Human Rad51 mediated DNA unwinding is facilitated by conditions that favour Rad51-dsDNA aggregation

RAD51 recombinase and its paralogs: Orchestrating homologous recombination and unforeseen functions in protozoan parasites

The DNA of protozoan parasites is highly susceptible to damage, either induced by environmental agents or spontaneously generated during cellular metabolism through reactive oxygen species (ROS). Certain phases of the cell cycle, such as meiotic recombination, and external factors like ionizing radiation (IR), ultraviolet light (UV), or chemical genotoxic agents further increase this susceptibility. Among the various types of DNA damage, double-stranded breaks (DSBs) are the most critical, as they are challenging to repair and can result in genetic instability or cell death. DSBs caused by environmental stressors are primarily repaired via one of two major pathways: non-homologous end joining (NHEJ) or homologous recombination (HR). In multicellular eukaryotes, NHEJ predominates, but in unicellular eukaryotes such as protozoan parasites, HR seems to be the principal mechanism for DSB repair. The HR pathway is orchestrated by proteins from the RAD52 epistasis group, including RAD51, RAD52, RAD54, RAD55, and the MRN complex. This review focuses on elucidating the diverse roles and significance of RAD51 recombinase and its paralogs in protozoan parasites, such as Acanthamoeba castellanii, Entamoeba histolytica (Amoebozoa), apicomplexan parasites (Chromalveolata), Naegleria fowleri, Giardia spp., Trichomonas vaginalis, and trypanosomatids (Excavata), where they primarily function in HR. Additionally, we analyze the diversity of proteins involved in HR, both upstream and downstream of RAD51, and discuss the implications of these processes in parasitic protozoa.

Expression of EhRAD54, EhRAD51, and EhBLM proteins during DNA repair by homologous recombination in Entamoeba histolytica

Entamoeba histolytica, the protozoan responsible for human amoebiasis, exhibits a great genome plasticity that is probably related to homologous recombination events. It contains the RAD52 epistasis group genes, including Ehrad51 and Ehrad54, and the Ehblm gene, which are key homologous recombination factors in other organisms. Ehrad51 and Ehrad54 genes are differentially transcribed in trophozoites when DNA double-strand breaks are induced by ultraviolet-C irradiation. Moreover, the EhRAD51 recombinase is overexpressed at 30 min in the nucleus. Here, we extend our analysis of the homologous recombination mechanism in E. histolytica by studying EhRAD51, EhRAD54, and EhBLM expression in response to DNA damage. Bioinformatic analyses show that EhRAD54 has the molecular features of homologous proteins, indicating that it may have similar functions. Western blot assays evidence the differential expression of EhRAD51, EhRAD54, and EhBLM at different times after DNA damage, suggesting their potential roles in the different steps of homologous recombination in this protozoan.

p53 suppresses BRCA2-stimulated ATPase and strand exchange functions of human RAD51

Although homologous recombination (HR) is an important pathway for DNA repair, it can also be a cause for deleterious genomic rearrangements leading to carcinogenesis. Therefore, cells have evolved elaborate mechanisms to regulate HR, positively as well as negatively. Among many molecular components that regulate HR are tumour suppressors p53, a negative regulator and breast cancer early-onset (BRCA)2, a positive regulator. Both the players not only interact with each other but also directly interact with human RAD51 (hRAD51), the key recombinase in HR. Here, for the first time we studied HR regulation by the combined action of p53 and BRCA2, in vitro. While BRC4 peptide inhibits ATP hydrolysis by hRAD51, BRCA2(BRC1-8) stimulates DNA-independent and double-stranded DNA-dependent ATPase several fold and only marginally single-stranded DNA-dependent ATPase. Pull down assays demonstrated the occurrence of complex comprising of all three proteins and DNA, where p53 tends to compete out hRAD51 and BRCA2(BRC1-8), leading to not only the decline in ATP hydrolysis but also the strand exchange function of hRAD51 that was stimulated by BRCA2(BRC1-8). Our findings suggest a rigorous p53-mediated regulation on hRAD51 functions in HR even in the presence of BRCA2.

BLM and RAD51 genes polymorphism and susceptibility to breast cancer

DNA repair by homologous recombination is one of the main processes of DNA double strand breaks repair. In the present work we performed a case-control study (304 cases and 319 controls) to check an association between the genotypes of the c.-61 G>T and the g.38922 C>G polymorphisms of the RAD51 gene and the g.96267 A>C and the g.85394 A>G polymorphisms of the BLM gene and breast cancer occurrence. Genotypes were determined in DNA from peripheral blood by PCR-RLFP and by PCR-CTPP. We observed an association between breast cancer occurrence and the T/G genotype (OR 4.41) of the c.-61 G>T-RAD51 polymorphism, the A/A genotype (OR 1.69) of the g.85394 A>G-BLM polymorphism and the A/A genotype (OR 2.49) of the g.96267 A>C-BLM polymorphism. Moreover, we demonstrated a correlation between intra- and intergenes genotypes combinations and breast cancer occurrence. We found a correlation between progesterone receptor expression and the T/G genotype (OR 0.57) of the c.-61 G>T- RAD51 polymorphism. We also found a correlation between the T/G genotype (OR 1.86) and the T/T genotype (OR 0.56) of the c.-61 G>T- RAD51 polymorphism and the lymph node metastasis. We showed an association between the A/A genotype (OR 2.45) and the A/C genotype (OR 0.41) of the g.96267 A>C-BLM polymorphism and G3 grade of tumor. Our results suggest that the variability of the RAD51 and BLM genes may play a role in breast cancer occurrence. This role may be underlined by a common interaction between these genes.

Widespread expressions of immunoglobulin superfamily proteins in cancer cells

RP215 monoclonal antibody (Mab) was shown to recognize a carbohydrate-associated epitope of cancer cell-expressed glycoproteins, known as CA215. Extensive MALDI-TOF MS analysis was performed to search for the molecular identity of CA215. Besides immunoglobulin (Ig) heavy chains, homology to human T-cell receptors (TCR) and Ig-like cell adhesion molecules was also detected. By using RT-PCR and cDNA sequencing, it was observed that as many as 80% of cancer cell lines showed significant levels of gene expressions of TCR-α and TCR-β. Selected Ig-like cell adhesion molecules such as CD47, CD54, CD58 and CD 147 were also highly expressed among all the cell lines tested. In contrast, co-receptors and co-stimulators of TCR such as CD3, CD4 and CD8 were rarely expressed demonstrating the non-functional nature of TCR in cancer cells. Results of immunohistochemical staining and Western blot assays of cancer cell lines as well as cancerous tissue sections were consistent with these observations. Anti-TCR and anti-human IgG antibodies were shown to induce complement-dependent cytotoxicity and apoptosis of cultured cancer cells indicating the surface nature of Ig-like proteins. Based on these experimental observations, it was hypothesized that the expressions of these immunoglobulin superfamily (IgSF) proteins may be relevant to the immune protection and proliferations of cancer cells during carcinogenesis or cancer progression. Surface-bound TCR-like proteins as well as immunoglobulins may be the potential targets for RP215-based anti-cancer drugs.