FANIN/Zfp148, the upstream regulator of pioneer factor paralogs Foxa1 and Foxa2, controls hepatocyte identity

Numerous attempts involving combinations of transcription factors to create functional hepatocytes to enable implantation into patients with chronic and end-stage liver disease have been promising but not yet successful. We identified the upstream regulator of pioneer factor paralogs Foxa1 and Foxa2 in Mus Musculus, critical to hepatogenesis and adult physiological function, we named FANIN. Winged-helix (forkhead) factor paralogs Foxa1 and Foxa2, originally named HNF3α and HNF3β, are called pioneer factors for their ability to independently bind closed chromatin and change chromatin accessibility allowing other factors to bind. Foxa1 and Foxa2, unlike a close homolog Foxa3, are required for life. Deletion of FANIN, encoded by Zfp148, leads to loss of Foxa1 and Foxa2 expression in the liver. FANIN affects global changes in chromatin accessibility, not observed in Foxa2 or Foxa1/Foxa2 double mutants, in particular at regions with enriched HNF4α motif. FANIN controls hepatocyte identity through regulating Foxa1 & Foxa2 expression, as well as activity/binding of HNF4α. Hence, FANIN modulates master regulators that control hepatocyte-specific gene expression and identity. In particular, reduced synergistic Foxa2/HNF4α function in FANIN mutants leads to destabilization of the hepatocyte, resulting in hepatic vacuolization, proliferation, and severe injury. We have uncovered a novel genetic circuit involving master regulators of the hepatocyte, which has significant implications for regeneration of these cells in patients with severe liver disease.