Genetic influence of the nonclassical major histocompatibility complex class I molecule MICB in multiple sclerosis susceptibility

It has been widely reported that the major histocompatibility complex (MHC) class II region provides the main genetic contribution to multiple sclerosis (MS) susceptibility. However, recent studies have suggested that the MHC class I region may also contribute to the development of MS. In this study, we investigated the possible association of the human leukocyte antigen (HLA)-B, MHC class I chain-related gene B (MICB) and MHC class I chain-related gene A (MICA) genes, located in the MHC class I region, with MS susceptibility. For this purpose, we analyzed the distribution of HLA-DR, HLA-B, MICB and MICA alleles in 121 MS patients and 156 healthy controls. Neither HLA-B nor MICA alleles were found to be associated with MS susceptibility, and only the frequency of HLA-DRB1*01 allele was found to be increased in controls (31% vs 14%, P(c) = 0.011). However, MICB*004 allele frequency was significantly increased in MS patients (46.3% vs 23.3%, P(c) < 0.001, odds ratio = 2.82, 95% confidence interval = 1.68-4.73). Although, MICB*004 and HLA-DRB1*15 belong to the AH 7.1 ancestral haplotype, the association of MICB*004 to MS susceptibility was found to be independent of HLA-DRB1*15 in our population. This and previous studies clearly suggest that the MHC class I, in addition to class II, could be involved in MS susceptibility.

Clinical behavior of multiple sclerosis is modulated by the MHC class I-chain-related gene A

It is well known that certain HLA class II alleles confer an increased risk for developing multiple sclerosis (MS). Recent studies have suggested HLA class I as a region that may also contribute to the development of MS. In this study, we investigated the association between HLA-DR, HLA-B alleles, and major histocompatibility complex (MHC) class I-chain-related gene A (MICA) transmembrane (MICA-TM) polymorphisms and disease progression in 104 MS patients and 116 healthy controls. DR1 was found to be decreased in patients when compared with controls (p(c) = 0.012). Neither HLA-B nor HLA-DR alleles were found to be associated with MS susceptibility. Furthermore, the prevalence of MICA-A5 in patients with relapsing MS was 9% while the prevalence in progressive forms was 42% (p(c) = 0.0015). The extended haplotypes related to MICA-TM5 that were found in our population were DR7-MICA5-B64 (EH 64.1, delta(s) = 0.38), DR4-MICA5-B62 (EH 62.1, delta(s) = 0.28), and DR11-MICA5-B35 (EH35.1, delta(s) = 0.10), but none of them were found to be associated to MS susceptibility or disease progression. Our data could indicate a possible role of MICA-TM in MS prognosis.

HLA class I variation in the West African Pygmies and their genetic relationship with other African populations

We have studied the polymorphism of HLA class I in two West African Pygmy populations, namely, the Bakola from Cameroon and the Mbenzele from the Central African Republic. A unique number of HLA alleles and haplotypes showed specific patterns of these populations. In this study, we identify two alleles (B*37, B*41) and three haplotypes (A*30-B*37, A*66-B*41 and A*68-B*58) that appear to be 'private' or typical of Western Pygmies. These data reflect similarities with the AKA Pygmies from the Central African Republic. On the other hand, we failed to identify alleles that are found at high frequencies among other sub-Saharan populations (B*42, B*51). Allelic and haplotypic frequency distributions show differences between the two Pygmy groups, e.g. B*35 was very common in the Mbenzele but has been found to be absent in the Bakola. In contrast, B*53, which is found in the Bakola, has been found to be rare in the Mbenzele Pygmies. In order to analyse the genetic relationships of the Bakola and Mbenzele Pygmies with other sub-Saharan populations, HLA gene frequencies were subjected to the Neighbour-Joining tree analysis. The Mbenzele, Bakola and AKA were found to be relatively close to each other and isolated from other sub-African populations. However, both the genetic distances and the within-group variation suggests that the Bakola are more admixed with Bantu farmers than Mbenzele.

MHC class I chain related gene A (MICA) modulates the development of coeliac disease in patients with the high risk heterodimer DQA1*0501/DQB1*0201

BACKGROUND AND AIMS

Coeliac disease (CD) is an enteropathic disorder characterised by a strong association with major histocompatibility complex (MHC) heterodimer HLA-DQ2. It has been suggested that other HLA class I genes in combination with DQ may also contribute to CD susceptibility. The aim of this study was to investigate whether other candidate genes modify the risk of developing different clinical forms of CD.

PATIENTS AND METHODS

We studied 133 Spanish coeliac patients, divided according to their clinical presentation into typical and atypical groups, and 116 healthy controls. All were typed by polymerase chain reaction-sequence specific primers (PCR-SSP) at HLA-B, DRB1, DQA1, and DQB1 loci and for exon 5 of the MHC class I chain related gene A (MICA).

