Lamin B1 sequesters 53BP1 to control its recruitment to DNA damage.

Double-strand breaks (DSBs) are harmful lesions and a major cause of genome instability. Studies have suggested a link between the nuclear envelope and the DNA damage response. Here, we show that lamin B1, a major component of the nuclear envelope, interacts directly with 53BP1 protein, which plays a pivotal role in the DSB repair. This interaction is dissociated after DNA damage. Lamin B1 overexpression impedes 53BP1 recruitment to DNA damage sites and leads to a persistence of DNA damage, a defect in nonhomologous end joining and an increased sensitivity to DSBs. The identification of interactions domains between lamin B1 and 53BP1 allows us to demonstrate that the defect of 53BP1 recruitment and the DSB persistence upon lamin B1 overexpression are due to sequestration of 53BP1 by lamin B1. This study highlights lamin B1 as a factor controlling the recruitment of 53BP1 to DNA damage sites upon injury.

Oxydative stress alters nuclear shape through lamins dysregulation: a route to senescence.

Progeroid phenotypes are mainly encountered in 2 types of syndromes: in laminopathies, which are characterized by nuclear shape abnormalities due to lamin A alteration, and in DNA damage response defect syndromes. Because lamin A dysregulation leads to DNA damages, it has been proposed that senescence occurs in both types of syndromes through the accumulation of damages. We recently showed that elevated oxidative stress is responsible for lamin B1 accumulation, nuclear shape alteration and senescence in the DDR syndrome, ataxia telangiectasia (A-T). Interestingly, overexpression of lamin B1 in wild type cells is sufficient to induce senescence without the induction of DNA damages. Here, we will discuss the importance of controlling the lamins level in order for maintenance nuclear architecture and we will comment the relationships of lamins with other senescence mechanisms. Finally, we will describe emerging data reporting redox control by lamins, leading us to propose a general mechanism by which reactive oxygen species can induce senescence through lamin dysregulation and NSA.

Partial complementation of a DNA ligase I deficiency by DNA ligase III and its impact on cell survival and telomere stability in mammalian cells.

DNA ligase I (LigI) plays a central role in the joining of strand interruptions during replication and repair. In our current study, we provide evidence that DNA ligase III (LigIII) and XRCC1, which form a complex that functions in single-strand break repair, are required for the proliferation of mammalian LigI-depleted cells. We show from our data that in cells with either dysfunctional LigI activity or depleted of this enzyme, both LigIII and XRCC1 are retained on the chromatin and accumulate at replication foci. We also demonstrate that the LigI and LigIII proteins cooperate to inhibit sister chromatid exchanges but that only LigI prevents telomere sister fusions. Taken together, these results suggest that in cells with dysfunctional LigI, LigIII contributes to the ligation of replication intermediates but not to the prevention of telomeric instability.

Oxidative stress induces an ATM-independent senescence pathway through p38 MAPK-mediated lamin B1 accumulation.

We report crosstalk between three senescence-inducing conditions, DNA damage response (DDR) defects, oxidative stress (OS) and nuclear shape alterations. The recessive autosomal genetic disorder Ataxia telangiectasia (A-T) is associated with DDR defects, endogenous OS and premature ageing. Here, we find frequent nuclear shape alterations in A-T cells, as well as accumulation of the key nuclear architecture component lamin B1. Lamin B1 overexpression is sufficient to induce nuclear shape alterations and senescence in wild-type cells, and normalizing lamin B1 levels in A-T cells reciprocally reduces both nuclear shape alterations and senescence. We further show that OS increases lamin B1 levels through p38 Mitogen Activated Protein kinase activation. Lamin B1 accumulation and nuclear shape alterations also occur during stress-induced senescence and oncogene-induced senescence (OIS), two canonical senescence situations. These data reveal lamin B1 as a general molecular mediator that controls OS-induced senescence, independent of established Ataxia Telangiectasia Mutated (ATM) roles in OIS.

Bcl-2 inhibits nuclear homologous recombination by localizing BRCA1 to the endomembranes.

