CMTM5 Exhibits Tumor Suppressor Activities and is Frequently Silenced by Methylation in Carcinoma Cell Lines

Purpose: CMTM5 (CKLF-like MARVEL transmembrane domain containing member 5) is located at 14q11.2, a locus associated with multiple cancers. It has six RNA splicing variants with CMTM5-v1 as the major one. We explored its expression pattern in normal tissues and tumor cell lines, as well as its functions in carcinoma cells.

Experimental Design: We evaluated CMTM5 expression by semiquantitative reverse transcription-PCR (RT-PCR) in normal tissues and carcinoma cell lines of cervical, breast, nasopharyngeal, lung, hepatocellular, esophageal, gastric, colon, and prostate. We further examined CMTM5 promoter methylation in these cell lines. We also analyzed CMTM5 expression after 5-aza-2'-deoxycytidine treatment and genetic demethylation and the functional consequences of restoring CMTM5 in HeLa and PC-3 cells.

Results: CMTM5-v1 is broadly expressed in human normal adult and fetal tissues, but undetectable or down-regulated in most carcinoma cell lines. Its promoter methylation was detected in virtually all the silenced or down-regulated cell lines. The silencing of CMTM5 could be reversed by pharmacologic demethylation or genetic double-knockout of DNMT1 and DNMT3B, indicating methylation-mediated mechanism. Restoration of CMTM5-v1 suppressed carcinoma cell proliferation, migration, and invasion.

Conclusions: These results indicate that CMTM5 exhibits tumor suppressor activities, but with frequent epigenetic inactivation in carcinoma cell lines.