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Disorders of the red cell membrane 

Disorders of the red cell membrane

Chapter:
Disorders of the red cell membrane
Author(s):

Patrick G. Gallagher

DOI:
10.1093/med/9780199204854.003.220510

July 30, 2015: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.

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Essentials

The integrity of the red-cell membrane depends on molecular interactions between proteins and protein–lipid interactions: vertical interactions stabilize the membrane lipid bilayer; horizontal interactions provide resistance against shear stress.

Hereditary spherocytosis

This disorder affects 1 in 25 000 individuals of northern European descent. There is typically a dominant family history, but the condition is genetically heterogeneous: combined spectrin and ankyrin deficiency is the most common defect observed, followed by band 3 deficiency, isolated spectrin deficiency, and protein 4.2 deficiency. These affect vertical membrane interactions with loss of surface area relative to red-cell volume.

Clinical features and diagnosis—the key clinical manifestations are anaemia and signs of persistent haemolysis, with jaundice and a marked propensity to gallstones. The best diagnostic test is probably the incubated osmotic fragility test, in which spherocytes burst at higher saline concentrations than normal.

Complications and treatment—parvovirus B19 infection of erythropoietic precursors may cause acute aplastic crises. Megaloblastic anaemia due to folate deficiency occurs in response to increased requirements during growth and pregnancy, but is preventable with supplementation. Splenectomy cures or alleviates the anaemia in most patients and reduces the risk of gallstones.

Hereditary elliptocytosis

This disorder occurs with a frequency of 1 in 2000 to 1 in 4000 worldwide, and is more frequent in parts of Africa. The inheritance is usually dominant, with defects in red-cell proteins such as α‎ and β‎ spectrin causing disturbances in horizontal interactions in the erythrocyte membrane

Clinical features, diagnosis, and treatment—most patients are asymptomatic and are typically diagnosed incidentally during testing for unrelated conditions, but about 10% experience haemolysis, anaemia, splenomegaly, and intermittent jaundice. Diagnosis is based on the presence of elliptocytes on peripheral blood smear. Treatment is rarely required.

Other conditions

These include (1) Hereditary pyropoikilocytosis—a rare cause of severe haemolytic anaemia, usually seen in patients of African descent. (2) South-East Asian (or Melanesian) ovalocytosis—an asymptomatic autosomal dominant condition due to band 3 protein abnormalities that confer resistance to invasion by malaria parasites. (3) Stomatocytosis—characterized by mouth-shaped red cells; a heterogeneous group of disorders that are often asymptomatic but may cause haemolysis and anaemia; may be hereditary (e.g. missense mutations in band 3) or acquired (e.g. cholestatic liver disease, alcoholism, vinca alkaloids). (4) Acanthocytosis—characterized by contracted red cells with spiky projections; may be hereditary (e.g. neuroacanthocytosis syndromes, abetalipoproteinaemia) or acquired (e.g. severe hepatic disease).

Acknowledgement: Supported in part by grants from the National Institutes of Health, NIDDK, and NHLBI.

The red cell membrane

Composition and function

Although the primary structure and a number of the important functions of the red cell membrane have been known for many years, its study continues to yield important insights into our understanding of membrane structure and function. The red cell membrane is composed of three major structural elements: a lipid bilayer primarily composed of phospholipids and cholesterol; integral proteins embedded in the lipid bilayer that span the membrane; and a membrane skeleton on the internal side of the red cell membrane.

The membrane and its skeleton provide the erythrocyte with the ability to undergo significant deformation without fragmentation or loss of integrity during its travel through the microcirculation. The membrane also assembles and organizes the proteins of the lipid bilayer and the membrane skeleton, allowing the red cell to participate in a wide range of functions. These include influencing cellular metabolism by selectively and reversibly binding and inactivating glycolytic enzymes, retaining organic phosphates and other vital compounds, removing metabolic waste, and sequestering the reductants required to prevent corrosion by oxygen. During erythropoiesis, the membrane responds to erythropoietin and imports the iron required for the synthesis of haemoglobin. The lipid bilayer provides an impermeable barrier between the cytoplasm and the external environment and helps maintain a slippery exterior so that erythrocytes do not adhere to endothelial cells or aggregate in the microcirculation. The membrane also participates in erythrocyte biogenesis and ageing. Finally, the membrane participates in the maintenance of pH homeostasis by participating in chloride–bicarbonate exchange.

