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DNA Methylation Dysregulates and Silences the HLA-DQ Locus by Altering Chromatin Architecture

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DNA Methylation Dysregulates and Silences the HLA-DQ Locus by Altering Chromatin Architecture Genes and Immunity (2011) 12, 291–299 & 2011 Macmillan Publishers Limited All rights reserved 1466-4879/11 www.nature.com/gene ORIGINAL ARTICLE DNA methylation dysregulates and silences the HLA-DQ locus by altering chromatin architecture P Majumder and JM Boss Department of Microbiology and Immunology, Emory University School Of Medicine, Atlanta, GA, USA The major histocompatibility complex class II (MHC-II) locus encodes a cluster of highly polymorphic genes HLA-DR, -DQ and -DP that are co-expressed in mature B lymphocytes. Two cell lines were established over 30 years ago from a patient diagnosed with acute lymphocytic leukemia. Laz221 represented the leukemic cells of the patient; whereas Laz388 represented the normal B cells of the patient. Although Laz388 expressed both HLA-DR and HLA-DQ surface and gene products, Laz221 expressed only HLA-DR genes. The discordant expression of HLA-DR and HLA-DQ genes was due to epigenetic silencing of the HLA-DQ region CCCTC transcription factor (CTCF)-binding insulators that separate the MHC-II sub-regions by DNA methylation. These epigenetic modifications resulted in the loss of binding of the insulator protein CTCF to the HLA-DQ flanking insulator regions and the MHC-II-specific transcription factors to the HLA-DQ promoter regions. These events led to the inability of the HLA-DQ promoter regions to interact with flanking insulators that control HLA-DQ expression. Inhibition of DNA methylation by treatment with 50-deoxyazacytidine reversed each of these changes and restored expression of the HLA-DQ locus. These results highlight the consequence of disrupting an insulator within the MHC-II region and may be a normal developmental mechanism or one used by tumor cells to escape immune surveillance. Genes and Immunity (2011) 12, 291–299; doi:10.1038/gene.2010.77; published online 17 February 2011 Keywords: MHC; gene regulation; transcription; DNA methylation; chromatin Introduction The class II transactivator (CIITA), a non-DNA-binding transcription factor, is recruited to the DNA bound X and Y The major histocompatibility complex class II (MHC-II) box transcription factors and mediates the interaction genes encode cell surface a/b heterodimeric glycopro- between cofactors, chromatin remodeling factors and teins that present foreign antigens to antigen-specific general transcription machinery.5,7,8 CIITA expression is CD4 þ T lymphocytes in order to initiate, control and/or limiting and is highly regulated.9–11 In addition to the maintain adaptive immune responses.1 There are three promoter proximal elements, distal elements contribute to MHC-II isotypes HLA-DR, -DQ and -DP encoded within the expression of the genes in this locus.12–14 The MHC-II the MHC-II locus. This process is aided by two other locus is punctuated by a series of CCCTC transcription MHC-II-associated molecules, HLA-DM and -DO, which factor (CTCF)-binding sites.15 CTCF binding to its target are also a/b heterodimers with sequence and structural DNA sequences is methylation sensitive,16,17 and CTCF has homology to MHC-II proteins.2,3 Although the levels of been shown to demarcate and insulate regions of regula- mRNA and protein from each of the above genes differ, tory activity within the genome by functioning as an all are typically coexpressed in a tissue-specific manner. enhancer blocker.18 or by preventing the spread of MHC-II genes are expressed constitutively in B-lympho- heterochromatin into active genes.19–21 CTCF has been cytes, macrophages, dendritic cells and thymic epithelia, proposed to function in part by forming the nexus of long- and can also be induced in most other cell types range chromatin loops. Recently, CTCF was found to be following exposure to interferon-g (reviewed in Ting required for the expression of all MHC-II genes.15,22 The and Trowsdale4). binding of CTCF to the insulator site between HLA-DR and The transcription of MHC-II genes is coordinated by a HLA-DQ,termedXL9, results in the formation of long- conserved group of transcriptional factors, regulatory range chromatin loops between XL9 and the promoter factor X (RFX), cyclic adenosine monophosphate res- proximal regulatory X–Y box regions of the flanking genes, ponse element-binding protein and nuclear factor-Y, HLA-DRB1 and HLA-DQA1. This interaction was coin- which bind to highly conserved, promoter–proximal cident with gene expression and dependent on CTCF and sequences termed the X1, X2 and Y boxes, respectively.4–6 CIITA binding to their respective sites.22 The depletion of CTCF by RNA interference, resulted in the loss of HLA- DRB1 and HLA-DQA1 gene expression. These studies Correspondence: Dr JM Boss, Department of Microbiology and predict that disruption of CTCF binding to one of the Immunology, Emory University School Of Medicine, 1510 Clifton MHC-II insulator sites may have a significant consequence Road, Atlanta, GA 30322, USA. E-mail: [email protected] on MHC-II gene expression. 