Abstract
Southeast Asian Ovalocytosis results from a heterozygous deletion of 9 amino acids in the erythrocyte anion exchange protein AE1 (band 3). The report of the first successful birth of an individual homozygous for this mutation showed an association with severe dyserythropoietic anemia. Imaging of the proband’s erythrocytes revealed the presence of band 3 at their surface, a reduction in Wr(b) antigen expression, and increases in glycophorin C, CD44, and CD147 immunoreactivity. Immunoblotting of membranes from heterozygous Southeast Asian Ovalocytosis red cells showed a quantitative increase in CD44, CD147, and calreticulin suggesting a defect in reticulocyte maturation, as well as an increase in phosphorylation at residue Tyr359 of band 3, and peroxiredoxin-2 at the membrane, suggesting altered band 3 trafficking and oxidative stress, respectively. In vitro culture of homozygous and heterozygous Southeast Asian Ovalocytosis erythroid progenitor cells produced bi- and multi-nucleated cells. Enucleation was severely impaired in the homozygous cells and reduced in the heterozygous cells. Large internal vesicular accumulations of band 3 formed, which co-localized with other plasma membrane proteins and with the autophagosome marker, LC3, but not with ER, Golgi or recycling endosome markers. Immunoprecipitation of band 3 from erythroblast cell lysates at the orthochromatic stage showed increased interaction of the mutant band 3 with heat shock proteins, ubiquitin and cytoskeleton proteins, ankyrin, spectrin and actin. We also found that the mutant band 3 forms a strong interaction with non-muscle myosins IIA and IIB, while this interaction could not be detected in wild type erythroblasts. Consistent with this, the localization of non-muscle myosin IIA and actin was perturbed in some Southeast Asian Ovalocytosis erythroblasts. These findings provide new insights toward understanding in vivo dyserythropoiesis caused by the expression of mutant membrane proteins.
Highlights
We previously reported on the only known case of homozygous Southeast Asian Ovalocytosis (SAO) (Picard et al, 2014), a condition caused by a mutation in SLC4A1 the gene encoding erythrocyte anion exchanger 1 (AE1, band 3)
In the clinical report of this patient we showed that homozygous SAO cells were clearly identifiable amongst the normal transfused donor red cells based on morphology and reduced reactivity with anti-band 3 monoclonal antibody BRIC6 (Smythe et al, 1995)
Anti-Rh-associated glycoprotein (RhAG) and anti-glycophorin A (GPA) showed slightly reduced immunoreactivity (Figure 1A), and reduced reactivity occurred with antibodies directed against the Wr(b) antigen which is formed by an interaction between GPA and band 3 (Bruce et al, 1995)
Summary
We previously reported on the only known case of homozygous Southeast Asian Ovalocytosis (SAO) (Picard et al, 2014), a condition caused by a mutation in SLC4A1 the gene encoding erythrocyte anion exchanger 1 (AE1, band 3). We found that some mature red blood cells (RBCs), containing SAO band 3 alone, were produced by the child’s bone marrow and survived in the circulation. These cells were very large, cigar-shaped and had an altered affinity for certain anti-band 3 antibodies. We were interested to further characterize these cells and examine the effect that the expression of SAO band 3 had on erythropoiesis
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