RESULTS

No differences were found in the frequency of the DQA1*0501/DQB1*0201 heterodimer in either group. The risk of typical CD was significantly associated with the DR7/DQ2 haplotype (p(c)=0.02, odds ratio (OR)=3.4, ethiological fraction (EF)=0.4). Extended haplotype (EH) 8.1 (B8/DR3/DQ2) was found to be overrepresented in the atypical form compared with the typical form (p(c)=0.001, OR=4.19, EF=0.56). The trinucleotide repeat polymorphism MICA-A5.1 was found to be increased in the atypical group of patients compared with the typical group (p(c)=0.00006, OR=8.63, EF=0.81). This association was independent of linkage disequilibrium with EH8.1 as this was also found to be increased in EH8.1 negative atypical patients compared with the typical group (p(c)=0.004, OR=6.66, EF=0.56).

CONCLUSIONS

Our results showed that the risk of developing typical forms of CD was associated with DR7/DQ2 haplotype, and the presence of B8/DR3/DQ2 was significantly increased in atypical patients. In these, the MICA-A5.1 allele confers an additive effect to the DR3/DQ2 haplotype that may modulate the development of CD.

New insights regarding HLA-B27 diversity in the Asian population

A polymerase chain reaction-sequence-specific primer (PCR-SSP) method which distinguishes all B27 alleles described at present (B*2701-23) has been developed. The distribution of B27 alleles was characterised in six different Asian populations. HLA-B*2705, 02, 04, 07, 22 (formerly B*2706) subtypes found in Asian populations differ in their ethnic distribution, which may be the result of different genetic and geographic origins. Furthermore, two novel B27 alleles were found in this study. B*2714 was identified in two Siberians, one of whom was a patient with ankylosing spondylitis. B*2715 was found in two patients with ankylosing spondylitis in Thais. These associations have not previously been reported in either ethnic group.

The OTF3 gene polymorphism confers susceptibility to psoriasis independent of the association of HLA-Cw*0602

Psoriasis has been strongly associated to HLA-Cw6, but it remains unclear whether Cw6 itself or a closely linked gene is associated with the disease. The aim of this study was to clarify whether the HLA-C itself determines disease susceptibility or whether it acts only as a marker for the susceptibility allele. We examined a sample of 95 type I psoriasis patients and 104 Spanish matched controls to investigate whether HLA-Cw*0602 or other closely related class I loci, such as HLA-B and MICA (which are centromeric to HLA-C), or corneodesmosin gene and octamer transcription factor-3 genes (which are telomeric to HLA-C), might play a part in disease development. DNA samples were genotyped by polymerase chain reaction/sequence-specific primers (HLA-C), polymerase chain reaction/sequence-specific primers (HLA-B), radioactive polymerase chain reaction (MICA-TM polymorphism in the transmembrane region), and polymerase chain reaction/restriction fragment length polymorphism (protein S and octamer transcription factor-3). Our results show a significant increase of Cw*0602 in psoriasis patients (odds ratio = 3.64; pc < 0.0006). A significant association between the beta allele of octamer transcription factor-3 (HindIII) and psoriasis was also detected (odds ratio = 3.76; pc < 0.0003). The allele octamer transcription factor-3B (etiologic fraction = 0.62) was found to be more strongly associated to psoriasis vulgaris than Cw*0602 (etiologic fraction = 0.35) and the increase of octamer transcription factor-3 B allele is independent of the linkage disequilibrium with Cw*0602 as this was also found in Cw*0602 negative patients (odds ratio = 3.63; pc < 0.015, etiologic fraction = 0.55). We did not detect an association between the corneodesmosin gene and psoriasis. This fact suggests that the psoriasis susceptibility gene is located within a critical region of 147 kb, telomeric to HLA-C and centromeric to the corneodesmosin gene, and the association of Cw6 to psoriasis may be secondary to linkage disequilibrium.

HLA-B27 alone rather than B27-related class I haplotypes contributes to ankylosing spondylitis susceptibility

Characterization of non-B27 susceptibility genes will be required to know the pathogenesis of AS. The aim of this study was to examine whether ankylosing spondylitis (AS) susceptibility is controlled by B27 alone rather than B27 haplotypes and, whether other closely related class I loci, such as MICA and TNFA genes might play a role in AS. Three hundred eleven B27-positive samples from Caucasoid, Asian, and African populations were selected and genotypes were carried out by PCR/SSOP (HLA-B27 and HLA-C), PCR/SSP (MICA-TM polymorphism in the transmembrane region), PCR/SSCP (MICA alleles), and PCR-RFLP (TNF-alpha). Of these, 161 were AS patients, chosen in order to investigate the contribution of TNFA and MICA loci to AS in HLA-B27 positive individuals. Some findings can be concluded from the study: (a) No significant differences of TNF-alpha promoter alleles at position -308 and -238 (A/G) were found between AS patients and B27 matched alleles from healthy controls; (b) strong linkage disequilibrium was found between the B27 and the MICA alleles. The MICA-A4 was found to be in association with B*2705,02,03 and 08; MICA-A5 with B*2704 and B*2707 and MICA-A.5.1 with B*2706; (c) no significant differences of MICA alleles were found between AS and controls carrying the B27-associated alleles, and therefore no evidence is provided for an additional role of MICA gene in AS susceptibility; (d) there are a striking correlations between the structure of B27 extended haplotypes (from MICA region to HLA-C) and the ethnic distribution of these subtypes. The results of differential linkage disequilibrium with HLA-B27 subtypes suggest that B27 itself remains the primary gene for AS susceptibility, and TNFA and MICA are not involved in the pathogenesis of the disease.

