Defective Connective Tissue Remodeling in Smad3 Mice Leads to Accelerated Aneurysmal Growth Through Disturbed Downstream TGF-ß Signaling.

Aneurysm-osteoarthritis syndrome characterized by unpredictable aortic aneurysm formation, is caused by SMAD3 mutations. SMAD3 is part of the SMAD2/3/4 transcription factor, essential for TGF-ß-activated transcription. Although TGF-ß-related gene mutations result in aneurysms, the underlying mechanism is unknown. Here, we examined aneurysm formation and progression in Smad3-/- animals. Smad3-/- animals developed aortic aneurysms rapidly, resulting in premature death. Aortic wall immunohistochemistry showed no increase in extracellular matrix and collagen accumulation, nor loss of vascular smooth muscle cells (VSMCs) but instead revealed medial elastin disruption and adventitial inflammation. Remarkably, matrix metalloproteases (MMPs) were not activated in VSMCs, but rather specifically in inflammatory areas. Although Smad3-/- aortas showed increased nuclear pSmad2 and pErk, indicating TGF-ß receptor activation, downstream TGF-ß-activated target genes were not upregulated. Increased pSmad2 and pErk staining in pre-aneurysmal Smad3-/- aortas implied that aortic damage and TGF-ß receptor-activated signaling precede aortic inflammation. Finally, impaired downstream TGF-ß activated transcription resulted in increased Smad3-/- VSMC proliferation. Smad3 deficiency leads to imbalanced activation of downstream genes, no activation of MMPs in VSMCs, and immune responses resulting in rapid aortic wall dilatation and rupture. Our findings uncover new possibilities for treatment of SMAD3 patients; instead of targeting TGF-ß signaling, immune suppression may be more beneficial.

Fibulin-4 deficiency increases TGF-ß signalling in aortic smooth muscle cells due to elevated TGF-ß2 levels.

Fibulins are extracellular matrix proteins associated with elastic fibres. Homozygous Fibulin-4 mutations lead to life-threatening abnormalities such as aortic aneurysms. Aortic aneurysms in Fibulin-4 mutant mice were associated with upregulation of TGF-ß signalling. How Fibulin-4 deficiency leads to deregulation of the TGF-ß pathway is largely unknown. Isolated aortic smooth muscle cells (SMCs) from Fibulin-4 deficient mice showed reduced growth, which could be reversed by treatment with TGF-ß neutralizing antibodies. In Fibulin-4 deficient SMCs increased TGF-ß signalling was detected using a transcriptional reporter assay and by increased SMAD2 phosphorylation. Next, we investigated if the increased activity was due to increased levels of the three TGF-ß isoforms. These data revealed slightly increased TGF-ß1 and markedly increased TGF-ß2 levels. Significantly increased TGF-ß2 levels were also detectable in plasma from homozygous Fibulin-4(R/R) mice, not in wild type mice. TGF-ß2 levels were reduced after losartan treatment, an angiotensin-II type-1 receptor blocker, known to prevent aortic aneurysm formation. In conclusion, we have shown increased TGF-ß signalling in isolated SMCs from Fibulin-4 deficient mouse aortas, not only caused by increased levels of TGF-ß1, but especially TGF-ß2. These data provide new insights in the molecular interaction between Fibulin-4 and TGF-ß pathway regulation in the pathogenesis of aortic aneurysms.

Rad54 is required for the normal development of male and female germ cells and contributes to the maintainance of their genome integrity after genotoxic stress.

Rad54 is an important factor in the homologous recombination pathway of DNA double-strand break repair. However, Rad54 knockout (KO) mice do not exhibit overt phenotypes at adulthood, even when exposed to radiation. In this study, we show that in Rad54 KO mouse the germline is actually altered. Compared with the wild-type (WT) animals, these mice have less premeiotic germ cells. This germ cell loss is found as early as in E11.5 embryos, suggesting an early failure during mutant primordial germ cells development. Both testicular and ovarian KO germ cells exhibited high radiation sensitivity leading to a long-term gametogenesis defect at adulthood. The KO female germline was particularly affected displaying decreased litter size or sterility. Spermatogenesis recovery after irradiation was slower and incomplete in Rad54 KO mice compared with that of WT mice, suggesting that loss of germ stem cell precursors is not fully compensated along the successive rounds of spermatogenesis. Finally, spermatogenesis recovery after postnatal irradiation is in part regulated by glial-cell-line-derived neurotrophic factor (GDNF) in KO but not in irradiated WT mice, suggesting that Sertoli cell GDNF production is stimulated upon substantial germ cell loss only. Our findings suggest that Rad54 has a key function in maintaining genomic integrity of the developing germ cells.

