Phd in HUMAN BIOLOGY AND MEDICAL GENETICS

Thesis title: Identification of malate dehydrogenase 2 (MDH2) as a new disease gene in familial diabetes of the adulthood

Type 2 diabetes (T2D), currently affecting over 450 million people, imposes a severe burden on health care systems. Despite its high prevalence, the exact definition of T2D is elusive as this disorder is diagnosed when other known forms of the disease are excluded. Clinical data indicate that at least 3% of the adult patients diagnosed with T2D in Italy in fact belong to families with multigenerational, presumably monogenic diabetes. Approximately 25% of such patients carry mutations in the genes involved in monogenic diabetes, thus leaving unraveled the genetic causes of the remaining 75% individuals. We have proposed to define these forms of diabetes as "familial diabetes of the adulthood" or FDA. Dissecting the genetic architecture of diabetes in these families is likely to provide insights into new mechanisms and pathways controlling glucose homeostasis as well as to enhance our ability to provide genetic testing and counseling to members of FDA families and possibly to set up tailored treatments as already possible for some other monogenic diabetes subtypes. By means of Whole Exome Sequencing, carried out in 60 FDA families (52 from the US and 8 from Italy), we identified two heterozygous pathogenic gain of function variants in the malate dehydrogenase 2 (MDH2) gene (namely p.Arg52Cys and p.Val160Met) that segregated in a dominant fashion with diabetes in two of them. In silico, both Arg52Cys and Val160Met were shown to affect MDH2 protein structure and function. In transfected HepG2 cells, both variants significantly increased MDH2 enzymatic activity, thereby decreasing the NAD+/NADH ratio - a change known to affect insulin signaling and secretion. Stable expression of human wild type MDH2 in MIN6-K8 cell lines enhanced glucose- and GLP-1-stimulated insulin secretion. This effect was blunted by the Cys52 or Met164 substitutions. Genetically-modified C.elegans carrying equivalent changes at the orthologous positions of the mdh-2 gene showed impaired glucose-stimulated insulin secretion. In all, these results suggest a key role of MDH2 in human glucose homeostasis and indicate that gain of function variants in this gene may be involved in the etiology of familial forms of diabetes.