Previous studies have demonstrated that the HLA loci are associated with IgAN in different populations but not consistent with any alleles [1, 15–20, 24]. A family-based and case–control association study using the genome-wide association study (GWAS) approach suggested that the HLA region contains the strongest common susceptibility alleles of primary IgAN in a small European cohort . HLA Imputation Analysis shows that the HLA-B, DRB1, DQA, and DQB loci are associated with IgAN, and the strongest association was observed at the HLA-DQ locus in the study . Recently, a GWAS for primary IgAN identified three independent loci in the major histocompatibility complex in a cohort of 3,144 primary IgAN cases of Chinese and European ancestry. The study showed that 27 SNPs exceeding genome-wide thresholds for significance were located in a 0.54-Mb interval within the MHC. Imputation of classical HLA alleles showed that the strongest association was located within a ~170-kb interval that includes HLA-DRB1, HLA-DQA1, and HLA-DQB1 . This further indicated that the HLA-DRB1 region is one of the major primary IgAN susceptibility loci.
The HLA-DRB1 locus is, by far, the most polymorphic MHC class II locus and has more than 540 alleles . Previous studies have found a significant association between IgAN and the HLA-DRB1 alleles in a Japanese population [17, 19, 28]. Recently, a study showed that HLA-DRB1 polymorphisms were related to the occurrence and disease progression of Han Chinese patients with primary IgAN. HLA-DRB1*140501 was reported to be a susceptible allele, and HLA-DRB1*070101 was reported to be a resistant allele. HLA-DRB1*030101 may serve as a predictor of disease progression and renal damage of primary IgAN in Han Chinese . However, previous studies did not substantiate the conclusion that HLA-DRB1 was significantly associated with IgAN because a small number of samples were included in these studies and there was no association between the HLA-DR antigen and primary IgAN in Taiwanese Chinese . In our studies, all of these samples were pre-typed at a low resolution using PCR –SSP, which provided information on the HLA-DRB1 alleles at serological recognition. Then, we typed the HLA-DRB1*04 subtype alleles using a high resolution sequence-based HLA-DRB1 typing of Exon 2. Compared with HLA-DRB1 typing using the SBT approach for these samples, it took less time and money to determine the allele frequencies of HLA-DRB1 using a two-stage typing approach combined with PCR-SSP and SBT. We further confirmed that the genetic variants at the HLA-DRB1 gene were significantly associated with IgAN in a large Han Chinese cohort composed of 1,127 cases and 2,295 controls. However, since our approach does not distinguish between the alleles DRB1*11 and *12, DRB1*13 and *14 and DRB1*15 and *16, it could be missing the true associations between other HLA-DRB1 alleles as well. In addition, the IgAN GWAS study demonstrated a strong protective effect of the DRB1*15 allele (OR=0.61, P=10-6) and potentially of other DRB1 alleles .
As for diseases of polygenic inheritance, there are different disease susceptibility genes in different populations. Primary IgAN, defined as predominant IgA mesangial deposits in the absence of clinical or laboratory evidence of other associated systemic diseases, has also been proposed to include at least two distinct clinical entities (those with and without macroscopic hematuria) [1, 30, 31], which may be determined by different susceptibility genes. Therefore, heterogeneity between populations may account for some of the controversy in HLA associations with IgAN.
The allelic frequency and diversity of HLA-DRB1 among different populations emphasizes the need for research using matched patient and control groups in genetic association studies. Some reports indicate that the population of Metropolitan Beijing, in the northern part of China, includes a mixture of individuals from both the northern and central provinces. However, these reports have also found that there are few people from the southern region. In addition, the reports indicate that the population mix of the Sichuan province, considered representative of southern China, could in fact have been affected by the mass migrations during the late Ming and early Qing dynasties. At those times, people came to Sichuan from both the southern and the central regions, such as Hunan and Hubei. As a result, the cases and controls for our studies have been recruited through geographic matching, to eliminate the adverse impact of stratification. We evaluated the allelic frequency of HLA-DRB1 in the Sichuan cohort (192 cases and 192 controls) and the Beijing cohort (935 cases and 2,103 controls). Subsequently, we analyzed the Sichuan and Beijing cohorts together and confirmed a strong association with primary IgAN susceptibility, that the allele frequency of HLA-DRB1*04 was significantly associated with primary IgAN susceptibility. HLA-DRB1*0405 and 0403 were the susceptible allele of pIgAN patients in Han Chinese population. However, there is no significant association between clinical factors and the frequency of the HLA-DRB1*04 alleles and its subtype in primary IgAN patients.
In summary, this study showed that the HLA-DRB1*04 had a strong association with primary IgAN in a Chinese population. In our disease association studies, all of these samples were pre-typed at a low resolution using PCR –SSP, which may limit our ability to detect rare alleles of HLA DRB1. The primary technology used to detect rare alleles of HLA-DRB1 is the high resolution SBT, which could contribute to the proper identification of susceptible or resistant alleles of IgAN. HLA-DRB1 has been shown to be associated with many autoimmune diseases, however, the mechanism of the HLA-DRB1 causing IgAN is poorly understood. Thus further research exploring the pathogenesis of HLA-DRB1 with IgAN is needed.