The MICA-A9 triplet repeat polymorphism in the transmembrane region confers additional susceptibility to the development of psoriatic arthritis and is independent of the association of Cw*0602 in psoriasis

OBJECTIVE

To investigate the relative contribution of HLA antigens in the susceptibility to psoriasis and to localize additional genetic factors involved in psoriatic arthritis (PsA).

METHODS

DNA from 45 patients with psoriasis, 65 with PsA, and 177 healthy control subjects was examined by polymerase chain reaction (PCR) using sequence-specific oligonucleotide probes to determine HLA-C. To examine whether MICA (class I major histocompatibility complex chain-related gene A) confers additional susceptibility, trinucleotide repeat polymorphism in the transmembrane region of the MICA gene was investigated by radioactive PCR. Further analysis of MICA was made by PCR-single-strand conformational polymorphism to determine the allelic variant corresponding to MICA transmembrane polymorphism.

RESULTS

Our results reveal new findings: 1) the frequency of the Cw*0602 allele was significantly increased in both patient groups: psoriasis (corrected P [Pcorr] < 10(-5), relative risk [RR] 6.2), PsA (Pcorr < 10(-6), RR 6.3), 2) the trinucleotide repeat polymorphism MICA-A9 was present at a significantly higher frequency in PsA patients (Pcorr < 0.00035, RR 3.2), whereas a similar distribution was found in both the control and psoriasis population, 3) this polymorphism corresponds to the MICA-002 allele and was found to be overrepresented in patients with the polyarticular form (Pcorr < 0.0008, RR 9.35), 4) the increase in MICA-A9 in PsA patients is independent of linkage disequilibrium with Cw*0602, 5) this allele confers additional relative risk (RR 3.27, etiologic fraction 0.44; etiologic fraction is the proportion of disease cases among the total population that are attributable to 1 allele when the relative risk is > 1) in PsA patients who carry Cw*0602.

CONCLUSION

The data obtained in this study are consistent with the polygenic inheritance of psoriasis. Cw*0602 appears to be the stronger genetic susceptibility factor for psoriasis. Independent of the HLA-C association, MICA-A9 polymorphism corresponding to the MICA-002 allele is a possible candidate gene for the development of PsA.

Susceptibility to ankylosing spondylitis is independent of the Bw4 and Bw6 epitopes of HLA-B27 alleles

We have characterized HLA-B27 alleles in a sample of the population from the Azores (n=46) with the aim of investigating the contribution of different subtypes to ankylosing spondylitis (AS). The study was carried out using PCR-SSOP and in some samples genomic sequencing was conducted. Some significant new finding have arisen from this study. First, B*2705,B*2702,B*2703,B*2707 and B*2708 alleles were found to be represented in this population. The polymorphism of B27 alleles found in a sample of the population from the Azores is higher than the Caucasian groups described. B*2703 and B*2707 have not previously been described to be represented in Caucasians and this could indicate admixtures with different populations of the world. In addition, the B*2708 allele was found to be associated with AS in a large family from the Azores. This association has not been previously reported in either ethnic group and needs to be confirmed in other population studies. This is of considerable interest since has only been described as a rare subtype underrepresented in the British population and has not been previously found to be associated with AS. B*2708 carries the sequence specifying the Bw6 epitope in contrast to most B27 alleles which carry a Bw4 sequence. Differences in this region (residues 77-83) can alter the F-pocket and affect T-cell recognition. The importance that these molecular changes can play in the pathogenesis of AS is discussed.

The role of HLA-B27 polymorphism and molecular mimicry in spondylarthropathy

Ankylosing spondylitis (AS), reactive arthritis (ReA) and other related spondyloarthropathies (SpAs) are characterized by a strong association with the major histocompatibility complex allele HLA-B27. Experimental evidence from humans and transgenic rodents suggests that HLA-B27 is itself involved in the pathogenesis of SpA. Population and peptide-specificity analysis of HLA-B27 suggest it has a pathogenic function related to antigen presentation. Putative roles for infectious agents have been proposed in ReA and suggested in AS. However, the mechanism by which HLA-B27 and bacteria interact to induce arthritis is not clear. Molecular mimicry between bacterial epitopes that cross-react with self-B27 peptides is the most persuasive explanation for the pathogenesis of SpA. The experimental studies reviewed here have greatly increased our knowledge of the structure, function and disease association of HLA-B27.