Regulation of the expression of class II genes of the human major histocompatibility complex in tumor cells

Methylation State of Cellular Genes and Oncogenes as a Marker of Malignancy in Human Carcinomas

The methylation pattern of the human HLA-DRα gene was analyzed in primary tumors and lymph node metastases isolated from patients with a variety of tumors, including thyroid, pancreas, breast and gastric carcinomas and melanomas. In normal tissues (including breast, muscle, brain, sperm, T-and B-lymphocytes) the HLA-DRα gene is hypermethylated at CCGG and GCGC sites. In all tissues studied, the only constantly unmethylated region was located in the 5′ portion of the gene. Our results indicate that the HLA-DRα gene is hypomethylated in metastatic lymph nodes, as well as in the carcinomas and melanomas studied. These findings lend support to the hypothesis that DNA hypomethylation of the human HLA-DRα gene may represent a molecular marker of malignant tumors.

Exon 6 is essential for invariant chain trimerization and induction of large endosomal structures

Invariant chain (Ii) is a transmembrane type II protein that forms a complex with the major histocompatibility complex (MHC) class II molecules in the endoplasmic reticulum (ER). The membrane proximal luminal region of Ii is responsible for the non-covalent association with MHC class II molecules. Chemical cross-linking in COS cells was used to study the effect of luminal and cytoplasmic deletions on trimerization of Ii. We demonstrate that trimerization of Ii is independent of the cytosolic tail of Ii, whereas residues 162-191 (the sequence encoded by exon 6) in the luminal part of Ii are essential for trimer formation. Immunofluorescence studies of the transfected luminal deletion constructs show that the amino acids encoded by exon 6 of Ii are also essential for the induction of large endosomal vesicles. The data suggest that Ii must be in a trimeric form to modify the endosomal pathway.

Sequencing of an upstream region of the human HLA-DRA gene containing X' and Y' boxes

In this paper we report the characterization of a newly sequenced 5' upstream region of the human HLA-DRA gene. We performed (i) search for transcription factor motifs, (ii) analysis of CpG display and observed/expected frequency ratios, (iii) search for regions homologous to the 5' upstream sequences of the murine EA gene, (iv) DNase I footprinting experiments and (v) electrophoretic mobility shift assays. Our results demonstrate the existence, in the HLA-DRA gene, of Y' and X' boxes highly homologous to the Y and X boxes present in MHC class II genes, but oriented in the opposite direction. These Y' and X' boxes have been conserved during the molecular evolution of both human HLA-DRA and murine EA genes. DNase I footprinting and gel retardation experiments suggest that the X' and Y' boxes of the HLA-DRA upstream gene region are specifically recognized by nuclear proteins that also bind to the X and Y boxes of the HLA-DRA proximal promoter, respectively.

First-trimester human trophoblast is class II major histocompatibility complex mRNA+/antigen

Lack of expression of the polymorphic class I and class II MHC antigens in the cytotrophoblast is one of the major factors determining the privileged immunologic status of the placenta. In this report, we show that first-trimester human placental cytotrophoblast cells display moderate to strong expression of class II MHC (HLA-DR alpha and -DR beta) and Ii chain transcripts, apparently in absence of detectable class II antigens and Ii chain. In addition, DR alpha, DR beta, and Ii mRNAs, but not antigens, are consistently upregulated by IFN-gamma. Constitutive expression and upregulation of mRNAs are detectable in trophoblast cells kept in short term as well as prolonged (2-3 weeks) culture. These results are reminiscent of an analogous mRNA+/antigen- dissociation occurring, in the case of class I MHC gene products, in a subpopulation of first-trimester cytotrophoblast cells. Thus, analogous mechanisms prevent the expression of potentially hazardous class I and II allodeterminants at early stages of semiallogeneic pregnancy.

Methylation state of the human HLA-DRA gene in T-lymphocytes and B-lymphocytes of transgenic mice. Lack of methylation at one 5'-GCGC site is not required for gene expression

A consistent lack of DNA methylation at one or both of two GCGC (Hha I) restriction sites in the 5' region of the HLA-DRA gene has been previously documented by the use of methyl-sensitive restriction enzymes in human cells and tissues, irrespectively of their expression of DR alpha products. Evidence presently available, however, does not exclude that a lack of methylation in this region, although not sufficient, might be necessary for gene expression. In this report, we show that only one of the 5'-GCGC sites is protected, although less efficiently than in man, from CG-->mCG modifications in tissues and cells of transgenic mice carrying an expressed single copy of the HLA-DRA gene/diploid genome. We demonstrate that the two 5' GCGC sites of the HLA-DRA transgene are fully methylated in DR alpha- splenocytes (more than 80% T-lymphocytes), while one of them (the most 5' site) is not methylated in a fraction of DR alpha+ splenocytes (more than 95% B-lymphocytes). These results provide evidence that absence of DNA methylation in the 5' region is not necessary for, but might be associated with and possibly secondary to the expression of the DRA gene.

Tissue-specific expression of the HLA-DRA gene in transgenic mice

Transgenic mice were produced containing a 33 kilobase (kb) DNA fragment encompassing the five exons and all the known regulatory regions of the class II HLA-DRA gene. The transgene displayed regulated expression [constitutive and interferon-gamma (IFN)-gamma induced] of the human products in most mouse tissues. The tissue distribution of the DRA transgene products more closely resembled that of their mouse homologues, the endogenous H-2 Ea products, than the wider distribution of DRA products in humans. This was evident in several tissues (endothelia of small vessels, especially those of glomerular capillaries, Kupffer cells, and epithelial cells lining the gastrointestinal tract), known to differentially express class II molecules in the two species. Thus, the wider human specific pattern of expression requires an exact cis/trans complementation which is incompletely reconstituted in transgenic mice, suggesting that human-specific cis-acting elements may have arisen during evolution to direct the expression of class II genes to those anatomical regions which usually lack them in the mouse. The only example of aberrant expression of the DRA gene in the present series of transgenic mice was in the dendritic and/or epithelial cells of the thymic cortex, which displayed greatly reduced DR alpha levels in spite of a normal expression of the endogenous E alpha molecules.