Genetic stability requires coordination of a network of pathways including DNA repair/recombination and apoptosis. In addition to its canonical anti-apoptotic role, Bcl-2 negatively impacts genome stability. In this study, we identified the breast cancer tumor suppressor BRCA1, which plays an essential role in homologous recombination (HR), as a target for Bcl-2 in the repression of HR. Indeed, ionizing radiation-induced BRCA1 foci assembly was repressed when Bcl-2 was expressed ectopically, in human SV40 fibroblasts, or spontaneously, in lymphoma t(14:18) cells and in HeLa and H460 cancer cell lines. Moreover, we showed that the transmembrane (TM) domain of Bcl-2 was required for both inhibition of BRCA1 foci assembly and the inhibition of HR induced by a double-strand break targeted into an intrachromosomal HR substrate by the meganuclease I-SceI. Fluorescence confocal microscopy, proximity ligation assay, and electron microscopy analyses as well as Western blot analysis of subcellular fractions showed that Bcl-2 and BRCA1 colocalized to mitochondria and endoplasmic reticulum in a process requiring the TM domain of Bcl-2. Targeting BRCA1 to the endomembranes depletes BRCA1 from the nucleus and, thus, accounts for the inhibition of HR. Furthermore, our findings support an apoptosis-stimulatory role for the cytosolic form of BRCA1, suggesting a new tumor suppressor function of BRCA1. Together, our results reveal a new mode of BRCA1 regulation and for HR in the maintenance of genome stability.

Comparative genomic hybridization analysis of hereditary swine cutaneous melanoma revealed loss of the swine 13q36-49 chromosomal region in the nodular melanoma subtype.

Genetic alterations implicated in malignant melanoma are still poorly understood. Malignant melanomas present highly variable histologic and cytologic patterns. The aim of the present study is to define genomic imbalances associated with the development of 2 histologic types of swine hereditary cutaneous melanoma. We have investigated 11 swine tumors by comparative genomic hybridization (CGH), 4 superficial spreading melanomas (SSMs) and 7 nodular melanomas (NMs). Following laser capture microdissection and degenerate oligonucleotide primed-polymerase chain reaction, we were able to isolate and then amplify DNA from the 2 histologic subtypes. Consensus regions of chromosome gains were identified on both histologic subtypes, on swine chromosomes 3p13-p17 (75% of the SSMs and 71% of the NMs), 12q (100% of the SSMs and 57% of the NMs) and 14q11-q21 (75% of the SSMs and 42% of NMs). Chromosomal loss was restricted to NM lesions and the swine 13q36-49 region was lost in 100% of the NMs. Interphase fluorescence in situ hybridization with a probe mapping to the 13q41-q42 region indicates loss of the corresponding region on NM lesions. Taking into account this CGH analysis and the comparative genomic data between swine and human genomes, we suggest that a role for the human chromosomes 3p11-qter and chromosome 21 losses should be investigated in human nodular melanoma progression.

Identification of five chromosomal regions involved in predisposition to melanoma by genome-wide scan in the MeLiM swine model.

In human familial melanoma, 3 risk susceptibility genes are already known, CDKN2A, CDK4 and MC1R. However, various observations suggest that other melanoma susceptibility genes have not yet been identified. To search for new susceptibility loci, we used the MeLiM swine as an animal model of hereditary melanoma to perform a genome scan for linkage to melanoma. Founders of the affected MeLiM stock were crossed with each other and with healthy Duroc pigs, generating MeLiM, F1 and backcross families. As we had previously excluded the MeLiM CDKN2A gene, we paid special attention to CDK4 and MC1R, as well as to other candidates such as BRAF and the SLA complex, mapping them on the swine radiation hybrid map and/or isolating close microsatellite markers to introduce them into the genome scan. The results revealed, first, that swine melanoma was inherited as an autosomal dominant trait with incomplete penetrance, preferably in black animals. Second, 4 chromosomal regions potentially involved in melanoma susceptibility were identified on Sus Scrofa chromosomes (SSC) 1, 2, 7 and 8, respectively, in intervals 44-103, 1.9-18, 59-73 and 47-62 cM. A fifth region close to MC1R was revealed on SSC 6 by analyzing an individual marker located at position 7.5 cM. Lastly, CDK4 and BRAF were unlikely to be melanoma susceptibility genes in the MeLiM swine model. The 3 regions on SSC 1, 6 and 7, respectively, have counterparts on human chromosomes (HSA) 9p, 16q and 6p, harboring melanoma candidate loci. The 2 others, on SSC 2 and 8, have counterparts on HSA 11 and 4, which might therefore be of interest for human studies.

