Naturally occurring mutations in the human HNF4 alpha gene impair the function of the transcription factor to a varying degree

The hepatocyte nuclear factor (HNF)4 alpha, a member of the nuclear recepto r superfamily, regulates genes that play a critical role in embryogenesis a nd metabolism. Recent studies have shown that mutations in the human HNF4 a lpha gene cause a rare form of type 2 diabetes, maturity onset diabetes of the young (MODY1), To investigate the properties of these naturally occurri ng HNF4a mutations we analysed five MODY1 mutations (R154X, R127W, V255M, Q 268X and E276Q) and one other mutation (D69A), which we found in HepG2 hepa toma cells. Activation of reporter genes in transfection assays and DNA bin ding studies showed that the MODY1-associated mutations result in a variabl e reduction in function, whereas the D69A mutation showed an increased acti vity on some promoters. None of the MODY mutants acted in a dominant negati ve manner, thus excluding inactivation of the wild-type factor as a critica l event in MODY development. A MODY3-associated mutation in the HNF1 alpha gene, a well-known target gene of HNF4 alpha, results in a dramatic loss of the HNF4 alpha binding site in the promoter, indicating that mutations in the HNF4 alpha gene might cause MODY through impaired HNF1 alpha gene funct ion. Based on these data we propose a two-hit model for MODY development.