BLM helicase determines chemotherapy-induced responses of human glioma cells and shifts between apoptosis, senescence or polyploidy

Abstract

Most of anti-tumour therapies eliminate neoplastic cells by introducing DNA damage which ultimately triggers cell death. These effects are counteracted by activated DNA repair pathways and RECQL family helicases, including BLM, participate in replication stress, DNA damage and repair. Glioblastoma (GBM) is a common, malignant brain tumour that inevitably recurs despite surgical resection, radiotherapy, and chemotherapy with temozolomide (TMZ). Expression and functions of the BLM helicase in therapy resistant GBMs have not been elucidated. We analysed expression and localisation of BLM in human gliomas and several glioma cell lines using TCGA datasets, immunostaining and Western blotting. BLM depleted human glioma cells were generated with CRISPR/Cas9. Effects of chemotherapeutics on cell proliferation, DNA damage and apoptosis were determined with flow cytometry, immunofluorescence Western blotting and RNA sequencing. We found upregulated BLM mRNA levels in malignant gliomas, increased cytosolic localisation and poor survival of BLM highly expressing GBM patients. BLM deficiency in LN18 and LN229 glioma cells resulted in profound transcriptomic alterations, reduced cell proliferation, and altered cell responses to chemotherapeutics. BLM-deficient glioma cells were resistant to the TMZ and PARP inhibitor treatment and underwent polyploidy or senescence depending on the TP53 activity. These dependencies were not detected in RECQL4-deficient cells. Our findings of high BLM expression in GBMs and its roles in responses to chemotherapeutics provide a rationale for targeting BLM helicase in those tumours. BLM deficiency affects responses of glioma cells to chemotherapeutics targeting PARP-1 dependent pathways, while these effects are not detected in RECQL4 deficient cells implying distinct functions of these helicases.