Lamin A, B and C, the nuclear intermediate-filament proteins, play a role in epigenetic regulation. While Lamin B is expressed in all nucleated cells studied, Lamin A/C are transcribed in most somatic cell types except mature B lymphocytes. Since Epstein-Barr virus (EBV), a human gammaherpesvirus, is associated with tumorigenic processes and is known to alter the epigenotype of its host cells, we studied the expression of the LMNA gene and its epigenetic marks in EBV-carrying human lymphoid cell lines. We observed a high lamin A/C mRNA and protein expression in EBV-immortalized lymphoblastoid cell lines (LCLs) and in group III Burkitt lymphoma (BL) lines where hypomethylated first exons were observed with activating histone marks. In most cell lines with low promoter activity a highly methylated first exon could be detected. Our data showed that methylation of the first exon of LMNA was associated with the downregulation of LMNA expression whereas euchromatic histone marks were enriched at active LMNA promoters in EBV-immortalized LCLs. These data suggest a role for viral latency products to activate LMNAp in EBV-infected latency type III B cells in vitro. Expression of lamin A/C may contribute to the establishment of activated B cell phenotype that needs further explorations.

Mutations in the LMNA gene (encoding lamin A/C proteins) cause several human cardiac diseases, including dilated cardiomyopathies (LMNA-DCM). The main clinical risks in LMNA-DCM patients are sudden cardiac death and progressive left ventricular ejection fraction deterioration, and therefore most human and animal studies have sought to define the mechanisms through which LMNA mutations provoke cardiac alterations, with particular focus on cardiomyocytes. To investigate if LMNA mutations also cause vascular alterations that might contribute to the etiopathogenesis of LMNA-DCM, we generated and characterized Lmnaflox/floxSM22αCre mice, which constitutively lack lamin A/C in vascular smooth muscle cells (VSMCs), cardiac fibroblasts, and cardiomyocytes. Like mice with whole body or cardiomyocyte-specific lamin A/C ablation, Lmnaflox/floxSM22αCre mice recapitulated the main hallmarks of human LMNA-DCM, including ventricular systolic dysfunction, cardiac conduction defects, cardiac fibrosis, and premature death. These alterations were associated with hyperactivation of Smad3 and elevated expression of the proapoptotic protein caspase 3 in the heart. Lmnaflox/floxSM22αCre mice also exhibited perivascular fibrosis in the coronary arteries, and a switch of aortic VSMCs from the ‘contractile’ to the ‘synthetic’ phenotype. Ex vivo wire myography in isolated aortic rings revealed impaired maximum contraction capacity and an altered response to vasoconstrictor and vasodilator agents in Lmnaflox/floxSM22αCre mice. To our knowledge, our results provide the first evidence of phenotypic alterations in VSMCs that might contribute significantly to the pathophysiology of some forms of LMNA-DCM. Future work addressing the mechanisms underlying vascular defects in LMNA-DCM may open new therapeutic avenues for these diseases.

Telomeres in epithelial tumours are usually maintained by telomerase, however, in the breast cancer MDA-MB-231 cell line, polyploid cells, induced by doxorubicin (DOX) were found, after mitotic slippage, transiently shifting to recombinatory alternative telomere lengthening (ALT) in PML bodies (APBs). The involvement of the meiotic recombination proteins was evidenced here by juxta-co-localisation of SPO11, DMC1 and RAD51/γH2AX with APBs in these and melanoma SkMel28-DOX-treated cells. Using sublethal doses of DOX we also observed the formation of the PML dimeric rod tandems linking γH2AX/TRF2 foci into ribbons discontinuously inserted into lamin B1 and circumventing cell nuclei. Next, we link these PML-lamin B1 insertions and peripheral rotation of the tandemly linked telomere repeats with the mobility of the nuclear envelope limited chromatin sheets (ELCS). ELCSs comprise two layers of the criss-crossed ~30 nm granules derived from surface heterochromatin (epichromatin) which contains telomere repeats enriched in GpC, PML, and ALU. ELCS protrude, circumvent, loop, split, fuse, and rejoin polyploid cell nuclei. We propose this gear-wheel-like turnover of telomere repeats to be used for the homology search. Interstitial and end-telomere repeats flanked by fragile ALU-Z flipons become recognised by SPO11 nuclease, attract TRF2, PML, and meiotic recombinases. Subsequently, they perform ALT recombining subtelomeres and telomeres or undergo non-homologous end-joining and chromosome rearrangements with ELCS. This mechanism increases the chance of cell survival and may be involved in chromothripsis.