Inherited disorders of the red cell membrane skeleton

Normocytic and macrocytic anemias

Differential diagnosis of normocytic and macrocytic anemias is guided by the patient history, physical examination results, reticulocyte count, and specific red blood cell abnormalities identified on the peripheral smear. In some cases this information is sufficient for diagnosis. When further tests are needed, a stepwise approach, as described by the author, ensures maximum cost-effectiveness without sacrificing diagnostic accuracy.

Evaluation of erythrocyte disorders with mean corpuscular volume (MCV) and red cell distribution width (RDW)

The red cell distribution width (RDW), which provides a quantitative measure of heterogeneity of red cells in the peripheral blood, and the mean corpuscular volume (MCV) are part of the routine red cell indices reported by automated blood analyses. This study evaluated 193 pediatric patients with a wide range of erythrocyte disorders and determined the diagnostic utility of the RDW in relation to the MCV. Six different groups of erythrocyte disorders by MCV and RDW values are described: low MCV/normal RDW, low MCV/high RDW, normal MCV/normal RDW, normal MCV/high RDW, high MCV/normal RDW, high MCV/high RDW. This combination established a useful differential diagnosis of erythrocyte disorders. The data provided a baseline against which future studies of infants and children can be compared, though each laboratory has to verify its own normals. It should be cautioned that different electronic counters yield different RDW values, so there have to be qualifications when reporting reference values. The RDW may find its best use as a guide in the differential diagnosis of anemia, rather than as a definitive test per se.

Hereditary ovalocytosis and reduced susceptibility to malaria in Papua New Guinea

Ovalocytosis, an hereditary condition in which most erythrocytes are oval in shape, is a polymorphism that occurs in up to 20% or more of the population in Papua New Guinea and Malaysia. Due to the geographical correlation of the trait with endemic malaria, the possibility of a selective advantage in resistance to malaria has been raised. In a study of 202 individuals with greater than or equal to 50% oval red cells matched by age, sex and village of residence with controls having less than or equal to 30% oval cells, ovalocytic subjects had blood films negative for Plasmodium vivax (P = 0.009), for P. falciparum (P = 0.044), and for all species of malaria parasites (P = 0.013), more often than controls. Among individuals parasitaemic at any time there were no clear differences in density of parasitaemia. However, in children 2 to 4 years old, parasite densities of both species were lower in ovalocytic subjects than in controls (0.01 less than P less than 0.025). The differential susceptibility to malaria infection suggested by this study has implications for the evaluation of interventions, including possible future vaccine field trials, in populations where high-frequency ovalocytosis is present.

Animal models of inherited hematologic disease

Inherited or acquired hematologic disease is the most prevalent of all human disease when we include the hematologic disorders which are secondary to disease of other systems. It follows that the study of the fundamental mechanisms of the disease processes affecting the hematopoietic system is of prime importance and much remains to be done when one considers that in only 25% of ail hemolytic anemias is the fundamental cause eventually discovered [150]. In the current climate of societal pressures on experimental animal research, animals with spontaneous inherited disease mimicking diseases of the various physiological systems assume proportionately greater importance. These animal models have been extremely valuable in the study of fundamental questions of molecular genetics, metabolic aberrations of the cell and its membrane, synthetic mechanisms of the cell as well as clinical questions of disease manifestations, pathogenetic mechanisms and management. Exploration of differences between normal animal species offer a secondary avenue of investigation into these same fundamental questions. New animal models are being uncovered constantly and this augurs well for the future of biomedical research and the ultimate benefit to humankind and to animals in their own right.

Clinical disorders of the red cell membrane skeleton

The lipid bilayer of the adult red cell is supported on its inner surface by a complex arrangement of proteins known as the membrane skeleton. This filamentous network, a major component of which is a multifunctional protein called spectrin, has an essential role in determining the shape, structural integrity, and deformability of the red cell. A significant achievement of modern biochemistry and hematology has been the elucidation of the organization of the components of the membrane skeleton and their relationship to other membrane proteins and lipids. This article reviews current concepts of membrane skeleton structure and function and emphasizes recent advances which have been made in characterizing and classifying molecular defects of the skeleton which manifest clinically with changes in the shape and stability of the red cell. The pathobiology of hereditary skeletal defects associated with hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary pyropoikilocytosis (HPP) are comprehensively discussed. Secondary defects of the membrane skeleton occurring in glucose-6-phosphate dehydrogenase deficiency and sickle cell anemia are also briefly considered.

Anemia in infancy and childhood. A systematic approach to evaluation

The evaluation of the infant or child with possible anemia requires accurate testing and use of age-specific reference standards for normal values. Simple laboratory tests such as a well-performed peripheral smear to establish specific red cell morphology and red cell indexes to determine red cell size are useful to classify anemias as microcytic hypochromic, normocytic normochromic, or macrocytic normochromic. Addition of the reticulocyte index to these tests allows subdivision of these anemias according to three different pathogenetic mechanisms: hypoproliferation, hyperproliferation, or abnormal maturation of red cells. Patient management depends on systematic determination of the pathogenesis of the anemia, because some anemias are of a genetic origin and no specific treatment is needed, some are best treated by specific replacement, and some are dependent on treatment of the primary disease.

