Hemoglobinopathies and Hemolytic Anemias Part 2

Protein Abnormalities

Hereditary Elliptocytosis

There are perhaps 250 to 500 cases of hereditary elliptocytosis per million population.10 Three morphologic variants are seen: (1) common hereditary elliptocytosis, (2) spherocytic hereditary elliptocytosis, and (3) stomatocytic hereditary elliptocytosis.12 Most patients with common hereditary elliptocytosis are heterozygous for this autosomal dominant disorder and have only elliptical RBCs or, at worst, compensated hemolysis. Homozygotes for the disorder may have severe uncompensated hemolytic anemia.

Under applied shear stress, erythrocytes assume an elliptical shape; when the stress is removed, the cell normally recoils to its discoid shape. It has been hypothesized that membrane defects in hereditary elliptocytosis interfere with normal recoil. The membrane defect appears to be a lesion in the membrane cy-toskeleton; RBC membranes from patients with hereditary ellip-tocytosis are almost invariably mechanically fragile.

The diagnosis is made in patients with extravascular intracor-puscular hemolysis who have elliptocytes on the peripheral smear. Elliptocytosis can also be seen in severe iron deficiency,myeloproliferative and myelodysplastic disorders, and, occasionally, cobalamin and folate deficiencies.12 Results of the osmotic fragility test are usually normal. Splenectomy has been useful in patients with severe common hereditary elliptocytosis.


Hereditary Propoikilocytosis

The syndrome of hereditary (autosomal recessive) pyropoi-kilocytosis, a variant of hereditary elliptocytosis, causes severe hemolysis in young children. It is caused by an abnormal a or | spectrin mutation. The blood smear shows extreme microcytosis and extraordinary variation in the size and shape of erythrocytes [see Figure 3]. Splenectomy may reduce the rate of hemolysis.

Hereditary Spherocytosis

Hereditary spherocytosis is usually inherited as an autosomal dominant trait and affects about 220 per million people worldwide. A rare autosomal recessive variant of hereditary spherocy-tosis has been described.

Because of a loss of surface membrane, RBCs assume a micro-spherocytic shape and thus cannot deform sufficiently to pass through the splenic vasculature; splenic trapping of RBCs, he-molysis, and a compensatory increase in RBC production result. The underlying membrane defects lead to budding of membrane vesicles under conditions of metabolic depletion. These membrane vesicles are enriched in phospholipids from the bilayer, as well as in associated transmembrane proteins [see Figure 2]. The underlying molecular lesions appear to consist of deficiencies of spectrin, spectrin-ankyrin, band 3, and band 4.2 (palladin).

About 25% of patients with hereditary spherocytosis have completely compensated hemolysis without anemia; their disorder is diagnosed only when a concomitant condition, such as infection or pregnancy, increases the rate of hemolysis or reduces the marrow’s compensatory capacity. In other patients, mild anemia, pig-mented gallstones, leg ulcers, and splenic rupture may develop. Aplastic crises may be precipitated by ordinary respiratory tract infections, especially by parvovirus infection.7 It is important to remember that this disease can become apparent during the first year of life, when increased splenic maturation resulting in RBC removal combined with a sluggish erythropoietic response can result in anemia severe enough to require RBC transfusion.

This diagnosis is suggested by a predominance of microsphe-rocytes on the peripheral smear [see Figure 3b], an MCHC of 35 g/dl or greater, reticulocytosis, mild jaundice, splenomegaly,and a positive family history, although at least half of newly diagnosed patients have no family history. Confirmation of the diagnosis is made by a 24-hour incubated osmotic fragility test. A negative Coombs test and a family history positive for hereditary spherocytosis rule against a diagnosis of acquired autoimmune hemolytic anemia. Splenectomy eradicates clinical manifestations of the disorder, including aplastic crises.

Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a somatic clonal disorder of hematopoietic stem cells. PNH involves the PIG-A gene, which maps to the short arm of the X chromosome.15 The mutation results in a deficiency of the membrane-anchoring protein phosphatidylinositol glycan class A; the resulting mature hematopoietic cells are usually chimeric. Normal human erythrocytes, and probably platelets and neutrophils, modulate complement attack by at least three GPI membrane-bound proteins: DAF (CD55), C8-binding protein (C8BP), and MIRL (CD59). In the absence of the GPI anchor, all of the proteins that use this membrane anchor will be variably deficient in the blood cells of persons with PNH.16 Because the defective synthesis of GPI affects all hematopoietic cells, patients with PNH may have variable degrees of anemia, neutropenia, or thrombocytopenia, or they may have complete bone marrow failure.17

Diagnosis

Classically, acute episodes of intravascular hemolysis are superimposed on a background of chronic hemolysis. The patient typically notes hemoglobinuria on voiding after sleep.18,19 Recurrent venous occlusions lead to pulmonary embolism and hepatic and mesenteric vein thrombosis, possibly resulting from release of procoagulant microparticles derived from platelets.20 A literature review found that thrombotic events accounted for 22% of deaths in patients with PNH.21 Occasionally, PNH patients with thrombosis are mistakenly thought to have psychosomatic disorders because they complain of recurrent severe pain in the abdomen and back that has no obvious cause. In these cases, the associated anemia and hemolysis may be very mild, and episodes of hemolysis do not necessarily correlate with bouts of pain.

A diagnosis of PNH should be considered in any patient with chronic or episodic hemolysis. The diagnosis should also be considered for patients with recurrent venous thromboembolism, particularly if the thrombus occurs in a site such as the inferior vena cava or the portal mesenteric system or if it produces Budd-Chiari syndrome. Evidence of intravascular hemolysis, such as hemoglobinemia; reduced serum haptoglobin; increased serum methemalbumin; hemoglobinuria; or hemosiderinuria, suggests the diagnosis. The combination of marrow hypoplasia and he-molysis is an important clue. PNH may occur in association with aplastic anemia. Erythrocyte morphology is usually normal. Diagnosis is made by specific tests based on fluorescence-activated cell sorter analysis using antibodies that quantitatively assess DAF (CD55) and particularly MIRL (CD59) on the erythrocyte or on the leukocyte surface.22

Treatment

In PNH, the anemia is occasionally so severe (hemoglobin level < 8 g/dl) that the patient needs transfusions regularly19; therefore, the choice of transfusion component is critical. It is believed that infusion of blood products containing complement may enhance hemolysis. Infusion of donor white blood cells (WBCs), which are ordinarily present in a unit of packed RBCs, into an HLA-immunized recipient may provide the antigen-antibody reaction that activates complement by the classical pathway. In such a case, the use of special leukocyte-poor units may be helpful [see 5:XTransfusion Therapy].

A trial of prednisone (e.g., 60 mg a day with rapid tapering, or 20 to 60 mg every other day) may reduce transfusion requirements and may be helpful in alleviating the anemia. Splenecto-my is of very questionable benefit. Surgery is risky in patients with PNH because stasis and trauma accentuate hemolysis and venous occlusion. If surgery is to be performed, prophylactic an-ticoagulation with warfarin in the perioperative period should be considered.

Patients with PNH are frequently iron deficient. The simple administration of iron to correct this defect, however, often aggravates hemolysis because iron therapy produces a cohort of new cells, many of which are susceptible to complement-mediated lysis. Transfusion before iron therapy may help circumvent this problem because it will decrease the erythropoietic stimulus to the marrow.

Thrombocytopenia resulting from poor platelet production may necessitate platelet transfusions [see 5:X Transfusion Thera-py].18 Budd-Chiari syndrome and inferior vena cava thrombosis must be diagnosed and treated quickly with heparin, followed by long-term administration of warfarin. If heparinization is ineffective, thrombolytic therapy (e.g., streptokinase) may be used.23 Children and adolescents with PNH that is complicated by aplastic anemia should be considered for allogeneic bone marrow transplantation.19,24 In case reports, the anemia associated with PNH responded to erythropoietin,25 and four patients with severe neutropenia and thrombocytopenia responded to combinations of granulocyte-colony-stimulating factor (G-CSF) and cyclosporine.26

Prognosis

A study of 80 patients with PNH indicated that median survival was 10 years.18 The causes of PNH-related death were thrombocytopenia, PNH hemolysis, thromboses, or PNH-associ-ated aplastic anemia [see 5:III Anemia: Production Defects]. Of interest is that 15% of patients experienced spontaneous remission.18 In rare instances, prolonged and severe iron loss may occur as a result of chronic hemosiderinuria, producing iron deficiency; some patients develop transfusion-associated hemochromatosis.

Acute myeloid leukemia may develop during the course of PNH. In one series, this occurred in three of 80 patients; in another series, of 220 patients, the incidence of myelodysplastic syndromes was 5% and the incidence of acute leukemia was 1%.

Abnormalities of Erythrocyte Metabolism

Defective reducing power

The reducing power of the erythrocyte is provided by reduced glutathione (GSH) and the reduced coenzymes nicotin-amide adenine dinucleotide (NADH) and nicotinamide-adenine dinucleotide phosphate (NADPH) [see Table 1]. When erythro-cytic stores of these materials are inadequate, hemoglobin and membrane-associated proteins can be oxidized, leading to the production of Heinz bodies, which consist predominantly of ox-idative degradation products of hemoglobin [see Figure 3b]. Ery-throcytes containing Heinz bodies are rigid and are therefore selectively removed by the reticuloendothelial system.

Defective Glutathione Synthesis

Deficiencies of certain enzymes involved in GSH synthesis lead to oxidative attacks on erythrocytes and to hemolysis. Several reports have described families whose members show almost negligible GSH synthesis and have hemolysis associated with the production of Heinz bodies. Glutathione peroxidase deficiency apparently contributes to hemolysis in newborn infants.

Glucose-6-Phosphate Dehydrogenises Deficiency

Glucose-6-phosphate dehydrogenase (G6PD) is the first enzyme in the pentose phosphate pathway, or hexose monophos-phate shunt. It catalyzes the conversion of NADP+ to NADPH, a powerful reducing agent. NADPH is a cofactor for glutathione reductase and thus plays a role in protecting the cell against ox-idative attack. RBCs deficient in G6PD are therefore susceptible to oxidation and hemolysis.27,28

G6PD deficiency is one of the most common disorders in the world; approximately 10% of male blacks in the United States are affected, as are large numbers of black Africans and some inhabitants of the Mediterranean littoral. This disorder confers some selective advantage against endemic malaria. For example, in a study in Ghana on pregnant women (who are highly susceptible to falciparum malaria and its consequences), the prevalence of infection was 66% in normal women, 58% in G6PD het-erozygotes, and 50% in homozygotes.29

The gene for G6PD is on the X chromosome at band q28; males carry only one gene for this enzyme, so those males that are affected by the disorder are hemizygous. Females are affected much less frequently because they would have to carry two defective G6PD genes to show clinical disease of the same severity as that in males. However, expression of a defective G6PD gene is not completely masked in heterozygous women; in fact, such women exhibit highly variable G6PD enzyme activity. According to the X-inactivation, or Lyon-Beutler, hypothesis,28 females heterozygous for G6PD have two cell lines: one that contains an active X chromosome with a gene for normal G6PD and another that contains an active X chromosome with a gene for deficient G6PD. Chance partly determines the relative proportions of the two cell lines, which in turn control the clinical severity of the defect.

Classification

There are three clinical classes of G6PD deficiency: class I, which is the uncommon chronic congenital nonspherocytic he-molytic anemia; class II, in which the enzyme deficiency is severe but hemolysis tends to be episodic; and class III, the most common variant, in which the enzyme deficiency is moderate and he-molysis is caused by oxidant attack. The severity of the hemolysis and the anemia is directly related to the magnitude of the enzyme deficiency, which is determined by the half-life of the enzyme. The normal G6PD half-life is 62 days; in class III G6PD deficiency, the enzyme has a half-life of 13 days; and in class II deficiency, G6PD has a half-life of several hours. The cloning and sequencing of the G6PD gene have clarified the classification of G6PD deficiency; before the sequencing of the G6PD gene, more than 300 variants of G6PD deficiency had been described.28

Etiology

Hemolysis occurs in persons with class III G6PD deficiency after exposure to a drug or substance that produces an oxidant stress. Ingestion of, or exposure to, fava beans may cause a devastating intravascular hemolysis (known as favism) in G6PD-de-ficient patients, but it usually occurs only in those with the Mediterranean variant of class II deficiency. Fava beans contain isouramil and divicine, two strong reducing agents whose actions eventuate in the oxidation of membrane proteins. This produces a rigid cell in which hemoglobin is confined to one part of the cytosol; the other part of the cytosol appears as a clear ghost (i.e., the classic bite, hemiblister, or cross-bonded cell) [see Figure 4]. These membrane defects cause extravascular and intravascular hemolysis.27 Severe infections, diabetic ketoacidosis, and renal failure also reportedly trigger hemolysis.

Bite, hemiblister, or cross-bonded cells are indicative of oxidative attack leading to oxidative hemolysis.

