Role of recombination enzymes in mammalian cell survival

Characterization of Drosophila Rad51/SpnA protein in DNA binding and embryonic development

The Rad51 is a highly conserved protein throughout the eukaryotic kingdom and an essential enzyme in DNA repair and recombination. It possesses DNA binding activity and ATPase activity, and interacts with meiotic chromosomes during prophase I of meiosis. Drosophila Rad51, Spindle-A (SpnA) protein has been shown to be involved in repair of DNA damage in somatic cells and meiotic recombination in female germ cells. In this study, DNA binding activity of SpnA is demonstrated by both agarose gel mobility shift assay and restriction enzyme protection assay. SpnA is also shown to interact with meiotic chromosomes during prophase I in the primary spermatocytes of hsp26-spnA transgenic flies. In addition, SpnA is highly expressed in embryos, and the depletion of SpnA by RNA interference (RNAi) leads to embryonic lethality implying that SpnA is involved in early embryonic development. Therefore, these results suggest that Drosophila SpnA protein possesses properties similar to mammalian Rad51 homologs.

Molecular mechanisms of individual radiosensitivity studied in normal diploid human fibroblasts

The molecular mechanisms of individual radiosensitivity were studied in normal diploid human fibroblasts. For fibroblasts irradiated with X-rays in G1-phase the individual radiosensitivity was shown to be correlated with the extent of double-strand break (dsb) repair. The number of residual dsbs (including both non- and mis-rejoined dsbs) varied between 2 and 5% of the initial number induced and was low for resistant and high for sensitive strains. In the G1-phase dsbs are considered to be mostly repaired via the non-homologous end-joining pathway (NHEJ). However, so far none of the parameters tested for this pathway was found to be correlated with the number of residual dsbs. The parameters tested were mRNA expression, protein level and localisation and activity of the DNA-PK, which is the central complex of NHEJ. The dsb-repair capacity is also not regulated by the differentiation status, which varies substantially among fibroblast strains, whereas there is some indication that dsb repair might depend on the chromatin structure, with more efficient repair in cells with condensed DNA. Residual dsbs are converted into lethal chromosome aberrations finally leading to the loss of clonogenic activity, when cells pass through mitosis. Beside this so-called mitotic death, X-irradiated human fibroblasts are also inactivated via the TP53-dependent permanent G1-arrest, while apoptosis appears to be not important. On average, mitotic death and G1-arrest are equally effective, but there is a broad variation from one strain to the other, with a negative correlation between these two pathways. Fibroblast strains exhibiting only a moderate G1-arrest showed a high number of lethal aberrations and vice versa. This result points to a common regulator of both G1-arrest and dsb repair, which is presently under investigation.

An antisense construct of full-length human RAD50 cDNA confers sensitivity to ionizing radiation and alkylating agents on human cell lines

In Saccharomyces cerevisiae, Rad50 is reported to participate in the repair of double-stranded DNA breaks, and most rad50 mutants are unable to repair gamma-ray-induced DNA damage. In this study, we examined whether human RAD50 is involved in the repair of DNA damage induced by gamma radiation, radiomimetic alkylating agents, or UVB radiation in cultured human cells. Because homozygous null RAD50 mutant cells could not be isolated, human 293 embryonic kidney cells and A431 epithelial tumor cells were transfected with antisense RAD50 cDNA to obtain viable cell lines which expressed reduced RAD50. Selected individual clones were subjected to PCR-Southern and Western blot analyses to confirm the integrity of the antisense RAD50 construct and the reduced RAD50 expression levels. The cells engineered to express reduced RAD50 levels showed significantly increased sensitivity to gamma radiation, mitomycin C and methylmethane sulfonate compared with control cells that were transfected with the vector alone. However, there were no differences in viability of cells with reduced RAD50 levels and control cells treated with UVB radiation. These results indicate that human RAD50 is involved in the repair of DNA damage induced by gamma radiation and alkylating agents in mammalian cells and suggest the possible application of antisense RAD50 cDNA transfection as a radiation sensitizer in radiation oncology.

Expression of E. coli RecA targeted to mitochondria of human cells

Mitochondrial DNA integrity is ensured by several nuclear-encoded proteins in vertebrates, and a number of mtDNA alterations in human diseases, including deletions and duplications, have been suspected to result from errors in the mitochondrial recombination pathway. However, the presence of the latter system is still a matter of controversy as RecA proteins display various functions in vitro. In Escherichia coli, RecA plays a central role in homologous recombination by pairing and transferring a single strand to a homologous duplex DNA. To address indirectly the issue of a mitochondrial recombination pathway in vivo, we have constructed a chimeric gene containing an N terminal mitochondrial targeting sequence and the E. coli RecA gene. Cells were transfected by the recombinant plasmid, then tested for their mtDNA repair upon bleomycin treatment. We found an increased repair rate of the mitochondrial DNA in cells expressing RecA as compared to control cells. These results indicate that the transfected cells display an improved mtDNA repair replication pathway due to the exogenous RecA, likely in synergy with an endogenous rate-limiting mitochondrial recombination pathway.

