Gene expression in Dicer-deficient mouse liver

Background & Aims: MiRNAs are small (~22 nucleotide), non-coding RNA molecules that regulate gene expression through imperfect complementarity with target messenger RNAs. The function of miRNA in mammalian organogenesis is largely unknown. Conditional loss-of-function of Dicer, the enzyme that processes precursor miRNA transcripts into their mature, active form, has been shown to cause severe defects in a number of organ systems. Here we address the role of Dicer in liver development and function. Methods: Mice lacking Dicer function in hepatocytes were generated using an Afp-Cre strain to drive deletion of a floxed Dicer allele. Deletion of the flox-dicer allele was confirmed by quantitative PCR. Decreased miRNA levels detected by quantitative RT-PCR and in situ hybridization confirmed loss of Dicer function. Gene expression microarray analysis was performed on liver RNA from P28 mutant and control mice. Liver sections from mutant and control mice ranging from embryonic stages through 3-4 months of age were examined and liver function tests were performed on adult mice. Results: Mice lacking hepatocyte Dicer function were born alive at the expected frequency, and had grossly normal appearance and behavior. Despite the loss of mature miRNA, hepatic function was normal, as reflected by normal blood gludose, albumin, cholesterol, and bilirubin. However, mutant mice between 2-4 months of age exhibit progressive hepatocyte damage, elevated ALT/AST, with evidence of balanced proliferation and apoptosis in the lobule. Microarray analysis indicates large-scale changes in gene expression, with increased expression of many miRNA targets, as well as imprinted genes. Conclusions: Loss of miRNA processing in the liver at late gestation has a remarkably mild phenotype, suggesting that miRNAs do not play an essential role in hepatic physiology. However, miRNA deficiency results in hepatocyte apoptosis and balanced hepatocyte regeneration. Finally, microarray analysis of gene expression in mutant liver suggests a previously unrecognized role for Dicer in the repression of imprinted genes.

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Hand NJ, Master ZR, Le Lay J, Friedman JR