Frequent and distinct aberrations of DNA methylation patterns in fibrolamellar carcinoma of the liver
Gene silencing due to aberrant DNA methylation is a frequent event in hepatocellular carcinoma (HCC) and also in hepatocellular adenoma (HCA). However, very little is known about epigenetic defects in fibrolamellar carcinoma (FLC), a rare variant of hepatocellular carcinoma that displays distinct clinical and morphological features. We analyzed the methylation status of the APC, CDH1, cyclinD2, GSTπ1, hsa-mir-9-1, hsa-mir-9-2, and RASSF1A gene in a series of 15 FLC and paired normal liver tissue specimens by quantitative high-resolution pyrosequencing. Results were compared with common HCC arising in non-cirrhotic liver (n = 10). Frequent aberrant hypermethylation was found for the cyclinD2 (19%) and the RASSF1A (38%) gene as well as for the microRNA genes mir-9-1 (13%) and mir-9-2 (33%). In contrast to common HCC the APC and CDH1 (E-cadherin) genes were found devoid of any DNA methylation in FLC, whereas the GSTπ1 gene showed comparable DNA methylation in tumor and surrounding tissue at a moderate level. Changes in global DNA methylation level were measured by analyzing methylation status of the highly repetitive LINE-1 sequences. No evidence of global hypomethylation could be found in FLCs, whereas HCCs without cirrhosis showed a significant reduction in global methylation level as described previously. FLCs display frequent and distinct gene-specific hypermethylation in the absence of significant global hypomethylation indicating that these two epigenetic aberrations are induced by different pathways and that full-blown malignancy can develop in the absence of global loss of DNA methylation. Only quantitative DNA methylation detection methodology was able to identify these differences.