RecE/RecT and Redalpha/Redbeta initiate double-stranded break repair by specifically interacting with their respective partners

The initial steps of double-stranded break (DSB) repair by homologous recombination mediated by the 5'-3' exonuclease/annealing protein pairs, RecE/RecT and Redalpha/Redbeta, were analyzed. Recombination was RecA-independent and required the expression of both components of an orthologous pair, even when the need for exonuclease activity was removed by use of preresected substrates. The required orthologous function correlated with a specific protein-protein interaction, and recombination was favored by overexpression of the annealing protein with respect to the exonuclease. The need for both components of an orthologous pair was observed regardless of whether recombination proceeded via a single-strand annealing or a putative strand invasion mechanism. The DSB repair reactions studied here are reminiscent of the RecBCD/RecA reaction and suggest a general mechanism that is likely to be relevant to other systems, including RAD52 mediated recombination.

RecE/RecT and Redα/Redβ initiate double-stranded break repair by specifically interacting with their respective partners

The initial steps of double-stranded break (DSB) repair by homologous recombination mediated by the 5′–3′ exonuclease/annealing protein pairs, RecE/RecT and Redα/Redβ, were analyzed. Recombination was RecA-independent and required the expression of both components of an orthologous pair, even when the need for exonuclease activity was removed by use of preresected substrates. The required orthologous function correlated with a specific protein–protein interaction, and recombination was favored by overexpression of the annealing protein with respect to the exonuclease. The need for both components of an orthologous pair was observed regardless of whether recombination proceeded via a single-strand annealing or a putative strand invasion mechanism. The DSB repair reactions studied here are reminiscent of the RecBCD/RecA reaction and suggest a general mechanism that is likely to be relevant to other systems, including RAD52 mediated recombination.

Laminin-rich extracellular matrix association with mammary epithelial cells suppresses Brca1 expression

Brca1 mRNA was detectable in female mouse mammary gland tissue from adult virgins, during pregnancy and early lactation. It was associated with phases of mammary epithelial cell proliferation and differentiation but the pattern of Brca1 expression was dissociable from that of a true differentiation marker, beta-casein, by virtue of its significant expression in the virgin gland and termination of its expression in early lactation. In a primary cell culture model, association of a laminin-rich extracellular matrix (ECM) with mammary epithelial cells was required for cell survival and cell differentiation and suppressed Brca1 expression in these cells. ECM-association may significantly contribute to the final restriction in Brca1 expression in the lactating gland in vivo. Interestingly, our results suggest that mammary epithelial cells undergo apoptosis both when expressing and when not expressing Brca1, depending on whether the dying cell populations had been terminally differentiated or not.

A role for RAD51 and homologous recombination in Trypanosoma brucei antigenic variation

Antigenic variation is an immune evasion strategy used by African trypanosomes, in which the parasites periodically switch the expression of VSG genes that encode their protective variant surface glycoprotein coat. Two main routes exist for VSG switching: changing the transcriptional status between an active and an inactive copy of the site of VSG expression, called the bloodstream VSG expression site, or recombination reactions that move silent VSGs or VSG copies into the actively transcribed expression site. Nothing is known about the proteins that control and catalyze these switching reactions. This study describes the cloning of a trypanosome gene encoding RAD51, an enzyme involved in DNA break repair and genetic exchange, and analysis of the role of the enzyme in antigenic variation. Trypanosomes genetically inactivated in the RAD51 gene were shown to be viable, and had phenotypes consistent with lacking functional expression of an enzyme of homologous recombination. The mutants had an impaired ability to undergo VSG switching, and it appeared that both recombinational and transcriptional switching reactions were down-regulated, indicating that RAD51 either catalyzes or regulates antigenic variation. Switching events were still detectable, however, so it appears that trypanosome factors other than RAD51 can also provide for antigenic variation.

Overexpression of kin17 protein disrupts nuclear morphology and inhibits the growth of mammalian cells

UVC or ionizing radiation of mammalian cells elicits a complex genetic response that allows recovery and cell survival. Kin17 gene, which is highly conserved among mammals, is upregulated during this response. Kin17 gene encodes a 45 kDa protein which binds to DNA and presents a limited similarity with a functional domain of the bacterial RecA protein. Kin17 protein is accumulated in the nucleus of proliferating fibroblasts and forms intranuclear foci. Using expression vectors, we show that overexpression of kin17 protein inhibits cell-cycle progression into S phase. Our results indicate that growth inhibition correlates with disruption of the nuclear morphology which seems to modify the intranuclear network required during the early steps of DNA replication. We report that a mutant encoding a protein deleted from the central domain of kin17 protein enhanced these effects whereas the deletion of the C-terminal domain considerably reduced them. These mutants will be used to elucidate the molecular mechanism by which kin17 protein alters cell growth and DNA replication.

