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Title: The role of ATM in the regulation of interferon-gamma responses
Author: Tomas dos Santos, Cristina Isabel
ISNI:       0000 0004 2728 914X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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Interferon (IFN)-γ is a key cytokine in the modulation of immune responses necessary for elimination of viral infections and some cancer cells. IFN-γ signals through the Janus kinase (JAK)/ signal transducers and activators of transcription (STAT) signalling pathways. Ataxia telangiectasia mutated (ATM) was identified by our group as a regulator of IFN-γ responses. Within DNA double-strand break (DSB) repair, ATM controls chromatin remodelling, which is also indispensable for transcription. We hypothesised, therefore, that ATM modulated IFN-γ responses at the transcriptional level by regulating STAT1 chromatin binding and/or chromatin-modifying proteins. ATM silencing impaired to varying degrees IFN-γ-induced expression of several IFN-stimulated genes (ISGs) at the mRNA and protein levels and delayed STAT1 serine 727 phosphorylation within the nucleus. These effects were independent of ATM enzymatic activity. ATM knockdown impaired IFN-γ- induced binding of STAT1 to class II transactivator (CIITA) and human leucocyte antigen (HLA)-DRα promoters, but not to IFN regulatory factor (IRF)-1 and antigen peptide transporter (TAP) 1 promoters. This could account for HLA Class II expression being more severely impaired than that of HLA Class I, even though both are classical IFN-γ target genes. ATM, however, bound to the promoters of all these ISGs. ATM and STAT1 interacted under basal conditions, with STAT1 becoming tyrosine 701 phosphorylated upon IFN-γ treatment. The siATM phenotype regarding HLA class II expression was partially or completely recapitulated by silencing of histone deacetylase (HDAC) 1 and 3, respectively, in contrast to phenotypes induced by RNAi against HDAC2 and Kruppel-associated box (KRAB) domain-associated protein 1 (KAP1). KAP1 also acted, directly or indirectly, as a negative regulator of IFN-γ-induced STAT1 phosphorylation and, consequently, of HLA class I and II HLA expression. Interestingly, KAP1 interaction with STAT1 decreased after IFN-γ treatment. Overall our data indicates that ATM plays an important role as a modulator of IFN-γ-mediated transcription probably by interacting with STAT1, regulating its binding to the promoters of a specific subset of ISGs and regulating chromatin-modifying proteins such as KAP1, HDAC1, HDAC2 and HDCA3. Understanding how ATM regulates IFN-γ signalling may provide new insights into important immune-mediated mechanisms, such as those involved in tumour suppression and thus have therapeutic value.
Supervisor: Seckl, Michael ; Costa-Pereira, Ana Sponsor: Fundacao para a Ciencia e a Tecnologia
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral