High throughput HLA genotyping using 454 sequencing and the Fluidigm Access Array™ System for simplified amplicon library preparation.

The human leukocyte antigen (HLA) class I and class II loci are the most polymorphic genes in the human genome; distinguishing the thousands of HLA alleles is challenging. Next generation sequencing of exonic amplicons with the 454 genome sequence (GS) FLX System and Conexio Assign ATF 454 software provides high resolution, high throughput HLA genotyping for eight class I and class II loci. HLA typing of potential donors for unrelated bone marrow donor registries typically uses a subset of these loci at high sample throughput and low cost per sample. The Fluidigm Access Array System enables the incorporation of 48 different multiplex identifiers (MIDs) corresponding to 48 genomic DNA samples with up to 48 different primer pairs in a microfluidic device generating 2304 parallel polymerase chain reactions (PCRs). Minimal volumes of reagents are used. During genomic PCR, in this 4-primer system, the outer set of primers containing the MID and the 454 adaptor sequences are incorporated into an amplicon generated by the inner HLA target-specific primers each containing a common sequence tag at the 5' end of the forward and reverse primers. Pools of the resulting amplicons are used for emulsion PCR and clonal sequencing on the 454 Life Sciences GS FLX System, followed by genotyping with Conexio software. We have genotyped 192 samples with 100% concordance to known genotypes using 8 primer pairs (covering exons 2 and 3 of HLA-A, B and C, and exon 2 of DRB1, 3/4/5 and DQB1) and 96 MIDs in a single GS FLX run. An average of 166 reads per amplicon was obtained. We have also genotyped 96 samples at high resolution (14 primer pairs covering exons 2, 3, and 4 of the class I loci and exons 2 of DRB1, 3/4/5, DQA1, DQB1, DPB1, and exon 3 of DQB1), recovering an average of 173 sequence reads per amplicon.

Use of class I and class II HLA loci for predicting age at onset of type 1 diabetes in multiple populations.

AIMS/HYPOTHESIS: The study aimed to assess, in multiple populations, the role of HLA alleles on early and late age at onset of type 1 diabetes. METHODS: Stepwise linear regression models were used to determine which HLA class I and class II risk alleles to include. High-resolution genotyping data for patients from the Type 1 Diabetes Genetics Consortium (T1DGC) collection (n = 2,278) and four independent cohorts from Denmark, Sardinia and the USA (Human Biological Data Interchange [HBDI] and Joslin Diabetes Center) (n = 1,324) (total n = 3,602) were used to assess the role of HLA variation on age of onset and predict early onset (age ≤ 5 years) and late onset (age ≥ 15 years) of type 1 diabetes. RESULTS: In addition to carriage of HLA class I alleles A*24:02, B*39:06, B*44:03 and B*18:01, HLA class II DRB1-DQB1 loci significantly contributed to age at onset, explaining 3.4% of its variance in the combined data. HLA genotypes, together with sex, were able to predict late onset in all cohorts studied, with AUC values ranging from 0.58 to 0.63. Similar AUC values (0.59-0.70) were obtained for early onset for most cohorts, except in the Sardinian study, in which none of the models tested had significant predictive power. CONCLUSIONS/INTERPRETATION: HLA associations with age of onset are consistent across most white populations and HLA information can predict some of the risk of early and late onset of type 1 diabetes. Considerable heterogeneity was observed between Sardinian and other populations, particularly with regard to early age of onset.

Two new HLA-B alleles, B*4422 and B*4704, identified in a study of families with autoimmunity.

HLA-B typing of approximately 1 262 individuals from a study of 372 simplex families with multiple sclerosis has led to the identification of two new alleles (HLA-B*4422 and HLA-B*4704). Sequencing confirmed that both of these new alleles represent novel combinations of previously described sequence motifs, reinforcing the notion that recombination and/or gene conversion-like events play an important role in generating HLA allelic diversity. The identification of these new alleles brings the total number of HLA-B alleles to 465.

Genetic variability and linkage disequilibrium within the HLA-DP region: analysis of 15 different populations.

In order to understand the forces governing the evolution of the genetic diversity in the HLA-DP molecule, polymerase chain reaction (PCR)-based methods were used to characterize genetic variation at the DPA1 and DPB1 loci encoding this heterodimer on 2,807 chromosomes from 15 different populations including individuals of African, Asian, Amerindian, Indian and European origin. These ethnically diverse samples represent a variety of population substructures and include small, isolated populations as well as larger, presumably admixed populations. Ten DPA1 and 39 DPB1 alleles were identified and observed on 87 distinct DP haplotypes, 34 of which were found to be in significant positive linkage disequilibrium in at least one population. Some haplotypes were found in all ethnic groups while others were confined to a single ethnic group or population. Strong positive global linkage disequilibrium (Wn) between DPA1 and DPB1 was present in all 15 populations. The African populations displayed the lowest values of Wn whereas the Amerindian populations displayed near absolute disequilibrium. Analysis of the distribution of haplotypes using the normalized deviate of the Ewens-Watterson homozygosity statistic, F, suggests that DP haplotypes encoding the functional heterodimer are subject to much lower degrees of balancing selection than other loci within the HLA region. Finally, neighbor joining tree analyses demonstrate the power of haplotype diversity for inferring the relationships between the different populations.

