DNA methylation constitutes the most stable type of epigenetic modifications modulating the transcriptional plasticity of mammalian genomes. Using bisulfite DNA sequencing, we report high-resolution methylation reference profiles of human chromosomes 6, 20 and 22, providing a resource of about 1.9 million CpG methylation values derived from 12 different tissues. Analysis of 6 annotation categories, revealed evolutionary conserved regions to be the predominant sites for differential DNA methylation and a core region surrounding the transcriptional start site as informative surrogate for promoter methylation. We find 17% of the 873 analyzed genes differentially methylated in their 5′-untranslated regions (5′-UTR) and about one third of the differentially methylated 5′-UTRs to be inversely correlated with transcription. While our study was controlled for factors reported to affect DNA methylation such as sex and age, we did not find any significant attributable effects. Our data suggest DNA methylation to be ontogenetically more stable than previously thought.

The major histocompatibility complex (MHC) is the most important region in the vertebrate genome with respect to infection and autoimmunity, and is crucial in adaptive and innate immunity. Decades of biomedical research have revealed many MHC genes that are duplicated, polymorphic and associated with more diseases than any other region of the human genome. The recent completion of several large-scale studies offers the opportunity to assimilate the latest data into an integrated gene map of the extended human MHC. Here, we present this map and review its content in relation to paralogy, polymorphism, immune function and disease.

The human major histocompatibility complex (MHC) is contained within about 4 Mb on the short arm of chromosome 6 and is recognised as the most variable region in the human genome. The primary aim of the MHC Haplotype Project was to provide a comprehensively annotated reference sequence of a single, human leukocyte antigen-homozygous MHC haplotype and to use it as a basis against which variations could be assessed from seven other similarly homozygous cell lines, representative of the most common MHC haplotypes in the European population. Comparison of the haplotype sequences, including four haplotypes not previously analysed, resulted in the identification of >44,000 variations, both substitutions and indels (insertions and deletions), which have been submit- Immunogenetics (2008) ted to the dbSNP database. The gene annotation uncovered haplotype-specific differences and confirmed the presence of more than 300 loci, including over 160 protein-coding genes. Combined analysis of the variation and annotation datasets revealed 122 gene loci with coding substitutions of which 97 were non-synonymous. The haplotype (A3-B7-DR15; PGF cell line) designated as the new MHC reference sequence, has been incorporated into the human genome assembly (NCBI35 and subsequent builds), and constitutes the largest single-haplotype sequence of the human genome to date. The extensive variation and annotation data derived from the analysis of seven further haplotypes have been made publicly available and provide a framework and resource for future association studies of all MHC-associated diseases and transplant medicine.

The future systematic mapping of variants that confer susceptibility to common diseases requires the construction of a fully informative polymorphism map. Ideally, every base pair of the genome would be sequenced in many individuals. Here, we report 4.75 Mb of contiguous sequence for each of two common haplotypes of the major histocompatibility complex (MHC), to which susceptibility to >100 diseases has been mapped. The autoimmune disease-associated-haplotypes HLA-A3-B7-Cw7-DR15 and HLA-A1-B8-Cw7-DR3 were sequenced in their entirety through a bacterial artificial chromosome (BAC) cloning strategy using the consanguineous cell lines PGF and COX, respectively. The two sequences were annotated to encompass all described splice variants of expressed genes. We defined the complete variation content of the two haplotypes, revealing >18,000 variations between them. Average SNP densities ranged from less than one SNP per kilobase to >60. Acquisition of complete and accurate sequence data over polymorphic regions such as the MHC from large-insert cloned DNA provides a definitive resource for the construction of informative genetic maps, and avoids the limitation of chromosome regions that are refractory to PCR amplification.

Here we report the first complete sequence and gene map of a human major histocompatibility complex (MHC), a region on chromosome 6 which is essential to the immune system. When it was discovered over 50 years ago the region was thought to specify histocompatibility genes, but their nature has been resolved only in the last two decades. Although many of the 224 identified gene loci (128 predicted to be expressed) are still of unknown function, we estimate that about 40% of the expressed genes have immune system function. Over 50% of the MHC has been sequenced twice, in different haplotypes, giving insight into the extraordinary polymorphism and evolution of this region. Several genes, particularly of the MHC class II and III regions, can be traced by sequence similarity and synteny to over 700 million years ago, clearly predating the emergence of the adaptive immune system some 400 million years ago. The sequence is expected to be invaluable for the identification of many common disease loci. In the past, the search for these loci has been hampered by the complexity of high gene density and linkage disequilibrium.