A chemical compound that stimulates the human homologous recombination protein RAD51.

RAD51 and other members of the RecA family of strand exchange proteins assemble on ssDNA to form presynaptic filaments, which carry out the central steps of homologous recombination. A microplate-based assay was developed for high-throughput measurement of hRAD51 filament formation on ssDNA. With this method, a 10,000 compound library was screened, leading to the identification of a small molecule (RS-1) that enhances hRAD51 binding in a wide range of biochemical conditions. Salt titration experiments showed that RS-1 can enhance filament stability. Ultrastructural analysis of filaments formed on ssDNA showed that RS-1 can increase both protein-DNA complex lengths and the pitch of helical filament turns. RS-1 stimulated hRAD51-mediated homologous strand assimilation (D-loop) activity by at least 5- to 11-fold, depending on the condition. This D-loop stimulation occurred even in the presence of Ca(2+) or adenylyl-imidodiphosphate, indicating that the mechanism of stimulation was distinct from that conferred by Ca(2+) and/or inhibition of ATPase. No D-loop activity was observed in the absence of a nucleotide triphosphate cofactor, indicating that the compound does not substitute for this requirement. These results indicate that RS-1 enhances the homologous recombination activity of hRAD51 by promoting the formation of active presynaptic filaments. Cell survival assays in normal neonatal human dermal fibroblasts demonstrated that RS-1 promotes a dose-dependent resistance to the cross-linking chemotherapeutic drug cisplatin. Given that RAD51-dependent recombination is a major determinant of cisplatin resistance, RS-1 seems to function in vivo to stimulate homologous recombination repair proficiency. RS-1 has many potential applications in both research and medical settings.

Pilot study examining tumor expression of RAD51 and clinical outcomes in human head cancers.

Radiation therapy and chemotherapy are commonly used treatments for head and neck cancer. RAD51 is a highly conserved DNA repair protein that serves a central function in the homologous recombination pathway. High levels of RAD51 protein expression have been reported in number of human cancer cell lines, and studies suggest that RAD51 overexpression can increase cellular resistance to radiation and some chemotherapeutic drugs. In this study, RAD51 protein levels were quantified by immunohistochemistry in tumor samples from twelve head and neck cancer patients who received identical treatment with induction chemotherapy (paclitaxel and carboplatinum) followed by radiation therapy given concurrently with additional chemotherapy (paclitaxel, fluorouracil, hydroxyurea). Patients with high RAD51 protein levels in their pre-treatment tumor biopsies demonstrated poorer cancer-specific survival rates than patients with lower RAD51 levels (33.3% vs. 88.9% at 2 years; p=0.025). In addition, within a subgroup of patients with normal tumor cell p53 expression, there was a non-significant trend toward better induction chemotherapy response rates observed in the tumors with lower RAD51 protein levels. These results suggest that tumor cell RAD51 expression levels may influence the outcome of patients with head and neck cancer treated with chemotherapy and radiotherapy.