A Japanese family with two sisters apparently homozygous for Mk

Large red cell-derived membrane particles are major contributors to hypercoagulability in sickle cell disease

Sickle cell disease (SCD) is one of the most common inherited single gene disorders. Polymerisation of sickle hemoglobin results in erythrocytes that are inflexible and adherent, leading to coagulation, vascular and cellular activation and resultant blood vessel blockage. Previous studies have observed elevated numbers of red cell-derived particles (RCDP), also denoted extracellular vesicles, in SCD plasma. Here, imaging flow cytometry was used to quantify all RCDP in SCD plasma. A more heterogenous population of RCDP was observed than previously reported. Significantly, large right side-out red cell macrovesicles (MaV), 7 µm in diameter, were identified. Most RCDP were right side-out but a minor population of inside-out vesicles was also present. Electron micrographs confirmed the heterogenous nature of the RCDP detected. All MaV are decorated with prothrombotic phosphatidylserine (PS) and their removal from plasma lengthened clotting times by more than three-fold. Removal of all right side-out RCDP from SCD patient plasma samples resulted in a seven-fold increase in clotting time. These results indicate that MaV comprise a large area of prothrombotic membrane and are thus major contributors to hypercoagulation in SCD. Consequently, controlled removal of MaV and PS exposed RCDP from plasma could provide a novel therapy for managing this disease.

A Jordanian family with three sisters apparently homozygous for Mk and evidence for clinical significance of antibodies produced by Mk Mk individuals

BACKGROUND

The rare M^k M^k phenotype is the result of a deletion of the coding regions of both GYPA and GYPB. Red blood cells (RBCs) of individuals homozygous for the rare M^k gene lack all MNS blood group antigens and have no glycophorin A or glycophorin B. This phenotype is extremely rare and only four families have been reported.

CASE REPORT

A 28-year-old woman was referred for assessment of recurrent early neonatal deaths. She was found to be apparently homozygous for M^k . With the presence of two M^k M^k sisters as matched donors, an intrauterine transfusion was performed to treat fetal anemia due to a strong maternal atypical antibody. Twins were delivered and subsequently transfused with RBCs from the proposita and her M^k M^k sisters.

CONCLUSION

This case reports the fifth family with members apparently homozygous for M^k and the second example of severe hemolytic disease of the fetus and newborn (HDFN) due to maternal antibodies produced by M^k M^k individuals. It also shows the importance of intensive monitoring and management of HDFN caused by alloantibodies to RBC antigens.

Autophagic vesicles on mature human reticulocytes explain phosphatidylserine-positive red cells in sickle cell disease

During maturation to an erythrocyte, a reticulocyte must eliminate any residual organelles and reduce its surface area and volume. Here we show this involves a novel process whereby large, intact, inside-out phosphatidylserine (PS)-exposed autophagic vesicles are extruded. Cell surface PS is a well-characterized apoptotic signal initiating phagocytosis. In peripheral blood from patients after splenectomy or in patients with sickle cell disease (SCD), the number of circulating red cells exposing PS on their surface is elevated. We show that in these patients PS is present on the cell surface of red cells in large (∼1.4 µm) discrete areas corresponding to autophagic vesicles. The autophagic vesicles found on reticulocytes are identical to those observed on red cells from splenectomized individuals and patients with SCD. Our data suggest the increased thrombotic risk associated with splenectomy, and patients with hemoglobinopathies is a possible consequence of increased levels of circulating mature reticulocytes expressing inside-out PS-exposed autophagic vesicles because of asplenia.

Studies on human red-cell membrane glycophorin A and glycophorin B genes in glycophorin-deficient individuals

1. Genomic DNA derived from individuals who lack glycophorin A (GPA), glycophorin B (GPB) or both of these proteins was subjected to Southern-blot analysis using GPA and GPB cDNA probes. 2. Bands on the Southern blots were assigned to the GPA gene, GPB gene or to a putative pseudogene. 3. Genomic DNA derived from an individual of the Mk phenotype was shown to have deletions in the GPA and GPB genes. The simplest model for the results obtained is that a single deletion spans the GPA and GPB genes in the individual studied.