A Recurrent Stop-Codon Mutation in Succinate Dehydrogenase Subunit B Gene in Normal Peripheral Blood and Childhood T-Cell Acute Leukemia

BACKGROUND: Somatic cytidine mutations in normal mammalian nuclear genes occur during antibody diversification in B lymphocytes and generate an isoform of apolipoprotein B in intestinal cells by RNA editing. Here, I describe that succinate dehydrogenase (SDH; mitochondrial complex II) subunit B gene (SDHB) is somatically mutated at a cytidine residue in normal peripheral blood mononuclear cells (PBMCs) and T-cell acute leukemia. Germ line mutations in the SDHB, SDHC or SDHD genes cause hereditary paraganglioma (PGL) tumors which show constitutive activation of homeostatic mechanisms induced by oxygen deprivation (hypoxia). PRINCIPAL FINDINGS: To determine the prevalence of a mutation identified in the SDHB mRNA, 180 samples are tested. An SDHB stop-codon mutation c.136C>T (R46X) is present in a significant fraction (average = 5.8%, range = less than 1 to 30%, n = 52) of the mRNAs obtained from PBMCs. In contrast, the R46X mutation is present in the genomic DNA of PBMCs at very low levels. Examination of the PBMC cell-type subsets identifies monocytes and natural killer (NK) cells as primary sources of the mutant transcript, although lesser contributions also come from B and T lymphocytes. Transcript sequence analyses in leukemic cell lines derived from monocyte, NK, T and B cells indicate that the mutational mechanism targeting SDHB is operational in T-cell acute leukemia. Accordingly, substantial levels (more than 3%) of the mutant SDHB transcripts are detected in five of 20 primary childhood T-cell acute lymphoblastic leukemia (T-ALL) bone marrow samples, but in none of 20 B-ALL samples. In addition, distinct heterozygous SDHB missense DNA mutations are identified in Jurkat and TALL-104 cell lines which are derived from T-ALLs. CONCLUSIONS: The identification of a recurrent, inactivating stop-codon mutation in the SDHB gene in normal blood cells suggests that SDHB is targeted by a cytidine deaminase enzyme. The SDHB mutations in normal PBMCs and leukemic T cells might play a role in cellular pre-adaptation to hypoxia

The E1A-Associated p400 Protein Modulates Cell Fate Decisions by the Regulation of ROS Homeostasis

The p400 E1A-associated protein, which mediates H2A.Z incorporation at specific promoters, plays a major role in cell fate decisions: it promotes cell cycle progression and inhibits induction of apoptosis or senescence. Here, we show that p400 expression is required for the correct control of ROS metabolism. Depletion of p400 indeed increases intracellular ROS levels and causes the appearance of DNA damage, indicating that p400 maintains oxidative stress below a threshold at which DNA damages occur. Suppression of the DNA damage response using a siRNA against ATM inhibits the effects of p400 on cell cycle progression, apoptosis, or senescence, demonstrating the importance of ATM–dependent DDR pathways in cell fates control by p400. Finally, we show that these effects of p400 are dependent on direct transcriptional regulation of specific promoters and may also involve a positive feedback loop between oxidative stress and DNA breaks since we found that persistent DNA breaks are sufficient to increase ROS levels. Altogether, our results uncover an unexpected link between p400 and ROS metabolism and allow deciphering the molecular mechanisms largely responsible for cell proliferation control by p400

Sirtuin 3, a New Target of PGC-1α, Plays an Important Role in the Suppression of ROS and Mitochondrial Biogenesis

