The islet-enriched transcription factor MAFA regulates the expression of genes critical to beta cell function and insulin secretion. We previously described anovel MAFA mutation (c.191C > T, p.S64F) causing familial insulinomatosis and diabetes mellitus, with male carriers more often developing diabetes and females more prone to insulinomatosis. The exact molecular mechanisms underlying these phenotypes are unclear. In this study, we assessed glucose metabolism and beta cell function in MAFAmutationcarriers and studied the effects of the S64F mutation in a beta cell line. Six seemingly unaffected carriers (3M, 3F) underwent oral glucose tolerance (oGTT) and mixed meal tolerance (MMTT) tests. Glucose, insulin, C-peptide, active GLP-1, GIP and glucagon were measured and results were compared to age-, BMI- and gender-matched healthy controls. Insulin secretion, cell viability, apoptosis and the transcriptome profile were investigated in INS-1 cells stably transduced with wild-type (WT) and S64F human MAFA.
The oGTT results were in keeping with diabetes or impaired glucose tolerance in five of the six carriers. During the oGTT and MMTT, glucose levels were increased in both male and female carriers compared to healthy controls, although males were more markedly hyperglycaemic. Insulin and C-peptide levels were reduced in male carriers while females showed higher insulin and C-peptide compared to controls. The insulinogenic index was reduced only in males, while females had higher HOMA-IR and lower Matsuda index suggesting insulin resistance. Active GLP-1 levels were significantly increased, while glucagon levels failed to suppress during the oGTT in MAFA mutation carriers.
In vitro, insulin secretion in response to glucose was significantly reduced in S64F-MAFA INS-1 compared to WT cells. S64F-MAFA cells were also more susceptible to apoptosis compared to WT. RNA-sequencing showed significant negative enrichment of genes involved with cell cycle regulation, cell division, DNA synthesis and repair in S64F cells. These include the transcription factor Foxm1, the centromere protein A-encoding Cenpa gene and several cyclins (Ccnd1, Ccnd2, Ccne1 and Ccna2) and cyclin-dependent kinases (Cdk1, Cdk14 and Cdk18). Among previously identified MAFA-regulated genes, Ins2, Glp1r and Kcnj11 were also downregulated in S64F-MAFA cells.
The S64F MAFA mutation leads to impaired insulin secretion and increased susceptibility of beta cells to apoptosis secondary to downregulation of genes involved in glucose-stimulated insulin secretion and cell cycle regulation. These findings likely underlie the hyperglycaemic phenotype and reduced insulin secretion observed in males. However, the mechanisms causing hyperinsulinism in female carriers remain to be elucidated.
05 Sep 2020 - 09 Sep 2020