DNA repair is limiting for haematopoietic stem cells during ageing.

Accumulation of DNA damage leading to adult stem cell exhaustion has been proposed to be a principal mechanism of ageing. Here we address this question by taking advantage of the highly specific role of DNA ligase IV in the repair of DNA double-strand breaks by non-homologous end-joining, and by the discovery of a unique mouse strain with a hypomorphic Lig4(Y288C) mutation. The Lig4(Y288C) mouse, identified by means of a mutagenesis screening programme, is a mouse model for human LIG4 syndrome, showing immunodeficiency and growth retardation. Diminished DNA double-strand break repair in the Lig4(Y288C) strain causes a progressive loss of haematopoietic stem cells and bone marrow cellularity during ageing, and severely impairs stem cell function in tissue culture and transplantation. The sensitivity of haematopoietic stem cells to non-homologous end-joining deficiency is therefore a key determinant of their ability to maintain themselves against physiological stress over time and to withstand culture and transplantation.

Werner protein cooperates with the XRCC4-DNA ligase IV complex in end-processing.

Werner syndrome is a rare human disease characterized by the premature onset of aging-associated pathologies, cancer predisposition, and genomic instability. The Werner protein (WRN), which is defective in Werner syndrome ( WS) patients, belongs to the RecQ family helicases and interacts with several DNA metabolic proteins, including DNA repair factors and telomere associated proteins. Nonhomologous end-joining (NHEJ) is an important pathway in the repair of DNA double strand breaks (DSBs), and the DNA-PK complex, composed of the heterodimer Ku 70/86 and the DNA-PK catalytic subunit (DNA-PKcs), together with the XRCC4-DNA ligase IV complex (X4L4), are major factors. One of the most prominent protein interactions of WRN is with Ku 70/86, and it is possible that WRN is involved in NHEJ via its associations with Ku 70/86 and DNA-PKcs. This study demonstrates that WRN physically interacts with the major NHEJ factor, X4L4, which stimulates WRN exonuclease but not its helicase activity. The human RecQ helicase, BLM, which possesses only helicase activity, does not bind to X4L4, and its helicase activity is not affected by X4L4. In a DNA end-joining assay, we find that a substrate, which is processed by WRN, is ligated by X4L4, thus further supporting the significance of their functional interaction.

Identification of a novel motif in DNA ligases exemplified by DNA ligase IV.

DNA ligase IV is an essential protein that functions in DNA non-homologous end-joining, the major mechanism that rejoins DNA double-strand breaks in mammalian cells. LIG4 syndrome represents a human disorder caused by mutations in DNA ligase IV that lead to impaired but not ablated activity. Thus far, five conserved motifs in DNA ligases have been identified. We previously reported G469E as a mutational change in a LIG4 syndrome patient. G469 does not lie in any of the previously reported motifs. A sequence comparison between DNA ligases led us to identify residues 468-476 of DNA ligase IV as a further conserved motif, designated motif Va, present in eukaryotic DNA ligases. We carried out mutational analysis of residues within motif Va examining the impact on adenylation, double-stranded ligation, and DNA binding. We interpret our results using the DNA ligase I:DNA crystal structure. Substitution of the glycine at position 468 with an alanine or glutamic acid severely compromises protein activity and stability. Substitution of G469 with an alanine or glutamic acid is better tolerated but still impacts upon activity and protein stability. These finding suggest that G468 and G469 are important for protein stability and provide insight into the hypomorphic nature of the G469E mutation identified in a LIG4 syndrome patient. In contrast, residues 470, 473 and 476 within motif Va can be changed to alanine residues without any impact on DNA binding or adenylation activity. Importantly, however, such mutational changes do impact upon double-stranded ligation activity. Considered in light of the DNA ligase I:DNA crystal structure, our findings suggest that residues 470-476 function as part of a molecular pincer that maintains the DNA in a conformation that is required for ligation.

Human, Drosophila, and C.elegans TDP43: nucleic acid binding properties and splicing regulatory function.

TAR DNA binding protein (TDP43), a highly conserved heterogeneous nuclear ribonucleoprotein, was found to down-regulate splicing of the exon 9 cystic fibrosis transmembrane conductance regulator (CFTR) through specific binding to a UG-rich polymorphic region upstream of the 3' splice site. Despite the emergence of new information regarding the protein's nuclear localization and splicing regulatory activity, TDP43's role in cells remains elusive. To investigate the function of human TDP43 and its homologues, we cloned and characterized the proteins from Drosophila melanogaster and Caenorhabditis elegans. The proteins from human, fly, and worm show striking similarities in their nucleic acid binding specificity. We found that residues at two different positions, which show a strong conservation among TDP43 family members, are linked to the tight recognition of the target sequence. Our three-dimensional model of TDP43 in complex with a (UG)(m) sequence predicts that these residues make amino acid side-chain to base contacts. Moreover, our results suggest that Drosophila TDP43 is comparable to human TDP43 in regulating exon splicing. On the other hand, C.elegans TDP43 has no effect on exon recognition. TDP43 from C.elegans lacks the glycine-rich domain found at the carboxy terminus of the other two homologues. Mutants of human and fly TDP43 devoid of the C-terminal domain are likewise unable to affect splicing. Our studies suggest that the glycine-rich domain is essential for splicing regulation by human and fly TDP43.

Genetic characterization of myeloperoxidase deficiency in Italy.

