Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the viral agent responsible for the global pandemic of coronavirus disease 2019 (COVID-19). First identified in December 2019, this highly transmissible and pathogenic coronavirus has rapidly spread, infecting over 771 million people worldwide and causing more than 6.9 million deaths as of reference number (1).
Virus Origin and Zoonotic Spillover
SARS-CoV-2 belongs to the Betacoronavirus genus and, like its predecessors, Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and Middle East Respiratory Syndrome Coronavirus (MERS-CoV), has undergone zoonotic spillover from animals to humans (2, 3). The virus primarily targets the cells of the lower respiratory tract in humans but has been found to infect various other organs, including the lungs, heart, brain, liver, kidneys, and intestines (4, 5–7).
The diverse range of organs affected contributes to the manifestation of severe complications such as arrhythmias, heart and renal failure, gastrointestinal disorders, multi-organ failure, and acute respiratory distress syndrome (8).
Prophylactic Measures and Variants of Concern
To mitigate the impact of the pandemic, numerous prophylactic SARS-CoV-2 vaccines have been developed and are in widespread use, with the aim of saving millions of lives globally (9–12). However, the emergence of serial variants of concern (VOCs) with notable immune evasion capabilities poses challenges to existing preventive measures (13). As of November 2021, the Delta (B.1.617.2) variant has been replaced by the Omicron (B.1.1.529) variant, which exhibits a higher capacity to escape protective immunity established through vaccination or previous infection by earlier VOCs (14, 15).
Anti-SARS-CoV-2 Therapies and Drug Repositioning
Given the evolving nature of the virus and the emergence of new variants, effective strategies for countering SARS-CoV-2 are crucial. The need for differential efficacy and safety of anti-SARS-CoV-2 therapies has led to the exploration of drug repositioning as a viable strategy for the treatment of COVID-19 (16–18).
RAD51 as a Potential Target
In a recent study, RAD51, a protein essential for DNA damage repair by homologous recombination (HR), was identified as a potential host factor crucial for SARS-CoV-2 propagation (19–21). RAD51 has previously been implicated in various viral propagation processes, including HIV, human papillomavirus (HPV), hepatitis B virus (HBV), and hepatitis C virus (HCV) (22–27).
The study demonstrated that knockdown of RAD51 impaired SARS-CoV-2 propagation, with RAD51 found to colocalize with SARS-CoV-2 RNA. This observation suggests that RAD51 inhibitors could block viral propagation. Multiple RAD51 inhibitors exhibited antiviral activities against SARS-CoV-2 both in vitro and in vivo. Remarkably, these RAD51 inhibitors displayed broad-spectrum antiviral activity against various SARS-CoV-2 variants.
Chemical Binding Site on RAD51 Dimerization Interface
Further investigations revealed that the chemical binding site on RAD51’s dimerization interface played a crucial role in the anti-SARS-CoV-2 activity of RAD51 inhibitors. These findings suggest that RAD51 may serve as a druggable host target for SARS-CoV-2 infection, making RAD51 inhibitors potential candidates for COVID-19 treatment.
RAD51, a crucial factor in homologous recombination (HR) and DNA repair, has previously been implicated in various virus propagation processes, including interactions with HIV-1 integrase, productive replication of human Papillomavirus 31, and regulation of HBV and HCV infections (22–27). Given its role in RNA virus infections, we investigated whether SARS-CoV-2, an RNA virus, utilizes RAD51 for its propagation.
The study demonstrated that silencing RAD51 expression impaired SARS-CoV-2 propagation in human lung cancer cells. Additionally, RAD51 was found to accumulate in the cytoplasm of SARS-CoV-2-infected cells, with colocalization of RAD51 protein and viral RNA observed in SARS-CoV-2-infected Vero E6 cells. The study suggested that RAD51 may be involved in the replication complex of SARS-CoV-2, possibly ensuring viral replication. The observed colocalization of RAD51 with viral RNA raises questions about potential mechanisms, such as nuclear membrane permeability or impaired nuclear import of RAD51 in infected cells.
RAD51 inhibitors were then evaluated for their potential as anti-SARS-CoV-2 agents. Six well-known RAD51 inhibitors were selected: B02, DIDS, IBR2, RI(dl)-2, RI-1, and RI-2. Among these, B02, DIDS, IBR2, and RI(dl)-2 displayed anti-SARS-CoV-2 activity in Vero E6 and Calu-3 cells. Notably, the inhibitors showed distinct mechanisms of action, targeting either RAD51-ssDNA or RAD51-dsDNA binding or inhibiting RAD51 filament formation.
Further analysis revealed that B02 significantly reduced SARS-CoV-2 propagation, while RI-1 exhibited no antiviral activity. This observation aligns with a recent study showing that B02 exhibits antiviral synergy with remdesivir. The differences in efficacy may be attributed to the diverse binding sites of RAD51 inhibitors on RAD51. Interestingly, DIDS, despite inhibiting RAD51 with a different mechanism, displayed anti-SARS-CoV-2 activity in a cell type-dependent manner.
To assess the in vivo efficacy of RAD51 inhibition, we selected DIDS and tested its impact on SARS-CoV-2-infected Syrian hamsters. The study showed a significant decrease in infectious virus titers in the lungs of DIDS-treated hamsters, emphasizing the potential of RAD51 inhibitors for COVID-19 treatment. Moreover, DIDS demonstrated modulation of proinflammatory cytokines, suggesting a potential role in preventing the aberrant innate immune response associated with COVID-19.
The study further highlighted the antiviral activity of RAD51 inhibitors against SARS-CoV-2 variants, including Delta and Omicron. This finding suggests that RAD51 inhibitors could serve as a novel class of broad-spectrum therapeutics for challenging COVID-19 cases. The potential for pangenotypic antiviral activity and a high genetic barrier to viral resistance make host-targeted agents an attractive avenue for drug development.
In conclusion, our study sheds light on the involvement of RAD51 in SARS-CoV-2 propagation and underscores the potential of RAD51 inhibitors as candidates for drug repositioning against COVID-19. Further investigations are warranted to unravel the molecular mechanisms underlying the antiviral activity of RAD51 inhibitors and to explore their therapeutic potential in clinical settings. The data presented here provide a foundation for advancing our understanding of virus-host interactions and developing effective strategies to combat SARS-CoV-2 and its variants.
reference link : https://journals.asm.org/doi/10.1128/jvi.01737-23