Characterization of HLA-A/HLA-B/HLA-C/HLA-DRB1 Haplotypes in Romanian Stem Cell Donors Through High-Resolution Next-Generation Sequencing

Human Leukocyte Antigen (HLA) genes are remarkable for their structural complexity and polymorphism. Located on chromosome 6 within the Major Histocompatibility Complex (MHC), these genes exhibit significant frequency variations across human populations and play a crucial role in immune responses, disease susceptibility, and transplant compatibility. This study aimed to assess the genetic profiles and HLA-A/HLA-B/HLA-C/HLA-DRB1 haplotype frequencies in a Romanian cohort. Whole venous blood samples were collected from 405 healthy, unrelated Romanian volunteers. Using next-generation sequencing (NGS), the study population was genotyped for HLA class I (HLA-A, HLA-B, and HLA-C) and class II (HLA-DRB1) loci. Haplotype frequencies were estimated using the expectation-maximization algorithm, addressing phase and allelic ambiguity. The Romanian cohort was compared with multiple populations sourced from the Allele Frequencies Net Database. The study identified 635 different HLA-A/HLA-B/HLA-C/HLA-DRB1 haplotypes. Among them, two haplotypes had frequencies close to 3%: HLA-A*01:01:01/HLA-B*08:01:01/HLA-C*07:01:01/HLA-DRB1*03:01:01, with a frequency of 3.33%, and HLA-A*02:01:01/HLA-B*18:01:01/HLA-C*17:01:01/HLA-DRB1*11:04:01, with a frequency of 2.84%. All other 633 haplotypes (approximately 99.7% of the total) had frequencies below 1%. The results of the current study underscore the extremely high diversity of HLA haplotypes in this population and the fact that even the most frequent haplotypes are relatively low in prevalence (each under 5% in this cohort). These findings and the great haplotypical diversity detected highlight the importance of NGS and high-resolution HLA typing in hematopoietic stem cell and solid organ transplantation, while also contributing to the better understanding of the area-specific population genetics resulting from historical regional dynamics. Further research with larger cohorts is necessary to validate these findings and expand upon their clinical implications.