RAD51 homolog C (S. cerevisiae), also known as RAD51C, is a protein which in humans is encoded by the RAD51C gene.[5][6]
RAD51 homolog C (S. cerevisiae), also known as RAD51C, is a protein which in humans is encoded by the RAD51C gene.[5][6]
The RAD51C protein is one of five paralogs of RAD51, including RAD51B (RAD51L1), RAD51C (RAD51L2), RAD51D (RAD51L3), XRCC2 and XRCC3. They each share about 25% amino acid sequence identity with RAD51 and each other.[7]
The RAD51 paralogs are all required for efficient DNA double-strand break repair by homologous recombination and depletion of any paralog results in significant decreases in homologous recombination frequency.[8]
RAD51C forms two distinct complexes with other related paralogs: BCDX2 (RAD51B-RAD51C-RAD51D-XRCC2) and CX3 (RAD51C-XRCC3). These two complexes act at two different stages of homologous recombinational DNA repair. The BCDX2 complex is responsible for RAD51 recruitment or stabilization at damage sites.[8] The BCDX2 complex appears to act by facilitating the assembly or stability of the RAD51 nucleoprotein filament.
The CX3 complex acts downstream of RAD51 recruitment to damage sites.[8] The CX3 complex was shown to associate with Holliday junction resolvase activity, probably in a role of stabilizing gene conversion tracts.[8]
The RAD51C gene is one of genes four localized to a region of chromosome 17q23 where amplification occurs frequently in breast tumors.[9] Overexpression of the four genes during amplification has been observed and suggests a possible role in tumor progression. Alternative splicing has been observed for this gene and two variants encoding different isoforms have been identified.[5]
A characteristic of many cancer cells is that parts of some genes contained within these cells have been recombined with other genes. One such gene fusion that has been identified in a MCF-7 breast cancer cell line is a chimera between the RAD51C and ATXN7 genes.[10][11] Since the RAD51C protein is involved in repairing double strand chromosome breaks, this chromosomal rearrangement could be responsible for the other rearrangements.[11]
RAD51C mutation increases the risk for breast and ovarian cancer, and was first established as a human cancer susceptibility gene in 2010.[12][13][14] Carriers of an RAD51C mutation had a 5.2-fold increased risk of ovarian cancer, indicating that RAD51C is a moderate ovarian cancer susceptibility gene.[15] A pathogenic mutation of RAD51C was present in approximately 1% to 3% of unselected ovarian cancers, and among mutation carriers, the lifetime risk of ovarian cancer was approximately 10-15%.[16][17][18][19]
In addition, there are three other causes of RAD51C deficiency that also appear to increase cancer risk. These are alternative splicing, promoter methylation and repression by over-expression of EZH2.
Three alternatively spliced RAD51C transcripts were identified in colorectal cancers. Variant 1 is joined from the 3' end of exon-6 to the 5' end of exon-8, variant 2 is joined at the 3' end of exon-5 to the 5' end of exon-8, and variant 3 is joined from the 3' end of exon-6 to the 5' end of exon-9.[20] Presence and mRNA expression of variant 1 RAD51C was found in 47% of colorectal cancers. Variant 1 mRNA was expressed about 5-fold more frequently in colorectal tumors than in non-tumor tissues, and when present, was expressed 8-fold more frequently than wild-type RAD51C mRNA. The authors concluded that variant 1 mRNA was associated with the malignant phenotype of colorectal cancers[20]
In the case of gastric cancer, reduced expression of RAD51C was found in about 40% to 50% of tumors, and almost all tumors with reduced RAD51C expression had methylation of the RAD51C promoter.[21] On the other hand, methylation of the RAD51C promoter was only found in about 1.5% of ovarian cancer cases.[17]
EZH2 protein is up-regulated in numerous cancers.[22][23] EZH2 mRNA is up-regulated, on average, 7.5-fold in breast cancer, and between 40% and 75% of breast cancers have over-expressed EZH2 protein.[24] EZH2 is the catalytic subunit of Polycomb Repressor Complex 2 (PRC2) which catalyzes methylation of histone H3 at lysine 27 (H3K27me) and mediates epigenetic gene silencing of target genes via local chromatin reorganization.[23] EZH2 targets RAD51C, reducing RAD51C mRNA and protein expression (and also represses other RAD51 paralogs RAD51B, RAD51D, XRCC2 and XRCC3).[25] Increased expression of EZH2, leading to repression of RAD51 paralogs and consequent reduced homologous recombinational repair, was proposed as a cause of breast cancer.[26]
RAD51C has been shown to interact with: