Tumor suppressive activities of the cohesin SA1/STAG2 and effects of PARP impairment during Drosophila brain development

The cohesin complex performs essential cellular functions including regulation of chromatin organization and DNA repair. Somatic pathogenetic variants in cohesin genes, such as STAG2, have been associated with cancer, but their contribution to brain tumorigenesis is unclear. Here, we report the presence of STAG2 variants in glioblastoma and medulloblastoma patients and determine the effects of loss of STAG2 in human cells and of the homolog SA1 in Drosophila tissues. Reduction of SA1 expression during fly brain development leads to defects in neural stem cells differentiation and promotion of tumorigenesis, both in the presence of oncogenic activity and per se. Treatment with inhibitors of the Poly ADP-ribose polymerase (PARP), which are used to treat forms of cancer with defects in DNA repair, in combination with STAG2/SA1 depletion resulted in apoptosis in vitro and in vivo. In flies, reduction of PARP activity ameliorated the tumor associated phenotypes of SA1-deficient tissue. Our in vivo and in vitro data suggest that impairment of PARP activity compensates for reduced cohesin activity, highlighting a vulnerability that might be pharmacologically exploited in brain tumors.