Defining the role of MafA in islet beta cells

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Much of our knowledge regarding the actions of islet-enriched transcription factors, key regulators of β cell differentiation and function, derive from studies in rodents, which is problematic because of differences in islet cell composition, cell architecture, and glucose responsive insulin secretion with humans. Unlike other islet- enriched transcription factors, there is a prominent difference between rodents and humans in the β cell composition of the MAFA (i.e. (V-Maf Avian Musculoaponeurotic Fibrosarcoma Oncogene Homolog A) and MAFB regulators. Thus, the dimeric activator in rodents is MafA2 and a heterodimer of MAFA and MAFB in humans. Notably, much evidence suggests that there are distinct functional differences between MafA and MafB, with MafB2 important during development and in the immature β cell. We will use human EndoCβH2 β cell lines, human embryonic stem derived β cells and human islets to elucidate how MAFB2 and MAFA/B control human β cell activity. Interestingly, a missence variant of human MAFA (S64F) was recently shown to co- segregate with diabetes and insulinomatosis by Drs. Sian Ellard, Andrew Hattersley, and Márta Korbonits in England. (Insulinomatosis is defined as multicentric insulin-secreting tumors that are not linked with mutations in MEN1 (Multiple Endocrine Neoplasia type 1), which often arises in a familial setting and is associated with diabetes). The phenotype of this variant suggests that MAFA is the newest member of the Maturity-Onset of Diabetes of the Young gene family, an autosomal dominant monogenic form of diabetes often observed in early adulthood. In collaboration with these investigators, we are determining how this mutation impacts MAFA, with our preliminary results supporting gain-of-function properties due to increased protein stability. The identification of this MAFA variant provides evidence for the important role of this transcription factor in human islet β cell function and diabetes pathogenesis. This idea is also supported by observations of compromised MAFA and MAFB mRNA and protein levels in human type 1 diabetes mellitus (T1D) and T2D β cells. These data will provide new mechanistic insights into how normal and pathological processes influence transcription factor actions. Our analyses will include the isolation and characterization of the coregulators recruited by MAFB2 and MAFA/B to mediate transcriptional control in human β cells. The questions being addressed in our experiments will provide knowledge relevant to those developing diagnostic and therapeutic approaches for diabetes.