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Biochemical Characterization of Aprataxin, the Protein Deficient in Ataxia with Oculomotor Apraxia Type 1

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Biochemical Characterization of Aprataxin, the Protein Deficient in Ataxia with Oculomotor Apraxia Type 1 BIOCHEMICAL CHARACTERIZATION OF APRATAXIN, THE PROTEIN DEFICIENT IN ATAXIA WITH OCULOMOTOR APRAXIA TYPE 1 This work was completed at the Queensland Institute of Medical Research by: JANELLE LOUISE HANCOCK B.App Sci (Biochem, Hons) and is submitted for the award of Doctor of Philosophy at the Queensland University of Technology September 2008 i ii STATEMENT OF ORIGINALITY I hereby declare that I am the sole author of this work and any content from other sources has been acknowledged and fully cited. The following material has not been submitted, either in part or whole, for a degree at this or any other institution. This thesis was prepared in accordance with the regulations outlined by the Queensland University of Technology, for the degree of Doctor of Philosophy. The research within this thesis was carried out under the principal supervision of Professor Martin Lavin and Dr Olivier Becherel. ……………………………………………….. Janelle Louise Hancock iii iv ABSTRACT Neurodegenerative disorders are heterogenous in nature and include a range of ataxias with oculomotor apraxia, which are characterised by a wide variety of neurological and ophthalmological features. This family includes recessive and dominant disorders. A subfamily of autosomal recessive cerebellar ataxias are characterised by defects in the cellular response to DNA damage. These include the well characterised disorders Ataxia-Telangiectasia (A-T) and Ataxia-Telangiectasia Like Disorder (A-TLD) as well as the recently identified diseases Spinocerebellar ataxia with axonal neuropathy Type 1 (SCAN1), Ataxia with Oculomotor Apraxia Type 2 (AOA2), as well as the subject of this thesis, Ataxia with Oculomotor Apraxia Type 1 (AOA1). AOA1 is caused by mutations in the APTX gene, which is located at chromosomal locus 9p13. This gene codes for the 342 amino acid protein Aprataxin. Mutations in APTX cause destabilization of Aprataxin, thus AOA1 is a result of Aprataxin deficiency. Aprataxin has three functional domains, an N-terminal Forkhead Associated (FHA) phosphoprotein interaction domain, a central Histidine Triad (HIT) nucleotide hydrolase domain and a C-terminal C2H2 zinc finger. Aprataxins FHA domain has homology to FHA domain of the DNA repair protein 5’ polynucleotide kinase 3’ phosphatase (PNKP). PNKP interacts with a range of DNA repair proteins via its FHA domain and plays a critical role in processing damaged DNA termini. The presence of this domain with a nucleotide hydrolase domain and a DNA binding motif implicated that Aprataxin may be involved in DNA repair and that AOA1 may be caused by a DNA repair deficit. This was substantiated by the interaction of Aprataxin with proteins involved in the repair of both single and double strand DNA breaks (X- Ray Cross-Complementing 1, XRCC4 and Poly-ADP Ribose Polymerase-1) and the hypersensitivity of AOA1 patient cell lines to single and double strand break inducing agents. At the commencement of this study little was known about the in vitro and in vivo properties of Aprataxin. Initially this study focused on generation of recombinant Aprataxin proteins to facilitate examination of the in vitro properties of Aprataxin. Using recombinant Aprataxin proteins I found that Aprataxin binds to double stranded DNA. Consistent with a role for Aprataxin as a DNA repair enzyme, this binding is v not sequence specific. I also report that the HIT domain of Aprataxin hydrolyses adenosine derivatives and interestingly found that this activity is competitively inhibited by DNA. This provided initial evidence that DNA binds to the HIT domain of Aprataxin. The interaction of DNA with the nucleotide hydrolase domain of Aprataxin provided initial evidence that Aprataxin may be a DNA-processing factor. Following these studies, Aprataxin was found to hydrolyse 5’adenylated DNA, which can be generated by unscheduled ligation at DNA breaks with non-standard termini. I found that cell extracts from AOA1 patients do not have DNA-adenylate hydrolase activity indicating that Aprataxin is the only DNA-adenylate hydrolase in mammalian cells. I further characterised this activity by examining the contribution of the zinc finger and FHA domains to DNA-adenylate hydrolysis by the HIT domain. I found that deletion of the zinc finger ablated the activity of the HIT domain against adenylated DNA, indicating that the zinc finger may be required for the formation of a stable enzyme-substrate complex. Deletion of the FHA domain stimulated DNA-adenylate hydrolysis, which indicated that the activity of the HIT domain may be regulated by the FHA domain. Given that the FHA domain is involved in protein-protein interactions I propose that the activity of Aprataxins HIT domain may be regulated by proteins which interact with its FHA domain. We examined this possibility by measuring the DNA-adenylate hydrolase activity of extracts from cells deficient for the Aprataxin-interacting DNA repair proteins XRCC1 and PARP-1. XRCC1 deficiency did not affect Aprataxin activity but I found that Aprataxin is destabilized in the absence of PARP-1, resulting in a deficiency of DNA-adenylate hydrolase activity in PARP-1 knockout cells. This implies a critical role for PARP-1 in the stabilization of Aprataxin. Conversely I found that PARP-1 is destabilized in the absence of Aprataxin. PARP-1 is a central player in a number of DNA repair mechanisms and this implies that not only do AOA1 cells lack Aprataxin, they may also have defects in PARP-1 dependant cellular functions. Based on this I identified a defect in a PARP-1 dependant DNA repair mechanism in AOA1 cells. vi Additionally, I identified elevated levels of oxidized DNA in AOA1 cells, which is indicative of a defect in Base Excision Repair (BER). I attribute this to the reduced level of the BER protein Apurinic Endonuclease 1 (APE1) I identified in Aprataxin deficient cells. This study has identified and characterised multiple DNA repair defects in AOA1 cells, indicating that Aprataxin deficiency has far-reaching cellular consequences. Consistent with the literature, I show that Aprataxin is a nuclear protein with nucleoplasmic and nucleolar distribution. Previous studies have shown that Aprataxin interacts with the nucleolar rRNA processing factor nucleolin and that AOA1 cells appear to have a mild defect in rRNA synthesis. Given the nucleolar localization of Aprataxin I examined the protein-protein interactions of Aprataxin and found that Aprataxin interacts with a number of rRNA transcription and processing factors. Based on this and the nucleolar localization of Aprataxin I proposed that Aprataxin may have an alternative role in the nucleolus. I therefore examined the transcriptional activity of Aprataxin deficient cells using nucleotide analogue incorporation. I found that AOA1 cells do not display a defect in basal levels of RNA synthesis, however they display defective transcriptional responses to DNA damage. In summary, this thesis demonstrates that Aprataxin is a DNA repair enzyme responsible for the repair of adenylated DNA termini and that it is required for stabilization of at least two other DNA repair proteins. Thus not only do AOA1 cells have no Aprataxin protein or activity, they have additional deficiencies in PolyADP Ribose Polymerase-1 and Apurinic Endonuclease 1 dependant DNA repair mechanisms. I additionally demonstrate DNA-damage inducible transcriptional defects in AOA1 cells, indicating that Aprataxin deficiency confers a broad range of cellular defects and highlighting the complexity of the cellular response to DNA damage and the multiple defects which result from Aprataxin deficiency. My detailed characterization of the cellular consequences of Aprataxin deficiency provides an important contribution to our understanding of interlinking DNA repair processes. vii KEYWORDS: Ataxia with Oculomotor Apraxia Type 1 (AOA1); Early onset ataxia with hypoalbuminemia (EAOH); Autosomal recessive cerebellar ataxia (ARCA); DNA repair; Aprataxin; Base Excision Repair (BER); single strand break repair (SSBR); APTX. viii LIST OF PUBLICATIONS Refereed Publications: Kijas, A. W., Harris, J. L., Harris, J. M., Lavin, M. F. (2006). Aprataxin forms a discrete branch in the HIT (histidine triad) superfamily of proteins with both DNA/RNA binding and nucleotide hydrolase activities. The Journal of Biological Chemistry. 281(20):13939-48. Poster Presentations: Queensland Institute of Medical Research Student Conference, 2005 Janelle L Harris, Amanda W Kijas, Martin F Lavin, The Histidine Triad domain of Aprataxin has novel DNA binding capability. East Coast Protein Meeting, 2005 Janelle L Harris, Amanda W Kijas, Martin F Lavin, The Histidine Triad domain of Aprataxin has novel DNA binding capability. Australian Society for Medical Research, Queensland Conference, 2007 Janelle L Harris, Amanda W Kijas, Martin F Lavin, The Histidine Triad domain of Aprataxin has novel DNA binding capability. Ataxia Telangiectasia Workshop, 2008. Janelle L Harris, Olivier Becherel, Martin F Lavin, Biochemical Characterization of the DNA repair protein Aprataxin. Australian Society for Medical Research, Queensland Conference, 2008. Janelle L Harris, Olivier Becherel, Martin F Lavin, Aprataxin has a unique role in DNA repair. ix x CONFERENCES AND INVITED SEMINARS East Coast Protein Meeting, 2007 Janelle L Harris, Martin F Lavin, Aprataxin has a Unique Role in DNA repair. Queensland Institute for Medical Research Student Conference, 2007 Janelle L Harris, Olivier Becherel, Martin Lavin. Queensland Institute for Medical Research Student Seminar, 2008
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