Disease relevance of Xrcc2

• To look at the causes of lethality, and possibly to allow Xrcc2(-/-) mice to survive to birth, we have produced double knockout mice deficient in either the p53 oncoprotein or Ataxia telangiectasia mutated (Atm) [1].
• The hypersensitivity of Xrcc2-/- cells to MNNG prompted us to investigate their killing and apoptotic response to various methylating, chloroethylating and crosslinking drugs used in anticancer therapy [2].
• Cells deficient in Xrcc2 are hypersensitive to X-rays and mitomycin C and display increased chromosomal aberration frequencies [2].
• Indeed, both high and low frequencies of HR as well as high and low levels of RAD51 expression have been reported in tumors and in precancerous conditions [3].
• RecA in Escherichia coli and its homolog, ScRad51 in Saccharomyces cerevisiae, are known to be essential for recombinational repair [4].

High impact information on Xrcc2

• RAD51/DMC1, RPA, and MSH4 foci (which mark early and intermediate steps in pairing/recombination) appear in similar numbers as in wild type, but do not all disappear, and MLH1 and MLH3 foci (which mark late steps in crossing over) are not formed [5].
• Although meiosis is initiated in Msh4 mutant male and female mice, as indicated by the chromosomal localization of RAD51 and COR1 during leptonema/zygonema, the chromosomes fail to undergo normal pairing [6].
• The mouse RecA-like gene Dmc1 is required for homologous chromosome synapsis during meiosis [7].
• At an acute double-strand break in the recombination substrate, Bcl-2 specifically inhibits RAD51-dependent gene conversion without affecting non-conservative recombination [8].
• In xrs6 as well as XR-1 cell lines, Rad51 foci accumulate more rapidly compared with their respective controls [9].

Biological context of Xrcc2

• Xrcc2(-/-) embryos surviving until later stages of embryogenesis commonly showed developmental abnormalities and died at birth [10].
• DNA damage-induced apoptosis resulting from inactivation of HR (Xrcc2 deficiency) only occurred in proliferating neural precursor cells, whereas disruption of NHEJ (DNA ligase IV deficiency) mainly affected differentiating cells at later developmental stages [11].
• Using spectral karyotyping, we find that aneuploidy and complex chromosome exchanges, including an unexpectedly high frequency of homologue exchanges, are hallmarks of Xrcc2 deficiency [12].
• In addition to growth arrest and sensitivity to agents causing severe DNA damage, we show that order-of-magnitude higher levels of chromosomal alterations are sustained in primary or immortal Xrcc2(-/-) embryonic fibroblasts [12].
• The data reveal that Xrcc2 plays a role in the protection against a wide range of anticancer drugs and, therefore, suggest Xrcc2 to be a determinant of anticancer drug resistance [2].

Anatomical context of Xrcc2

• In parallel, we used CHO cell lines expressing a RAD51 dominant-negative form that specifically inhibits gene conversion without affecting cell viability (Hypo-rec lines) [3].
• The transformed Brca1-deficient OSE cells display an increased number of centrosomes, acquire complex chromosome aberrations, and lack Rad51 nuclear foci in the presence of DNA-damaging agents, such as mitomycin C and cisplatin [13].
• RESULTS: In the leptotene and zygotene stages, the Rad51 protein was present on chromatin loops of mouse testis chromosomes then the protein left the loops [14].
• By using a dominant negative approach, we generated a mouse embryonic stem cell line that expresses an ATP hydrolysis-defective RAD51 protein, hRAD51-K133R, at comparable levels to the endogenous wild-type RAD51 protein, whose expression is retained in these cells [15].

Associations of Xrcc2 with chemical compounds

• We show that Xrcc2-/- cells are hypersensitive to the killing effect of the simple methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) [2].
• Xrcc2 deficient cells were found to be hypersensitive to temozolomide, fotemustine and mafosfamide [2].
• Xrcc2 deficiency sensitizes cells to apoptosis by MNNG and the alkylating anticancer drugs temozolomide, fotemustine and mafosfamide [2].
• In order to definitively show which of apoB's carboxyl-terminal cysteines is essential in interacting with apo(a) we have used RecA-assisted restriction enzyme digestion coupled with site-specific mutagenesis to convert Cys-3734 and Cys-4326 to serine within separate 90-kilobase pair apoB P1 phagemid clones [16].
• To test if the increase in RAD51 foci in p53-/- MEFs was due to an increased level of damage during replication, we measured the formation of DNA double-strand breaks (DSBs) in p53+/+ and p53-/- MEFs following treatments with HU [17].

Other interactions of Xrcc2

• Thus, loss of Ercc1, Csb or Ku70 leads to increased fragment formation, but loss of Xrcc2 promotes exchanges between chromosomes [18].
• Xrcc2 is one of a family of five Rad51-like genes with important roles in the repair of DNA damage by homologous recombination (HR) in mammals [1].
• The Rad51 gene is the mammalian homologue of the bacterial RecA gene and catalyses homologous recombination in mammalian cells [19].
• A novel role for the Bcl-2 protein family: specific suppression of the RAD51 recombination pathway [8].
• These paralogs form two complexes (Rad51C/Xrcc3 and Rad51B/C/D/Xrcc2), which play roles in somatic recombination, DNA repair and chromosome stability [20].

Analytical, diagnostic and therapeutic context of Xrcc2

• In addition, Rad51 has been localized by immunofluorescence in abundant foci that may correspond to early nodules in yeast, lily, and mouse [21].
• The recombinants can thus be amplified in mammalian cells, isolated by plasmid rescue in an Escherichia coli RecA- host, and identified by in situ hybridization, by using mammalian vector sequences as probes [22].
• The gene family cloning method is fast, efficient, and free from PCR errors; moreover, it exploits the abilities of RecA protein to pair homologous or partially homologous DNA sequences stably in kinetically trapped, multistranded DNA hybrids that can be used for subsequent gene clone enrichment [23].
• RecA-assisted restriction endonuclease cleavage of genomic DNA showed that the cloned fragments correspond to telomere-terminal genomic DNA, and restriction enzyme mapping of the YACs shows that the smaller clone (175 kb) corresponds exactly to the telomeric end of the larger one (300 kb) [24].
• Overexpression of bacterial RecA protein in somatic mammalian cells increases the frequency of gene targeting [25].

References