High Expression of Casein Kinase 2 Alpha Is Responsible for Enhanced Phosphorylation of DNA Mismatch Repair Protein MLH1 and Increased Tumor Mutation Rates in Colorectal Cancer

Abstract

DNA mismatch repair (MMR) deficiency plays an essential role in the development of colorectal cancer (CRC). We recently demonstrated in vitro that the serine/threonine casein kinase 2 alpha (CK2α) causes phosphorylation of the MMR protein MLH1 at position serine 477, which significantly inhibits the MMR. In the present study, CK2α-dependent MLH1 phosphorylation was analyzed in vivo. Using a cohort of 165 patients, we identified 88 CRCs showing significantly increased nuclear/cytoplasmic CK2α expression, 28 tumors with high nuclear CK2α expression and 49 cases showing a general low CK2α expression. Patients with high nuclear/cytoplasmic CK2α expression demonstrated significantly reduced 5-year survival outcome. By immunoprecipitation and Western blot analysis, we showed that high nuclear/cytoplasmic CK2α expression significantly correlates with increased MLH1 phosphorylation and enriched somatic tumor mutation rates. The CK2α mRNA levels tended to be enhanced in high nuclear/cytoplasmic and high nuclear CK2α-expressing tumors. Furthermore, we identified various SNPs in the promotor region of CK2α, which might cause differential CK2α expression. In summary, we demonstrated that high nuclear/cytoplasmic CK2α expression in CRCs correlates with enhanced MLH1 phosphorylation in vivo and seems to be causative for increased mutation rates, presumably induced by reduced MMR. These observations could provide important new therapeutic targets.

CK2 and the Hallmarks of Cancer

“…Furthermore, we were able to verify the importance of CK2α-mediated MLH1 phosphorylation in vivo [ 496 ]. In a cohort of 165 CRC patients, we could show that enhanced MLH1 phosphorylation was significantly associated with high nuclear/cytoplasmic CK2α expression.…”

“…In addition, high nuclear/cytoplasmic CK2α levels could be significantly correlated with a reduced 5-year survival outcome of patients. Furthermore, we detected enriched somatic mutation rates in tumors with high nuclear/cytoplasmic CK2α expression and speculated that enriched somatic mutation rates are induced by the reduction of MMR, caused by CK2-mediated phosphorylation of MLH1 [ 496 ]. Recently, MMR deficiency and high rates of microsatellite instability have been shown to be predictive of anti-programmed cell death 1 (PD-1)/PD-L1 immunotherapy efficacy.…”

Personalised neoantigen‐based therapy in colorectal cancer

“…Notably, nearly 3% of CRC is classified as MSS with high TMB. These MSS patients with high TMB may respond favourably to ICI‐based therapy 120 . The underlying mechanism for the generation of high TMB in MSS CRC is still not clear.…”

“…These MSS patients with high TMB may respond favourably to ICI-based therapy. 120 The underlying mechanism for the generation of high TMB in MSS CRC is still not clear. Neoantigen-derived epitopes (neoepitopes) on HLA-I have also been observed in MSS CRC.…”

Key role of phosphorylation sites in ATPase domain and Linker region of MLH1 for DNA binding and functionality of MutLα

“…We previously showed that MLH1 can be phosphorylated by casein kinase 2 α (CK2α) at amino acid position serine 477 (S477) and we identified that phosphorylation of MLH1 at position S477 can switch off MMR activity in vitro 14 . Very recently, we were able to demonstrate in vivo, that CK2α is frequently overexpressed in CRCs and that a high nuclear/cytoplasmic expression of CK2α is responsible for enhanced phosphorylation of MLH1 and increased tumor mutation rates in these tumors 15 . Moreover, affected patients demonstrated a significantly reduced 5-year survival outcome 15 .…”