Interactions of membrane proteins and disorders of red cell shape

Membrane protein–protein and protein–lipid interactions have been classified into two categories, vertical and horizontal interactions (Fig. 22.5.10.1). Vertical interactions stabilize the lipid bilayer membrane while horizontal interactions support the structural integrity of erythrocytes after their exposure to shear stress. The interactions between proteins and lipids of the erythrocyte membrane are more complex than this simplistic model, but it serves as a useful starting point for understanding red cell membrane interactions, particularly in membrane-related disorders. According to this model, hereditary spherocytosis (HS) is a disorder of vertical interactions. Although the primary molecular defects in HS are heterogeneous (see below), one common feature of HS erythrocytes is a weakening of the vertical contacts between the skeleton and the lipid bilayer. As a result, the lipid bilayer membrane is destabilized, leading to release of lipids in the form of skeleton-free lipid vesicles, which in turn results in membrane surface area deficiency and spherocytosis. In this model, hereditary elliptocytosis is a defect of horizontal interactions, primarily those involving spectrin dimer self-association. Defects of horizontal interactions disrupt the membrane skeletal lattice leading to elliptocytic shape in mild cases and skeletal instability and cell fragmentation in severe cases.

Fig. 22.5.10.1 Schematic diagram of the red cell membrane (not to scale). Membrane–protein and membrane–lipid interactions can be divided into two categories: (1) vertical interactions, which are perpendicular to the plane of the membrane and involve spectrin–ankyrin–Band 3 interactions, spectrin–Protein 4.1–glycophorin C interactions, and weak interactions between spectrin and the lipid bilayer, and (2) horizontal interactions, which are parallel to the plane of the membrane.

Fig. 22.5.10.1
Schematic diagram of the red cell membrane (not to scale). Membrane–protein and membrane–lipid interactions can be divided into two categories: (1) vertical interactions, which are perpendicular to the plane of the membrane and involve spectrin–ankyrin–Band 3 interactions, spectrin–Protein 4.1–glycophorin C interactions, and weak interactions between spectrin and the lipid bilayer, and (2) horizontal interactions, which are parallel to the plane of the membrane.

(From Tse WT, Lux SE (1999). Red blood cell membrane disorders. Br J Haematol, 104, 2–13, with permission.)

Hereditary spherocytosis (HS)

This group of inherited disorders is characterized by the presence of spheroidal erythrocytes on peripheral blood smear. HS occurs in all racial and ethnic groups. It is the most common inherited anaemia in individuals of northern European descent, affecting approximately 1 in 2500 individuals in the United States of America and England. It is much more common in whites than in individuals of African descent. Clinical, laboratory, biochemical, and genetic heterogeneity characterize the spherocytosis syndromes.

Aetiology and pathogenesis

The primary defect in HS is loss of membrane surface area relative to intracellular volume, accounting for the spheroidal shape and decreased deformability of the red cell. This loss of surface area results from increased membrane fragility due to defects in erythrocyte membrane proteins. Increased fragility leads to membrane vesiculation and membrane loss. Splenic destruction of these nondeformable erythrocytes is the primary cause of haemolysis experienced by HS patients. Physical entrapment of erythrocytes in the splenic microcirculation and ingestion by phagocytes have been proposed as mechanisms of destruction. Furthermore, the splenic environment is hostile to erythrocytes. Low pH, glucose, and ATP concentrations, and high local concentrations of toxic free radicals produced by adjacent phagocytes, all contribute to membrane damage.

Membrane loss is due to defects in several membrane proteins, including ankyrin, band 3, α‎-spectrin, β‎-spectrin, and protein 4.2. Combined spectrin and ankyrin deficiency is the most common defect observed, followed by band 3 deficiency, isolated spectrin deficiency, and protein 4.2 deficiency. The genetic defects underlying HS are heterogeneous. Multiple genetic loci are implicated and various abnormalities, including point mutations, defects in mRNA processing, and gene deletions, have been described. Except for a few rare exceptions, HS mutations are private, i.e. each individual kindred has a unique mutation.

Clinical features

The clinical manifestations of the spherocytosis syndromes vary widely. The typical picture of HS combines evidence of haemolysis (anaemia, jaundice, reticulocytosis, gallstones, and splenomegaly) with spherocytosis (spherocytes on peripheral blood smear, positive osmotic fragility) and a positive family history (Box 22.5.10.1). Mild, moderate, and severe forms of HS have been defined according to differences in haemoglobin, bilirubin, and reticulocyte counts correlated with the degree of compensation for the haemolysis (Table 22.5.10.1). Initial assessment of a patient with suspected HS should include a family history and questions about history of anaemia, jaundice, gallstones, and splenectomy. Physical examination should seek signs such as scleral icterus, jaundice, and splenomegaly. After diagnosing a patient with HS, family members should be examined for the presence of HS.