23 Received 14 July 2010; revised 30 August 2010; accepted 31 August In 1978, Lazarus et al. established the Laz221 cell 2010; published online 17 February 2011 line from a patient with acute lymphoblastic leukemia. DNA methylation silences HLA-DQ expression P Majumder and JM Boss 292 Laz221 cells were similar in phenotype to the acute Laz388 Laz221 lymphocytic leukemia cells of the patient and were consi- 300 dered to represent null cells at the time. Later Laz221 200 cells were determined to be most similar to a pre-B Isotype/Control lymphocyte and were thought to arise from these 100 cells.24,25 The cell line Laz388 was established from the same patient by transforming with Epstein–Barr 0 virus23,26 and represented a normal peripheral B cell. 150 One curious difference between the cells was that, 100 although Laz388 expressed both HLA-DR and HLA- HLA-DQ DQ proteins, Laz221 only expressed the HLA-DR 50 proteins.24,25 Analysis of the cells showed that the HLA-DR genes were expressed in Laz221 but neither 0 24 HLA-DQA or HLA-DQB was expressed. Examination 200 of the cis-regulatory elements by transfection of DNA 150 reporter constructs into each of the cell lines and the use HLA-DR 100 of electrophoretic mobility shift assays suggested that the # Cells HLA-DQB1 gene was regulated differently and that the 50 24,27 0 Laz221 cell line was lacking a critical component. 0 1 2 3 4 1 2 3 4 Because Laz221 expressed HLA-DR molecules at normal 10 10 10 10 10 10 10 10 10 levels, this meant that each of the MHC-II-specific FL2-H transcription factors RFX5, RFX-B, RFXAP and CIITA 1.2 were present and fully active and that some other aspect Laz388 Laz221 of the system was defective. On discovery of XL9 and its 1 role in HLA-DRB1 and HLA-DQA1 transcription, we hypothesized that the differential expression of the 0.8 HLA-DQ locus in Laz221 might involve the use of XL9. 0.6 We found that the region between and including the two insulators XL9 and C2 was hypermethylated in Laz221 0.4 but not Laz388 cells. DNA methylation resulted in the expression mRNA 0.2 loss of CTCF binding and long distance interactions 0 between the XL9 and MHC-II gene promoters. Critically, DRB1 DQA1 DQB1 treatment with 50-deoxyazacytadine (5-azaC) reverted each of the molecular defects in Laz221 cells, resulting in Figure 1 Laz221 cells express HLA-DR but not HLA-DQ gene the reexpression of the HLA-DQ genes. Thus, these data products. (a) Surface MHC-II expression patterns were assessed using flow cytometry with isotype control, anti-HLA-DQ and anti- point to DNA methylation of an MHC-II sub-region and HLA-DR antibodies. This figure represents one of two independent insulator as a potential mechanism by which cells may experiments. (b) The mRNA levels for HLA-DRB1, -DQA1 and attempt to avoid detection by the immune system -DQB1 genes were determined by the real-time RT-PCR using through the loss of an MHC-II isotype. RNAs prepared from the Laz388 and Laz221 cells. These data represent an average derived from three biological replicates plotted with respect to the expression in Laz388. Standard error is shown for Laz221 cells. Results CTCF does not bind to the MHC insulators that surround the surface and that HLA-DRB1 mRNA is present in Laz221 HLA-DQ sub-region in Laz221 cells cells argues that the RFX subunits, CIITA, and other Previous analyses demonstrated differential expression necessary transcription factors for MHC-II transcription of HLA-DR and HLA-DQ surface protein levels between are fully functional in Laz221 cells. the Laz221 and Laz388 cell lines.24,25 This data is Our recent discovery that HLA-DQA1 used the XL9 recapitulated in Figure 1a, which shows two distinct insulator as a distal regulator of HLA-DQA1 expression,22 expression profiles for these cells. Previous work also led us to examine if XL9 was mutated in Laz221 cells. showed that HLA-DQB mRNA was not expressed in However, DNA sequencing of genomic clones isolated Laz221.24 Confirmation of that result showed that from Laz221 and Laz388 showed identical sequences compared with Laz388, Laz221 cells had extremely low (data not shown). We also tested whether CTCF, the levels of HLA-DQB1 mRNA. Additionally in Laz221 insulator factor at XL9 that is responsible for its MHC-II cells, HLA-DQA1 mRNA levels were very low, suggest- regulatory activity, was normally expressed Laz221 cells. ing that the cause of expression may be related to the Western blotting of both Laz388 and Laz221 revealed that HLA-DQ locus. Similar levels of HLA-DRB1 transcripts the overall levels of CTCF are similar with Laz221 were observed between the cell lines (Figure 1b).
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