Aprataxin, a novel protein that protects against genotoxic stress

Ataxia-oculomotor apraxia (AOA1) is a neurological disorder with symptoms that overlap those of ataxia-telangiectasia, a syndrome characterized by abnormal responses to double-strand DNA breaks and genome instability. The gene mutated in AOA1, APTX, is predicted to code for a protein called aprataxin that contains domains of homology with proteins involved in DNA damage signalling and repair. We demonstrate that aprataxin is a nuclear protein, present in both the nucleoplasm and the nucleolus. Mutations in the APTX gene destabilize the aprataxin protein, and fusion constructs of enhanced green fluorescent protein and aprataxin, representing deletions of putative functional domains, generate highly unstable products. Cells from AOA1 patients are characterized by enhanced sensitivity to agents that cause single-strand breaks in DNA but there is no evidence for a gross defect in single-strand break repair. Sensitivity to hydrogen peroxide and the resulting genome instability are corrected by transfection with full-length aprataxin cDNA. We also demonstrate that aprataxin interacts with the repair proteins XRCC1, PARP-1 and p53 and that it co-localizes with XRCC1 along charged particle tracks on chromatin. These results demonstrate that aprataxin influences the cellular response to genotoxic stress very likely by its capacity to interact with a number of proteins involved in DNA repair.

Neural progenitor number is regulated by nuclear factor-kappa B p65 and p50 subunit-dependent proliferation rather than cell survival

The number of cells generated by a proliferating stem or precursor cell can be influenced both by proliferation and by the degree of cell death/survival of the progeny generated. In this study, the extent to which cell survival controls progenitor number was examined by comparing the growth characteristics of neurosphere cultures derived from mice lacking genes for the death inducing Bcl-2 homologue Hara Kiri (Hrk), apoptosis-associated protein 1 (Apaf1), or the prosurvival nuclear factor-kappa B (NF kappa B) subunits p65, p50, or c-rel. We found no evidence that Hrk or Apaf1, and by inference the mitochondrial cell death pathway, are involved in regulating the number of neurosphere-derived progeny. However, we identified the p65p50 NF kappa B dimer as being required for the normal growth and expansion of neurosphere cultures. Genetic loss of both p65 and p50 NF kappa B subunits resulted in a reduced number of progeny but an increased proportion of neurons. No effect on cell survival was observed. This suggests that the number and fate of neural progenitor cells are more strongly regulated by cell cycle control than survival. (c) 2005 Wiley-Liss, Inc.

The Mre11 complex and ATM: a two-way functional interaction in recognising and signaling DNA double strand breaks

Mutations in components of the Mre 11/Rad50/Nbs1 complex give rise to genetic disorders characterized by neurological abnormalities, radiosensitivity, cell cycle checkpoint defects, genomic instability and cancer predisposition. Evidence exists that this complex associates with chromatin during DNA replication and acts as a sensor of double strand breaks (dsbs) in DNA after exposure to radiation. A series of recent reports provides additional support that the complex senses breaks in DNA and relays this information to ATM, mutated in ataxia-telangiectasia (A-T), which in turn activates pathways for cell cycle checkpoint activation. Paradoxically members of the Mre11 complex are also downstream of ATM in these pathways. Here, Lavin attempts to make sense of this sensing mechanism with reference to a series of recent reports on the topic. (C) 2004 Elsevier B.V. All rights reserved.

ATM signaling and genornic stability in response to DNA damage

DNA double strand breaks represent the most threatening lesion to the integrity of the genome in cells exposed to ionizing radiation and radiomimetic chemicals. Those breaks are recognized, signaled to cell cycle checkpoints and repaired by protein complexes. The product of the gene (ATM) mutated in the human genetic disorder ataxia-telangietasia (A-T) plays a central role in the recognition and signaling of DNA damage. ATM is one of an ever growing number of proteins which when mutated compromise the stability of the genome and predispose to tumour development. for recognising double strand breaks in DNA, maintaining genome stability and minimizing risk of cancer are discussed. (C) 2004 Published by Elsevier B.V.