Variações na expressão fenotípica dos antígenos do sistema ABO

Abstract

Introduction: The ABO gene is considerably polymorphic, with more than 200 alleles that account for the expression of glycosyltransferases (GTs) that synthesize A and B antigens. These allelic variants may affect the specificity and/or activity of GTs, resulting in antigenic variations. In addition, the composition and expression of A and B antigens are also associated with enzymatic regulation factors, such as type and availability of precursors, degree of branching of their chains, types of glycoconjugates, glycoproteins or glycosphingolipids, as well as transcriptional and/or translational factors. Although antigenic variations are attributed to several known factors, these cannot always be identified or are insufficient to support a population pattern. Objective: The general objective of this study was to identify the causes of antigenic variations of the ABO blood group system in cases with unexpected phenotypic results. Material and Method: Samples of blood donors, healthy individuals, and patients who presented alterations in ABO phenotyping in routine tests were analyzed. ABO phenotyping was performed in a tube and gel column, with reagents from different clones. Red blood cells suspected of presenting weak A or B antigens were tested against a panel of anti-A or anti-B monoclonal antisera (titers 128 to 2048), respectively. Anti-A,B antiserum (titers 64 to 2048) was also used, except in cases with AB phenotype. The cases that presented mixed field (MF) with the different titers of antisera were evaluated by a technique of separation of red blood cells from MF for phenotyping of the two populations. They were also evaluated by PCR-RFLP and sequencing of the ABO and FUT2 genes. Results: All cases with weak A or B antigens, detected in direct ABO phenotyping, presented divergent results between methods and/or antisera, from non-reactive to reactive, with varying intensities (w to 4+) or in MF. The results obtained with monoclonal antisera of higher titers favored both the detection of antigens as well as the increase in the intensity of the agglutination reaction. The technique of separating the reactive from the non-reactive red blood cells of the MF allowed the phenotypic characterization of the two populations present in the case of hematopoietic mosaicism. Molecular analyses of the ABO gene by PCR-RFLP allowed the detection of the c.261delG mutation and the characterization of the O allele. The careful analysis of electropherograms obtained with sequencing favored the identification of mutations present in the ABO gene. Conclusions: Monoclonal antisera with higher titers favored the detection of weakly expressed A or B antigens in the red blood cells, as well as eliminating reactions of MF resulting from weak A or B phenotypes. In addition, anti-A,B antiserum presented better resolution when compared to anti-A antiserum of the same title. The technique of separating red blood cells from the MF, although simple, proved efficient in resolving cases of mosaicism, containing populations of red blood cells with distinct ABO phenotypes. The molecular analyses have corroborated the interpretation of the results achieved.