Abstract
The BRAF V600E mutation plays an important role in the tumorigenesis of papillary thyroid cancer (PTC). To explore an epigenetic mechanism involved in this process, we performed a genome-wide DNA methylation analysis using a methylated CpG island amplification (MCA)/CpG island microarray system to examine gene methylation alterations after shRNA knockdown of BRAF V600E in thyroid cancer cells. Our results revealed numerous methylation targets of BRAF V600E mutation with a large cohort of hyper- or hypo-methylated genes in thyroid cancer cells, which are known to have important metabolic and cellular functions. As hypomethylation of numerous genes by BRAF V600E was particularly a striking finding, we took a further step to examine the selected 59 genes that became hypermethylated in both cell lines upon BRAF V600E knockdown and found them to be mostly correspondingly under-expressed (i.e. they were normally maintained hypomethylated and over-expressed by BRAF V600E in thyroid cancer cells). We confirmed the methylation status of selected genes revealed on MCA/CpG microarray analysis by performing methylation-specific PCR. To provide proof of concept that some of the genes uncovered here may play a direct oncogenic role, we selected six of them to perform shRNA knockdown and examined its effect on cellular functions. Our results demonstrated that the HMGB2 gene played a role in PTC cell proliferation and the FDG1 gene in cell invasion. Thus, this study uncovered a prominent epigenetic mechanism through which BRAF V600E can promote PTC tumorigenesis by altering the methylation and hence the expression of numerous important genes.