Missense mutations in PIEZO1, which encodes the Piezo1 mechanosensor protein, define Er red blood cell antigens

Despite the identification of the high-incidence red cell antigen Era nearly 40 years ago, the molecular background of this antigen, together with the other 2 members of the Er blood group collection, has yet to be elucidated. Whole exome and Sanger sequencing of individuals with serologically defined Er alloantibodies identified several missense mutations within the PIEZO1 gene, encoding amino acid substitutions within the extracellular domain of the Piezo1 mechanosensor ion channel. Confirmation of Piezo1 as the carrier molecule for the Er blood group antigens was demonstrated using immunoprecipitation, CRISPR/Cas9-mediated gene knockout, and expression studies in an erythroblast cell line. We report the molecular bases of 5 Er blood group antigens: the recognized Era, Erb, and Er3 antigens and 2 novel high-incidence Er antigens, described here as Er4 and Er5, establishing a new blood group system. Anti-Er4 and anti-Er5 are implicated in severe hemolytic disease of the fetus and newborn. Demonstration of Piezo1, present at just a few hundred copies on the surface of the red blood cell, as the site of a new blood group system highlights the potential antigenicity of even low-abundance membrane proteins and contributes to our understanding of the in vivo characteristics of this important and widely studied protein in transfusion biology and beyond.

Tazobactam-induced haemolytic anaemia, possibly caused by non-immunological adsorption of IgG onto patient's red cells

A patient with pneumonia was treated with Tazocin (piperacillin/tazobactam). However, the expected haemoglobin (Hb) increment after transfusion was not achieved. Plasma bilirubin and lactate dehydrogenase were raised. The direct antiglobulin test (DAT) was positive (4+) for immunoglobulin G (IgG) only, but no RBC antibodies were demonstrable in the plasma or an eluate from the patient's RBCs. Drug-induced haemolysis was suspected. After discontinuing Tazocin administration, Hb and bilirubin levels returned to expected values. The patient's plasma gave a positive (3+) indirect antiglobulin reaction only with RBCs pretreated with tazobactam. However, random patient plasmas also gave weak (+/- to 1+) reactions, indicating non-immunological adsorption of IgG onto RBCs rather than specific anti-tazobactam antibodies. Subsequently, plasma samples with varying IgG levels (0.8-89.7 g L(-1)) were tested against RBCs pretreated with tazobactam. The amount of plasma IgG non-immunologically adsorbed onto the drug-coated RBCs was found to correlate directly with the plasma IgG level. The patient had a high plasma IgG level (41.6 g L(-1)) which explains why the antiglobulin test was stronger with the patient's plasma than with random plasma samples. Previous reports (Garratty & Arndt, (1998) British Journal of Haematology, 100, 777-783; Arndt & Garratty (2000) Transfusion, 40, 29S) suggested that non-immunological coating of RBCs with IgG may affect RBC survival; our results would support that suggestion. This is the first reported case of haemolytic anaemia associated with tazobactam.