Chromatin mobility is increased at sites of DNA double-strand breaks.

DNA double-strand breaks (DSBs) can efficiently kill cancer cells, but they can also produce unwanted chromosome rearrangements when DNA ends from different DSBs are erroneously joined. Movement of DSB-containing chromatin domains might facilitate these DSB interactions and promote the formation of chromosome rearrangements. Therefore, we analyzed the mobility of chromatin domains containing DSBs, marked by the fluorescently tagged DSB marker 53BP1, in living mammalian cells and compared it with the mobility of undamaged chromatin on a time-scale relevant for DSB repair. We found that chromatin domains containing DSBs are substantially more mobile than intact chromatin, and are capable of roaming a more than twofold larger area of the cell nucleus. Moreover, this increased DSB mobility, but not the mobility of undamaged chromatin, can be reduced by agents that affect higher-order chromatin organization.

Opposite modifying effects of HR and NHEJ deficiency on cancer risk in Ptc1 heterozygous mouse cerebellum.

Heterozygous Patched1 (Ptc1(+/-)) mice are prone to medulloblastoma (MB), and exposure of newborn mice to ionizing radiation dramatically increases the frequency and shortens the latency of MB. In Ptc1(+/-) mice, MB is characterized by loss of the normal remaining Ptc1 allele, suggesting that genome rearrangements may be key events in MB development. Recent evidence indicates that brain tumors may be linked to defects in DNA-damage repair processes, as various combinations of targeted deletions in genes controlling cell-cycle checkpoints, apoptosis and DNA repair result in MB in mice. Non-homologous end joining (NHEJ) and homologous recombination (HR) contribute to genome stability, and deficiencies in either pathway predispose to genome rearrangements. To test the role of defective HR or NHEJ in tumorigenesis, control and irradiated Ptc1(+/-) mice with two, one or no functional Rad54 or DNA-protein kinase catalytic subunit (DNA-PKcs) alleles were monitored for MB development. We also examined the effect of Rad54 or DNA-PKcs deletion on the processing of endogenous and radiation-induced double-strand breaks (DSBs) in neural precursors of the developing cerebellum, the cells of origin of MB. We found that, although HR and NHEJ collaborate in protecting cells from DNA damage and apoptosis, they have opposite roles in MB tumorigenesis. In fact, although Rad54 deficiency increased both spontaneous and radiation-induced MB development, DNA-PKcs disruption suppressed MB tumorigenesis. Together, our data provide the first evidence that Rad54-mediated HR in vivo is important for suppressing tumorigenesis by maintaining genomic stability.

Effect of the BRCA2 CTRD domain on RAD51 filaments analyzed by an ensemble of single molecule techniques.

Homologous recombination is essential for the preservation of genome stability, thereby preventing cancer. The recombination protein RAD51 drives DNA strand exchange, which requires the assembly, rearrangement and disassembly of a RAD51 filament on DNA, coupled to ATP binding and hydrolysis. This process is facilitated and controlled by recombination mediators and accessory factors. Here, we have employed a range of single molecule techniques to determine the influence of the C-terminal RAD51 interaction domain (CTRD) of the breast cancer tumor suppressor BRCA2 on intrinsic aspects of RAD51-DNA interactions. We show that at high concentration the CTRD entangles RAD51 filaments and reduces RAD51 filament formation in a concentration dependent manner. It does not affect the rate of filament disassembly measured as the loss of fluorescent signal due to intrinsic RAD51 protein dissociation from double-stranded DNA (dsDNA). We conclude that, outside the context of the full-length protein, the CTRD does not reduce RAD51 dissociation kinetics, but instead hinders filament formation on dsDNA. The CTRDs mode of action is most likely sequestration of multiple RAD51 molecules thereby rendering them inactive for filament formation on dsDNA.

Visualizing RAD51-mediated joint molecules: implications for recombination mechanism and the effect of sequence heterology.

