RHD zygosity assignments based on most probable genotype and hybrid Rhesus box detection in Tunisia

Determination of the RHD zygosity is important for genetic counselling and risk evaluation of hemolytic disease of the newborn HDN in women with D iso-immunisation.

We proposed to determine the genotype frequencies of the RHD locus using a PCR-SSP method and assignment of the most probable genotype (MPG) and analyse the concordance between the two methods.

The complete Rh phenotype and the frequencies of RH haplotypes were determined on 506 blood donors. RHD zygosity was determined by both assignment of the MPG and PCR-SSP specific for the hybrid Rhesus box. For RH:-1 samples, analysis of the RHD exon 10 was done to detect eventual RHD aberrant alleles.

Among the 466 RH:1 samples, 54.08% were hemizygous and 45.92% homozygous by PCR-SSP, and 64.16% hemizygous and 35.84% homozygous by the MPG. The comparison between the methods showed discordant results in 135 RH:1 samples. For the 40 RH:-1 samples, hybrid Rhesus box was detected in all samples and RHD exon 10 was detected in three samples suggesting unequivocal alleles identified as one RHDψ, one (C)ce(s) and one weak D type 4.

The PCR-SSP should replace the MPG. However, studying of aberrant RHD alleles and aberrant Rhesus boxes could confirm the accuracy of this method in Tunisian population.