Table 22.5.10.1 Clinical classification of hereditary spherocytosis

Trait

Mild spherocytosis

Moderate spherocytosisa

Severe spherocytosis

Haemoglobin (g/dl)

Normal

11–15

8–12

≤ 8

Reticulocytes (%)

1–3

3–8

≥ 8

≥ 10

Bilirubin (mg/dl)

0–1

1–2

> 2

> 3

Spectrin contentb (% of normal)

100

80–100

50–80

20–80

Peripheral smear

Normal

Mild spherocytosis

Spherocytosis

Spherocytosis and poikilocytosis

Osmotic fragility

Fresh

Normal

Slightly increased

Distinctly increased

Distinctly increased

Incubated

Slightly increased

Distinctly increased

Distinctly increased

Markedly increased

a Values in untransfused patients.

b In most patients ankyrin content is decreased to a comparable degree. A minority of hereditary spherocytosis patients lack band 3 or protein 4.2 and may have mild to moderate spherocytosis with normal amounts of spectrin and ankyrin.

HS typically presents in childhood, but may present at any age. In children, anaemia is the most frequent presenting complaint (50%), followed by splenomegaly, jaundice, or a positive family history. Two-thirds to three-quarters of HS patients have incompletely compensated haemolysis and mild to moderate anaemia. The anaemia is often asymptomatic except for fatigue and mild pallor. Jaundice is seen at some time in about 50% of patients, usually in association with viral infections. When present, it is acholuric, that is there is unconjugated hyperbilirubinaemia without detectable bilirubinuria. Palpable splenomegaly is detectable in most (75–95%) older children and adults. Typically, the spleen is modestly enlarged but it may be massive.

About 20–30% of HS patients have ‘compensated haemolysis,’ i.e. erythrocyte production and destruction are balanced. Although the erythrocyte lifespan may only be about 20–30 days, these patients adequately compensate for their haemolysis with increased marrow erythropoiesis. They are not anaemic and are usually asymptomatic. Many of these individuals escape detection until adulthood, when they are being evaluated for unrelated disorders or when complications related to anaemia or chronic haemolysis occur. Haemolysis may become severe with illnesses that cause splenomegaly, such as infectious mononucleosis, or may be exacerbated by other factors such as pregnancy. Because of the asymptomatic course of HS in these patients, diagnosis of HS should be considered during evaluation of splenomegaly, gallstones at a young age, or anaemia from viral infection.

Approximately 5 to 10% of HS patients have moderate to severe anaemia. Patients with ‘moderately severe’ disease typically have a haemoglobin of 6 to 8 g/dl, reticulocytes about 10%, bilirubin 2 to 3 mg/dl, and 40 to 80% of the normal red cell spectrin content. This category includes patients with both dominant and recessive HS and a variety of molecular defects. Patients with ‘severe’ disease, by definition, have life-threatening anaemia and are transfusion-dependent. They almost always have recessive HS. Most have isolated severe spectrin deficiency. In addition to the risks of recurrent transfusions, these patients often suffer from haemolytic and aplastic crises and may develop complications of severe uncompensated anaemia including growth retardation, delayed sexual maturation, or aspects of thalassaemic faces.

The parents of patients with recessive HS are clinically asymptomatic and do not have anaemia, splenomegaly, hyperbilirubinaemia, or spherocytosis on peripheral blood smears (‘Trait’, Table 22.5.10.1). Most have subtle laboratory signs of HS including: slight reticulocytosis and slightly elevated osmotic fragility. The incubated osmotic fragility test is probably the most sensitive measure of this condition, particularly the 100% lysis point (0.43 ± 0.05 g NaCl/dl compared to control 0.23 ± 0.07). It has been estimated that at least 1.4% of the population are silent carriers.

Inheritance

The genes responsible for HS include ankyrin, β‎-spectrin, band 3 protein, α‎-spectrin, and protein 4.2. In approximately two-thirds to three-quarter of HS patients, inheritance is autosomal dominant. In the remaining patients, inheritance is nondominant due to autosomal recessive inheritance or a de novo mutation. Cases with autosomal recessive inheritance are due to defects in either α‎-spectrin or protein 4.2. A surprising number of de novo mutations have been reported in the HS genes. A few cases of ‘double dominant’ HS due to defects in band 3 or spectrin that result in fetal death or severe haemolytic anaemia presenting in the neonatal period have been reported. In general, affected individuals of the same kindred experience similar degrees of haemolysis.

Complications

Gallbladder disease

Chronic haemolysis leads to the formation of bilirubinate gallstones, the most frequently reported complication in HS patients. Although gallstones have been detected in infancy, most occur between 10 and 30 years of age. Management should include interval ultrasonography to detect gallstones, as many patients with cholelithiasis and HS are asymptomatic. Timely diagnosis and treatment will help prevent complications of symptomatic biliary tract disease including biliary obstruction, cholecystitis, and cholangitis.

Haemolytic, aplastic, and megaloblastic crises

Haemolytic crises are usually associated with viral illnesses and typically occur in childhood. They are generally mild and are characterized by jaundice, increased splenomegaly, decreased haematocrit, and reticulocytosis. Intervention is rarely necessary. When severe haemolytic crises occur, there is marked jaundice, anaemia, lethargy, abdominal pain, and tender splenomegaly. Hospitalization and erythrocyte transfusion may be required.

