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:
Breast cancer type 1 susceptibility protein is a protein that in humans is encoded by the BRCA1 gene. Orthologs are common in other vertebrate species, whereas invertebrate genomes may encode a more distantly related gene. BRCA1 is a human tumor suppressor gene and is responsible for repairing DNA.
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids.
DNA repair protein RAD51 homolog 1 is a protein encoded by the gene RAD51. The enzyme encoded by this gene is a member of the RAD51 protein family which assists in repair of DNA double strand breaks. RAD51 family members are homologous to the bacterial RecA, Archaeal RadA and yeast Rad51. The protein is highly conserved in most eukaryotes, from yeast to humans.
Enhancer of zeste homolog 2 (EZH2) is a histone-lysine N-methyltransferase enzyme encoded by EZH2 gene, that participates in histone methylation and, ultimately, transcriptional repression. EZH2 catalyzes the addition of methyl groups to histone H3 at lysine 27, by using the cofactor S-adenosyl-L-methionine. Methylation activity of EZH2 facilitates heterochromatin formation thereby silences gene function. Remodeling of chromosomal heterochromatin by EZH2 is also required during cell mitosis.
DNA repair protein XRCC3 is a protein that in humans is encoded by the XRCC3 gene.
Fanconi anemia group D2 protein is a protein that in humans is encoded by the FANCD2 gene. The Fanconi anemia complementation group (FANC) currently includes FANCA, FANCB, FANCC, FANCD1, FANCD2, FANCE, FANCF, FANCG, FANCI, FANCJ, FANCL, FANCM, FANCN and FANCO.
RAD52 homolog , also known as RAD52, is a protein which in humans is encoded by the RAD52 gene.
DNA repair protein RAD51 homolog 2 is a protein that in humans is encoded by the RAD51L1 gene.
DNA repair protein RAD51 homolog 4 is a protein that in humans is encoded by the RAD51L3 gene.
DNA repair protein XRCC2 is a protein that in humans is encoded by the XRCC2 gene.
DNA repair and recombination protein RAD54-like is a protein that in humans is encoded by the RAD54L gene.
Fanconi anemia group J protein is a protein that in humans is encoded by the BRCA1-interacting protein 1 (BRIP1) gene.
E3 ubiquitin-protein ligase FANCL is an enzyme that in humans is encoded by the FANCL gene.
DNA repair and recombination protein RAD54B is a protein that in humans is encoded by the RAD54B gene.
Partner and localizer of BRCA2, also known as PALB2 or FANCN, is a protein which in humans is encoded by the PALB2 gene.
Synthetic lethality is defined as a type of genetic interaction where the combination of two genetic events results in cell death or death of an organism. Although the foregoing explanation is wider than this, it is common when referring to synthetic lethality to mean the situation arising by virtue of a combination of deficiencies of two or more genes leading to cell death, whereas a deficiency of only one of these genes does not. In a synthetic lethal genetic screen, it is necessary to begin with a mutation that does not result in cell death, although the effect of that mutation could result in a differing phenotype, and then systematically test other mutations at additional loci to determine which, in combination with the first mutation, causes cell death arising by way of deficiency or abolition of expression.
PARP inhibitors are a group of pharmacological inhibitors of the enzyme poly ADP ribose polymerase (PARP).
RI-1 is a selective inhibitor of RAD51, which is a central gene molecule of homologous recombination, with IC50 ranging from 5 to 30 μM.
GT198 is a human oncogene located within the BRCA1 locus at chromosome 17q21. It encodes protein product named GT198, Hop2 or TBPIP. The GT198 gene is found to be mutated with its protein overexpressed in human cancers including breast and ovarian cancers.
A double-strand break repair model refers to the various models of pathways that cells undertake to repair double strand-breaks (DSB). DSB repair is an important cellular process, as the accumulation of unrepaired DSB could lead to chromosomal rearrangements, tumorigenesis or even cell death. In human cells, there are two main DSB repair mechanisms: Homologous recombination (HR) and non-homologous end joining (NHEJ). HR relies on undamaged template DNA as reference to repair the DSB, resulting in the restoration of the original sequence. NHEJ modifies and ligates the damaged ends regardless of homology. In terms of DSB repair pathway choice, most mammalian cells appear to favor NHEJ rather than HR. This is because the employment of HR may lead to gene deletion or amplification in cells which contains repetitive sequences. In terms of repair models in the cell cycle, HR is only possible during the S and G2 phases, while NHEJ can occur throughout whole process. These repair pathways are all regulated by the overarching DNA damage response mechanism. Besides HR and NHEJ, there are also other repair models which exists in cells. Some are categorized under HR, such as synthesis-dependent strain annealing, break-induced replication, and single-strand annealing; while others are an entirely alternate repair model, namely, the pathway microhomology-mediated end joining (MMEJ).