The epicenter of chromosomal fragility of Fra14A2, the mouse ortholog of human FRA3B common fragile site,is largely attached to the nuclear matrix in lymphocytes but not in other cell types that do not express such a fragility

Common fragile sites (CFSs) correspond to chromosomal regions susceptible to present breaks, discontinuities or constrictions in metaphase chromosomes from cells subjected to replication stress. They are considered as genomic regions intrinsically difficult to replicate and they are evolutionary conserved at least in mammals. However, the recent discovery that CFSs are cell-type specific indicates that DNA sequence by itself cannot account for CFS instability. Nevertheless, the large gene FHIT that includes FRA3B, the most highly expressed CFS in human lymphocytes, is commonly deleted in a variety of tumors suggesting a tumor suppressor role for its product. Here, we report that the epicenter of fragility of Fra14A2/Fhit, the mouse ortholog of human FRA3B/FHIT that like its human counterpart is the most highly expressed CFS in mouse lymphocytes, is largely attached to the nuclear matrix compartment in naive B lymphocytes but not in primary hepatocytes or cortical neurons that do not express such a CFS. Our results suggest a structural explanation for the difficult-to-replicate nature of such a region and so for its common fragility in lymphocytes, that is independent of the possible tumor suppressor role of the gene harboring such CFS.     

Identification of copy number variation hotspots in human populations

Copy number variants (CNVs) in the human genome contribute to both Mendelian and complex traits as well as to genomic plasticity in evolution. The investigation of mutational rates of CNVs is critical to understanding genomic instability and the etiology of the copy number variation (CNV)-related traits. However, the evaluation of the CNV mutation rate at the genome level poses an insurmountable practical challenge that requires large samples and accurate typing. In this study, we show that an approximate estimation of the CNV mutation rate could be achieved by using the phylogeny information of flanking SNPs. This allows a genome-wide comparison of mutation rates between CNVs with the use of vast, readily available data of SNP genotyping. A total of 4187 CNV regions (CNVRs) previously identified in HapMap populations were investigated in this study. We showed that the mutation rates for the majority of these CNVRs are at the order of 10⁻⁵ per generation, consistent with experimental observations at individual loci. Notably, the mutation rates of 104 (2.5%) CNVRs were estimated at the order of 10⁻³ per generation; therefore, they were identified as potential hotspots. Additional analyses revealed that genome architecture at CNV loci has a potential role in inciting mutational hotspots in the human genome. Interestingly, 49 (47%) CNV hotspots include human genes, some of which are known to be functional CNV loci (e.g., CNVs of C4 and β-defensin causing autoimmune diseases and CNVs of HYDIN with implication in control of cerebral cortex size), implicating the important role of CNV in human health and evolution, especially in common and complex diseases.     

Initiation of Genome Instability and Preneoplastic Processes through Loss of Fhit Expression

Genomic instability drives tumorigenesis, but how it is initiated in sporadic neoplasias is unknown. In early preneoplasias, alterations at chromosome fragile sites arise due to DNA replication stress. A frequent, perhaps earliest, genetic alteration in preneoplasias is deletion within the fragile FRA3B/FHIT locus, leading to loss of Fhit protein expression. Because common chromosome fragile sites are exquisitely sensitive to replication stress, it has been proposed that their clonal alterations in cancer cells are due to stress sensitivity rather than to a selective advantage imparted by loss of expression of fragile gene products. Here, we show in normal, transformed, and cancer-derived cell lines that Fhit-depletion causes replication stress-induced DNA double-strand breaks. Using DNA combing, we observed a defect in replication fork progression in Fhit-deficient cells that stemmed primarily from fork stalling and collapse. The likely mechanism for the role of Fhit in replication fork progression is through regulation of Thymidine kinase 1 expression and thymidine triphosphate pool levels; notably, restoration of nucleotide balance rescued DNA replication defects and suppressed DNA breakage in Fhit-deficient cells. Depletion of Fhit did not activate the DNA damage response nor cause cell cycle arrest, allowing continued cell proliferation and ongoing chromosomal instability. This finding was in accord with in vivo studies, as Fhit knockout mouse tissue showed no evidence of cell cycle arrest or senescence yet exhibited numerous somatic DNA copy number aberrations at replication stress-sensitive loci. Furthermore, cells established from Fhit knockout tissue showed rapid immortalization and selection of DNA deletions and amplifications, including amplification of the Mdm2 gene, suggesting that Fhit loss-induced genome instability facilitates transformation. We propose that loss of Fhit expression in precancerous lesions is the first step in the initiation of genomic instability, linking alterations at common fragile sites to the origin of genome instability.     