Aplastic crises following virally induced bone-marrow suppression are uncommon, but may result in severe anaemia with serious complications including congestive heart failure or even death. The most common aetiological agent in these cases is parvovirus B19. Parvovirus selectively infects erythropoietic progenitor cells and inhibits their growth. Parvovirus infections are frequently associated with mild neutropenia, thrombocytopenia, or even pancytopenia. During the aplastic phase, the haemoglobin and the production of new red cells fall, the cells that remain age, and microspherocytosis and osmotic fragility increase. Aplastic crises usually last 10 to 14 days (about half the lifespan of HS red cells), and the haemoglobin typically falls to half its usual level before recovery occurs. In patients with severe HS, the anaemia may be profound, requiring hospitalization and transfusion. As the marrow recovers, granulocytes, platelets, and, finally, reticulocytes return to the peripheral blood. Aplastic crisis brings many patients to medical attention, particularly asymptomatic HS patients with normally compensated haemolysis. Because parvovirus may infect several members of a family simultaneously, leading to aplastic crises, there have been reports of ‘outbreaks’ of HS.

Megaloblastic crisis occurs in HS patients with increased folate demands, for example the pregnant patient, growing children, or patients recovering from an aplastic crisis. With appropriate folate supplementation, this complication is preventable.

Diagnosis

The laboratory findings in HS are heterogeneous. Initial laboratory investigation should include a complete blood count with peripheral smear, reticulocyte count, Coombs’ test, and serum bilirubin. When the peripheral smear or family history is suggestive of HS, an incubated osmotic fragility should be obtained. Rarely, additional, specialized testing is required to confirm the diagnosis.

Peripheral blood smear

Erythrocyte morphology is quite variable. Typical HS patients have blood smears with obvious spherocytes lacking central pallor (Fig. 22.5.10.2a). Less commonly, patients present with only a few spherocytes on peripheral smear or, at the other end of the spectrum, with numerous small, dense spherocytes and bizarre erythrocyte morphology with anisocytosis and poikilocytosis (Fig. 22.5.10.2b). Specific morphological findings have been identified in patients with certain membrane protein defects such as pincered erythrocytes (band 3) or spherocytic acanthocytes (β‎-spectrin).

Fig. 22.5.10.2 Peripheral blood smears: (a) typical hereditary spherocytosis; (b) severe, recessively-inherited spherocytosis; (c) hereditary elliptocytosis; (d) hereditary pyropoikilocytosis.

Fig. 22.5.10.2
Peripheral blood smears: (a) typical hereditary spherocytosis; (b) severe, recessively-inherited spherocytosis; (c) hereditary elliptocytosis; (d) hereditary pyropoikilocytosis.

Erythrocyte indices

Most patients have mild to moderate anaemia. The mean corpuscular volume (MCV) is normal except in severe HS cases, when it is slightly decreased despite reticulocytosis, reflecting membrane loss and cellular dehydration. The mean corpuscular haemoglobin concentration (MCHC) is increased (≥35 g/dl) due to relative cellular dehydration in around 50% of patients. Strategies using erythrocyte indices have combined MCHC and red cell distribution width (>35.4 g/dl and >14, respectively) or utilized histograms of hyperdense erythrocytes (MCHC>40 g/dl) obtained from laser-based cell counters, sometimes combined with elevated MCHC, in attempts to rapidly identify HS patients.

Osmotic fragility

In the normal erythrocyte, membrane redundancy gives the cell its characteristic discoid shape and provides it with abundant surface area. In spherocytes, there is a decrease in surface area relative to cell volume, resulting in their abnormal shape. This change is reflected in the increased osmotic fragility found in these cells (Fig. 22.5.10.3). Osmotic fragility is tested by adding increasingly hypotonic concentrations of saline to red cells. The normal erythrocyte is able to increase its volume by swelling, but spherocytes, which are already at maximum volume for surface area, burst at higher saline concentrations than normal. Approximately 25% of HS individuals will have a normal osmotic fragility on freshly drawn red cells, with the osmotic fragility curve approximating the number of spherocytes seen on peripheral smear. However, after incubation at 37°C for 24 h, HS red cells lose membrane surface area more readily than normal because their membranes are leaky and unstable. Thus incubation accentuates the defect in HS erythrocytes and brings out the defect in osmotic fragility, making incubated osmotic fragility the standard test for diagnosing HS. When the spleen is present, a subpopulation of very fragile erythrocytes, which have been conditioned by the spleen, form the ‘tail’ of the osmotic fragility curve; this disappears after splenectomy (Fig. 22.5.10.3). Osmotic fragility testing suffers from poor sensitivity as about 20% of mild cases of HS are missed after incubation. It is unreliable in patients with small numbers of spherocytes, including those who have been recently transfused. It is abnormal in other conditions where spherocytes are present.

