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

Background: 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.

Objectives: 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.

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.

Results: 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.

Conclusion: 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.