Clinical and histopathological characterization of cutaneous melanomas in the melanoblastoma-bearing Libechov minipig model.

Spontaneous animal tumors appear to be highly suitable models to study human oncology and cancer therapy. The aim of this study was to characterize the clinical and histological features of hereditary melanocytic lesions found in the French herd of melanoblastoma-bearing Libechov minipigs (MeLiM) and their Duroc crossbreeds. Clinically, we discriminated between three types of melanocytic skin lesions, which offer a lesion continuum from lentigo to metastatic melanomas. More than 70% of these lesions appear on piglets before they are 3 months old and preferentially on homogeneous black coat piglets. The incidence of melanoma reaches 50% in MeLiM. Most of the highly invasive melanomas regressed spontaneously in the first year of the piglet's life and the regression was followed by hair, skin and iris depigmentation. A histopathological study was conducted according to the human melanoma classification. Except for lentigo maligna, we observed the three main types of human melanoma in swine [superficial spreading melanoma (SSM), nodular or unclassified melanoma] with an excess of SSM (59-67%). The histological events leading to total spontaneous regression are chronologically described. The genetic predisposition, the high incidence of melanoma, the clinical and histopathological features similar to the human disease and the high rate of spontaneous regression offer an opportunity to use this model for studying genetic events controlling melanoma development and regression and the biological mechanisms involved in oncogenesis and anti-cancerous self-defense.

A new animal model for the imaging of melanoma: correlation of FDG PET with clinical outcome, macroscopic aspect and histological classification in Melanoblastoma-bearing Libechov Minipigs.

The aim of this study was to evaluate the Melanoblastoma-bearing Libechov Minipigs (MeLiM) as an animal model of melanoma for in vivo imaging. Serial whole-body 2-deoxy-2-[(18)F]fluoro- d-glucose positron emission tomography (FDG PET) scans were conducted on five MeLiM. In order to explore different clinical stages of the tumoural lesions, each animal was scanned two to four times, at intervals of 30-155 days. PET images were analysed by a semiquantitative method based on the tumour to muscle metabolic ratio. Histology was performed on biopsies taken between or after the scans and the histological grading of the tumours was compared with the FDG uptake. The overall sensitivity of FDG PET for the detection of cutaneous melanoma was 75%; 62.5% of involved lymph nodes were positive. Sensitivity was better for tumours with vertical growth than for flat lesions. FDG PET did not detect tumours with epidermal involvement only, nor did it detect small metastatic foci. The metabolic ratio was correlated with the evolution of the melanoma. FDG PET is effective in the staging of cutaneous melanoma and the follow-up of tumoural extension and regression in Melanoblastoma-bearing Libechov Minipigs. The results obtained in this animal model correlate well with those described in human melanoma. Accordingly, this model may be useful in testing new tracers specific for melanoma and in helping to detect molecules expressed early during tumoural regression.

CDKN2A region polymorphism and genetic susceptibility to melanoma in the melim swine model of familial melanoma.

Some herds of miniature swine are genetically predisposed to cutaneous melanoma. To test if swine melanoma susceptibility could be linked to the CDKN2A gene, which is involved in a proportion of 9p21-linked human familial melanoma, we performed a genetic analysis of miniature pigs of the MeLiM strain. F(1) and backcross animals were generated by crossing 1 MeLiM boar with healthy Duroc sows. We isolated the swine CDKN2A gene and characterized a linked informative microsatellite marker, the S0644 marker. Using this marker and 2 flanking markers, we analyzed the segregation of the CDKN2A gene in a 3-generation pedigree. Allelic association, linkage analysis and haplotype analysis of these data led to exclusion of the CDKN2A gene as a candidate for melanoma susceptibility. Nonetheless, this analysis suggests an association with the swine 1q25 chromosomal region, which is homologous to the human 9p21 region.