Decreased membrane deformability in Melanesian ovalocytes from Papua New Guinea

We examined the ability of Melanesian ovalocytes from Papua New Guinea to be deformed in order to probe the resistance of these cells to invasion by several species of malaria parasite. We found ovalocytes were refractile to drug-induced endocytosis, that they formed abnormal rouleaux, showed reduced deformability when aspirated into 0.6-micron diameter pores in polycarbonate sieves, and failed to crenate when mounted under a glass coverslip. No substantial differences were found between normocytes and ovalocytes in their initial rate of filtration through 4.5-micron pore polycarbonate sieves, their membrane fluidity as measured by the rate of depolarization of fluorescent probes or the rate of extraction of cytoskeletal proteins in low ionic strength buffers. We conclude that the resistance of ovalocytes to undergo localized deformation might be significant in explaining the resistance of these cells to invasion by malarial merozoites.

Molecular and functional changes in spectrin from patients with hereditary pyropoikilocytosis

The structural and functional properties of spectrin from normal and hereditary pyropoikilocytosis (HPP) donors from the two unrelated families were studied. The structural domains of the spectrin molecule were generated by mild tryptic digestion and analyzed by two-dimensional electrophoresis (isoelectric focusing; sodium dodecyl sulfate-polyacrylamide gel electrophoresis). The alpha I-T80 peptide (Mr 80,000) is not detectable in two related HPP donors; instead, two new peptides (Mr 50,000 and 21,000) are generated and have been identified as fragments of the normal alpha I-T80. A third sibling has reduced levels of both the normal alpha I-T80 and the two new peptides. A similar analysis of spectrin from another HPP family indicates that their spectrins contain reduced amounts of the alpha I-T80 and the 50,000 and 21,000 fragments of the alpha I domain. The HPP donor also has other structural variations in the alpha I, alpha II, and alpha III domains. The alpha I-T80 domain of normal spectrin has been shown to be an important site for spectrin oligomerization (J. Morrow and V.T. Marchesi. 1981. J. Cell Biol. 88: 463-468), and in vitro assays indicate that HPP spectrin has an impaired ability to oligomerize. Ghost membranes from HPP donors are also more fragile than membranes from normal erythrocytes when measured by ektacytometry. In both the oligomerization and fragility assays, the degree of impairment is correlated with the amount of normal alpha I-T80 present in the spectrin molecule. We believe that a structural alteration in the alpha I-T80 domain perturbs normal in vivo oligomerization of spectrin, producing a marked decrease in erythrocyte stability.

Hereditary elliptocytosis: morphologic abnormalities during acute hepatitis

A 10-year-old black girl with hereditary elliptocytosis had a transient increase in hemolysis and unusual red cell morphologic changes during an episode of acute hepatitis. Changes in membrane lipids of these elliptocytes with defective membrane skeletal protein and abnormal distribution of cholesterol may be responsible for this phenomenon.

Abnormalities in membrane phospholipid organization in sickled erythrocytes

In contrast to the wealth of information concerning membrane phospholipid asymmetry in normal human erythrocytes, very little is known about membrane phospholipid organization in pathologic erythrocytes. Since the spectrin-actin lattice, which has been suggested to play an important role in stabilizing membrane phospholipid asymmetry, is abnormal in sickled erythrocytes, we determined the effects of sickling on membrane phospholipid organization. We used two enzymatic probes: been venom phospholipase A2 and Staphylococcus aureus sphingomyelinase C, which do not penetrate the membrane and react only with phospholipids located in the outer leaflet of the bilayer. Our results suggest that the distribution of glycerophospholipids within the membrane of sickled cells is different from that in nonsickled cells. Compared with the normal erythrocyte, the outer membrane leaflet of the deoxygenated, reversibly sickled cells (RSC) and irreversibly sickled cells (ISC) was enriched in phosphatidyl ethanolamine in addition to containing phosphatidyl serine. These changes were compensated for by a decrease in phosphatidyl choline in that layer. The distribution of sphingomyelin over the two halves of the bilayer was unaffected by sickling. In contrast to ICS, where the organization of phospholipids was abnormal under both oxy and deoxy conditions, reoxygenation of RSC almost completely restored the organization of membrane phospholipids to normal. These results indicate that the process of sickling induces an abnormality in the organization of membrane phospholipids to normal. These results indicate that the process of sickling induces an abnormality in the organization of membrane lipids in RSC which become permanent in ISC.

Tryptic digestion of spectrin in variants of hereditary elliptocytosis

Spectrin, either in the form of unfractionated low ionic strength extracts of erythrocyte membranes or purified by chromatography on Sepharose (CL)4B, was subjected to tryptic digestion at 0 degrees C. Four patients, each with a different variant of hereditary elliptocytosis, were studied. In one patient, whose erythrocytes showed significant fragmentation on heating on 45 degrees C, such preparations generated a remarkably different pattern of polypeptide fragments on tryptic digestion at low ionic strength. In this patient 32P was released at a slower rate on tryptic digestion of labeled band 2, and an unusual 32P-labeled peptide fragment was also generated, in contrast to control preparations in which such a peptide could not be easily distinguished. There was increased susceptibility of this patient's spectrin to tryptic digestion at physiological ionic strength, but the qualitative pattern of polypeptide fragments was normal. Phosphorylation of spectrin by membrane protein kinase was markedly impaired in this patient, whereas phosphorylation of casein ws unimpaired. However, the phosphorylation of spectrin in her intact erythrocytes was normal. Our findings suggest an abnormality of spectrin structure which we postulate is causally related to the predisposition to hemolysis in this patient, but do not distinguish whether this is a primary abnormality or a post-translational modification of the spectrin molecule. The other three patients showed normal tryptic digestion of spectrin.