Identification of Two HLA Haplotypes Harbouring the DRB4*01:03:01:02N Null Allele and DRB1*04:02~DQ8 in the Spanish Population.

The DRB4*01:03:01:02N null allele, which abolishes surface expression of the DR53 molecule, has been primarily reported in association with the HLA-DRB1*07~DQB1*03:03(DQ9) haplotype. Its linkage with DRB1*04 remains poorly defined. In this study, 242 Spanish deceased donors carrying DRB1*04, DRB1*07, or DRB1*09 alleles were analysed to determine the distribution of DRB4*01:03:01:02 N and identify predictive haplotypes. HLA typing of loci A, B, C, DRB1, DQB1, and DRB3/4/5 was performed using low-resolution PCR-SSP, PCR-SSOP or PCR-NGS methods accordingly, with specific amplification for DRB4*01:03:01:02N. The null allele was detected in 17.2% of DRB1*07~DQ9 and in 6.1% of DRB1*04-positive donors, exclusively in association with DRB1*04:02~DQ8. Extended haplotype analysis revealed two combinations strongly linked to the null allele: H1 (HLA-A*02:01~C*05~B*44:02~DRB1*04:02~DQ8), present in 50% of null carriers, and H2 (HLA-A*02:05~C*07~B*58~DRB1*04:02~DQ8), consistently associated with the null allele (100%). These findings confirm the association between DRB4*01:03:01:02N and DRB1*04:02 in Spain and identify two extended haplotypes-particularly H2-as novel predictive markers. Their recognition may improve the accuracy of HLA typing and support clinical decision-making in transplantation settings.