Abstract
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 5'-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.