In vivo role of the HNF4α AF-1 activation domain revealed by exon swapping

The gene encoding the nuclear receptor hepatocyte nuclear factor 4 alpha (HNF4 alpha) generates isoforms HNF4 alpha 1 and HNF4 alpha 7 from usage of alternative promoters. In particular, HNF4 alpha 7 is expressed in the pancreas whereas HNF4 alpha 1 is found in liver, and mutations affecting HNF4 alpha function cause impaired insulin secretion and/or hepatic defects in humans and in tissue-specific 'knockout' mice. HNF4 alpha 1 and alpha 7 isoforms differ exclusively by amino acids encoded by the first exon which, in HNF4 alpha 1 but not in HNF4 alpha 7, includes the activating function (AF)-1 transactivation domain. To investigate the roles of HNF4 alpha 1 and HNF4 alpha 7 in vivo, we generated mice expressing only one isoform under control of both promoters, via reciprocal swapping of the isoform-specific first exons. Unlike Hnf4 alpha gene disruption which causes embryonic lethality, these 'alpha 7-only' and 'alpha 1-only' mice are viable, indicating functional redundancy of the isoforms. However, the former show dyslipidemia and preliminary results indicate impaired glucose tolerance for the latter, revealing functional specificities of the isoforms. These 'knock-in' mice provide the first test in vivo of the HNF4 alpha AF-1 function and have permitted identification of AF-1-dependent target genes.