DNA repair protein RAD51 homolog 1 is a protein encoded by the geneRAD51. 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 bacterialRecA, ArchaealRadA and yeast Rad51.[4][5] The protein is highly conserved in most eukaryotes, from yeast to humans.[6]
The name RAD51 derives from RADiation sensitive protein 51.[7]
Variants
Two alternatively spliced transcript variants of this gene, which encode distinct proteins, have been reported. Transcript variants utilizing alternative polyA signals exist.
In humans, RAD51 is a 339-amino acid protein that plays a major role in homologous recombination of DNA during double strand break repair. In this process, an ATP dependent DNA strand exchange takes place in which a template strand invades base-paired strands of homologous DNA molecules. RAD51 is involved in the search for homology and strand pairing stages of the process.
Unlike other proteins involved in DNA metabolism, the RecA/Rad51 family forms a helical nucleoprotein filament on DNA.[9]
This protein can interact with the ssDNA-binding protein RPA, BRCA2, PALB2[10] and RAD52.
The structural basis for Rad51 filament formation and its functional mechanism still remain poorly understood. However, recent studies using fluorescent labeled Rad51[11] have indicated that Rad51 fragments elongate via multiple nucleation events followed by growth, with the total fragment terminating when it reaches about 2 μm in length. Disassociation of Rad51 from dsDNA, however, is slow and incomplete, suggesting that there is a separate mechanism that accomplishes this.
RAD51 expression in cancer
In eukaryotes, RAD51 protein has a central role in homologous recombinational repair. RAD51 catalyses strand transfer between a broken sequence and its undamaged homologue to allow re-synthesis of the damaged region (see homologous recombination models).
Numerous studies report that RAD51 is over-expressed in different cancers (see Table 1). In many of these studies, elevated expression of RAD51 is correlated with decreased patient survival. There are also some reports of under-expression of RAD51 in cancers (see Table 1).
Where RAD51 expression was measured in conjunction with BRCA1 expression, an inverse correlation was found.[12][13] This was interpreted as selection for increased RAD51 expression and thus increased homologous recombinational repair (HRR) (by the HRR RAD52-RAD51 back-up pathway[14]) to compensate for the added DNA damages remaining when BRCA1 was deficient.[12][13][15]
Many cancers have epigenetic deficiencies in various DNA repair genes (see Frequencies of epimutations in DNA repair genes in cancers), likely causing increased unrepaired DNA damages. The over expression of RAD51 seen in many cancers may reflect compensatory RAD51 over expression (as in BRCA1 deficiency) and increased HRR to at least partially deal with such excess DNA damages.
Under-expression of RAD51 would itself lead to increased unrepaired DNA damages. Replication errors past these damages (see translesion synthesis), would lead to increased mutations and cancer.
Double-strand break (DSB) repair by homologous recombination is initiated by 5' to 3' strand resection (DSB resection). In humans, the DNA2 nuclease cuts back the 5'-to-3' strand at the DSB to generate a 3' single-strand DNA overhang strand.[26][27]
A number of paralogs (see Figure) of RAD51 are essential for RAD51 protein recruitment or stabilization at damage sites in vertebrates.
In vertebrates and plants, five paralogs of RAD51 are expressed in somatic cells, including RAD51B (RAD51L1), RAD51C (RAD51L2), RAD51D (RAD51L3), XRCC2 and XRCC3. They each share about 25% amino acid sequence identity with RAD51 and with each other.[28]
Outside of plants and vertebrates, a much broader diversity of Rad51 recombinase paralog proteins exists. In budding yeast, Saccharomyces cerevisiae, the paralogs Rad55 and Rad57 are present, which form a complex that associates with yeast Rad51 to ssDNA. The recombinase paralog rfs-1 is found in the round worm Caenorhabditis elegans, where it is not essential for homologous recombination. Among archaea the RadB and RadC recombinase paralogs are found in many organisms belonging to Euryarchaeota while a broader diversity of related recombinase paralogs seem to be found in the Crenarchaea including Ral1, Ral2, Ral3, RadC, RadC1, and RadC2.
