Heterozygous Mutations Affecting the Sonic Hedgehog Pathway Predispose to Non-Alcoholic Fatty Liver Disease
Maria J. Guillen1, Ariel F. Martinez1, Robert Lipinski4, Benjamin Solomon1, Joshua L. Everson5, Niraj S. Trivedi2, Abdel G. Elkahloun3, Ramón Bataller6, Kathleen Sulik5, Maximilian Muenke1;
1Medical Genetics Branch, National Human Genome Research Institute, Bethesda, MD; 2Genome Technology Branch, National Human Genome Research Institute, Bethesda, MD; 3Cancer Genetics Branch, National Human Genome Research Institute, Bethesda, MD; 4Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI; 5The Bowles Center for Alcohol Studies, University of North Carolina, Chapel Hill, NC; 6Division of Gastroenterology and Hepatology, University of North Carolina, Chapel Hill, NC
The Sonic Hedgehog (SHH) pathway is fundamental in early embryonic development and while relatively quiescent in the adult healthy liver, the SHH pathway becomes active in alcoholic and non-alcoholic fatty liver disease. In humans, heterozygous mutations in the SHH pathway cause non-syndromic holoprosencephaly (HPE), a rare disorder of forebrain development. In a cohort of patients with HPE studied in our center, we observed a 6-fold higher prevalence of fatty liver disease when compared to the general population (pediatric: p=0.0023; adult: p=0.0135). The majority of these patients have no environmental/medical risk factors for fatty liver. Steatosis was also seen in liver sections from affected individuals who succumbed in the fetal/neonatal period. To test the hypothesis that mutations in the SHH pathway predispose to steatohepatitis, we compared heterozygous knockout mice (Gli2+/-) bred back to the C57Bl/6J strain to their wild-type (WT) littermates. Animals were fed a high-fat (HF) diet for 12 weeks, after which their livers were harvested. Weight gain, gross liver morphology, and liver fat accumulation were analyzed. We then performed whole-transcript mRNA expression profiling of liver tissues. Gli2+/- mice showed increased susceptibility to fatty liver when fed HF diet compared to controls.
All mice fed the HF diet showed an increase in body mass, but Gli2+/- animals gained significantly more weight than WT. Gli2+/- mice fed control diet also demonstrated a differential expression of genes involved in carbohydrate and lipid metabolism, immune function, oxidative stress and cell cycle regulation, which suggests an inherent metabolic defect affecting liver function. The most dysregulated molecules include Ctse, Taf1d, Gadd45g, Snora74a, Lcn2, Hsph1, Sult2a1, Sult2a2, Cyp3a5, Cyp2c9, Rnf1 86, Pdk4 and Rsad2. SHH pathway genes did not show significant differences in gene expression. Although all animals exposed to the HF diet showed similar changes in genes involved in the above mentioned cellular processes, Gli2+/- mice had more genes with aberrant expression involved in these pathways and a specific fingerprint of molecules involved in liver cirrhosis. OurGli2+/- mouse model shows that SHH signaling defects predispose to the development of fatty liver disease causing a more severe phenotype when exposed to environmental factors (diet). We propose that mutations interfering with SHH signaling be considered a target for investigation as a risk factor for idiopathic fatty liver in the general population. We are currently working on a Shh+/-mouse model and using a different diet that induces fatty liver through an alternate pathway.
The following people have nothing to disclose: Maria J. Guillen, Ariel F. Martinez, Robert Lipinski, Benjamin Solomon, Joshua L. Everson, Niraj S. Trivedi, Abdel G. Elkahloun, Ramón Bataller, Kathleen Sulik, Maximilian Muenke