas excessive secretion results in hypoglycemia. Insulin biosynthesis and secretion are therefore tightly regulated to maintain blood glucose levels within a narrow physiological range. Extensive studies have dissected an array of cis sequence elements in the INS promoter region and their cognate DNA binding factors, which together ensure the cellular specificity and rate of INS transcription (15)(16)(17)(18)(19)(20)(21)(22). In addition, insulin biosynthesis is strongly dependent on posttranscriptional regulatory mechanisms, including the modulation of translation and stability (23-25). The latter is largely mediated through sequences located in the untranslated regions of INS transcripts (26-28).

Inefficient Translocation of Preproinsulin Contributes to Pancreatic β Cell Failure and Late-onset Diabetes

“…It has been shown that insulin gene mutations causing low gene expression are recessive, rather than dominant, causes of diabetes (31). Therefore, for R6C or R6H, a 50% reduction in insulin production alone (Fig.…”

“…1-4) showing that ϳ50% of R6C molecules are translocated into the ER. However, this partial defect alone is insufficient to account for insulin-deficient diabetes (31). Given that R6C and R6H are associated with autosomal domi- The ability of exogenously added SRP and SRP receptor to target preproinsulins to salt-washed and trypsin-digested ER microsomes (TKRM) was assayed by cleavage of the SP upon successful translocation (A) and by protection of the translocated protein from PK digestion (B), as described under "Experimental Procedures."…”

Diabetes caused by insulin gene (INS) deletion: clinical characteristics of homozygous and heterozygous individuals

“…The two boys with biallelic INS deletion had been reported recently (12) and we now add mapping data of the 646 bp deletion resulting in loss of half of the preproinsulin protein and proximal parts of the insulin promoter. Furthermore, we report the phenotype of subjects with a heterozygous INS deletion and their risk of developing adult-onset and insulin-dependent diabetes.…”

“…The molecular mechanism behind these cases with a MODY-like presentation was suggested to be defective trafficking of proinsulin and increased endoplasmic reticulum (ER) stress (11). Recessive mutations of the preproinsulin (INS) gene have also been identified as a novel cause of neonatal diabetes and were the commonest cause of isolated PND in the offspring of consanguineous parents (12). In humans, complete loss or inactivation of the two insulin alleles clearly leads to insulin-deficient diabetes from birth, as it has been reported previously from our cases (12).…”

“…Recessive mutations of the preproinsulin (INS) gene have also been identified as a novel cause of neonatal diabetes and were the commonest cause of isolated PND in the offspring of consanguineous parents (12). In humans, complete loss or inactivation of the two insulin alleles clearly leads to insulin-deficient diabetes from birth, as it has been reported previously from our cases (12). But whether allelic depletion of INS has any human phenotype at all, remains controversial since inactivation of three of four mouse INS alleles (Ins1 K/K /Ins2 K/C ) does not lead to diabetes during mouse lifespan (13).…”