The RAD51 paralogs contribute to efficient DNA double-strand break repair by homologous recombination and depletion of any paralog often results in significant decreases in homologous recombination frequency.[29]
The paralogs form two identified complexes: BCDX2 (RAD51B-RAD51C-RAD51D-XRCC2) and CX3 (RAD51C-XRCC3). These two complexes act at two different stages of homologous recombinationalDNA repair. The BCDX2 complex is responsible for RAD51 recruitment or stabilization at damage sites.[29] 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.[29]
Another complex, the BRCA1-PALB2-BRCA2 complex, and the RAD51 paralogs cooperate to load RAD51 onto ssDNA coated with RPA to form the essential recombination intermediate, the RAD51-ssDNA filament.[30]
In mice and humans, the BRCA2 complex primarily mediates orderly assembly of RAD51 on ssDNA, the form that is active for homologous pairing and strand invasion.[31] BRCA2 also redirects RAD51 from dsDNA and prevents dissociation from ssDNA.[31] However, in the presence of a BRCA2 mutation, human RAD52 can mediate RAD51 assembly on ssDNA and substitute for BRCA2 in homologous recombinational DNA repair,[32] though with lower efficiency than BRCA2.
Rad51 has a crucial function in meiotic prophase in mice and its loss leads to depletion of late prophase I spermatocytes.[33]
During meiosis, the two recombinases, Rad51 and Dmc1, interact with single-stranded DNA to form specialized filaments that are adapted for facilitating recombination between homologous chromosomes. Both Rad51 and Dmc1 have an intrinsic ability to self-aggregate.[34] The presence of Dmc1 stabilizes the adjacent Rad51 filaments suggesting that cross-talk between these two recombinases may affect their biochemical properties.
Chemotherapy and aging
In aged and chemotherapy treated females, oocytes and follicles are depleted by apoptosis (programmed cell death) leading to ovarian failure. DNA damage-induced oocyte apoptosis depends on the efficiency of the DNA repair machinery that in turn declines with age. Survival of oocytes following chemotherapy or aging can be enhanced by increased expression of Rad51.[35] The Rad51-induced oocyte resistance to apoptosis is likely due to Rad51’s central role in homologous recombinational repair of DNA damage.
MicroRNA control of RAD51 expression
In mammals, microRNAs (miRNAs) regulate about 60% of the transcriptional activity of protein-encoding genes.[36] Some miRNAs also undergo methylation-associated silencing in cancer cells.[37][38] If a repressive miRNA is silenced by hypermethylation or deletion, then a gene it is targeting becomes over-expressed.
At least eight miRNAs have been identified that repress RAD51 expression, and five of them appear to be important in cancer. For instance, in triple negative breast cancers (TNBC), over-expression of miR-155 occurs together with repression of RAD51.[39] Further tests directly showed that transfecting breast cancer cells with a vector over-expressing miR-155 represses RAD51, causing decreased homologous recombination and increased sensitivity to ionizing radiation.[39]
Four further miRNAs that repress RAD51 (miR-148b* and miR-193b*,[40] miR-506,[41] and miR-34a[42]) are under-expressed in cancers, presumably leading to induction of RAD51.
Under-expression of miR-148b* and miR-193b* cause an observed induction of RAD51 expression.[40] Deletions of 148b* and miR-193b* in serous ovarian tumors correlate with increased incidences[spelling?] of (possibly carcinogenic) losses of heterozygosity (LOH). This excess LOH was thought to be due to excess recombination caused by induced expression of RAD51.[40]
Under-expression of miR-506 is associated with early time to recurrence (and reduced survival) for epithelial ovarian cancer patients.[43]
Methylation of the promoter of miR-34a, resulting in under-expression of miR-34a, is observed in 79% of prostate cancers and 63% of primary melanomas.[44] Under-expressed levels of miR-34a are also seen in 63% of non-small cell lung cancers,[45] and 36% of colon cancers.[46] miR-34a is also generally under-expressed in primary neuroblastoma tumors.[47]
Table 2 summarizes these five microRNAs, their over or under expression, and the cancers in which their altered expression was noted to occur.
