CHEK2 gene alterations independently increase the risk of death from breast cancer in Bulgarian patients

Checkpoint kinase 2 (CHEK2) is a DNA damage-activated protein kinase implicated in cell cycle checkpoint control. The significance of CHEK2 alterations for breast cancer incidence and clinical behavior is not clear. In this study we determined the mutational spectrum and the level of promoter hypermethylation of CHEK2 gene in a group of 145 Bulgarian patients with breast cancer. A special emphasis was put on the clinical impact of CHEK2 alterations for breast cancerogenesis. PCR-SSCP-sequencing analysis of the entire coding sequence of CHEK2 gene was performed to estimate the mutational profile of tumor samples. Methylation-sensitive SSCP was applied to determine the methylation status in CpG clusters implicated in CHEK2 silencing. Clinical significance of CHEK2 alterations was evaluated using standard statistical methods. Mutations in CHEK2 were identified in 9.65 % of the patients. Two novel missense substitutions Thr476Met (C > T) and Ala507Gly (C > G), and a novel silent variant Glu79Glu (A > G) were registered. However, hypermethylation was not found in any of the studied cases. Comparison with clinical characteristics showed that CHEK2 positive women have predominantly lobular type of breast carcinoma (p=0.04) and PR+ status (p=0.092). CHEK2 mutations correlated significantly with ATM(+) status (p=-0.046). All patients with the Glu79Glu variant were progesterone receptor positive (p=0.004). A decrease in overall survival (p = 0.6301) and a threefold increased independent risk of death (HR = 3.295, 95%CI 0.850-12.778, p = 0.085) in CHEK2(+)patients was found. Our data indicate the significance of CHEK2 gene alterations in contrast to promoter hypermethylation in breast cancerogenesis. Specificity of CHEK2 mutational profile for the Bulgarian population was found. Though CHEK2 mutational status correlated with more favorable clinical characteristics, including positive progesterone receptor and lobular histological type, it independently increased the risk of death in these patients.