Loss of human SMC5/6 induces spindle multipolarity and genome stress exacerbated by BLM helicase deficiency

Mutations of the human SMC5/6 genome maintenance complex can lead to several genome instability syndromes and are associated with tumorigenesis. However, the molecular consequences of SMC5/6 deficiencies in human cells are only partly known, preventing a better understanding of disease-causing mechanisms. To address this issue, we employed the dTAG technology to induce rapid loss of SMC5 or SMC6 in a panel of human cell lines. While SMC5/6 depletion confers gradual cessation of cell proliferation in all tested cell lines, distinct molecular consequences are seen in p53-deficient verses -proficient cells. In p53-deficient HEK293A and HeLa cells or p53 knockout RPE1 cells, SMC5/6 depletion reveals a mitotic phenotype of increases in multipolar spindle and abnormal chromosome segregation. In contrast, in p53-proficient RPE1 and HCT116 cells, SMC5/6 loss favors G1 accumulation and thereby masking mitotic defects. The mitotic phenotype and cell proliferation defect caused by SMC5/6 loss is exacerbated by knockout of the BLM helicase, a tumor suppressor mutated in Bloom syndrome patients. In comparison, SMC5/6 did not show genetic interaction in cell proliferation assays with the MUS81 endo nuclease, the FANCM DNA helicase, or the PrimPol polymerase. Collectively, our data reveal that SMC5/6 is required to prevent multipolar spindle and associated mitotic defects in conjunction with BLM, thus providing new perspectives in understanding etiologies of diseases caused by their deficiencies.