Checkpoint-dependent and independent roles of the Werner syndrome protein in preserving genome integrity in response to mild replication stress

Werner syndrome (WS) is a human chromosomal instability disorder associated with cancer predisposition and caused by mutations in the WRN gene. WRN helicase activity is crucial in limiting breakage at common fragile sites (CFS), which are the preferential targets of genome instability in precancerous lesions. However, the precise function of WRN in response to mild replication stress, like that commonly used to induce breaks at CFS, is still missing. Here, we establish that WRN plays a role in mediating CHK1 activation under moderate replication stress. We provide evidence that phosphorylation of CHK1 relies on the ATR-mediated phosphorylation of WRN, but not on WRN helicase activity. Analysis of replication fork dynamics shows that loss of WRN checkpoint mediator function as well as of WRN helicase activity hamper replication fork progression, and lead to new origin activation to allow recovery from replication slowing upon replication stress. Furthermore, bypass of WRN checkpoint mediator function through overexpression of a phospho-mimic form of CHK1 restores fork progression and chromosome stability to the wild-type levels. Together, these findings are the first demonstration that WRN regulates the ATR-checkpoint activation upon mild replication stress, preventing chromosome fragility.     

DNA polymerases eta and kappa exchange with the polymerase delta holoenzyme to complete common fragile site synthesis

Common fragile sites (CFSs) are inherently unstable genomic loci that are recurrently altered in human tumor cells. Despite their instability, CFS are ubiquitous throughout the human genome and associated with large tumor suppressor genes or oncogenes. CFSs are enriched with repetitive DNA sequences, one feature postulated to explain why these loci are inherently difficult to replicate, and sensitive to replication stress. We have shown that specialized DNA polymerases (Pols) η and κ replicate CFS-derived sequences more efficiently than the replicative Pol δ. However, we lacked an understanding of how these enzymes cooperate to ensure efficient CFS replication. Here, we designed a model of lagging strand replication with RFC loaded PCNA that allows for maximal activity of the four-subunit human Pol δ holoenzyme, Pol η, and Pol κ in polymerase mixing assays. We discovered that Pol η and κ are both able to exchange with Pol δ stalled at repetitive CFS sequences, enhancing Normalized Replication Efficiency. We used this model to test the impact of PCNA mono-ubiquitination on polymerase exchange, and found no change in polymerase cooperativity in CFS replication compared with unmodified PCNA. Finally, we modeled replication stress in vitro using aphidicolin and found that Pol δ holoenzyme synthesis was significantly inhibited in a dose-dependent manner, preventing any replication past the CFS. Importantly, Pol η and κ were still proficient in rescuing this stalled Pol δ synthesis, which may explain, in part, the CFS instability phenotype of aphidicolin-treated Pol η and Pol κ-deficient cells. In total, our data support a model wherein Pol δ stalling at CFSs allows for free exchange with a specialized polymerase that is not driven by PCNA.     

Rev3, the catalytic subunit of Polζ, is required for maintaining fragile site stability in human cells

It has been long speculated that mammalian Rev3 plays an important, yet unknown role(s) during mammalian development, as deletion of Rev3 causes embryonic lethality in mice, whereas no other translesion DNA synthesis polymerases studied to date are required for mouse embryo development. Here, we report that both subunits of Polζ (Rev3 and Rev7) show an unexpected increase in expression during G₂/M phase, but they localize independently in mitotic cells. Experimental depletion of Rev3 results in a significant increase in anaphase bridges, chromosomal breaks/gaps and common fragile site (CFS) expression, whereas Rev7 depletion primarily causes lagging chromosome defect with no sign of CFS expression. The genomic instability induced by Rev3 depletion seems to be related to replication stress, as it is further enhanced on aphidicolin treatment and results in increased metaphase-specific Fanconi anemia complementation group D type 2 (FANCD2) foci formation, as well as FANCD2-positive anaphase bridges. Indeed, a long-term depletion of Rev3 in cultured human cells results in massive genomic instability and severe cell cycle arrest. The aforementioned observations collectively support a notion that Rev3 is required for the efficient replication of CFSs during G₂/M phase, and that the resulting fragile site instability in Rev3 knockout mice may trigger cell death during embryonic development.