CSK-homologous kinase(CHK/MATK) is a potential colorectal cancer tumour suppressor gene epigenetically silenced by promoter methylation

Hyperactivation of SRC-family protein kinases (SFKs) contributes to the initiation and progression of human colorectal cancer (CRC). Since oncogenic mutations of SFKgenes are rare in human CRC, we investigated if SFK hyperactivation is linked to dysregulation of their upstream inhibitors, C-terminal SRC kinase (CSK) and its homolog CSK-homologous kinase (CHK/MATK). We demonstrate that expression of CHK/MATKbut not CSKwas significantly downregulated in CRC cell lines and primary tumours compared to normal colonic tissue. Investigation of the mechanism by which CHK/MATKexpression is down-regulated in CRC cells uncovered hypermethylation of the CHK/MATKpromoter in CRC cell lines and primary tumours. Promoter methylation of CHK/MATKwas also observed in several other tumour types. Consistent with epigenetic silencing of CHK/MATK, genetic deletion or pharmacological inhibition of DNA methyltransferases increased CHK/MATKmRNA expression in CHK/MATK-methylated colon cancer cell lines. SFKs were hyperactivated in CHK/MATK-methylated CRC cells despite expressing enzymatically active CSK, suggesting loss of CHK/MATK contributes to SFK hyperactivation. Re-expression of CHK/MATK in CRC cell lines led to reduction in SFK activity via a non-catalytic mechanism, a reduction in anchorage-independent growth, cell proliferation and migration in vitro, and a reduction in tumour growth and metastasis in a zebrafish embryo xenotransplantation model in vivo, collectively identifying CHK/MATKas a novel putative tumour suppressor gene in CRC. Furthermore, our discovery that CHK/MATKhypermethylation occurs in the majority of tumours warrants its further investigation as a diagnostic marker of CRC.