Table 2. Altered expression of microRNAs that affect RAD51 expression in sporadic cancers
The information summarized in Table 2 suggests that under-expression of microRNAs (causing induction of RAD51) occurs frequently in cancers. Over-expression of a microRNA that causes repression of RAD51 appears to be less frequent. The data in Table 1 (above) indicates that, in general, over-expression of RAD51 is more frequent in cancers than under-expression.
Three other microRNAs were identified, by various criteria, as likely to repress RAD51 (miR-96,[48] miR-203,[49] and miR-103/107[50]). These microRNAs were then tested by over-expressing them in cells in vitro, and they were found to indeed repress RAD51. This repression was generally associated with decreased HR and increased sensitivity of the cells to DNA damaging agents.
Pathology
This protein is also found to interact with PALB2[10] and BRCA2, which may be important for the cellular response to DNA damage. BRCA2 is shown to regulate both the intracellular localization and DNA-binding ability of this protein. Loss of these controls following BRCA2 inactivation may be a key event leading to genomic instability and tumorigenesis.[51]
Several alterations of the Rad51 gene have been associated with an increased risk of developing breast cancer. The breast cancer susceptibility protein BRCA2 and PALB2 controls the function of Rad51 in the pathway for DNA repair by homologous recombination.[10][52] In addition to the data listed in Table 1, increased RAD51 expression levels have been identified in metastatic canine mammary carcinoma, indicating that genomic instability plays an important role in the carcinogenesis of this tumor type.[53][54][55][56]
Fanconi anemia (FA) is a hereditary condition characterized by cellular hypersensitivity to DNA cross-linking agents. A dominant negative mutation in the Rad51 gene has been reported to give rise to an FA-like phenotype with features of mental retardation.[57][58] This report included evidence that Rad51-mediated homologous recombinational repair likely has an important role in neurodevelopment.
Genetic recombination is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes, genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: (1) interchromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes ; & (2) intrachromosomal recombination, occurring through crossing over.
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.
BRCA2 and BRCA2 are a human gene and its protein product, respectively. The official symbol and the official name are maintained by the HUGO Gene Nomenclature Committee. One alternative symbol, FANCD1, recognizes its association with the FANC protein complex. Orthologs, styled Brca2 and Brca2, are common in other vertebrate species. BRCA2 is a human tumor suppressor gene, found in all humans; its protein, also called by the synonym breast cancer type 2 susceptibility protein, 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.
Recombinases are genetic recombination enzymes.
DNA repair protein XRCC3 is a protein that in humans is encoded by the XRCC3 gene.
RAD52 homolog , also known as RAD52, is a protein which in humans is encoded by the RAD52 gene.
RAD51 homolog C , also known as RAD51C, is a protein which in humans is encoded by the RAD51C 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.
Meiotic recombination protein DMC1/LIM15 homolog is a protein that in humans is encoded by the DMC1 gene.
E3 ubiquitin-protein ligase FANCL is an enzyme that in humans is encoded by the FANCL gene.
BRCA1-A complex subunit BRE is a protein that in humans is encoded by the BRE gene.
DNA repair and recombination protein RAD54B is a protein that in humans is encoded by the RAD54B gene.
Lys-63-specific deubiquitinase BRCC36 is an enzyme that in humans is encoded by the BRCC3 gene.
Partner and localizer of BRCA2, also known as PALB2 or FANCN, is a protein which in humans is encoded by the PALB2 gene.
Protein ZGRF1 is a protein encoded in the human by the ZGRF1 gene also known as C4orf21, that has a weight of 236.6 kDa. The ZGRF1 gene product localizes to the cell nucleus and promotes DNA repair by stimulating homologous recombination. This gene shows relatively low expression in most human tissues, with increased expression in situations of chemical dependence. ZGRF1 is orthologous to nearly all eukaryotes. Functional domains of this protein link it to a series of helicases, most notably the AAA_12 and AAA_11 domains.
Lumír Krejčí is a Czech biochemist. His research is focused on regulatory processes involved in maintaining genome integrity. Currently, as Associate Professor in biochemistry, he leads the laboratory of recombination and DNA repair (LORD) at the Department of Biology, Faculty of Medicine, at Masaryk University in Brno.
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).
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