Fig. 22.5.10.3 Osmotic fragility curves in hereditary spherocytosis. The shaded region is the normal range. Results representative of both typical and severe spherocytosis are shown. A tail, representing very fragile erythrocytes that have been conditioned by the spleen, is common in many spherocytosis patients prior to splenectomy.

Fig. 22.5.10.3
Osmotic fragility curves in hereditary spherocytosis. The shaded region is the normal range. Results representative of both typical and severe spherocytosis are shown. A tail, representing very fragile erythrocytes that have been conditioned by the spleen, is common in many spherocytosis patients prior to splenectomy.

Additional testing

Other investigations, such as the autohaemolysis test and the acidified glycerol test, suffer from lack of specificity and are not widely used. Flow cytometry analysis of the relative amounts of eosin-5-maleimide binding to band 3 and Rh-related proteins in the erythrocyte membrane has recently been developed for the diagnosis of HS. Although utilization has been limited, it appears to have high predictive value, but suffers from some lack of specificity, as other erythrocyte abnormalities such as congenital dyserythropoietic anaemia, sickle cell disease, and abnormalities of erythrocyte hydration may yield positive results. Because it is a flow-based technique, it may become a rapid, reproducible strategy for HS screening. Specialized testing, such as ektacytometry, membrane protein quantitation, and genetic analyses, are available for studying difficult cases or cases where additional information is desired.

Other laboratory manifestations in HS are markers of ongoing haemolysis. Reticulocytosis, increased bilirubin, increased lactate dehydrogenase, increased urinary and faecal urobilinogen, and decreased haptoglobin reflect increased erythrocyte production or destruction.

Differential diagnosis

HS should be able to be distinguished from other haemolytic anaemias by additional diagnostic testing, such as autoimmune haemolytic anaemia via a Coombs’ test. Other causes of haemolytic anaemia (with spherocytes on peripheral smear) (Table 22.5.10.2) should be viewed in the appropriate clinical context. Occasional spherocytes are also seen in patients with a large spleen (such as in cirrhosis, myelofibrosis) or in patients with microangiopathic anaemias, but the differentiation of these conditions from HS is not usually difficult.

Table 22.5.10.2 Conditions with spherocytes on peripheral blood smear

  • Hereditary spherocytosis

  • Autoimmune haemolytic anaemia

  • Liver disease

  • Thermal injury

  • Microangiopathic and macroangiopathic haemolytic anaemias

  • Transfusion reaction with haemolysis

  • Clostridial sepsis

  • Severe hypophosphataemia

  • Poisoning from certain snake, spider, bee, and wasp venoms

  • Heinz body anaemias

  • Hypersplenism

  • ABO incompatibility (neonates)

Treatment

Splenectomy

Splenic sequestration is the primary determinant of erythrocyte survival in HS patients. Thus splenectomy cures or alleviates the anaemia in the overwhelming majority of patients, reducing or eliminating the need for transfusions and decreasing the incidence of cholelithiasis. Postsplenectomy, spherocytosis and altered osmotic fragility persist, erythrocyte lifespan nearly normalizes, and reticulocyte counts fall to normal or near normal levels. Typical postsplenectomy changes, including Howell–Jolly bodies, target cells, and acanthocytes, become evident on peripheral smear. Postsplenectomy, patients with the most severe forms of HS still suffer from shortened erythrocyte survival and haemolysis, but their clinical improvement is striking.

Early complications of splenectomy include local infection, bleeding, and pancreatitis due to injury to the tail of the pancreas incurred during surgery. Overwhelming postsplenectomy infection (OPSI), typically from encapsulated organisms, is an uncommon but significant late complication of splenectomy, especially in the first few years of life. The introduction of pneumococcal vaccines and the promotion of early antibiotic therapy for febrile children who have had a splenectomy have led to decreases in the incidence of OPSI. Increasing rates of penicillin-resistant pneumococci has raised concerns about the potential increases in this feared complication. Another postsplenectomy complication is the increased risk of cardiovascular disease, particularly thrombosis and pulmonary hypertension. Finally, as global travel increases, increasing importance has been placed on the critical role of the spleen in protection from parasitic diseases such as malaria or babesiosis.

Indications for splenectomy

In the past, splenectomy was considered routine in HS patients. However, the risk of OPSI with penicillin-resistant pneumococci, increased recognition of postsplenectomy cardiovascular disease, and increased international travel, have led to a re-evaluation of the role of splenectomy in the treatment of HS. The risks and benefits of splenectomy should be reviewed and discussed between health care providers, patient, and family when splenectomy is considered. Considering the risks and benefits, a reasonable approach would be to splenectomize all patients with severe spherocytosis and all patients who suffer from significant signs or symptoms of anaemia including growth failure, skeletal changes, leg ulcers, and extramedullary haematopoietic tumours. Other candidates for splenectomy are older HS patients who suffer vascular compromise of vital organs.

