The D– phenotype, which is extremely rare, has been linked to hemolytic disease of the fetus and newborn (HDFN), as well as to the occurrence of severe hemolytic transfusion reactions. Hereditary large inversion and recombination events have been reported between RHD and RHCE genes, which can be associated with a lack of RhCE expression, according to findings from a case study conducted in Shanghai, China, and published in the journal Blood Advances.
It has been well recognized that the Rhesus (Rh) blood group is one of the most immunogenic and polymorphic among all of the human blood group systems. In fact, the Rh blood group comprises >50 independent antigens that are encoded by RHD and RHCE genes, with c, C, D, e, and E considered to be the most important antigens in the Rh blood group system.
RHD and RHCE, which are closely linked genes, are thought to have been derived from “ancient gene duplication events based on the accumulation of mutations between them.” These two genes are arranged in a tail-to-tail configuration that is separated by transmembrane protein 50A (TMEM50A). Thus, RHD and RHCE are regarded as being extremely homologous, with only a 3% disparity in their coding regions being observed.
The D– phenotype is depicted by the total absence of c, C, e and E antigens. Typically, the D– phenotype exhibits elevated expression of D antigens in red blood cells (RBCs). There have been case reports published on the presence of the D– phenotype in diverse ethnic groups. The common mechanism observed in the D– phenotype has been described as genomic rearrangements between the closely linked homologous genes RHD and RHCE, which results in the formation of RHCE*CE-D-CE hybrid alleles.
Read more about HDFN prognosis
Individuals who carry the D– phenotype can generate a variety of alloantibodies, such as anti-Rh17, during pregnancy, transplantation, or blood transfusion. This alloantibody can react with all of the common Rh phenotypes, thus rendering its management quite difficult at times of pregnancy or transfusion. Anti-Rh17 can be associated with mild to severe hemolytic reactions, which include HDFN and severe hemolytic transfusion reactions.
The current case involves a proband with the D– phenotype and six family members in whom Rh phenotypes were confirmed. Based on results of Sanger sequencing of all exons of the RHCE gene, no apparent single-nucleotide mutations, segment mutations, or deletions were detected. To some degree, these findings show that the exons of the RHCE gene are integrated.
Samples of peripheral whole blood were collected from the proband and her family at the Weifang People’s Hospital; random samples were collected at the Shanghai Blood Center.
Inconsistent results were observed between the phenotypic analysis and the Sanger sequencing, however, with intact RHCE exons with no mutations in the D– proband observed, although the protein was not expressed. Additional testing revealed that the proband exhibited two novel recombinant RHCE haplotypes, in which clear-cut breakpoints were recognized. The RH haplotypes of the family members were identified and confirmed as well.
“In summary, we made a novel discovery of hereditary large inversions and recombination events occurring between RHD and RHCE genes, leading to [a] lack of RhCE expression,” the authors explained. “This [study] highlights the advantages of using integrated genetic analyses and also provides new insights into RH genotyping,” they concluded.
