Project 4: The BRCA Network in Medulloblastoma Responses to Replication Stress

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The BRCA Network in Medulloblastoma Responses to Replication Stress (Project-4) Abstract: BRCA1 and BRCA2 are critical genes for the repair of DNA strand breaks (DSB) via homologous recombination (HR). PALB2 functions to bridge BRCA1 with BRCA2. Due to the intimate relationship between BRCA1, PALB2, and BRCA2, these genes and their coded proteins are considered the core of a “BRCA network.” Cancers with BRCA mutations display unique molecular features of “BRCAness,” including hypersensitivity to replication stress drugs. In addition to the core BRCA proteins, defects of other BRCA1/2- associated genes often share the “BRCAness” characteristics. Brain tumors are the leading cause of childhood cancer-related deaths, and medulloblastomas (MBs) account for more than 60% of pediatric tumors. Current treatments of MBs mainly rely on local control by radiation in combination with generic chemotherapy. Targeted therapy has not been vigorously considered for MBs. Recently, it has been recognized that germline mutations in BRCA2 and PALB2, two of the core genes in the “BRCA Network,” are among the top causes of MBs. Furthermore, widespread basal activation of DNA damage signaling and excessive replication stress were common among different groups of MBs. Based on genomic signatures, 20-30% of sporadic MBs, including those from MB groups 2, 3, and 4, likely bear HR defects (HRD). These recent developments raise an important question of whether DNA damage repair defects and replication stress represent common vulnerabilities of MBs to new therapeutic developments targeting HRD. If so, how to efficiently identify the individual cases with HRD remains an issue. In the preliminary studies, we established conditional knockouts of Brca1, Palb2, and Brca2 in embryonic multi-potential stem and progenitor cells using the glial fibrillary acidic protein (GFAP) promoter-driven Cre expression. For the first time, we demonstrated that conditional loss of Palb2 and Brca1 are effective inducers of MBs, with a similar potency as that of Brca2 loss. An initial genomic characterization of these MBs exhibiting “BRCAness” had identified several unique structural features that can be used to define the HRD status. By treating the de novo MBs with a developing topoisomerase 1 inhibitor, we found it to be very effective to extend the survival of mice with MBs. Thus, we hypothesize that MBs originated from BRCA defects have unique genomic signatures to define the MBs with HRD, and targeted therapy of MBs with HRD will be more effective than the current treatment regimens. In Project-4, we take advantage of our recent success in establishing BRCA-related mouse MB models and strive to identify the genomic signatures associated with HRD and to validate the MB “BRCAness” response to replication stress drugs. Aim 1 will identify the structural and chromosome-context signatures that accurately reflect the HDR status in MBs exhibiting “BRCAness”. Aim 2 will experimentally target the vulnerability of HRD in MBs and investigate the potential mechanism of tumor recurrence after the treatment. The success of this study would effectively establish a new remedy for MB treatment.