Whether patients with moderate HS and compensated asymptomatic anaemia should have a splenectomy remains controversial. Patients with mild HS and compensated haemolysis can be followed and referred for splenectomy if clinically indicated. The treatment of patients with mild to moderate HS and gallstones is also debatable, particularly since new treatments for cholelithiasis, including laparoscopic cholecystectomy, endoscopic sphincterotomy, and extracorporal choletripsy, lower the risk of this complication.

When splenectomy is warranted, laparoscopic splenectomy is the method of choice as it results in less postoperative discomfort, shorter hospitalization, and decreased costs. Partial splenectomy has been advocated for infants and young children with significant anaemia associated with HS and it may be of benefit in typical HS patients. The goals of this procedure are to allow for the palliation of haemolysis and anaemia while maintaining some residual splenic immune function. Long-term follow-up data for this procedure are lacking.

Before splenectomy, preferably several weeks preoperatively, patients should be immunized with vaccines against pneumococcus, Haemophilus influenzae type b, and meningococcus. The use and duration of prophylactic antibiotics postsplenectomy is controversial. Presplenectomy, and in severe cases, postsplenectomy, HS patients should take folic acid to prevent folate deficiency.

Elliptocytosis, pyropoikilocytosis, and related disorders

Hereditary elliptocytosis (HE) is characterized by the presence of elliptical or cigar-shaped erythrocytes on peripheral blood smears of affected individuals. The worldwide incidence of HE has been estimated to be 1 in 2000 to 1 in 4000 individuals. The true incidence of HE is unknown because most patients are asymptomatic. It is common in individuals of African and Mediterranean ancestry, presumably because elliptocytes confer some resistance to malaria. In parts of Africa, the incidence of HE approaches 1 in 100. HE is typically inherited in an autosomal dominant pattern. Rare cases of de novo mutations have been described.

Hereditary pyropoikilocytosis (HPP) is a rare cause of severe haemolytic anaemia with erythrocyte morphology reminiscent of that seen in severe burns. Initial studies of erythrocytes from these patients revealed abnormal thermal sensitivity compared to normal erythrocytes. HPP occurs predominantly in patients of African descent. There is a strong relationship between HPP and HE. Approximately one-third of parents or siblings of patients with HPP have typical HE. Many patients with HPP experience severe haemolysis and anaemia in infancy that gradually improves, evolving toward typical HE later in life.

Aetiology and pathogenesis

The principle defect in HE/HPP erythrocytes is an intrinsic mechanical weakness or fragility of the erythrocyte membrane skeleton due to a defect of horizontal interactions (see above). This is due to defects in the red cell membrane proteins α‎-spectrin, β‎-spectrin, protein 4.1, or glycophorin C. The majority of defects occur in spectrin, the principal structural protein of the membrane skeleton. A variety of mutations in the genes encoding these proteins have been described, with several mutations identified in a number of individuals on the same genetic background, suggesting a ‘founder effect’ for these mutations.

Clinical features

The clinical presentation of HE is heterogeneous, ranging from asymptomatic carriers to patients with severe, transfusion-dependent anaemia. Most patients with HE are asymptomatic and are typically diagnosed incidentally during testing for unrelated conditions. The erythrocyte lifespan is normal in most patients. The 10% of patients with decreased red-cell lifespan are the ones who experience haemolysis, anaemia, splenomegaly, and intermittent jaundice. Many of these symptomatic patients have parents with typical HE and thus are homozygotes or compound heterozygotes for defects inherited from each of the parents. Symptomatology may vary between members of the same family, indeed, it may vary in the same individual at different times. To explain these observations, modifier alleles have been hypothesized to influence spectrin expression and clinical severity. One such allele, α‎LELY (low expression Lyon), has been identified and characterized.

Diagnosis

The hallmark of HE is the presence of elliptocytes on peripheral blood smear (Fig. 22.5.10.2c). These normochromic, normocytic elliptocytes number from a few to 100%. The degree of haemolysis and anaemia do not correlate with the number of elliptocytes present. A few ovalocytes, spherocytes, stomatocytes, and fragmented cells may also be seen. Elliptocytes may be seen in association with several disorders including megaloblastic anaemias, hypochromic microcytic anaemias (iron deficiency anaemia and thalassaemia), myleodysplasic syndromes, and myelofibrosis; however, elliptocytes generally make up less than one-third of red cells in these conditions. History and additional laboratory testing usually clarify the diagnosis of these disorders. In addition to the peripheral blood smear findings found in HE, HPP erythrocytes are bizarre-shaped with fragmentation and budding (Fig. 22.5.10.2d). Microspherocytosis is common and the MCV is frequently decreased (50–65 mm3).

