Gain of function mutations in Malate Dehydrogenase 2 (MDH2) cause familial diabetes of the adulthood

Background and aims We recently described adult-onset forms of monogenic diabetes, collectively termed as familial diabetes of the adulthood (FDA), showing a mode of inheritance similar to MODY but characterized by later onset and higher body weight, being often misdiagnosed as type 2 diabetes. Genetics of FDA is still mostly unknown. To fill this knowledge gap, we investigated 60 FDA families (52 from the US and 8 from Italy) with an apparent autosomal dominant inheritance, in which mutations in the six most common MODY genes (US families) or in any of the known monogenic diabetes genes (Italian families) had been previously excluded. Among these families we have already identified APPL1 as a disease-gene responsible for hyperglycemia in two of them. Here we report on MDH2 as a new FDA diabetogenes. Methods Whole Exome Sequencing was carried out in a total of 128 subjects from the 60 families. In order to characterize the two identified MDH2 missense mutations as derived from WES study, both bioinformatic and functional studies have been carried out by using molecular dynamics simulations as well in vitro (by using transfected HepG2 cells for the enzymatic activity evaluation or human pancreata for expression studied) and in vivo studies (by using C.Elegans CRISPR-edited strains) respectively. Results We identified two missense variants in the malate dehydrogenase 2 gene (MDH2, NM_005918.2), namely c.154C>T; p.Arg52Cys and c.478G>A; p.Val160Met co-segregating with hyperglycemia in two families from US and Italy, respectively. By operating in the Krebs cycle, MDH2 catalyzes the reversible oxidation of malate to oxaloacetate using the NAD+/NADH cofactor system. This enzyme is involved, also, in the malate-aspartate NADH shuttle, which, particularly in β-cells, is instrumental in the metabolic coordination between cytosol and mitochondria. The two amino acid changes were predicted to be deleterious by several bioinformatics tools and showed altered in silico dynamic properties. When exploring the in vitro effect in human transfected HepG2 cells, variants behaved as gain of function mutations significantly increasing MDH2 enzymatic activity, as compared to wild-type, and decreasing the NAD+/NADH ratio, a redox change known to be deleterious for both insulin signaling and secretion. C. elegans carrying the corresponding mutation at the orthologous mdh-2 gene recapitulated several features reported in worms with genetically-induced defective insulin signaling, also showed impaired glucose-stimulated insulin secretion. In addition, MDH2 mRNA levels were significantly correlated with glucose-induced insulin release in human islets from multi-organ non-diabetic donors, suggesting that MDH2 plays a direct role on human insulin secretion. Conclusions We have identified two MDH2 gain-of-function mutations segregating with hyperglycemia in two FDA families. Our findings suggest a central role of MDH2 in human glucose homeostasis and point to this gene as a new diabetogene contributing to FDA.