The defining event in homologous recombination is the exchange of base-paired partners between a single-stranded (ss) DNA and a homologous duplex driven by recombinase proteins, such as human RAD51. To understand the mechanism of this essential genome maintenance event, we analyzed the structure of RAD51-DNA complexes representing strand exchange intermediates at nanometer resolution by scanning force microscopy. Joint molecules were formed between substrates with a defined ssDNA segment and homologous region on a double-stranded (ds) partner. We discovered and quantified several notable architectural features of RAD51 joint molecules. Each end of the RAD51-bound joints had a distinct structure. Using linear substrates, a 10-nt region of mispaired bases blocked extension of joint molecules in all examples observed, whereas 4 nt of heterology only partially blocked joint molecule extension. Joint molecules, including 10 nt of heterology, had paired DNA on either side of the heterologous substitution, indicating that pairing could initiate from the free 3'end of ssDNA or from a region adjacent to the ss-ds junction. RAD51 filaments covering joint ss-dsDNA regions were more stable to disassembly than filaments covering dsDNA. We discuss how distinct structural features of RAD51-bound DNA joints can play important roles as recognition sites for proteins that facilitate and control strand exchange.

BLM has early and late functions in homologous recombination repair in mouse embryonic stem cells.

BLM is a RecQ family helicase that is defective in individuals with the cancer predisposition disorder, Bloom's syndrome (BS). At the cellular level, BS is characterized by hyper-recombination manifested as excessive sister chromatid exchange and loss of heterozygosity. However, the precise function of BLM remains unclear. Multiple roles have been proposed for BLM in the homologous recombination (HR) repair pathway, including 'early' functions, such as the stimulation of resection of DNA double-strand break ends or displacement of the invading strand of DNA displacement loops, and 'late' roles, such as dissolution of double Holliday junctions. However, most of the evidence for these putative roles comes from in vitro biochemical data. In this study, we report the characterization of mouse embryonic stem cells with disruption of Blm and/or Rad54 genes. We show that Blm has roles both upstream and downstream of the Rad54 protein, a core HR factor. Disruption of Rad54 in the Blm-mutant background reduced the elevated level of gene targeting and of sister chromatid exchanges, implying that Blm primarily functions downstream of Rad54 in the HR pathway. Conversely, however, mutation of Blm in Rad54(-/-) cells rescued their mitomycin C (MMC) sensitivity, and decreased both the level of DNA damage and cell cycle perturbation induced by MMC, suggesting an early role for Blm. Our data are consistent with Blm having at least two roles in HR repair in mammalian cells.

Disruption of maternal DNA repair increases sperm-derived chromosomal aberrations.

Male and female germ cells can transmit genetic defects that lead to pregnancy loss, infant mortality, birth defects, and genetic diseases in offspring; however, the parental origins of transmitted defects are not random, with de novo mutations and chromosomal structural aberrations transmitted predominantly by sperm. We tested the hypotheses that paternal mutagenic exposure during late spermatogenesis can induce damage that persists in the fertilizing sperm and that the risk of embryos with paternally transmitted chromosomal aberrations depends on the efficiency of maternal DNA repair during the first cycle after fertilization. We show that female mice with defective DNA double-strand break repair had significantly increased frequencies of zygotes with sperm-derived chromosomal aberrations after matings with wild-type males irradiated 7 days earlier with 4 Gy of ionizing radiation. These findings demonstrate that mutagenic exposures during late spermatogenesis can induce damage that persists for at least 7 days in the fertilizing sperm and that maternal genotype plays a major role in determining the risks for pregnancy loss and frequencies of offspring with chromosomal defects of paternal origin.

Human Rad50/Mre11 is a flexible complex that can tether DNA ends.

The human Rad50 protein, classified as a structural maintenance of chromosomes (SMC) family member, is complexed with Mre11 (R/M) and has important functions in at least two distinct double-strand break repair pathways. To find out what the common function of R/M in these pathways might be, we investigated its architecture. Scanning force microscopy showed that the complex architecture is distinct from the described SMC family members. R/M consisted of two highly flexible intramolecular coiled coils emanating from a central globular DNA binding domain. DNA end-bound R/M oligomers could tether linear DNA molecules. These observations suggest that a unified role for R/M in multiple aspects of DNA repair and chromosome metabolism is to provide a flexible, possibly dynamic, link between DNA ends.

