In addition to allowing immune cells to signal tissue damage, HMBG1 is secreted by senescent cells to initiate inflammatory cytokine secretion.

No abstract

Females with germline mutations in BRCA1 are predisposed to develop breast and ovarian cancers. A previous report indicated that BRCA1 colocalizes with and is necessary for the correct localization of XIST, a noncoding RNA that coats the inactive X chromosome (Xi) to mediate formation of facultative heterochromatin. A model emerged from this study suggesting that loss of BRCA1 in female cells could reactivate genes on the Xi through loss of the XIST RNA. However, our independent studies of BRCA1 and XIST RNA revealed little evidence to support this model. We report that BRCA1 is not enriched on XIST RNA-coated chromatin of the Xi. Neither mutation nor depletion of BRCA1 causes significant changes in XIST RNA localization or X-linked gene expression. Together, these results do not support a role for BRCA1 in promoting XIST RNA localization to the Xi or regulating XIST-dependent functions in maintaining the stability of facultative heterochromatin.

{These authors contributed equally to this study.During B and T lymphocyte maturation, V(D)J recombination is initiated by creation of DNA doublestrand breaks. Artemis is an exonuclease essential for their subsequent repair by nonhomologous end-joining. Mutations in DCLRE1C, the gene encoding Artemis, cause T NK+ severe combined immunodeficiency (ART-SCID) and also confer heightened sensitivity to ionizing radiation and alkylating chemotherapy. Although allogeneic hematopoietic cell transplantation can treat ART-SCID, conditioning regimens are poorly tolerated, leading to early mortality and/or late complications, including short stature, endocrinopathies, and dental aplasia. However, without alkylating chemotherapy as preconditioning, patients usually have graft rejection or limited T cell and no B cell recovery. Thus, addition of normal DCLRE1C cDNA to autologous hematopoietic stem cells is an attractive strategy to treat ART-SCID. We designed a self-inactivating lentivirus vector containing human Artemis cDNA under transcriptional regulation of the human endogenous Artemis promoter (AProArt). Fibroblasts from ART-SCID patients transduced with AProArt lentivirus showed correction of radiosensitivity. Mobilized peripheral blood CD34+ cells from an ART-SCID patient as well as hematopoietic stem cells from Artemis-deficient mice demonstrated restored T and B cell development following AProArt transduction. Murine hematopoietic cells transduced with AProArt exhibited no increase in replating potential in an in vitro immortalization assay, and analysis of AProArt lentivirus insertions showed no predilection for sites that could activate oncogenes. These efficacy and safety findings support institution of a clinical trial of gene addition therapy for ART-SCID.

Deficiency in Artemis is associated with lack of V(D)J recombination, sensitivity to radiation and radiomimetic drugs, and failure to repair a subset of DNA double-strand breaks (DSBs). Artemis harbors an endonuclease activity that trims both 5′- and 3′-ends of DSBs. To examine whether endonucleolytic trimming of terminally blocked DSBs by Artemis is a biologically relevant function, Artemis-deficient fibroblasts were stably complemented with either wild-type Artemis or an endonuclease-deficient D165N mutant. Wild-type Artemis completely restored resistance to γ-rays, bleomycin and neocarzinostatin, and also restored DSB-repair proficiency in G0/G1 phase as measured by pulsed-field gel electrophoresis and repair focus resolution. In contrast, cells expressing the D165N mutant, even at very high levels, remained as chemo/radiosensitive and repair deficient as the parental cells, as evidenced by persistent γ-H2AX, 53BP1 and Mre11 foci that slowly increased in size and ultimately became juxtaposed with promyelocytic leukemia protein nuclear bodies. In normal fibroblasts, overexpression of wild-type Artemis increased radioresistance, while D165N overexpression conferred partial repair deficiency following high-dose radiation. Restoration of chemo/radioresistance by wild-type, but not D165N Artemis suggests that the lack of endonucleolytic trimming of DNA ends is the principal cause of sensitivity to double-strand cleaving agents in Artemis-deficient cells.

Rapid phosphorylation of histone variant H2AX proximal to DNA breaks is an initiating event and a hallmark of eukaryotic DNA damage responses. Three mammalian kinases are known to phosphorylate H2AX in response to DNA damage. However, the mechanism(s) for damage-localized phosphorylation remains incompletely understood. The DNA-dependent protein kinase (DNA-PK) is the most abundant H2AX-modifying kinases and uniquely activated by binding DNA termini. Here, we have developed a novel approach to examine enzyme activity and substrate properties by executing biochemical assays on intact cellular structures. We apply this approach to examine the mechanisms of localized protein modification in chromatin within fixed cells. DNA-PK retains substrate specificity and independently generates break-localized γH2AX foci in chromatin. In situ DNA-PK activity recapitulates localization and intensity of in vivo H2AX phosphorylation and requires no active cellular processes. Nuclease treatments or addition of exogenous DNA resulted in genome-wide H2AX phosphorylation, showing that DNA termini dictated the locality of H2AX phosphorylation in situ. DNA-PK also reconstituted focal phosphorylation of structural maintenance of chromatin protein 1, but not activating transcription factor 2. Allosteric regulation of DNA-PK by DNA termini protruding from chromatin constitutes an autonomous mechanism for break-localized protein phosphorylation that generates sub-nuclear foci. We discuss generalized implications of this mechanism in localizing mammalian DNA damage responses.

No abstract

DNA ligase 4 deficiency (LIG4-SCID) causes lymphopenia (T-B-NK) and a radiosensitive SCID (RS-SCID) phenotype. We demonstrate, for the first time, flow cytometric-based kinetic analysis of phosphorylated H2AX (γH2AX) in lymphocyte subsets, especially NK cells, for the assessment of LIG4-SCID. Measurement of phosphorylated (p) ATM, SMC1, and H2AX (γH2AX) was performed by flow cytometry to assess DNA repair defects in a 3-year-old girl. Functional assessment (phosphorylation) was measured in T and NK cells (B cells were absent) before irradiation (background control) or after low-dose (2Gy) irradiation (1 and 24 hours). We observed maximal γH2AX at 1 hour postirradiation, with dephosphorylation at 24 hours postirradiation in healthy control patients. The patient showed normal frequencies (percentage) of T cells and NK cells for γH2AX, but increased levels of γH2AX compared with control patients at 1 hour postirradiation. At 24 hours postirradiation, there was a lack of dephosphorylation in a substantial proportion of lymphocytes (with differences observed between T and NK cells) compared with healthy control patients. Although there was dephosphorylation of γH2AX at 24 hours in patient lymphocytes compared with 1 hour, the amount remained elevated at 24 hours compared with in control patients. The data from pATM and pSMC1 were uninformative. Flow-based kinetic analysis of γH2AX is a useful marker for the diagnosis of LIG4-SCID.