Transgenic mice mimic the methylation pattern of the human HLA-DR alpha gene

The methylation pattern of the human HLA-DR alpha gene has been studied in different tissues of transgenic mice. Offspring from two transgenic lines was selected for this analysis, carrying the integrated HLA-DR alpha gene in either single or multiple (8-10) copies per diploid genome. In transgenic animals two distinct methylation patterns of the HLA-DR alpha gene are generated, due to a complete methylation of all the GCGC and CCGG sites the former, and to unmethylation restricted to one or both the GCGC sites located in the 5' portion of the HLA-DR alpha gene, the latter. Unmethylation restricted to the 5' portion of the HLA-DR alpha gene is a highly conserved feature in human tissues and in vitro cultured cell lines; therefore, it is concluded that the methylation pattern of the human HLA-DR alpha transgene may be faithfully reconstituted in transgenic animals. Northern blotting analysis of the RNA isolated from tissues of the transgenic mouse carrying single-copy HLA-DR alpha transgene demonstrates its tissue specific expression, suggesting that transgenic mice may represent an "in vivo" experimental system to study the relationship between methylation state and transcriptional activation.

Distamycin inhibits the binding of a nuclear factor to the -278/-256 upstream sequence of the human HLA-DR alpha gene

In this study we analyse the effects of the anti-tumor compound distamycin on the binding of nuclear factor(s) to a synthetic oligonucleotide (GTATA/IFN-gamma) mimicking a putative regulatory region of the human HLA-DR alpha gene. This region contains the sequence (GTATA), that is required for nuclear protein binding and is likely to interact with distamycin. The present results, by showing that distamycin inhibits the interaction between nuclear factors and the GTATA/IFN-gamma oligonucleotide, suggest that distamycin might alter the binding of transacting factors to cis-elements containing AT/TA sequences. Alterations of nuclear protein binding to specific target sequences could be one of the molecular mechanism(s) by which distamycin exerts its antiproliferative activity on living cells.

Human HLA-DR alpha gene: a rare oligonucleotide (GTATA) identifies an upstream sequence required for nuclear protein binding

Synthetic oligonucleotides containing putative regulatory sequences are currently employed to identify and isolate genes coding for nuclear binding factors. Upstream DNA sequences of eukaryotic genes required for transcriptional activity and tissue specificity can be identified by means of biochemical techniques as well as computer analysis using homology searching. An alternative approach has been recently proposed by our research group. Scanning DNA sequences 1.8 megabases in length from a Genetic Sequence Data Bank, we have identified rare oligonucleotides 5 base pairs (bp) long, which are localized within or close to regulatory segments in mammalian promoters. In this paper we demonstrate that the rare GTATA sequence identifies an upstream region of the HLA-DR alpha gene which operates in conjunction with the sequence AGAAGTCAG, homologous to a box found in many interferon-inducible genes, in binding nuclear proteins.

Methylation pattern of the HLA-DR alpha gene in human tissues

The CCGG and GCGC sites of the human HLA-DR alpha gene are hypermethylated in human tissues (including B-lymphocytes, T-lymphocytes, muscle, brain, sperm, skin, kidney, suprarenal and mammary glands) and three B-lymphoid cell lines. Therefore, the HLA-DR alpha gene can be transcribed even though extensively methylated. The only exception to the hypermethylated state of the HLA-DR alpha gene is represented by one or both of the two HhaI sites (H1 and H2) localized in the 5' portion of the gene. Analysis of the computer-generated secondary structure of the HLA-DR alpha mRNA suggests that the H1 and H2 sites belong to a region (5'-GAGCGCCCA-3'/5'-UGAGCGCUC-3') exhibiting extensive base pairing. Therefore, unmethylation of these CG sites can contribute in preventing mCG----TG/CA changes in this region, which would lead to extensive alterations of the secondary structure of the 5' portion of the HLA-DR alpha MRNA. On the other hand, the selective pressure to maintain unaltered the methylated CG dinucleotides in the coding regions of the HLA-DR alpha gene could be due to codon restrictions, since the majority of the methylation-related CG----TG or CG----CA variations would generate aminoacid changes. Accordingly, the analysis of different HLA-DR alpha genomic sequences indicates that variations of the CpG dinucleotides occur only in the non-coding portions of the HLA-DR alpha gene.

Hypomethylation of the human HLA-DR alpha gene in breast carcinomas and autologous metastases

The methylation pattern of the human HLA-DR alpha gene was analyzed in normal breast tissues, breast primary tumors and lymphonodal metastases isolated from patients carrying breast carcinomas. In breast adenomas and also in normal tissues (including breast, muscle, brain, sperm and T- and B-lymphocytes), the HLA-DR alpha gene is hypermethylated at the CCGG and GCGC sites. In all tissues studied, the only constantly unmethylated region is located in the 5' portion of the gene, near the promoter sequence. Further, the results indicate that the HLA-DR alpha gene is hypomethylated in carcinomas and in the relative metastatic lymph nodes. It is suggested that hypomethylation of the human HLA-DR alpha gene could be proposed as a molecular marker of malignant breast tumors.