Band 4.2 Shiga: 317 CGC-->TGC in compound heterozygotes with 142 GCT-->ACT results in band 4.2 deficiency and microspherocytosis

Investigating the key membrane protein changes during in vitro erythropoiesis of protein 4.2 (-) cells (mutations Chartres 1 and 2)

BACKGROUND

Protein 4.2 deficiency caused by mutations in the EPB42 gene results in hereditary spherocytosis with characteristic alterations of CD47, CD44 and RhAG. We decided to investigate at which stage of erythropoiesis these hallmarks of protein 4.2 deficiency arise in a novel protein 4.2 patient and whether they cause disruption to the band 3 macrocomplex.

DESIGN AND METHODS

We used immunoprecipitations and detergent extractability to assess the strength of protein associations within the band 3 macrocomplex and with the cytoskeleton in erythrocytes. Patient erythroblasts were cultured from peripheral blood mononuclear cells to study the effects of protein 4.2 deficiency during erythropoiesis.

RESULTS

We report a patient with two novel mutations in EPB42 resulting in complete protein 4.2 deficiency. Immunoprecipitations revealed a weakened ankyrin-1-band 3 interaction in erythrocytes resulting in increased band 3 detergent extractability. CD44 abundance and its association with the cytoskeleton were increased. Erythroblast differentiation revealed that protein 4.2 and band 3 appear simultaneously and associate early in differentiation. Protein 4.2 deficiency results in lower CD47, higher CD44 expression and increased RhAG glycosylation starting from the basophilic stage. The normal downregulation of CD44 expression was not seen during protein 4.2(-) erythroblast differentiation. Knockdown of CD47 did not increase CD44 expression, arguing against a direct reciprocal relationship.

CONCLUSIONS

We have established that the characteristic changes caused by protein 4.2 deficiency occur early during erythropoiesis. We postulate that weakening of the ankyrin-1-band 3 association during protein 4.2 deficiency is compensated, in part, by increased CD44-cytoskeleton binding.

Protein 4.2 Komatsu (D175Y) associated with the lack of interaction with ankyrin in human red blood cells

Membrane skeletal proteins play an important role in regulating the shape and function of the human red blood cell. Protein 4.2 interacts with cytoplasmic domain of band 3 (CDB3) and ankyrin for association between the skeleton network and the membrane. The deficiency of protein 4.2 may result in hereditary spherocytosis. In order to explore the molecular mechanism of the linkage of protein 4.2 Komatsu (D175Y) and protein 4.2 Nippon (A142T) with hereditary spherocytosis, a series of protein 4.2-derived mutants were designed and expressed in Escherichia coli. Their interactions with ankyrin and CDB3 were investigated by Far Western blot and pull-down assay in vitro. The results showed that the mutant D175Y of protein 4.2 cannot interact with ankyrin while mutant A142T, just like normal protein 4.2, can bind to ankyrin directly and can associate with CDB3 in the presence of ankyrin. Based on comparing the binding abilities of the protein 4.2 mutants D175F, D175A, D175K and D175Y with ankyrin and CDB3, we suggested that defective binding of protein 4.2 Komatsu to ankyrin is resulted from the charge effect of amino acid residue 175 substitution (D-->Y), which leads to significant structural change in protein 4.2 function domain.

DNA methylation in promoter regions of red cell membrane protein genes in healthy individuals and patients with hereditary membrane disorders

The methylation state of 5'-CG-3' sites is known to be linked to the regulation of promoter function by modulating DNA-protein interactions and to the structure of chromatin. As part of a project to determine methylation patterns in the human genome, we examined the methylation profiles of several genes for human erythroid membrane proteins: ELB42 (protein 4.2), EPB3 (band 3), SPTB gene (beta-spectrin), and ANK1 (ankyrin). The bisulfite protocol of the genomic sequencing method was applied. The number of 5'-CG-3' dinucleotides was the most abundant in SPTB and ANK1, much less in EPB3, and the least in ELB42. In the DNA of peripheral blood mononuclear cells from healthy individuals, the promoter regions of EPB3 and ELB42 were extensively methylated, but the SPTB and ANK1 promoters were totally unmethylated. We also investigated methylation profiles in peripheral blood mononuclear cells from patients with red cell membrane diseases, such as complete protein 4.2 deficiency due to ELB42 mutations, hereditary spherocytosis with EPB3 mutations, and hereditary elliptocytosis with SPTB mutations. The DNA methylation states in these genes of erythroid cells, which we obtained at the second phase of the 2-phase liquid culture of erythroid precursor cells in the peripheral blood, were essentially identical or very similar to those of peripheral blood mononuclear cells. In disease states, the DNA methylation profiles of these red cell membrane protein genes were essentially not different from those in healthy individuals (statistically not significant).

