Excess admixture proportion of extended major histocompatability complex haplotypes of Caucasian origin among rheumatoid arthritis associated haplotypes in African Americans and Afro-Caribbeans

HLA polymorphisms in African Americans with idiopathic inflammatory myopathy: allelic profiles distinguish patients with different clinical phenotypes and myositis autoantibodies

OBJECTIVE

To investigate possible associations of HLA polymorphisms with idiopathic inflammatory myopathy (IIM) in African Americans, and to compare this with HLA associations in European American IIM patients with IIM.

METHODS

Molecular genetic analyses of HLA-A, B, Cw, DRB1, and DQA1 polymorphisms were performed in a large population of African American patients with IIM (n = 262) in whom the major clinical and autoantibody subgroups were represented. These data were compared with similar information previously obtained from European American patients with IIM (n = 571).

RESULTS

In contrast to European American patients with IIM, African American patients with IIM, in particular those with polymyositis, had no strong disease associations with HLA alleles of the 8.1 ancestral haplotype; however, African Americans with dermatomyositis or with anti-Jo-1 autoantibodies shared the risk factor HLA-DRB1*0301 with European Americans. We detected novel HLA risk factors in African American patients with myositis overlap (DRB1*08) and in African American patients producing anti-signal recognition particle (DQA1*0102) and anti-Mi-2 autoantibodies (DRB1*0302). DRB1*0302 and the European American-, anti-Mi-2-associated risk factor DRB1*0701 were found to share a 4-amino-acid sequence motif, which was predicted by comparative homology analyses to have identical 3-dimensional orientations within the peptide-binding groove.

CONCLUSION

These data demonstrate that North American IIM patients from different ethnic groups have both shared and distinct immunogenetic susceptibility factors, depending on the clinical phenotype. These findings, obtained from the largest cohort of North American minority patients with IIM studied to date, add additional support to the hypothesis that the myositis syndromes comprise multiple, distinct disease entities, perhaps arising from divergent pathogenic mechanisms and/or different gene-environment interactions.

Heterogeneity of HLA-DRB1*04 and its associated haplotypes in the North Indian population

HLA-DR4 has been implicated in several diseases including rheumatoid arthritis (RA) and type I diabetes, the strength of associations being ethnically variable. Unusually high level of heterogeneity in DR4-DQB1 haplotypes has been reported in the Indian population. The present study is an attempt to determine the genetic diversity of the HLA-DR4 allelic family and its associated DQA1-DQBI haplotypic combinations in the healthy North Indian population. Using PCR-SSP and PCR-SSOP techniques, nine subtypes of DR4 were encountered of which DRB1*0403 was the most predominant allele (34.8%) followed by *0404 (27%), *0401 (14.6%), and *0405 (11%). No examples of *0402, *0409, *0411, *0413-*0417, and *0419-23 were encountered, although a few other subtypes, *0410 (three examples), *0406 and *0418 (two examples each), and *0407, *0408, and *0412 (single example each) occurred infrequently in a cohort of 85 HLA-DR4 positive samples studied. Most of these subtypes occurred in combination with DQA1*03-DQB1*0302 (69.5%). DRB1*0403 and *0404 exhibited maximum heterogeneity of DQB1 combinations. Haplotype data revealed the presence of 15 different DR4-DQ haplotypes, four of which were found to be "unique" to Asian Indians, not reported in any other population. These results help to explain the observed variability in DR4 associations in autoimmune diseases in Asian Indians and provide support for scientific and historical documentation of extensive admixture in the Indian subcontinent.

Update on the epidemiology of systemic lupus erythematosus: new spins on old ideas

Systemic lupus erythematosus is a serious autoimmune disease of obscure etiology with a propensity for widespread organ involvement. Its manifestations range from ephemeral symptoms to life-threatening events such as thromboses and strokes. Although once considered rare, SLE now appears to be relatively common in certain subsets of the population. Considerable publicity for this disorder as a "silent killer" has heightened public awareness and may have contributed to the increases in the number of individuals reporting this diagnosis in surveys. Epidemiologic studies face numerous challenges in disease classification and case ascertainment. Despite this, such studies have increased in size and number, and have generated novel etiologic hypotheses, ranging from environmental pollutants to viral infections.

Risk for rheumatic disease in relation to ethnicity and admixture

Risk of systemic lupus erythematosus (SLE) is high in west Africans compared with Europeans, and risk of rheumatoid arthritis (RA) is high in Native Americans compared with Europeans. These differences are not accounted for by differences in allele or haplotype frequencies in the human leucocyte antigen (HLA) region or any other loci known to influence risk of rheumatic disease. Where there has been admixture between two or more ethnic groups that differ in risk of disease, studies of the relationship of disease risk to proportionate admixture can help to distinguish between genetic and environmental explanations for ethnic differences in disease risk and to map the genes underlying these differences.

HLA-Cw alleles associated with HLA extended haplotypes and C2 deficiency

There are four MHC-linked complement genes, BF, C2, C4A and C4B, that are inherited as single DNA units, known as complotypes. Extended haplotypes were initially defined by studying the distribution of complotypes in relation to HLA-B and HLA-DR loci in Caucasian families. In order to analyze the distribution of HLA-Cw alleles in relation to extended haplotypes, we studied a large panel of MHC homozygous and heterozygous cell lines representing previously described Caucasian-derived extended haplotypes and 14 patients with complete C2 deficiency. HLA alleles were assigned using sequence-specific oligonucleotide probe hybridization (SSOP). Family analysis served to assign haplotypes for heterozygous samples. We found distinctive HLA-Cw alleles for each independent extended haplotype. Their association in each instance was statistically significant. All patients with C2 deficiency carrying the haplotype [HLA-B18, S042, DR2] were associated with HLA-Cw*1203. These conserved allelic combinations may become an important tool for the study of human evolution and may contribute to the expeditious selection of prospective donors in clinical transplantation.