Sirtuin 3 (SIRT3) is one of the seven mammalian sirtuins, which are homologs of the yeast Sir2 gene. SIRT3 is the only sirtuin with a reported association with the human life span. Peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) plays important roles in adaptive thermogenesis, gluconeogenesis, mitochondrial biogenesis and respiration. PGC-1alpha induces several key reactive oxygen species (ROS)-detoxifying enzymes, but the molecular mechanism underlying this is not well understood.Here we show that PGC-1alpha strongly stimulated mouse Sirt3 gene expression in muscle cells and hepatocytes. Knockdown of PGC-1alpha led to decreased Sirt3 gene expression. PGC-1alpha activated the mouse SIRT3 promoter, which was mediated by an estrogen-related receptor (ERR) binding element (ERRE) (-407/-399) mapped to the promoter region. Chromatin immunoprecipitation and electrophoretic mobility shift assays confirmed that ERRalpha bound to the identified ERRE and PGC-1alpha co-localized with ERRalpha in the mSirt3 promoter. Knockdown of ERRalpha reduced the induction of Sirt3 by PGC-1alpha in C(2)C(12) myotubes. Furthermore, Sirt3 was essential for PGC-1alpha-dependent induction of ROS-detoxifying enzymes and several components of the respiratory chain, including glutathione peroxidase-1, superoxide dismutase 2, ATP synthase 5c, and cytochrome c. Overexpression of SIRT3 or PGC-1alpha in C(2)C(12) myotubes decreased basal ROS level. In contrast, knockdown of mSIRT3 increased basal ROS level and blocked the inhibitory effect of PGC-1alpha on cellular ROS production. Finally, SIRT3 stimulated mitochondrial biogenesis, and SIRT3 knockdown decreased the stimulatory effect of PGC-1alpha on mitochondrial biogenesis in C(2)C(12) myotubes.Our results indicate that Sirt3 functions as a downstream target gene of PGC-1alpha and mediates the PGC-1alpha effects on cellular ROS production and mitochondrial biogenesis. Thus, SIRT3 integrates cellular energy metabolism and ROS generation. The elucidation of the molecular mechanisms of SIRT3 regulation and its physiological functions may provide a novel target for treating ROS-related disease

Star Cluster Catalogs for the LEGUS Dwarf Galaxies

We present the star cluster catalogs for 17 dwarf and irregular galaxies in the $HST$ Treasury Program "Legacy ExtraGalactic UV Survey" (LEGUS). Cluster identification and photometry in this subsample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogs with accurate fluxes due to low cluster statistics. The colors and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogs are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of $-2$ independent of age and global SFR binning. The age distribution declines as a power law, with an index of $\approx-0.80\pm0.15$, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cutoff ($<2\sigma$) in the composite cluster mass function, and can rule out a truncation mass below $\approx10^{4.5}$M$_{\odot}$ but cannot rule out the existence of a truncation at higher masses

Star cluster catalogues for the LEGUS dwarf galaxies

We present the star cluster catalogues for 17 dwarf and irregular galaxies in the HST Treasury Program ‘Legacy ExtraGalactic UV Survey’ (LEGUS). Cluster identification and photometry in this sub-sample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogues with accurate fluxes due to low cluster statistics. The colours and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogues are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of −2 independent of age and global SFR binning. The age distribution declines as a power law, with an index of ≈− 0.80 ± 0.15, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cut-off (<2σ) in the composite cluster mass function, and can rule out a truncation mass below ≈104.5M⊙ but cannot rule out the existence of a truncation at higher masses

DNA Oxidative Damage and Cancer

Cancer cells have undergone several distinct transformational events leading to alterations of the normal growth and proliferation regulatory pathways. The probabilities of these events are increased by agents that damage and mutate DNA. It is now well established that oxygen is one such mutagen. Within the cell, the metabolic conversion of oxygen to reactive forms, particularly the hydroxyl radical, is an initial step in this process. Hydroxyl radical-mediated oxidative damage to DNA results in a variety of mutagenic lesions. However, this is a normal occurrence in every cell, and a host of proteins is involved in surveillance of the genome and removal of the damage. The link between DNA oxidative damage and cancer is evident from animal models lacking these DNA repair and antioxidant proteins. Mice deficient in repair of DNA oxidative damage or reactive oxygen species (ROS) detoxification typically are susceptible to cancer. Mitochondria, as a major source of intracellular ROS, and organelles essential for the maintenance of metabolic homeostasis, can also play critical roles in the initiation and promotion of cancer. Important mechanistic details of mitochondrial participation in tumori-genesis have recently been uncovered