Protein-4.2 association with band 3 (AE1, SLCA4) in Xenopus oocytes: effects of three natural protein-4.2 mutations associated with hemolytic anemia

We have investigated the effects of coexpression of protein 4.2 and three protein-4.2 variants with band 3 in the Xenopus oocyte expression system. Normal protein 4.2 increased band-3–specific chloride transport in the oocytes. Protein 4.2 also coimmunoprecipitated with band 3 and colocalized with band 3 at the oocyte plasma membrane. The increase in band-3–mediated chloride transport and coimmunoprecipitation of protein 4.2 required the presence of the N-terminal cytoplasmic domain of band 3. Protein 4.2 also localized to the oocyte plasma membrane in the absence of band 3. The protein-4.2 variants 4.2 Tozeur (R310Q) and 4.2 Komatsu (D175Y) had impaired ability to bind to band 3 and these variants did not localize to the oocyte plasma membrane when expressed on their own or when coexpressed with band 3. Unexpectedly, 4.2 Nippon (A142T) behaved similarly to normal protein 4.2. In the absence of a crystal structure of protein 4.2, we propose a homology model of protein 4.2 based on the structure of the sequence-related protein transglutaminase. Using our results in oocytes and this homology model we speculate how these mutations affect protein 4.2 and result in hereditary spherocytosis.

Induction of erythrocyte protein 4.2 gene expression during differentiation of murine erythroleukemia cells

Protein 4.2 (P4.2) is an important component in the erythrocyte membrane skeletal network that regulates the stability and flexibility of erythrocytes. Recently, we provided the evidence for specific P4.2 expression in erythroid cells during development (L. Zhu et al., 1998, Blood 91, 695-705). Using dimethyl sulfoxide (DMSO)-induced differentiation of murine erythroleukemia (MEL) cells as a model, transcription of the P4.2 gene was found to be induced during erythroid differentiation. To examine the mechanism for this induction, we isolated the mouse P4.2 genomic DNA containing the 5' flanking sequence and defined the location of the P4.2 promoter. Transcription of the mouse P4.2 gene initiates at multiple sites, with the major initiation site mapped at 174 nucleotides upstream of the ATG start codon. The mouse P4.2 promoter is TATA-less and contains multiple potential binding sites for erythroid transcription factors GATA-1, NF-E2, EKLF, and tal-1/SCL. Transient transfection experiments demonstrated that a 1.7-kb mouse P4.2 promoter fused with the luciferase coding regions was induced in DMSO-treated MEL cells. Deletion analysis showed that a 259-bp P4.2 promoter DNA (nucleotide position -88 to +171 relative to the major transcription initiation site designated +1), containing a GATA-binding site at position -29 to -24, could still respond to the induction in differentiated MEL cells. Importantly, mutations in the -29/-24 GATA motif rendered the promoter unresponsive to DMSO induction. Electrophoretic mobility shift assay revealed that GATA-1 could bind to the -29/-24 GATA motif and this was confirmed by the observation that the nuclear protein bound to the motif was supershifted by an anti-GATA-1 monoclonal antibody. Taken together, these results suggest that the erythroid transcription factor GATA-1 plays an important role in the induction of P4.2 gene expression during erythroid cell differentiation.

Late expression of red cell membrane protein 4.2 in normal human erythroid maturation with seven isoforms of the protein 4.2 gene

The expression of protein 4.2 in normal human erythroid cells was studied utilizing erythroblasts from bone marrow and erythroid cells cultured by the two-phase liquid culture method from burst-forming unit erythroid (BFU-E) in peripheral blood. As opposed to spectrin, which was expressed in erythroid progenitors or very early erythroblasts, protein 4.2 was first detected in late erythroblasts with a morphology nearly identical to orthochromatic erythroblasts. Among the various major membrane proteins, the expression of protein 4.2 was the latest. At the gene level, protein 4.2 gene mRNA was expressed in early erythroblasts. During normal erythroid maturation, the expression of seven different protein 4.2 gene products was observed by Southern blot analysis. These seven gene products appeared to be derived from protein 4.2 gene in the presence or absence of skipping of the 90 bp in exon 1, exon 3, and/or exon 5, as judged by deduction from the protein 4.2 sequence. Therefore, it can be speculated that protein 4.2 is expressed after the cytoskeletal network has been constructed and assembled with integral proteins in the membrane lipid bilayer.

Red blood cell membrane disorders

The recent discovery of the specific molecular defects in many patients with hereditary spherocytosis and hereditary elliptocytosis/pyropoikilocytosis partially clarifies the molecular pathology of these diseases. HE and HPP are caused by defects in the horizontal interactions that hold the membrane skeleton together, particularly the critical spectrin self-association reaction. Single gene defects cause red cells to elongate as they circulate, by a unknown mechanism, and are clinically harmless. The combination of two defective genes or one severe alpha spectrin defect and a thalassaemia-like defect in the opposite allele (alphaLELY) results in fragile cells that fragment into bizarre shapes in the circulation, with haemolysis and sometimes life-threatening anaemia. A few of the alpha spectrin defects are common, suggesting they provide an advantage against malaria or some other threat. HS, in contrast, is nearly always caused by family-specific private mutations. These involve the five proteins that link the membrane skeleton to the overlying lipid bilayer: alpha and beta spectrin, ankyrin, band 3 and protein 4.2. Somehow, perhaps through loss of the anchorage band 3 provides its lipid neighbours (Peters et al, 1996), microvesiculation of the membrane surface ensues, leading to spherocytosis, splenic sequestration and haemolysis. Future research will need to focus on how each type of defect causes its associated disease, how the spleen aggravates membrane skeleton defects (a process termed 'conditioning'), how defective red, cells are recognized and removed in the spleen, and why patients with similar or even identical defects can have different clinical severity. Emphasis also needs to be given to improving diagnostic tests, particularly for HS, and exploring new options for therapy, like partial splenectomy, which can ameliorate symptoms while better protecting patients from bacterial sepsis and red cell parasites, and perhaps from atherosclerosis (Robinette & Franmeni, 1977) and venous thrombosis (Stewart et al, 1996).