The structure-specific endonuclease Ercc1-Xpf is required for targeted gene replacement in embryonic stem cells.

The Ercc1-Xpf heterodimer, a highly conserved structure-specific endonuclease, functions in multiple DNA repair pathways that are pivotal for maintaining genome stability, including nucleotide excision repair, interstrand crosslink repair and homologous recombination. Ercc1-Xpf incises double-stranded DNA at double-strand/single-strand junctions, making it an ideal enzyme for processing DNA structures that contain partially unwound strands. Here we demonstrate that although Ercc1 is dispensable for recombination between sister chromatids, it is essential for targeted gene replacement in mouse embryonic stem cells. Surprisingly, the role of Ercc1-Xpf in gene targeting is distinct from its previously identified role in removing nonhomologous termini from recombination intermediates because it was required irrespective of whether the ends of the DNA targeting constructs were heterologous or homologous to the genomic locus. Our observations have implications for the mechanism of gene targeting in mammalian cells and define a new role for Ercc1-Xpf in mammalian homologous recombination. We propose a model for the mechanism of targeted gene replacement that invokes a role for Ercc1-Xpf in making the recipient genomic locus receptive for gene replacement.

Growth inhibition and DNA damage induced by Cre recombinase in mammalian cells.

The use of Cre/loxP recombination in mammalian cells has expanded rapidly. We describe here that Cre expression in cultured mammalian cells may result in a markedly reduced proliferation and that this effect is dependent on the endonuclease activity of Cre. Chromosome analysis after Cre expression revealed numerous chromosomal aberrations and an increased number of sister chromatid exchanges. Titration experiments in mouse embryo fibroblasts with a ligand-regulatable Cre-ER(T) show that toxicity is dependent on the level of Cre activity. Prolonged, low levels of Cre activity permit recombination without concomitant toxicity. This urges for a careful titration of Cre activity in conditional gene modification in mammalian cells.

Repair of DNA interstrand cross-links.

DNA interstrand cross-links (ICLs) are very toxic to dividing cells, because they induce mutations, chromosomal rearrangements and cell death. Inducers of ICLs are important drugs in cancer treatment. We discuss the main properties of several classes of ICL agents and the types of damage they induce. The current insights in ICL repair in bacteria, yeast and mammalian cells are reviewed. An intriguing aspect of ICLs is that a number of multi-step DNA repair pathways including nucleotide excision repair, homologous recombination and post-replication/translesion repair all impinge on their repair. Furthermore, the breast cancer-associated proteins Brca1 and Brca2, the Fanconi anemia-associated FANC proteins, and cell cycle checkpoint proteins are involved in regulating the cellular response to ICLs. We depict several models that describe possible pathways for the repair or replicational bypass of ICLs.

The architecture of the human Rad54-DNA complex provides evidence for protein translocation along DNA.

Proper maintenance and duplication of the genome require accurate recombination between homologous DNA molecules. In eukaryotic cells, the Rad51 protein mediates pairing between homologous DNA molecules. This reaction is assisted by the Rad54 protein. To gain insight into how Rad54 functions, we studied the interaction of the human Rad54 (hRad54) protein with double-stranded DNA. We have recently shown that binding of hRad54 to DNA induces a change in DNA topology. To determine whether this change was caused by a protein-constrained change in twist, a protein-constrained change in writhe, or the introduction of unconstrained plectonemic supercoils, we investigated the hRad54--DNA complex by scanning force microscopy. The architecture of the observed complexes suggests that movement of the hRad54 protein complex along the DNA helix generates unconstrained plectonemic supercoils. We discuss how hRad54-induced superhelical stress in the target DNA may function to facilitate homologous DNA pairing by the hRad51 protein directly. In addition, the induction of supercoiling by hRad54 could stimulate recombination indirectly by displacing histones and/or other proteins packaging the DNA into chromatin. This function of DNA translocating motors might be of general importance in chromatin metabolism.

Homologous recombination: from model organisms to human disease.

Recent experiments show that properly controlled recombination between homologous DNA molecules is essential for the maintenance of genome stability and for the prevention of tumorigenesis.

DNA repair: spot(light)s on chromatin.

