G-quadruplexes as a source of vulnerability in BRCA2 <i>-</i> deficient granule cell progenitors and medulloblastoma

Biallelic pathogenic variants in the essential DNA repair gene <i>BRCA2</i> cause Fanconi anemia complementation group D1. Patients in this group are highly prone to develop embryonal tumors, most commonly medulloblastoma arising from the cerebellar granule cell progenitors (GCPs). GCPs undergo high proliferation in the postnatal cerebellum under Sonic Hedgehog (SHH) activation, but the type of DNA lesions that require the function of the BRCA2 to prevent tumorigenesis remains unknown. To identify such lesions, we assessed both GCP neurodevelopment and tumor formation using a mouse model with deletion of exons three and four of <i>Brca2</i> in the central nervous system, coupled with global <i>Trp53</i> loss. <i>Brca2^Δex3-4;Trp53^-/-</i> animals developed SHH subgroup medulloblastomas with complete penetrance. Whole-genome sequencing of the tumors identified structural variants with breakpoints enriched in areas overlapping putative G-quadruplexes (G4s). <i>Brca2</i>-deficient GCPs exhibited decreased replication speed in the presence of the G4-stabilizer pyridostatin. <i>Pif1</i> helicase, which resolves G4s during replication, was highly upregulated in tumors, and <i>Pif1</i> knockout in primary medulloblastoma tumor cells resulted in increased genome instability upon pyridostatin treatment. These data suggest that G4s may represent sites prone to replication stalling in highly proliferative GCPs and without BRCA2, G4s become a source of genome instability. Tumor cells upregulate G4-resolving helicases to facilitate rapid proliferation through G4s highlighting PIF1 helicase as a potential therapeutic target for treatment of BRCA2-deficient medulloblastomas.