Evolution of Pacific/Asian populations inferred from HLA class II allele frequency distributions.

The allele frequency distributions for the HLA class II loci, DRB1, DQB1 and DPB1, in eight Pacific/Asian populations: Hawaiian, Samoan, Malay, Papua New Guinea (PNG) Highlands, and two Indonesian and PNG Lowland groups, were determined using high-resolution polymerase chain reaction/sequence-specific oligonucleotide probe (PCR/SSOP) typing methods. The allele frequency distributions for the HLA-DRB1 locus were determined for a third Indonesian population as well as for an additional Filipino population. DRB1 alleles in the DR2 serogroup (or allelic lineage) are very common in this region; in some populations, more than 50% of the alleles belong to this serogroup. The DRB1*1502 allele is frequent in nine of the ten populations studied, reaching a frequency of 0.48 in one Indonesian population and among Filipinos. Extensive DR-DQ haplotype diversity was detected in these populations. Seven different DR2-DQB1 haplotypes were observed in the Indonesian and PNG Lowland populations, eight in the PNG Highlands and ten in Malays and Filipinos. The DRB1*0410 allele, commonly observed in Australia, is observed in the PNG Highlands at a low frequency (f=0.03) and is absent in the other populations. Two additional DRB1 alleles commonly observed in Australia, DRB1*0405 and *1407, are also observed in the PNG Highlands at high frequencies (f=0.132 and 0.126), while they are rare in the PNG Lowlands (f=0.039 and 0.013). These alleles are generally rare or absent in the other populations. The DPB1*0501 allele, common in Chinese and Japanese populations, is most frequent in the Samoan, Hawaiian, Indonesian, and Malay populations, and the *0401 allele is the most frequent DPB1 allele in the PNG Lowlands. Both of these alleles have the same very high frequency (f=0.34) in the PNG Highlands. Analyses of homozygosity (the Ewens-Watterson F statistic) in these and other populations indicate that, while most allele frequency distributions are consistent with balancing selection, values of F for the Indonesian and Javan populations may reflect positive directional selection. Phylogenetic trees constructed using the allele frequencies at the DRB1 locus of the populations reported here, as well as those for additional Pacific, Asian, and Australian populations, indicate that the PNG Highland population is more closely related to Australian populations than to PNG Lowland populations, while the PNG Lowlands are more closely related to other Melanesian populations.

Genetic variability and linkage disequilibrium within the DP region in the CEPH families.

A PCR-based SSO-assay has been developed to characterize the allelic polymorphism at the HLA-DPA1 locus. To validate the performance of this assay, 77 samples were typed side by side in a blinded fashion by the SSO assay and sequencing-based typing (SBT); 100% concordance was seen between the two methods. To address questions of genetic variability and linkage disequilibrium within the class II region, 478 members of the 37 original Caucasian Centre d'Etude du Polymorphisme Humain (CEPH) families were typed for DPA1 using the SSO assay providing information on 247 independent chromosomes. Six of the eight known DPA1 alleles were detected in this population; DPA1*0103 was the most frequent allele. Analysis of the distribution of allele and haplotype frequencies using the homozygosity statistic suggests that balancing selection does not appear to be acting on the DPA1 locus nor on the functional DP heterodimer in this population. Family data permits the unambiguous assignment of haplotypes. Of the 247 independent chromosomes analyzed, 24 distinct DPA1-DPB1 haplotypes were identified with DPA1*0103-DPB1*0401 being the most common. Twelve of the 18 DPB1 alleles identified in this population have an exclusive association with one DPA1 allele. Of the remaining six DPB1 alleles, four are present at a frequency of >3% and show preferential association with just one DPA1 allele. Calculation of the normalized disequilibrium parameter (D') shows 13 DP haplotypes to be in significant positive disequilibrium. These data suggest there is strong linkage disequilibrium between the DPA1 and DPB1 loci in this Caucasoid population and provide a basis with which to study linkage disequilibrium in other ethnic groups as well as analyze the evolutionary forces which govern allelic and haplotypic variation.

Genes within the HLA class II region confer both predisposition and resistance to primary biliary cirrhosis.

Nonradioactive sequence-specific oligonucleotide probes for the polymorphic HLA class II genes have been used to type samples from 51 Caucasian patients with the autoimmune liver disease, primary biliary cirrhosis, and 240 Caucasian controls. Although the allelic distribution at the DPB1 locus showed no significant variation between patients and controls, there was heterogeneity in the distribution of DR-DQ haplotypes where the frequency of the DRB1*0801-DQA1*0401/0601-DQB1*04 haplotype was significantly increased in the patients, suggesting it confers susceptibility to this disease. Two other haplotypes, DRB1*1501-DQA1*0102-DQB1*0602 and DRB1*1302-DQA1*0102-DQB1*0604, were significantly reduced in the patients, suggesting they confer protection. Tests of the individual loci show that resistance to this disease is most strongly associated with the DQA1*0102 allele shared by both protective haplotypes. Due to linkage disequilibrium it is unclear whether multiple genes or a single locus on the susceptible DR8 haplotype are needed for predisposition. These data show that distinct HLA class II alleles confer both predisposition and resistance to PBC and provide insight into the role that these genes may play in the immunopathogenesis of this disease.