CHK2 PHOSPHORYLATION OF SURVIVIN-DEX3 CONTRIBUTES TO A DNA DAMAGE-SENSING CHECKPOINT IN CANCER

Survivin is a pivotal cancer gene with multiple roles in cell viability and mitosis, but the function(s) of its alternatively spliced isoforms has remained elusive. Here, we show that a survivin spliced variant that lacks exon 3 and contains a unique -COOH terminus sequence, survivin-DEx3, is differentially expressed in cancer, compared to normal tissues, and correlates with aggressive disease and markers of unfavorable prognosis, by genome-wide bioinformatics analysis. Unlike other survivin variants, survivin-DEx3 localizes exclusively to nuclei in tumor cells, and is phosphorylated by the DNA damage checkpoint kinase, Chk2, on residues located in its unique -COOH terminus. Ala mutagenesis of the Chk2 phosphorylation sites prolongs survivin-DEx3 stability in tumor cells, inhibits the expression of Ser139 phosphorylated H2AX in response to double strand DNA breaks, and impairs colony formation in soft agar after DNA damage. Active Chk2 was detected at the earliest stages of the colorectal adenoma-to-carcinoma transition, persisted in advanced tumors, and correlated with increased survivin expression, in vivo. These data suggest that Chk2 phosphorylation of survivin-DEx3 contributes to a DNA damage-sensing checkpoint in tumor cells, which may affect sensitivity to genotoxic therapies.