Human POMp75 is identified as the pro-oncoprotein TLS/FUS: both POMp75 and POMp100 DNA homologous pairing activities are associated to cell proliferation

We have previously developed an assay to measure DNA homologous pairing activities in crude extracts: The POM blot. In mammalian nuclear extracts, we detected two major DNA homologous pairing activities: POMp100 and POMp75. Here, we present the purification and identification of POMp75 as the pro-oncoprotein TLS/FUS. Because of the pro-oncogene status of TLS/FUS, we studied in addition, the relationships between cell proliferation and POM activities. We show that transformation of human fibroblasts by SV40 large T antigen results in a strong increase of both POMpl00 and TLS/POMp75 activities. Although detectable levels of both POMp100 and TLS/POMp75 are observed in non-immortalized fibroblasts or lymphocytes, fibroblasts at mid confluence or lymphocytes stimulated by phytohaemaglutinin, show higher levels of POM activities. Moreover, induction of differentiation of mouse F9 line by retinoic acid leads to the inhibition of both POMp100 and TLS/POMp75 activities. Comparison of POM activity of TLS/FUS with the amount of TLS protein detected by Western blot, suggests that the POM activity could be regulated by post-translation modification. Taken together, these results indicate that POMp100 and TLS/POMp75 activities are present in normal cells but are connected to cell proliferation. Possible relationship between cell proliferation, response to DNA damage and DNA homologous pairing activity of the pro-oncoprotein TLS/FUS are discussed.

Recombination-based mechanisms for somatic hypermutation

We review some experiments designed to test recombination-based mechanisms for somatic hypermutation in mice, particularly mechanisms involving templated mutation or gene conversion. As recombination and repair functions are highly conserved among prokaryotes and eukaryotes, pathways of mutation in microorganisms may prove relevant to the mechanism of somatic hypermutation. Escherichia coli initiates a recombination-based pathway of mutation in response to environmental stimuli, and this "adaptive" pathway of mutation has striking similarities with somatic hypermutation, as does a process of mutagenic repair that occurs at double-strand breaks in Saccharomyces cerevisiae. We present a model for recombination-based hypermutation of the immunoglobulin loci which could result in either templated or non-templated mutation.

Identification, characterization, and genetic mapping of Rad51d, a new mouse and human RAD51/RecA-related gene

Homologous DNA recombination occurs in all organisms and is important for repair of DNA damage during mitosis. One of the critical genes for DNA repair and meiotic recombination in yeast is RAD51, and homologs of RAD51 have been identified in several species, including mouse and human. Here we describe a new RAD51-related mammalian gene, named Rad51d, identified by searching the EST database with the yeast RAD55 and human RAD51B/REC2 genes. A full-length 1.5-kb mouse cDNA clone that encodes a predicted 329-amino-acid protein was isolated. Rad51d mRNA was present in every mouse tissue examined. Four different transcript sizes were detected, one of which was specific to testis. Human cDNA clones that predicted 71% amino acid identity to the mouse protein were also isolated. Interestingly, the sequences of these human clones and of RT-PCR-derived products provided evidence for alternative splicing. These mRNAs are predicted to encode proteins that are truncated relative to the mouse and lack the ATP-binding motif characteristic of RecA-related proteins. Using an interspecific backcross mapping panel, Rad51d was mapped to mouse Chromosome 11, 48.5 cM from the centromere. By radiation hybrid mapping, the human ortholog RAD51D was mapped to chromosome 17q11, which is a region syntenic to mouse Chromosome 11. Due to its expression pattern and sequence similarity to other RAD51 family members, it is likely that Rad51d is part of a complex of proteins required for DNA repair and meiotic recombination.

RAB22 and RAB163/mouse BRCA2: proteins that specifically interact with the RAD51 protein

The human RAD51 protein is a homologue of the bacteria RecA and yeast RAD51 proteins that are involved in homologous recombination and DNA repair. RAD51 interacts with proteins involved in recombination and also with tumor suppressor proteins p53 and breast cancer susceptibility gene 1 (BRCA1). We have used the yeast two-hybrid system to clone murine cDNA sequences that encode two RAD51-associated molecules, RAB22 and RAB163. RAB163 encodes the C-terminal portion of mouse BRCA2, the homologue of the second breast cancer susceptibility gene protein in humans, demonstrating an in vitro association between RAD51 and BRCA2. RAB22 is a novel gene product that also interacts with RAD51 in vitro. To detect RAD51 interactions in vivo, we developed a transient nuclear focus assay that was used to demonstrate a complete colocalization of RAB22 with RAD51 in large nuclear foci.