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Suppression of PROX1‐mediated TERT expression in hepatitis B viral hepatocellular carcinoma

Somatic mutations in the telomerase reverse transcriptase (TERT) promoter are related to telomerase activation and frequently occur at two hot spots located at −124 and −146 bp relative to the start codon in various cancers. Here, we investigated the occurrence and implications of genetic alterations in the TERT promoter in hepatitis B viral hepatocellular carcinoma (B viral HCC). TERT promoter mutations, especially −124C>T, clearly enhanced transcriptional activity in HCC cell lines. In contrast, TERT mRNA expression was lower in B viral HCC patients with TERT promoter mutations than in those without. We identified prospero homeobox protein 1 (PROX1) as a novel transcriptional activator of TERT; this protein was shown to have particularly strong binding affinity for the mutant TERT promoter. However, stable expression of the hepatitis B virus X (HBx) protein inhibited PROX1‐mediated TERT expression in vitro. Our data suggest that TERT promoter mutations can enhance the promoter activity in HCC cell lines expressing PROX1 but are not the predominant mechanism of TERT upregulation in B viral HCC patients, based on the inhibition of PROX1‐dependent transcriptional activation by HBx.