RAD50/51

  • Radiation Sensitive 50 (RAD50)
  • Radiation Sensitive 51 (RAD51)
  • Gene Location(s):
    • RAD50, chromosome 5 (5q31.1)
    • RAD51, chromosome 15 (15q15.1)

Biology

RAD50

  • RAD50, located on chromosome 5q31.1, encodes RAD50, an ATP-dependent DNA-binding protein that is a central component of the MRN complex (MRE11-RAD50-NBS1), which plays a critical role in detecting and repairing DNA double-strand breaks.1,2
  • Loss of RAD50 function or MRN complex integrity impairs double-strand break repair and checkpoint signaling, driving genomic instability and contributing to cancer development, including pancreatic ductal adenocarcinoma (PDAC).2

RAD51

  • RAD51, encoded by the RAD51 gene on chromosome 15q15.1, functions as a recombinase enzyme that plays a central role in the homologus repair HR DNA repair pathway.2
  • Its activity is tightly regulated by key HR mediators—including BRCA1, BRCA2, and PALB2—which stabilize the RAD51 nucleoprotein filament and ensure repair fidelity.3
  • RAD51 binds to long 3-inch single-stranded DNA (ssDNA), forming a nucleoprotein filament that facilitates homology search and strand invasion.3
  • Upon locating a homologous DNA sequence, the RAD51-ssDNA filament invades the duplex, creating a displacement loop (D-loop) that stabilizes strand pairing and enables DNA polymerase–mediated repair synthesis.3
  • Impaired RAD51 function disrupts homologous recombination, promoting genomic instability and contributing to cancer development.2

Etiology and Epidemiology

  • Mutations in RAD50 and RAD51 have been observed across a range of solid tumors, including breast, ovarian, colorectal, prostate, and pancreatic cancers, with a reported prevalence of approximately 1% to 3% across all cancer types.4
  • Although less common than BRCA1/2 mutations, alterations in RAD50 and RAD51 are emerging as clinically relevant in the broader context of homologous recombination deficiency (HRD).4
  • Mutations in RAD50 and RAD51 can be either germline or somatic and may influence tumor response or resistance to DNA repair-targeted therapies, such as poly ADP-ribose polymerase (PARP) inhibitors.5
  • In PDAC, alterations in RAD50 and RAD51 are relatively rare, occurring in approximately 0.3% and 0.1% of patients, respectively. Despite the low frequency, these alterations may still confer a clinically relevant HRD phenotype in selected patients.4,6,7

Testing

When to Test:

  • Homologous recombination deficiency (HRD) testing (genomic signatures, gene panels, functional assays like RAD51 foci) is specifically employed to determine HRD and thus predict sensitivity to platinum chemotherapy and PARP inhibitors.8
  • Loss-of-function mutations in RAD50 or RAD51 can impair HR repair, leading to HRD. Tumors with HRD become more dependent on PARP-mediated DNA repair pathways, making them more susceptible to synthetic lethality with PARP inhibitors and improving treatment response.8
  • Overexpression or gain-of-function alterations in RAD50 or RAD51 may promote aberrant recombination and genomic instability. This can reduce reliance on PARP-mediated repair and diminish the efficacy of PARP inhibitor therapy by allowing tumor cells to bypass DNA damage checkpoints and tolerate accumulating mutations.9

Available Testing Methods:

  • Somatic and germline mutations are generally identified through next-generation sequencing, where sequencing technology is used to analyze multiple genes simultaneously. It is highly sensitive, and can detect a wide range of mutations in genes.11,12

Guideline Recommendations:

  • National Comprehensive Cancer Network guidelines (version 2.2025) recommend germline testing in any patient with confirmed pancreatic cancer and in those in whom there is a clinical suspicion for inherited susceptibility.11,12

Targeted Therapy

Approved Agents:

  • As of mid-2025, there are no agents with FDA-approved indications for patients with PDAC and RAD50 or RAD51 aberrations.

Investigational Agents:

  • Olaparib13,14
    • Mechanism of Action:
      • Olaparib is an inhibitor of PARP enzymes, which are involved in normal cellular functions, such as DNA transcription and DNA repair.
    • Investigational Studies:
      • Phase 2 Study
        • ClinicalTrials.gov: NCT02677038
        • Study Design: A phase 2 trial evaluating the safety and antitumor activity of pembrolizumab and olaparib (POLAR) maintenance for patients with metastatic PDAC and HRD and/or exceptional treatment response to platinum-based therapy.
        • Cohort B includes patients with either pathogenic somatic or germline non-core 15 HR-gene alterations (ATM, BAP1, BARD1, BLM, BRIP1, CHEK2, FAM175A, FANCA, FANCC, MUTYH, NBN, RAD50, RAD51, RAD51C, RTEL1) who have stable or responding disease on first-line or second-line platinum therapy in 2 consecutive imaging assessments over at least 4 months.
  • RS-35d15
    • Mechanism of Action:
      • RS-35d is a novel small-molecule inhibitor that disrupts the protein-protein interaction between RAD51 and BRCA2, a critical step in HR DNA repair.
      • By inhibiting this interaction, RS-35d mimics the effect of a BRCA2 loss-of-function mutation and induces HRD.
    • Preclinical Evidence:
      • In preclinical models, RS-35d demonstrated activity as both monotherapy and in combination with olaparib (a PARP inhibitor), with synergistic effects observed in PDAC cell lines.
      • RS-35d may offer a promising approach to targeting tumors with RAD51 overexpression and overcoming PARP inhibitor resistance by inducing synthetic lethality in HR-proficient tumors through simultaneous disruption of HR and DNA damage response signaling.

