C4B gene polymorphisms among African and African-American HLA-Bw42-DRw18 haplotypes

The Path to Conserved Extended Haplotypes: Megabase-Length Haplotypes at High Population Frequency

This minireview describes the history of the conceptual development of conserved extended haplotypes (CEHs): megabase-length haplotypes that exist at high (≥0.5%) population frequency. My career began in internal medicine, shifted to pediatrics, and clinical practice changed to research. My research interest was initially in hematology: on plasma proteins, their metabolism, synthesis, and function. This narrowed to a focus on proteins of the human complement system, their role in immunity and their genetics, beginning with polymorphism and deficiency of C3. My group identified genetic polymorphisms and/or inherited deficiencies of C2, C4, C6, and C8. After defining glycine-rich beta glycoprotein as factor B (Bf) in the properdin system, we found that the genes for Bf (CFB), C2, C4A, and C4B were inherited as a single haplotypic unit which we named the "complotype." Complotypes are located within the major histocompatibility complex (MHC) between HLA-B and HLA-DRB1 and are designated (in arbitrary order) by their CFB, C2, C4A, and C4B types. Pedigree analysis revealed long stretches (several megabases) of apparently fixed DNA within the MHC that we referred to as "extended haplotypes" (later as "CEHs"). About 10 to 12 common CEHs constitute at least 25 - 30% of MHC haplotypes among European Caucasian populations. These CEHs contain virtually all the most common markers of MHC-associated diseases. In the case of type 1 diabetes, we have proposed a purely genetic and epigenetic model (with a small number of Mendelian recessive disease genes) that explains all the puzzling features of the disease, including its rising incidence.

HLA-Cw*1701 is associated with two sub-Saharan African-derived HLA haplotypes: HLA-B*4201, DRB1*03 and HLA-B*4202 without DRB1*03

Different extended haplotypes have been described for many ethnic groups, such as African-Americans. The complotype FC(1,90)0 is in linkage disequilibrium with HLA-B42, DRB1*0302 in African-Americans and Southern African Xhosa individuals, suggesting a common ancestry. In order to analyze the distribution of Cw*17 alleles (Cw*1701, 1702) in relation to this African-derived extended haplotype, we studied a large panel of samples from African-American individuals and additionally a group of selected samples carrying HLA-B42, DR3 and HLA-B42, non-DR3 antigens. HLA alleles were assigned using sequence-specific amplification (SSP) and sequence-specific oligonucleotide probe hybridization (SSOP). We have found that all haplotypes (10 in total) carrying the extended haplotypes [HLA-B42, FC(1,90)0, DRB1*0302] were positive for HLA-Cw*1701. Interestingly, HLA B*4201 was found in all samples (17 in total) carrying HLA-B42, DR3, Cw*1701, whereas HLA-B*4202 was found in 10 out of 13 samples from individuals carrying HLA B42, Cw*1701 non-DR3. These findings suggest that HLA-Cw*17 polymorphism is conserved in different ethnic populations and that HLA-B42 alleles seem to separate at least different African-derived haplotypes. The historical context of these findings are important for the study of human evolution and they may be useful for the development of strategies in the search for possible donors in organ transplantation for African-derived populations.

Soluble HLA-I in rheumatic diseases

OBJECTIVE

To study serum levels of Class I soluble HLA (sHLA-I) in patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), polymyositis or dermatomyositis (PM/DM) or scleroderma and to assess the possible influence of ethnic factors on concentration in each disease group.

METHODS

Solid-phase enzyme linked immunoassay was used to measure sHLA-I in the serum of 385 patients with varied ethnic backgrounds (American-Caucasians, African-Americans, Georgian-Caucasians) with rheumatic diseases. Studies on patients were compared to similar measurements of 189 healthy individuals.

RESULTS

Mean sHLA-I levels were significantly higher in patients with SLE than those observed in healthy individuals or other rheumatic diseases. Highest concentrations were present in Georgian-Caucasian patients with SLE. American-Caucasian patients with RA or scleroderma had higher sHLA-I levels than normal Caucasian individuals. The majority of patients with PM/DM in all ethnic subgroups were low secretors of sHLA-I.

CONCLUSION

Mechanisms underlying the secretion of sHLA-I appear to differ among the rheumatic diseases studied and various ethnic groups. These genetic differences in sHLA-I secretion could be associated with ethnic and pathophysiologic differences among these rheumatic diseases.

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.

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

Several extended major histocompatability complex (MHC) haplotypes are associated with susceptibility to autoimmune disease in Caucasian populations. It is known that African Americans and Afro-Caribbeans are ethnic groups descended from west, central and southern black African populations which are admixed with Caucasians. To examine the possible association of some marker of Caucasian MHC genes and susceptibility to rheumatoid arthritis (RA) in African Americans, we studied extended MHC haplotypes (HLA-B, complement and DR) in a sample of 18 African American and Afro-Caribbean probands with RA, their first degree relatives and in 15 non-RA families. We defined 36 disease-associated RA haplotypes among the probands and 96 normal haplotypes in normal individuals. To obtain the most conservative estimate, we excluded recognized Caucasian, DR4-bearing, extended MHC haplotypes from the analysis. Admixture proportions for non-HLA-DR4 extended MHC haplotypes of known Caucasian origin among RA-associated and normal haplotypes were computed (0.40 versus 0.163 respectively). When we compared the difference in proportions between RA and normal haplotypes, the proportion of extended MHC haplotypes of known Caucasian origin was significantly increased among RA-associated haplotypes (Z = 3.16, p (one sided) < 0.001, p (adjusted) < 0.008). Our results suggest that racial admixture with Caucasian MHC genes may augment RA susceptibility and thus may be one mechanism to explain the higher prevalence of RA in African Americans and Afro-Caribbeans than in black African populations.

C4 polymorphism and extended HLA haplotypes in Namibian San and Khoi and in South African Xhosa

We studied C4A and C4B polymorphisms and HLA-B and -DR associations in the San, Khoi and Xhosa. C4A and C4B alleles were determined using conventional protein allotyping methods. The C4A*3, C4B*1 haplotype had a high frequency (30-55%) in all populations. The frequency of C4A*3, C4B*Q0 was 7-19%. The C4A*Q0, C4B*1 haplotype was frequent (15%) in the Khoi but very rare in the San (P < 0.001). C4A*12 A*91, C4B*Q0 was frequent in the Xhosa (15%) but rare in the San and Khoi (P < 0.001). Alleles C4A*5 and C4A*6, and the C4B*2 B*92 duplication were only found in the Xhosa. C4A alleles A*4, A*45, A*58, A*12, A*14, A*19 and the C4A*3 A*91 duplication were only found in the San/Khoi population group. In the San, fourteen extended haplotypes were found in a relatively high frequency (2-7%). In the Xhosa, one extended haplotype (B42, C4A*12 A*91, C4B*Q0, DR18) was found in a very high frequency (13%) and was characteristic for this group; five other extended haplotypes were found with a low frequency (< 3%).