Biology subjects

Burma, S.

Publications and source records attributed to Burma, S..

4 recordsLinked to original sources

bioRxiv2026

BRCA1 promotes homologous recombination through separable DNA and RAD51 binding activities within its disordered region.

The breast and ovarian tumor suppressor BRCA1 heterodimerizes with BARD1 to promote DNA double-strand break repair by homologous recombination (HR) and to protect stressed DNA replication forks against nuclease attack. The large, intrinsically disordered central region of BRCA1 harbors binding sites for DNA and multiple repair factors, but its lack of stable structure has hindered mechanistic dissection of these activities. Using biochemical mapping and NMR spectroscopy, we delineate the DNA binding and RAD51 interaction interfaces within this region and construct separation-of-function mutants that selectively ablate each activity. Both DNA binding and RAD51 interaction are required for BRCA1-BARD1 to promote RAD51-mediated DNA strand invasion, and DNA binding also contributes to BLM-DNA2 end resection. These findings provide mechanistic insights into how individual ligand binding activities within BRCA1 contribute to genome maintenance.

biochemistry↗

bioRxiv2024

Distinct roles of the two BRCA2 DNA binding domains in DNA damage repair and replication fork preservation

Homologous recombination (HR) is a highly conserved tool for the removal of DNA double-strand breaks (DSBs) and the preservation of stalled and damaged DNA replication forks. Successful completion of HR requires the tumor suppressor BRCA2. Germline mutations in BRCA2 lead to familial breast, ovarian, and other cancers, underscoring the importance of this protein for maintaining genome stability. BRCA2 harbors two distinct DNA binding domains, one that possesses three oligonucleotide/oligosaccharide binding (OB) folds (known as the OB-DBD), and with the other residing in the C-terminal recombinase binding domain (termed the CTRB-DBD) encoded by the last gene exon. Here, we employ a combination of genetic, biochemical, and cellular approaches to delineate contributions of these two DNA binding domains toward HR and the maintenance of stressed DNA replication forks. We show that OB-DBD and CTRB-DBD confer ssDNA and dsDNA binding capabilities to BRCA2, respectively, and that BRCA2 variants mutated in either DNA binding domain are impaired in the ability to load the recombinase RAD51 onto ssDNA pre-occupied by RPA. While the CTRB-DBD mutant is modestly affected for HR, it exhibits a strong defect in the protection of stressed replication forks. In contrast, the OB-DBD is indispensable for both BRCA2 functions. Our study thus defines the unique contributions of the two BRCA2 DNA binding domains in genome maintenance.

biochemistry↗

bioRxiv2020

Impact of the radiated brain microenvironment on a panel of human patient-derived xenografts.

ObjectiveRadiotherapy, combined with surgical resection and chemotherapy, remains a first-line treatment for infiltrative gliomas. However, these tumor are not surgically curable, and often recur, even within the prior radiation field, and may demonstrate a more aggressive phenotype. We recently demonstrated that the radiated brain tumor microenvironment promotes tumor aggressiveness in an orthotopic patient-derived xenograft (PDX) model of glioblastoma (Mayo GBM 143). Importantly, high grade gliomas display diverse molecular phenotypes, and whether this genetic variability leads to divergent behaviour in the radiated tumor microenvironment is unknown. Herein, we characterize the effects of the irradiated brain microenvinroment on nine additional unique GBM cell lines to better understand the nuances of how tumor molecular phenotypes influence cellular dynamics. MethodsFemale athymic nude mice were randomly divided into cranial radiation (15 Gy) and non-radiated groups. Mice then underwent intracranial implantation with one of the selected PDX GBM cell lines (GBM 6, 10, 12, 39, 46, 76, 123, 164, 196; total n=8-15, per group, per line). GBM 6 cells were additionally implanted 6 months after completion of fractionated radiation (4Gy x 10 fractions or 2Gy x 30 fractions) vs sham radiation. Kaplan-Meyer (K-M) and log-rank tests were performed to compare the survival between irradiated and non-irradiated groups. ResultOf nine previously untested human GBM lines, we found that five demonstrated shorter survival in the pre-radiated brain (GBM 6, 46, 76, 164, 196); similar to previous observations with GBM 143. GBM 6 was also evaluated 6 months after fractionated radiation yielding similar results. However, two lines yielded prolonged survival in the pre-radiated brain (GBM 10, 12); GBM12 and 10 demonstrated the fastest baseline growth in the non-radiated brain; GBM 39, 123 whose rate of growth was not impacted by the radiated brain, demonstrated a an intermediate baseline growth rate between that of those positively and negatively impacted by the radiated brain microenvironment. No other clinical or molecular phenotype was found to consistently correlate with response to the radiated microenvironment. ConclusionAmong a total of 10 total human GBM lines evaluated to date, 60% induce faster mortality in a radiated microenvironment, and 20% induce slower mortality. These results highlight the likely critical impact of the irradiated microenvironment on tumor behaviour, yet illustrate that different tumors may exhibit opposing responses. Although further evaluation will be needed to understand mechanisms of divergent behavior, our data suggest the increased rate of growth in the radiated microenvironment may not apply to the fastest-growing tumor lines, which could instead demonstrate a paradoxical response.

cancer biology↗

bioRxiv2020

Selective Vulnerability of Senescent Glioblastoma Cells to Bcl-XL Inhibition

Despite decades of research and numerous basic science advances, there have only marginal gains in improving glioblastoma multiforme survival. Therefore, new ideas and approaches for treating this aggressive disease are essential to drive progress forward. Conventional therapies, such as radiation and Temozolomide (TMZ), function to cause oxidative stress and DNA damage yielding a senescent-like state of replicative arrest in susceptible cells. However, increasing evidence demonstrates malignant cells can escape senescence leading to tumor recurrence. Ablation of non-replicating senescent tumor cells after radiation and chemotherapy may be an avenue to reduce the rates of tumor recurrence. Senolytic agents have been developed that selectively target senescent cells, but it remains unknown whether senolytics might be utilized against senescent-like glioma cells. We employed radiation or TMZ to induce a functionally senescent state in human glioblastoma cells. Viable cells that survive these treatments were then utilized to screen candidate senolytic drugs, to identify those selectively effective at ablating senescent-like cells over naive non-tumor and proliferative cells. Among 10 candidate senolytic drugs evaluated, only Bcl-XL inhibitors demonstrated reproducible senolytic activity in radiated or TMZ-treated glioma across the majority of GBM cell lines evaluated. Conversely, Bcl-2 inhibitors and other established senolytic drugs failed to show any consistent senolytic activity. In agreement with these data, Bcl-XL knockdown selectively reduced the viability of senescent-like GBM cells, whereas knockdown of Bcl-2 or Bcl-W yielded no senolytic effect. These findings demonstrate the potential to harness radiation-induced biology to ablate latent surviving cells and highlight Bcl-XL dependency as a potential vulnerability of surviving GBM cells after exposure to radiation or TMZ.

cancer biology↗