Chromatin modifications regulate many nuclear processes. Recent studies on the phosphorylation of a histone 2A variant have revealed that this chromatin modification is a general and evolutionarily conserved cellular response to DNA double-strand breaks.

Chromosomal stability and the DNA double-stranded break connection.

Genome stability is of primary importance for the survival and proper functioning of all organisms. Double-stranded breaks in DNA are important threats to genome integrity because they can result in chromosomal aberrations that can affect, simultaneously, many genes, and lead to cell malfunctioning and cell death. These detrimental consequences are counteracted by two mechanistically distinct pathways of double-stranded break repair: homologous recombination and non-homologous end-joining. Recently, unexpected links between these double-stranded break-repair systems, and several human genome instability and cancer predisposition syndromes, have emerged. Now, interactions between both double-stranded break-repair pathways and other cellular processes, such as cell-cycle regulation and replication, are being unveiled.

DNA-binding and strand-annealing activities of human Mre11: implications for its roles in DNA double-strand break repair pathways.

DNA double-strand breaks (DSBs) in eukaryotic cells can be repaired by non-homologous end-joining or homologous recombination. The complex containing the Mre11, Rad50 and Nbs1 proteins has been implicated in both DSB repair pathways, even though they are mechanistically different. To get a better understanding of the properties of the human Mre11 (hMre11) protein, we investigated some of its biochemical activities. We found that hMre11 binds both double- and single-stranded (ss)DNA, with a preference for ssDNA. hMre11 does not require DNA ends for efficient binding. Interestingly, hMre11 mediates the annealing of complementary ssDNA molecules. In contrast to the annealing activity of the homologous recombination protein hRad52, the activity of hMre11 is abrogated by the ssDNA binding protein hRPA. We discuss the possible implications of the results for the role(s) of hMre11 in both DSB repair pathways.

ATP-dependent chromatin remodeling by the Cockayne syndrome B DNA repair-transcription-coupling factor.

The Cockayne syndrome B protein (CSB) is required for coupling DNA excision repair to transcription in a process known as transcription-coupled repair (TCR). Cockayne syndrome patients show UV sensitivity and severe neurodevelopmental abnormalities. CSB is a DNA-dependent ATPase of the SWI2/SNF2 family. SWI2/SNF2-like proteins are implicated in chromatin remodeling during transcription. Since chromatin structure also affects DNA repair efficiency, chromatin remodeling activities within repair are expected. Here we used purified recombinant CSB protein to investigate whether it can remodel chromatin in vitro. We show that binding of CSB to DNA results in an alteration of the DNA double-helix conformation. In addition, we find that CSB is able to remodel chromatin structure at the expense of ATP hydrolysis. Specifically, CSB can alter DNase I accessibility to reconstituted mononucleosome cores and disarrange an array of nucleosomes regularly spaced on plasmid DNA. In addition, we show that CSB interacts not only with double-stranded DNA but also directly with core histones. Finally, intact histone tails play an important role in CSB remodeling. CSB is the first repair protein found to play a direct role in modulating nucleosome structure. The relevance of this finding to the interplay between transcription and repair is discussed.

The Rad51 paralog Rad51B promotes homologous recombinational repair.

The highly conserved Saccharomyces cerevisiae Rad51 protein plays a central role in both mitotic and meiotic homologous DNA recombination. Seven members of the Rad51 family have been identified in vertebrate cells, including Rad51, Dmc1, and five Rad51-related proteins referred to as Rad51 paralogs, which share 20 to 30% sequence identity with Rad51. In chicken B lymphocyte DT40 cells, we generated a mutant with RAD51B/RAD51L1, a member of the Rad51 family, knocked out. RAD51B(-/-) cells are viable, although spontaneous chromosomal aberrations kill about 20% of the cells in each cell cycle. Rad51B deficiency impairs homologous recombinational repair (HRR), as measured by targeted integration, sister chromatid exchange, and intragenic recombination at the immunoglobulin locus. RAD51B(-/-) cells are quite sensitive to the cross-linking agents cisplatin and mitomycin C and mildly sensitive to gamma-rays. The formation of damage-induced Rad51 nuclear foci is much reduced in RAD51B(-/-) cells, suggesting that Rad51B promotes the assembly of Rad51 nucleoprotein filaments during HRR. These findings show that Rad51B is important for repairing various types of DNA lesions and maintaining chromosome integrity.