Rh komplex R0Har and Rh protein D HAR – RhD antigen variant with themost atypical genotypic background and phenotypic expression

Overview

Rh system belongs to the most important blood group systems. Its molecular basis are two highlyhomologous genes (RHD a RHCE) localized in closed proximity and in opposite orientation on the firstchromozome (1p34-36). Products of these genes are two proteins, RhD and RhCcEe, carrying the Rhantigens. RhD protein is the most immunogenic antigen complex of the red cell membrane – RhDnegative phenotype is characterized by its total absence – that iswhy the immune system of RhD- peopleis very sensitive to contact with RhDpositive erythrocytes (frequent antibody production in pregnancyor after transfusion of RhD+ red cells). Other antigenic differences (C+/-, c+/-, E+/-, e+/- aj.) on the otherhand are represented by only one or few aminoacid substitutions in one protein and therefore are muchless immunogenic. The detection of RhD in cca 1–2% RhD positive samples is complicated by theabnormal expression of the protein, which can be divided into quantitative weakening (less proteincopies on the membrane) and into qualitative variants. First type (weak D) is based on RHD(weak) genemutations which do not affect the extramembranousportion of the protein.The second type (D variants)is based on RHD(variant) gene with alteration(s) resulting in the loss of one or more D epitopes – thisfeature is used the serologic characterisation of variants and is the cause of their clinical importance(carriers of D-epitope deficient phenotype are able to form antibodies against „missing“ part of theprotein when contacted with „normal“ RhD positive red cells). This article is describing one uniqueRhD variant – RoHar, which dramatically differs from other types due to its molecular background(presence of only one hybrid RHCE-RHD(5)-RHCE gene) as well as serologic characteristics (unusualnegative reactions with most IgG anti-D in the antiglobulin test), causing frequent discrepancies anddiagnostic complications.

Key words:
Rh system, RHD and RHCE genes, RhD and RhCcEe proteins, weak and variant RhDantigens, D epitopes, R0Har variant