The osmotic fragility is abnormal in severe HE and HPP. Other laboratory findings in HE are similar to those found in other haemolytic anaemias and are nonspecific markers of increased erythrocyte production and destruction. When indicated, specialized testing, such as membrane protein quantitation, ektacytometry, spectrin analyses, and genetic studies can be performed.

Treatment

Therapy is rarely necessary. In rare cases, occasional red blood cell transfusions may be required. In cases of severe HE and HPP, splenectomy has been palliative. The same indications for splenectomy in HS can be applied to patients with symptomatic HE or HPP. Postsplenectomy, patients with HE or HPP experience increased haemoglobin, decreased haemolysis, and improvement in clinical symptoms.

During acute illnesses, patients should be followed for signs of haematological decompensation. Ultrasonography at regular intervals to detect gallstones should be performed. In patients with significant haemolysis, folate should be administered daily.

South-East Asian ovalocytosis (SAO)

SAO is characterized by the presence of oval erythrocytes with a central longitudinal slit or transverse bar on peripheral blood smears of affected individuals. It is common in parts of the Philippines, Indonesia, Malaysia, and New Guinea and is inherited in an autosomal dominant fashion. Incredibly rigid, SAO erythrocytes are resistant to invasion by malaria parasites. The underlying defect is a mutation in a critical region of band 3. Haematologically, patients with SAO are asymptomatic, with little or no evidence of haemolysis or anaemia. Osmotic fragility is normal. The finding of characteristic ovalocytes in the peripheral blood of an asymptomatic individual from one of the above mentioned ethnic backgrounds is highly suggestive of the diagnosis. Biochemical and DNA diagnostic techniques are available to detect this condition.

Stomatocytosis

The hereditary stomatocytosis syndromes are a heterogeneous group of disorders characterized by mouth-shaped (stomatocytic) erythrocyte morphology on peripheral blood smear (Fig. 22.5.10.4). The clinical severity of stomatocytosis patients is variable; some patients experience haemolysis and anaemia, while others are asymptomatic. An unusual feature of the stomatocytosis syndromes is a dramatically increased predisposition to thrombosis or pulmonary hypertension post splenectomy. Fortunately, anaemia is well compensated in most patients and splenectomy is not required.

Fig. 22.5.10.4 Peripheral blood smears: (a) dehydrated stomatocytosis. (b) overhydrated stomatocytosis.

Fig. 22.5.10.4
Peripheral blood smears: (a) dehydrated stomatocytosis. (b) overhydrated stomatocytosis.

(From Lande WM, Mentzer WC (1985). Haemolytic anaemia associated with increased cation permeability. Clin Haematol, 14, 89–103, with permission.)

The red blood cell membranes of stomatocytosis patients usually exhibit abnormal permeability to the cations sodium and potassium, with consequent modification of intracellular water content, ranging from dehydrated (xerocytosis) to overhydrated (hydrocytosis) erythrocytes. The molecular basis of stomatocytosis is poorly understood. The variable clinical, laboratory, and pathophysiologic findings associated with the stomatocytosis syndromes suggest these are a complex collection of syndromes caused by various molecular defects. A locus for both hereditary xerocytosis maps to a region of chromosome 16, but the causative gene has not been identified. In one subgroup of stomatocytosis patients, some with a spherocytic phenotype, missense mutations in band 3 have been identified that convert it from an anion exchanger to a nonselective cation leak channel. In some hydrocytosis patients, mutations in the RHAG gene have been found.

Other conditions

Other conditions associated with hereditary stomatocytosis include the Rh deficiency syndromes, sitosterolemia, and familial deficiency of high-density lipoproteins. The rare Rh deficiency syndromes are associated with mild to moderate haemolytic anaemia and absent (Rhnull) to decreased (Rhmod) erythrocyte expression of Rh antigens associated with mutation in the RHD and RHAG genes. Sitosterolemia is associated with early onset atheroscleosis, anaemia, and macrothrombocytopenia associated with mutation of the ABCG5/ABCG8 cotransporters, leading to increased intestinal absorption and decreased biliary elimination of sterols, particularly those derived from plants. Familial deficiency of high-density lipoproteins (Tangier disease, OMIM 205 400) is due to mutation of ABCA1, a protein critical for cellular cholesterol export, leading to accumulation of tissue cholesterol esters manifest as enlarged orange-yellow tonsils, hepatosplenomegaly, cloudy corneas, neuropathy, and premature atherosclerosis. Affected patients exhibit mild to moderate haemolytic anaemia.

Acquired stomatocytosis has been observed in a large number of conditions, particularly hepatobiliary disease and acute alcoholism. Acquired stomatocytosis has also been seen in patients with various malignant neoplasms, cardiovascular disease, and after the administration of vinca alkaloids.