Figure 4 Bite, hemiblister, or cross-bonded cells are indicative of oxidative attack leading to oxidative hemolysis.

Diagnosis

Hemolytic anemia characterized by the appearance of bite cells and Heinz bodies after administration of certain drugs suggests the possibility of G6PD deficiency [see Table 2]. Dapsone, which is capable of inducing oxidant-type hemolysis, has increasingly come into use as prophylaxis for Pneumocystis carinii pneumonia in patients infected with HIV [see 7:XXXIH HIV and AIDS]. Therefore, it is important to screen potential users of dap-sone for G6PD deficiency with the standard enzymatic tests. Other agents with oxidative potential, such as amyl nitrite ("poppers"), can cause hemolysis.30

Other disorders to be considered in the differential diagnosis of oxidative hemolysis include unstable hemoglobinopathy, he-moglobin M disease, and deficiencies of other enzymes essential to glutathione metabolism. A G6PD screening test or direct enzyme assay usually resolves the question. Patients with A-type G6PD (class III) deficiency and brisk reticulocytosis, however, may have a near-normal G6PD level because young RBCs have relatively high G6PD levels. In such cases, it is best to repeat the tests when the reticulocyte count returns to normal. Information on genetic testing for G6PD deficiency can be found on the Internet at http://www.geneclinics.org.

Table 2 Drugs That Produce Hemolysis in G6PD-Deficient Patients

Class

Example

Antimalarials

Primaquine Chloroquine

Sulfonamides

Sulfamethoxazole Sulfapyridine

Sulfones

Dapsone

Acetanilid

Analgesics

Phenacetin

Acetylsalicylic acid (10 g/day)

Nitrofurans

Nitrofurantoin

Furazolidone

Water-soluble vitamin K derivatives

Menadiol

Treatment

Avoidance of drugs that may produce hemolysis is critical in management. Acute favism requires circulatory support, maintenance of good renal blood flow, and transfusions with erythrocytes that are not G6PD deficient. The physician must also be alert to the possible onset of disseminated intravascular coagulation.


Defects in Glycolysis

The series of reactions constituting the glycolytic pathway generates several products, such as ATP, that have various essential functions in erythrocyte metabolism [see Table 1]. Defects involve the major glycolytic pathway (Embden-Meyerhof pathway) and generally interfere with ATP production.

Pyruvate kinase (PK) catalyzes the formation of pyruvate, a reaction associated with ATP synthesis. After G6PD deficiency, PK deficiency (autosomal recessive) is the second most common hereditary enzymopathy. Hemolysis, mild jaundice, and, occasionally, palpable splenomegaly are the presenting problems. The peripheral smear usually reveals normal RBCs, but in a few cases, the RBCs show extreme spiculation. Aplastic crises may occur.7

Congenital nonspherocytic hemolysis raises the possibility of PK deficiency. An enzyme assay establishes the diagnosis. Splenectomy should be considered for patients who require transfusions.

Glucose-6-phosphate isomerase deficiency is the third most common enzymopathy that leads to hemolysis. Other enzy-mopathies are quite rare. Screening tests and specific assays are available for deficiencies of such enzymes as hexokinase, phos-phofructokinase, triose phosphate isomerase, phosphoglycerate kinase, and aldolase.

Defects in Nucleotide

Metabolism

In hemolytic anemia associated with pyrimidine 5′-nucleoti-dase deficiency, coarse basophilic stippling persists in mature erythrocytes, presumably because the enzyme deficiency prevents degradation of reticulocyte RNA. This accumulation results in expansion of the total RBC nucleotide pool to a level five times normal. Pyrimidine nucleotides accumulate, and adenine nucleotides are decreased. Glycolysis is impaired by an undetermined mechanism.

Disorders Involving Hemoglobin

Classification of The Hemoglobinopathies

The clinically important hemoglobinopathies are classified into five categories on the basis of the underlying defect. The defects are as follows:

1. Hemoglobin tends to gel or crystallize (e.g., sickle cell anemia or hemoglobin C disease).

2. Hemoglobin is unstable (e.g., the congenital Heinz body anemias).

3. Hemoglobin has abnormal oxygen-binding properties (e.g., the disorder caused by hemoglobin Chesapeake).

4. Hemoglobin is readily oxidized to methemoglobin (e.g., methemoglobinemia).

5. Hemoglobin chains are synthesized at unequal rates (e.g., the thalassemias).

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