QTL mapping of murine hepatocellular damage in vivo and in vitro delineates two candidate genes within a 6 megabase locus on chromosome 11

Aims: Over the past two decades diverse molecular mechanisms of fibrogenesis have been identified. There is however still a substantial amount of unknown contributors, indicated by the so-called „missing heritability“, i.e. the lack of predictive value using known genetic risk factors.

The aim of our study was to identify novel risk factors by dissecting genetic loci underlying differenzial susceptibility of hepatocytes to fibrosis-associated cytokine signalling.

Methods: We performed two genome-wide quantitative trait loci (QTL) mapping experiments: One in vitro experiment using cultured hepatocytes of BXD recombinant inbred mouse lines, measuring TGF-beta-induced total cell death as trait; a second QTL experiment used the histological fibrosis stage after CCl₄ challenge in vivo as trait. Loci within the resulting common candidate regions were investigated for their impact on gene expression during short-term liver damage by ethanol or CCl₄ in vivo.

Results: The correlation analysis identifies chromosome 11q 83–89 Mb as modifier of both traits in BXD mice. Gene expression analysis in BXD livers in response to short-term ethanol damage or CCl₄ injections reveals five single nucleotide variants within the peak area that appear to have a regulatory impact on various other genes during acute liver damage. Non-conservative coding variants near the most proximal and distal expression-associated polymorphisms in the candidate area delineate Expi and Msi2 as candidate genes. Expi is in close proximity to a SNP showing regulatory activity following ethanol injury. Its sequence reveals functional homology with two neighbouring genes, both involved in inflammation and innate immune response. Msi2 is a haematopoietic stem cell marker.

Conclusions: The combination of QTL mapping in vivo and in vitro with eQTL analysis in silico has identified two candidate loci and adjacent genes within a 6 Mb chromosomal region on chromosome 11 that have an impact on the regulation of other genes in the region during liver stress. These genes represent creedal candidates for conveying differenzial susceptibility to fibrogenesis by virtue of genetic variation.