Genetic and epigenetic changes in fibrosis‐associated hepatocarcinogenesis in mice

Abstract

Hepatocellular carcinoma (HCC) is one of the most prevalent cancers and is rising in incidence worldwide. The molecular mechanisms leading to the development of HCC are complex and include both genetic and epigenetic events. To determine the relative contribution of these alterations in liver tumorigenesis, we evaluated epigenetic modifications at both global and gene specific levels, as well as the mutational profile of genes commonly altered in liver tumors. A mouse model of fibrosis-associated liver cancer that was designed to emulate cirrhotic liver, a prevailing disease state observed in most humans with HCC, was used. Tumor and non-tumor liver samples from B6C3F1 mice treated with N-nitrosodiethylamine (DEN; a single ip injection of 1 mg/kg at 14 days of age) and carbon tetrachloride (CCl4; 0.2 ml/kg, 2 times/week ip starting at 8 weeks of age for 14 weeks), as well as corresponding vehicle control animals, were analyzed for genetic and epigenetic alterations. H-ras, Ctnnb1, and Hnf1α genes were not mutated in tumors in mice treated with DEN+CCl4. In contrast, the increased tumor incidence in mice treated with DEN+CCl4 was associated with marked epigenetic changes in liver tumors and non-tumor liver tissue, including demethylation of genomic DNA and repetitive elements, a decrease in histone 3 lysine 9 trimethylation (H3K9me3), and promoter hypermethylation and functional down-regulation of Riz1, a histone lysine methyltransferase tumor suppressor gene. Additionally, the reduction in H3K9me3 was accompanied by increased expression of long interspersed nucleotide elements (LINE) 1 and short interspersed nucleotide elements (SINE) B2, which is an indication of genomic instability. In summary, our results suggest that epigenetic events, rather than mutations in known cancer-related genes, play a prominent role in increased incidence of liver tumors in this mouse model of fibrosis-associated liver cancer.

Characterization of copy number alterations in a mouse model of fibrosis‐associated hepatocellular carcinoma reveals concordance with human disease

“…In relation to our previous findings of epigenetic alterations in both tumor and fibrotic surrounding tissue, as well as a lack of mutations commonly observed in HCC in both humans and mice in the same mouse tissues discussed herein , we posit that epigenetic alterations indicative of genomic instability precede CNAs. Further, these findings suggest that CNAs occur relatively early in the carcinogenic process compared to common activating or inactivating mutations, which is consistent with other reports of chromosomal instability as an early event in tumorigenesis .…”

“…In concordance with the findings of a recent study that evaluated the association between chromosomal instability and global DNA hypomethylation in human HCC samples , we observed CNAs in the same tissues in which epigenetic alterations that are indicative of genomic instability were observed. Although the mechanistic link between these two phenotypes is not clear, one potential explanation that has been proposed suggests that activation of hypomethylated repetitive DNA elements within genes may cause chromosomal rearrangements , a supposition that is further supported by the fact that both chromosomal instability and epigenetic alterations are generally considered early events in carcinogenesis .…”

“…The combination of genotoxic, for example, DEN, and nongenotoxic, for example, CCl 4 , insults has been used to emulate the comorbidity features often observed in human HCC patients, and offers a relevant model to study the mechanisms involved in hepatocellular carcinogenesis . Using this mouse model, we found that epigenetic alterations indicative of genomic instability were prevalent in both tumors and nontumor fibrotic tissues of mice treated with DEN+CCl 4 compared with liver tissue of vehicle‐treated control animals . Chromosomal instability, a condition that results in DNA copy number alterations (CNAs), can occur simultaneously with and promote genomic instability , one of the “hallmarks of cancer” .…”

Histone lysine methyltransferase SUV39H1 is a potent target for epigenetic therapy of hepatocellular carcinoma

“…Expressions of H3K9me3 and Suv39h1 gradually increased with the progression from pre‐neoplastic nodules to established tumors in a methyl‐deficient model of rat hepatocarcinogenesis . In contrast, an increased incidence of liver tumors in mice that were treated with diethylnitrosamine and carbon tetrachloride was associated with a decreased level of H3K9me3 . These findings indicate that SUV39H1 plays an important role in HCC development and progression through H3K9 trimethylation.…”

LINE-1 in cancer: multifaceted functions and potential clinical implications

“…37 Many studies have investigated the associations between L1 methylation levels (or L1 activity) and cancer risk, progression, and prognosis, with a majority of them supporting correlations between tissue L1 hypomethylation and increased cancer risk or poor prognosis. 32,33,[40][41][42] In people with CRC family history, for example, colonic L1 hypomethylation confers a higher CRC risk. 42 Another study 43 revealed that L1 hypomethylation in normal colon tissue predicts predisposition to multiple colonic tumors.…”