Homozygous missense mutation (band 3 Fukuoka: G130R): a mild form of hereditary spherocytosis with near-normal band 3 content and minimal changes of membrane ultrastructure despite moderate protein 4.2 deficiency

The characteristics of phenotypic expression were studied in a Japanese family with hereditary spherocytosis and an extremely rare homozygous missense mutation of the band 3 gene (band 3 Fukuoka: G130R). The homozygous unsplenectomized proband was a 29-year-old male with compensated haemolytic anaemia (red cell count 4.21 x 10(12)/l, reticulocytes 278 x 10(9)/l, and indirect bilirubin 44 micromol/l). His red cell band 3 (B3) protein demonstrated a 9.3% reduction and his protein 4.2 (P4.2) level was substantially reduced (45.0%), compared to normal subjects. P4.2 protein was composed mostly of a wild type (72 kD) with a trace of 68 kD peptide. The binding properties of the mutated B3 to normal P4.2 were significantly impaired, which probably resulted in the substantial reduction of P4.2 in this proband, since no abnormalities were detected on the P4.2 gene. Electron microscopy (EM) using the freeze-fracture method demonstrated a mild decrease in intramembrane particles (IMPs) of near-normal size (8 nm in diameter) with no substantial increases in their oligomerization. Their distribution on the membrane P face was almost normal, although most of the IMPs could represent the homozygously mutated B3 protein. EM (quick-freeze deep-etching method) disclosed a skeletal network of near-normal size and size distribution of the skeletal units, suggesting that the mutated B3 protein itself did not have much effect on the skeletal network in situ. Therefore the reduced P4.2 content (45% of that of normal subjects), which remained on the red cell membrane of this proband, appeared to be nearly sufficient for maintaining the normal structure of the skeletal network and IMPs in situ, contrary to the marked abnormalities in both IMPs and the skeletal network in complete P4.2 deficiencies.

Hereditary spherocytosis: a review of the clinical and molecular aspects of the disease

Hereditary spherocytosis is a common and very heterogeneous hemolytic anemia caused by defects of the red cell membrane proteins. In recent years, major advances in our understanding of the red cell membrane skeleton and a better characterization of its individual components have allowed a brighter insight into the pathogenesis of the disease. In this article, we present an overview of the erythrocyte skeleton and its individual constituents. We also review the clinical aspects of the disease and describe the currently known molecular defects involving the membrane proteins which have been shown to play an essential role in the underlying mechanism of hereditary spherocytosis. Finally we examine several models that have been proposed in an attempt to clarify the mechanism leading from the initial molecular insult to the clinical phenotype.

Electron microscopic evidence of impaired intramembrane particles and instability of the cytoskeletal network in band 4.2 deficiency in human red cells

To obtain direct evidence of impaired intramembrane particles (IMPs) and a deranged cytoskeletal network in situ in human red cells of band 4.2 deficiency, electron microscopic studies were performed utilizing the freeze fracture method for IMPs and the quick-freeze deep-etching method for the cytoskeletal network. Three patients with three different previously identified mutations of the band 4.2 gene, i.e., band 4.2 Komatsu (homozygous; codon 175 GAT --> TAT), band 4.2 Nippon (homozygous; codon 142 GCT --> ACT), and band 4.2 Shiga (compound heterozygous; codon 317 CGC --> TGC and codon 142 GCT --> ACT), were selected for this study. The decrease in the number of IMPs with increase in their size was most marked in band 4.2 Komatsu, which was clinically most severe with no band 4.2 protein. In this regard, in band 4.2 Nippon, which showed moderate severity in clinical hematology with a nearly missing band 4.2 protein, increased sizing was less marked. The abnormalities in IMPs were the least in band 4.2 Shiga, which demonstrated compensated hemolysis with band 4.2 protein in a trace amount. The extent of the impairment of IMPs may be reflected by the total absence or the presence of band 4.2 protein even in a trace amount and/or by the specific site(s) of the mutation of the band 4.2 gene. Derangement of the cytoskeletal network was also observed in these three patients. It was most abnormal in band 4.2 Komatsu, and less so in band 4.2 Nippon and in band 4.2 Shiga. These results clearly indicate that 1) band 4.2 plays an important role not only in its binding to band 3 but also to the skeletal network (mostly to spectrins) vertically, and 2) its deficiency produces critical abnormality in maintenance of the structural and functional integrity of the integral proteins (such as band 3), as well as the cytoskeletal network.