Heme Regulates Exocrine Peptidase Precursor Genes in Zebrafish

We previously determined that yquem harbors a mutation in the gene encoding uroporphyrinogen decarboxylase (UROD), the fifth enzyme in heme biosynthesis, and established zebrafish yquem (yqe(tp61)) as a vertebrate model for human hepatoery-thropoietic porphyria (HEP). Here we report that six exocrine peptidase precursor genes, carboxypeptidase A, trypsin precursor, trypsin like, chymotrypsinogen B1, chymotrypsinogen 1-like, and elastase 2 like, are downregulated in yquem/urod (-/-), identified initially by microarray analysis of yquem/urod zebrafish and, subsequently, confirmed by in situ hybridization. We then determined downregulation of these six zymogens specifically in the exocrine pancreas of sauternes (sau(tb223)) larvae, carrying a mutation in the gene encoding delta-amino-levulinate synthase (ALAS2), the first enzyme in heme biosynthesis. We also found that ptf1a, a transcription factor regulating exocrine zymogens, is downregulated in both yquem/urod (-/-) and sau/alas2 (-/-) larvae. Further, hemin treatment rescues expression of ptf1a and these six zymogens in both yquem/urod (-/-) and sauternes/alas2 (-/-) larvae. Thus, it appears that heme deficiency downregulates ptf1a, which, in turn, leads to downregulation of exocrine zymogens. Our findings provide a better understanding of heme deficiency pathogenesis and enhance our ability to diagnose and treat patients with porphyria or pancreatic diseases.