Diversity of the <scp>HLA</scp> 8.1 Extended Haplotype in the Croatian Population and Identification of a Novel Allele <i>HLA‐DRB3*01:132N</i>

The HLA 8.1 extended haplotype is a well-defined haplotype within the HLA region, characterised by strong linkage disequilibrium (LD) and recognised as most prevalent among populations of European ancestry. Despite its significance, comprehensive data on the extended 8.1 haplotype, encompassing all nine HLA loci with high-resolution typing, remains limited. In this study we examined the structure of the 8.1 haplotype in a cohort of 146 unrelated Croatian individuals (HLA-A*01; C*07; B*08; DRB1*03 homozygous at low resolution level), who were additionally NGS typed for HLA-A, -C, -B, -DRB1, -DRB3/4/5, -DQA1, -DQB1, -DPA1 and -DPB1 loci. All individuals remained homozygous at the HLA-DRB1 locus (100%) with only a single allele observed, while the HLA-DPB1 locus showed the lowest homozygosity (31.5%), with 18 distinct alleles identified. Four novel alleles were detected in this cohort, of which the HLA-DRB3*01:132N allele is reported here for the first time. The analysis of 4th field allelic differences revealed 30 distinct haplotypes, with HLA-A*01:01:01:01~C*07:01:01:01~B*08:01:01:01~DRB1*03:01:01:01~DRB3*01:01:02:01~ DQA1*05:01:01:02~DQB1*02:01:01:01~DPA1*02:01:02:02~DPB1*01:01:01:01 being the most frequent one (47.9%). Out of 146 individuals, 43 (29.5%) were homozygous for all nine HLA loci, encompassing three distinct haplotypes. In addition, the two (out of 21 different combinations) most frequent HLA-DPA1~DPB1 haplotypes were HLA-DPA1*02:01:02:02~DPB1*01:01:01:01 and HLA-DPA1*01:03:01:02~DPB1*04:01:01:01 which were significantly more prevalent (P_corr < 0.0001 each). Our study provides valuable insights on the polymorphism of nine HLA loci within 8.1 extended haplotype, highlighting 4th-field allelic variations and potential haplotype formation preferences. Additionally, it underscores the genetic diversity of HLA-DPA1~DPB1 within 8.1 extended haplotype, which may influence its role in disease associations.