References

  1. Marsden CG, Jensen RB, Gruber JJ, et al. The tumor-associated variant RAD51 G151D induces a hyper-recombination phenotype. PLoS Genet. 2016;12(8):e1006208. doi:10.1371/journal.pgen.1006208
  2. Tisi R, Vertemara J, Zampella G, Longhese MP. Functional and structural insights into the MRX/MRN complex, a key player in recognition and repair of DNA double-strand breaks. Comput StructBiotechnol J. 2020;18:1137-1152. doi:10.1016/j.csbj.2020.05.013
  3. Li X, Heyer WD. Homologous recombination in DNA repair and DNA damage tolerance. Cell Res. 2008;18(1):99-113. doi:10.1038/cr.2008.1
  4. Mekonnen N, Yang H, Shin YK. Homologous recombination deficiency in ovarian, breast, colorectal, pancreatic, non-small cell lung and prostate cancers, and the mechanisms of resistance to PARP inhibitors. Front Oncol. 2022;12:880643. doi:10.3389/fonc.2022.880643
  5. RAD50 gene mutation: cancer risk and health implications. Biology Insights. Published June 21, 2025. Accessed July 9, 2025. https://biologyinsights.com/rad50-gene-mutation-cancer-risk-and-health-implications/
  6. Rapposelli IG, Zampiga V, Cangini I, et al. Comprehensive analysis of DNA damage repair genes reveals pathogenic variants beyond BRCA and suggests the need for extensive genetic testing in pancreatic cancer. BMC Cancer. 2021;21(1):611. doi:10.1186/s12885-021-08368-5
  7. Yadav S, Kasi PM, Bamlet WR, et al. Effect of germline mutations in homologous recombination repair genes on overall survival of patients with pancreatic adenocarcinoma. Clin Cancer Res. 2020;26(24):6505-6512. doi:10.1158/1078-0432.CCR-20-1788
  8. Korsholm LM, Kjeldsen M, Perino L, et al. Combining homologous recombination-deficient testing and functional RAD51 analysis enhances the prediction of poly (ADP-ribose) polymerase inhibitor sensitivity. JCO Precis Oncol. 2024;8:e2300483. doi:10.1200/PO.23.00483
  9. Cruz C, Castroviejo-Bermejo M, Gutiérrez-Enríquez S, et al. RAD51 foci as a functional biomarker of homologous recombination repair and PARP inhibitor resistance in germline BRCA-mutated breast cancer. Ann Oncol. 2018;29(5):1203-1210. doi:10.1093/annonc/mdy099
  10. Xu J, Dai Y, Gao Y, et al. RAD51D secondary mutation-mediated resistance to PARP-inhibitor-based therapy in HGSOC. Int J Mol Sci. 2023;24(19):14476. doi:10.3390/ijms241914476
  11. NCCN. Clinical Practice Guidelines in Oncology. Pancreatic adenocarcinoma, version 2.2025. February 3, 2025. Accessed July 28, 2025. https://www.nccn.org/professionals/physician_gls/pdf/pancreatic.pdf
  12. Crowley F, Gandhi S, Rudshteyn M, Sehmbhi M, Cohen DJ. Adherence to NCCN genetic testing guidelines in pancreatic cancer and impact on treatment. Oncologist. 2023;28(6):486-493. doi:10.1093/oncolo/oyad044
  13. LYMPARZA (olaparib). Prescribing information. AstraZeneca, 2025. Accessed July 28, 2025. https://drd9vrdh9yh09.cloudfront.net/50fd68b9-106b-4550-b5d0-12b045f8b184/00997c3f-5912-486f-a7db-930b4639cd51/00997c3f-5912-486f-a7db-930b4639cd51_viewable_rendition__v.pdf
  14. A Study of pembrolizumab and olaparib for people with metastatic pancreatic ductal adenocarcinoma and homologous recombination deficiency or exceptional treatment response to platinum-based therapy. ClinicalTrials.gov. Updated February 28, 2025. Accessed July 28, 2025. https://clinicaltrials.gov/study/NCT04666740
  15. Masi M, Poppi L, Previtali V, et al. Investigating synthetic lethality and PARP inhibitor resistance in pancreatic cancer through enantiomer differential activity. Cell DeathDiscov. 2025;11(1):106. doi:10.1038/s41420-025-02382-3