Acanthocytosis

Acanthocytes are dense, contracted erythrocytes with irregular ‘thorny’ projections. Acanthocytes may also been found on the peripheral smears of patients with abetalipoproteinemia, the McLeod phenotype, or one of the neuroacanthocytosis syndromes. Abetalipoproteinemia (OMIM 200 100) is associated with hypolipidemia, fat malabsorption, progressive ataxia, retinitis pigmentosa, and poor growth in childhood due to the inability to produce or secrete the B apoproteins B100 and B48, or defects in the microsomal triglyceride transfer protein (MTTP), required for production of apoprotein B-containing β‎-lipoproteins. The X-linked McLeod phenotype (OMIM 314 850) is due to mutation of XK, necessary for Kell antigen expression. Affected individuals experience compensated anaemia, susceptibility to Kell D antigen alloimmunization, and late-onset myopathy and nervous system abnormalities.

The neuroacanthocytosis syndromes are a heterogeneous group of neurodegenerative disorders including the McLeod syndrome, chorea-acanthocytosis due to mutation of chorein or VPS13A, Huntington’s Disease Like 2 due to mutation of junctophilin-3, and pantothenate kinase-associated neurodegeneration (formerly known as Hallervorden–Spatz syndrome and its allelic variant HARP syndrome–hypobetalipoproteinemia, acanthocytosis, retinitis pigmentosa, pallidal degeneration) due to mutations of pantothenate kinase 2. The cause of the acanthocytosis in these disorders is unknown.

Acquired acanthocytosis may be seen in patients with severe hepatic disease (commonly known as spur cell anaemia), hypothyroidism, malnutrition, and after splenectomy.

Further reading

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Bennett V, Healy J (2008). Organizing the fluid membrane bilayer: diseases linked to spectrin and ankyrin. Trends Mol Med, 14, 28–36.Find this resource:

Bolton-Maggs PH, et al. (2004). Guidelines for the diagnosis and management of hereditary spherocytosis. Br J Haematol, 126, 455–74.Find this resource:

Bonderman D, et al. (2005). Medical conditions increasing the risk of chronic thromboembolic pulmonary hypertension. Thromb Haemost, 93, 512–6.Find this resource:

Bruce LJ, et al. (2009). The monovalent cation leak in overhydrated stomatocytic red blood cells results from amino acid substitutions in the Rh-associated glycoprotein. Blood, 113, 1350–57.Find this resource:

Delaunay J (2004). The hereditary stomatocytoses: genetic disorders of the red cell membrane permeability to monovalent cations. Semin Hematol, 41, 165–72.Find this resource:

Delaunay J (2007). The molecular basis of hereditary red cell membrane disorders. Blood Rev, 21, 1–20.Find this resource:

Dhermy D, et al. (2007). Spectrin-based skeleton in red blood cells and malaria. Curr Opin Hematol, 14, 198–202.Find this resource:

Eber S, Lux SE (2004). Hereditary spherocytosis—defects in proteins that connect the membrane skeleton to the lipid bilayer. Semin Hematol, 41, 118–41.Find this resource:

Gallagher PG (2004). Hereditary elliptocytosis: spectrin and protein 4.1R. Semin Hematol, 41, 142–64.Find this resource:

Johnson CP, et al. (2007). Pathogenic proline mutation in the linker between spectrin repeats: disease caused by spectrin unfolding. Blood, 109, 3538–43.Find this resource:

Lusher JM, Barnhart MI (1980). The role of the spleen in the pathoophysiology of hereditary spherocytosis and hereditary elliptocytosis. Am J Pediatr Hematol Oncol, 2, 31.Find this resource:

    Nicolas V, et al. (2006). Functional interaction between Rh proteins and the spectrin-based skeleton in erythroid and epithelial cells. Transfus Clin Biol, 13, 23–8.Find this resource:

    Pasini EM, et al. (2006). In-depth analysis of the membrane and cytosolic proteome of red blood cells. Blood, 108, 791–801.Find this resource:

    Perrotta S, et al. (2008). Hereditary spherocytosis. Lancet, 372, 1411–26.Find this resource:

    Rees DC, et al. (2005). Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia. Br J Haematol, 130, 297–309.Find this resource:

    Schilling RF. (2009). Risks and benefits of splenectomy versus no splenectomy for hereditary spherocytosis--a personal view. Br J Haematol, 145, 728–32.Find this resource:

    Tracy ET, Rice HE (2008). Partial splenectomy for hereditary spherocytosis. Pediatr Clin North Am, 55, 503–19.Find this resource:

    Walker RH, et al. (2007). Neurologic phenotypes associated with acanthocytosis. Neurology, 68, 92–8.Find this resource:

    Yawata Y, et al. (2000). Characteristic features of the genotype and phenotype of hereditary spherocytosis in the Japanese population. Int J Hematol, 71, 118–35.Find this resource:

    Young NS (2006). Hematologic manifestations and diagnosis of parvovirus B19 infections. Clin Adv Hematol Oncol, 4, 908–10.Find this resource: