Anatomy of a homeoprotein revealed by the analysis of human MODY3 mutations

Hepatocyte nuclear factor 1 alpha (HNF1 alpha) is an atypical dimeric homeo domain-containing protein that is expressed in liver, intestine, stomach, k idney, and pancreas. Mutations in the HNF1 alpha gene are associated with a n autosomal dominant form of non-insulin-dependent diabetes mellitus called maturity-onset diabetes of the young (MODY3). More than 80 different mutat ions have been identified so far, many of which involve highly conserved am ino acid residues among vertebrate HNF1 alpha. In the present work, we inve stigated the molecular mechanisms by which MODY3 mutations could affect HNF 1 alpha function. For this purpose, we analyzed the properties of 10 mutant s resulting in amino acid substitutions or protein truncation. Some mutants have a reduced protein stability, whereas others are either defective in t he DNA binding or impaired in their intrinsic trans-activation potential. T hree mutants, characterized by a complete loss of trans-activation, behave as dominant negatives when transfected with the wildtype protein. These dat a define a clear causative relationship between MODY3 mutations and functio nal defects in HNF1 alpha trans-activation. In addition, our analysis sheds new light on the structure of a homeoprotein playing a key role in pancrea tic beta cell function.