Hereditary myeloperoxidase (MPO) deficiency (MPOD) is the most common neutrophil biochemical defect, and is characterized by a lack of peroxidase activity. In order to extend the epidemiological studies on hereditary MPOD in Italy, a population screening was carried out to detect mutations in the MPO gene. Of approximately 40,000 individuals analyzed, seven partial and eight total MPO-deficient subjects were identified. The genetic characterization of the subjects showed the presence of three already-known mutations (c.752T>C, c.1705C>T, and c.1566_1579del14) and six novel mutations: four missense mutations (c.995C>T, c.1112A>G, c.1715T>G, and c.1927T>C), then a deletion of an adenine within exon 3 (c.325delA) and a mutation within the 3' splice site of intron 11 (c.2031-2A>C). The novel missense mutations cause the substitution of the residues p.A332V, p.D371G, p.L572W, and p.W643R, respectively, and the potential structural changes are discussed. The c.325delA deletion causes a shift of the reading frame with the occurrence of a premature stop codon within the propeptide. Then, considering the difficulty in obtaining bone marrow samples from MPO-deficient subjects to study MPO mRNA splicing in vivo, we set up an eukaryotic expression system to investigate how the c.2031-2A>C mutation alters the MPO pre-mRNA splicing. The activation of a cryptic 3' splice site located 109nt upstream of the authentic 3' splice site was observed. The 109nt-insertion causes a shift in the reading frame that should lead to the generation of an abnormal MPO precursor lacking the enzymatic activity.

Genetic studies on myeloperoxidase deficiency in Italy.

Hereditary myeloperoxidase (MPO) deficiency is the most common neutrophil biochemical defect characterized by the lack of peroxidase activity. In order to extend the epidemiological studies on hereditary MPO deficiency in Italy, approximately 40,000 individuals were analyzed and 7 partial and 8 total MPO deficient subjects were identified. The genetic characterization of the subjects showed the presence of 3 already-known mutations (c.752T>C, c.1705C>T and c.1566_1579del14) and 6 novel mutations: four missense mutations (c.995C>T, c.1112A>G, c.1715T>G and c.1927T>C), then a deletion of an adenine within exon 3 (c.325delA) and a mutation within the 3' splice site of intron 11 (c.2031-2A>C). The novel missense mutations cause the substitution of residues the p.A332V, p.D371G, p.L572W and p.W643R, respectively, and can cause potential structural changes. The c.325delA deletion causes a shift of the reading frame with the occurrence of a premature stop codon within the pro-peptide. An eukaryotic expression system was set up to investigate how the c.2031-2A>C mutation alters the MPO pre-mRNA splicing. The activation of a cryptic 3' splice site located 109nt upstream of the authentic 3' splice site was observed. The 109nt-insertion might cause the rapid degradation of the MPO mRNA or, alternatively, might lead to the generation of an abnormal MPO precursor lacking the enzymatic activity.

DNA-PK autophosphorylation facilitates Artemis endonuclease activity.

The Artemis nuclease is defective in radiosensitive severe combined immunodeficiency patients and is required for the repair of a subset of ionising radiation induced DNA double-strand breaks (DSBs) in an ATM and DNA-PK dependent process. Here, we show that Artemis phosphorylation by ATM and DNA-PK in vitro is primarily attributable to S503, S516 and S645 and demonstrate ATM dependent phosphorylation at serine 645 in vivo. However, analysis of multisite phosphorylation mutants of Artemis demonstrates that Artemis phosphorylation is dispensable for endonuclease activity in vitro and for DSB repair and V(D)J recombination in vivo. Importantly, DNA-dependent protein kinase catalytic subunit (DNA-PKcs) autophosphorylation at the T2609-T2647 cluster, in the presence of Ku and target DNA, is required for Artemis-mediated endonuclease activity. Moreover, autophosphorylated DNA-PKcs stably associates with Ku-bound DNA with large single-stranded overhangs until overhang cleavage by Artemis. We propose that autophosphorylation triggers conformational changes in DNA-PK that enhance Artemis cleavage at single-strand to double-strand DNA junctions. These findings demonstrate that DNA-PK autophosphorylation regulates Artemis access to DNA ends, providing insight into the mechanism of Artemis mediated DNA end processing.

Kainate-induced currents in rat cortical neurons in culture are modulated by riluzole.

The action of the neuroprotective and anticonvulsant agent riluzole on kainate-induced currents was studied in rat cortical neurons in primary culture by using the whole-cell configuration of the patch-clamp technique. Kainate elicited macroscopic, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive inward currents in all the patched cells and the amplitude of the current was concentration-dependent (EC50= 106 microM). Riluzole decreased the inward currents induced by 100 microM kainate at all holding potentials and the reduction was dose-dependent (IC50= 101 microM). The maximal response to kainate decreased in the presence of 50 microM riluzole, without changing its EC50, indicating a noncompetitive mechanism of inhibition. The amplitude of the responses induced by kainate under control conditions and during riluzole was a linear function of the membrane potential and the reversal potential of the currents was not significantly different in the two experimental conditions. Instead, the total conductance of the cell membrane for the currents induced by 100 microM kainate was significantly reduced in the presence of 50 microM riluzole (P < 0.05). The analysis of the kainate membrane current noise performed under control conditions and during perfusion of 100 microM riluzole revealed that riluzole reduced the probability of kainate-activated ionic channels to be in the open state. Conversely, the unitary conductance of channels, as well as their characteristic time constant, seemed to be unchanged. These results reveal an additional mechanism by which riluzole can interact with glutamatergic neurotransmission and provides further support for the idea that riluzole may prove beneficial in the treatment of central nervous system injuries involving the excitotoxic actions of glutamate.