Genetic Diversity Of HLA Polymorphism And New Genes.

ABSTRACT

DNA level studies, based on SSOP hybridization of HLA class II PCR products have revealed considerable diversity in HLA among Asian Indians. High resolution typing of specific alleles such as DR2 and DR4 in the HLA class II region and their DR-DQ haplotypes have helped to detect unique haplotypes and novel alleles which have subsequently been confirmed by sequencing. Incidentally, remarkable stability is maintained in several other DRBI alleles, viz DR1, DR7, DR9 and DR10. Further characterization of new alleles will be carried out by sequencing of mRNA. The ARMS-PCR technology has been found to be particularly useful for typing difficult HLA-A, HLA-B and all HLA-Cw alleles. These technologies are remarkably superior over serological methods. Our studies have shown appreciable heterogeneity of common HLA-A and B alleles in Asian Indians. Molecular subtypes of HLA-A2 revealed that subtype A*0211 is found only in the Indian population and may be the result of a selection pressure in this population. Investigations into polymorphism in the HLA-B27 gene revealed that subtypes common both to the western Caucasoids and orientals occur in the Indian population. It is apparent that the population of the Indian subcontinent, placed as it is between the Caucasoids and Negroids on one hand and Australoids and Mongoloids on the other, provides a rich source of many HLA haplotypes. While the most frequent Caucasian haplotypes occurred with a reasonable frequency in Asian Indians, those found predominantly in other ethnic groups (e.g., Australian Aborigines and populations of Oceania, China and Japan) are also detected. Knowledge on this is most important for donor selection during organ and bone marrow transplantation and for designing MHC targeted vaccines in specific diseases. The major histocompatibility complex (MHC) encompasses two major classes of molecules the MHC classes I and class II, both of which appear to have originated from a common ancestor gene. The major biological function of the MHC is to bind peptide fragments derived from protein antigens (viruses, peptides etc) and display them on the surface of antigen presenting cells (APCs), evoking effector responses upon recognition by the antigen specific receptors on T lymphocytes. This process requires an efficient intracellular machinery to fragment the protein antigens into smaller peptides capable of binding to a host MHC molecule. Since the number of peptides that can theoretically be generated is very large, there is need for an extensive MHC gene pool. Thus the HLA system which is the MHC of man is extremely polymorphic. Although the general rules for peptide-MHC interactions for both classes of MHC molecules are essentially similar; there are two fundamental differences i) MHC class I ligands originate from endogenous sources, mainly from proteins of the cytosol or the nucleus and are delivered by the 'endogenous processing pathway In contrast, the MHC class II ligands are generated by the degradation of proteins from the extracellular compartment. These distinctions are also reflected in the responding T cells CD8 positive T cells being restricted by class I-peptide complexes, and CD4 positive T cells by class II-peptide complexes. ii) In accordance with the distribution of hydrogen bonds in the peptide binding groove of the MHC, the anchor residues are placed at the terminal ends of the class I groove. Contrarily, the binding forces are distributed throughout the class II groove and ensure bonds between the peptide's backbone and class II molecule (Madden et al, 1993; Stern et al, 1994; Stern and Wiley 1994). The specificity of the interaction is determined by pockets in the MHC groove that have a fixed spacing from each other and which also have specificity for anchoring particular side chains of the peptide's amino acids.