Amplicon Sequencing-Based Noninvasive Fetal Genotyping for RHD-Positive D Antigen-Negative Alleles

Background: To avoid hemolytic disease of the fetus and newborn resulting from maternal alloantibodies against fetal Rh antigens, anti-D immunoglobulin is routinely administered to RhD-negative pregnant women in Japan. Fetal RHD genotyping using cell-free DNA may prevent unnecessary antibody administration; however, current PCR-based methods, which detect RHD deletion, do not address the higher rates of RHD-positive D antigen-negative alleles in nonwhite populations without additional inspections. Methods: We developed an amplicon-sequencing method that could estimate the type of paternally inherited fetal RHD allele from 4 major RHD alleles in the Japanese population: the D antigen-positive allele (RHD*01, 92.9%) and 3 D antigen-negative alleles (RHD*01N.01, 6.6%; RHD*01EL.01, 0.3%; RHD* 01N.04, 0.1%) using cell-free DNA obtained from the blood plasma of pregnant women. Results: The method correctly determined the fetal RhD type even when RhD-negative pregnant women possessed an RHD-positive D antigen-negative allele: RHD*01EL.01 or RHD*01N.04. Conclusions: This method is a reliable noninvasive fetal RHD genotyping method for Japanese and other East Asian populations. The genotyping principle of amplifying 2 different regions using the same primer pair and distinguishing them by their sequence difference during the subsequent mapping procedure is also theoretically applicable to RHD-positive D antigen-negative alleles prevalent in Africans. Therefore, this method offers an opportunity to consider targeted administration of anti-D immunoglobulin to RhD-negative pregnant women in East Asian and African countries and to increase the specificity of the fetal RHD genotyping implemented nationwide in several European countries.