Novel P2 promoter-derived HNF4α isoforms with different N-terminus generated by alternate exon insertion

Hepatocyte nuclear factor 4 alpha (HNF4 alpha) is a critical transcription factor for pancreas and liver development and functions in islet beta cells to maintain glucose homeostasis. Mutations in the human HNF4A gene lead to maturity onset diabetes of the young (MODY1) and polymorphisms are associated with increased risk for type 2 diabetes mellitus (T2DM). Expression of six HNF4 alpha variants, three each from two developmentally regulated promoters, has been firmly established. We have now detected a new set of HNF4 alpha variants designated HNF4 alpha 10-12 expressed from distal promoter P2. These variants, generated by inclusion of previously undetected exon 1E (human =222 nt, rodent =136 nt) following exon 1D have an altered N-terminus but identical remaining reading frame. HNF4 alpha 10-alpha 12 are expressed in pancreatic islets (and liver) and exhibit transactivation potentials similar to the corresponding alpha 7-alpha 9 isoforms. DNA-binding analyses implied much higher protein levels of HNF4 alpha 10-alpha 12 in liver than expected from the RT-PCR data. Our results provide evidence for a more complex expression pattern of HNF4 alpha than previously appreciated. We recommend inclusion of exon 1E and nearby DNA sequences in screening for HNF4 alpha mutations and polymorphisms in genetic analyses of MODY1 and T2DM. (C) 2009 Elsevier Inc. All rights reserved.