MARKED BILE DUCT PROLIFERATION IN MICE HETEROZYGOUS FOR LUNATIC FRINGE AND JAG1 MUTATIONS

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Background: The Notch signaling pathway is involved in cell fate determination in many organs. JAG1, encoding a ligand in the Notch pathway, has been identified as the disease gene for Alagille Syndrome (AGS), characterized by bile duct paucity and other manifestations. Mice heterozygous for mutations in both the Notch2 receptor and Jag1 ligand have a phenotype similar to AGS. Notch signaling is regulated by three mammalian Fringe genes. These genes encode glycosyltransferases that modify the activation of the Notch receptor by the ligand. To examine the role of the Lunatic Fringe (Lfng) gene in modifying Jag1 ligand function, we carried out genetic analysis of double heterozygous mice (Jag1tm1Grid/+; Lfngtm1Rjo/+, abbreviated as J1LF). Methods: Liver sections from both J1LF and control mice were examined by H&E staining. Bile ducts were identified by cytokeratin staining, and blood vessels were stained with Factor VIII antibody. Mice were evaluated as newborns and 5-week old adults. Quantitative real time RT-PCR for Hes1, a downstream effector in the Notch pathway, was performed to measure Notch signaling. Results: Adult J1LF mice had a significant increase in the number of bile ducts in the portal tracts, when compared to their single heterozygous and wild type littermates. Numbers of blood vessels in portal tracts were not significantly different between J1LF and control mice. Gross examination revealed that the extrahepatic biliary tree was intact. In adult J1LF livers with bile duct proliferation, real time PCR demonstrated a 4 to 8 fold increase in Hes1 expression levels over wild type and single heterozygous controls. However, in the newborn period, J1LF mice did not display a significant difference in the numbers of bile ducts. Conclusions: Jag1/Lfng double heterozygous mice have a striking proliferation of bile ducts in the adult liver. These differences arise postnatally, pointing to a role for Lfng in regulating Jag1-Notch signaling in bile duct growth and maturation and LFNG as a potential modifier gene for AGS.