Characterization of promoter regulatory elements involved in downexpression of the DNA polymerase κ in colorectal cancer

The low-fidelity DNA polymerases thought to be specialized in DNA damage processing are frequently misregulated in cancers. We show here that DNA polymerase kappa (pol kappa), prone to replicate across oxidative and aromatic adducts and known to function in nucleotide excision repair (NER), is downregulated in colorectal tumour biopsies. Contrary to the replicative pol delta and pol alpha, for which only activating domains were described, we identified an upstream 465-bp-long repressor region in the promoter of POLK. We also found an activating 237-bp region that includes stimulating protein-1 (SP1) and cyclic AMP-responsive element (CRE)-binding sites. Mutations at one CRE-binding site led to a dramatic 80% decrease in promoter activity. Alterations of the SP1-binding site also affected, to a lesser extent, the transcription. Gel shift assays confirmed the role played by CRE/SP1 recognition sequences. Moreover, ectopic expression of SP1 or CRE-binding protein (CREB) protein favoured Pol kappa transcription. Finally, we found that Pol kappa down-expression in colorectal biopsies correlated with a decreased level of CREB and SP1 transcripts. This work shows that the promoter of POLK is cis-controlled and suggests that silencing of CREB and SP1 proteins could contribute to downregulation of this repair polymerase in colorectal tumours.