CLINICAL AND LABORATORY FEATURES
CDA II is an autosomal, recessively inherited condition in which the severity of anemia varies from mild to severe and in which approximately 7 percent of cases are transfusion-dependent.2,8,30,31 This disorder becomes variably manifest in infancy, childhood, or adolescence. Very few cases are characterized by clinical manifestations during intrauterine life, but hydrops fetalis caused by severe anemia has been reported.32,33 More commonly, anemia is mild and, in several cases, diagnosis has been based on the appearance of complications (mainly iron overload) during adulthood.2,8
Erythrocytes of CDA II patients lyse in acidified serum (Ham test; Chap. 40) because of a naturally occurring immunoglobulin M class antibody that recognizes an antigen present on CDA II red cells but which is absent on normal cells. Thus, the acronym HEMPAS (hemolytic anemia with a positive acidified serum test) is commonly used as a synonym for CDA II. The technical difficulty of this test, and the fact that cross-testing of more than 30 normal sera is needed to obtain a reliable result, has undermined its usefulness.34
The clinical picture of CDA II includes hemolytic anemia with marrow erythroid expansion, commonly with splenomegaly, hepatomegaly, intermittent jaundice, and cholelithiasis.30,31 The blood film exhibits moderate to marked anisocytosis and anisochromia and a number of spherocytes. This, along with the patient’s clinical appearance, may lead to confusion of CDA II with hereditary spherocytosis (HS; Chap. 46). However, typically in HS, the reticulocyte count in comparison to hemoglobin level is higher and the serum transferrin receptor level is lower. Moreover, the majority of HS cases are inherited as autosomal dominant and, thus, a parent is likely to have findings of spherocytosis on blood examination, whereas CDA II is invariably an autosomal recessive condition. Despite these differentiating features, CDA II is at times only diagnosed after the failure of splenectomy to normalize anemia when performed for suspected HS. In the marrow, 10 to 30 percent of intermediate and late erythroblasts have two or more nuclei or lobulated nuclei (see Fig. 39–2A). Karyorrhexis (fragmentation of the nucleus) is common. Gaucher-like cells may develop as a result of phagocytosis of erythroblasts by macrophages. Ringed sideroblasts are present in severe forms.12 Electron microscopy shows structures that have been misnamed as “double membrane” (see Fig. 39–2B). These are cisternae of the endoplasmic reticulum (ER) that run along the red cell plasma membrane inner surface, and which contain ER-specific proteins, as shown by immunochemistry labeling.35 Sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE), followed by immunoblots, reveals the presence of calreticulin, glucose-regulated protein 78, and disulfide isomerase; these are specific for the ER and are not detected in normal individuals.35
The diagnostic hallmark of CDA II is analysis of erythrocyte membrane proteins by SDS-PAGE identifying narrower band size and faster migration of erythrocyte anion transporters (AE1 or band 3) and band 4.5 proteins.36,37 Increased destruction of red blood cells in CDA II is associated with hypoglycosylation of AE1, which causes clustering of this protein on the red cell surface and contributes to erythrocyte destruction in the spleen.38 Rare patients have been reported without this characteristic SDS-PAGE pattern; it is recommended that these should be classified as CDA II–like conditions.
Pathognomonic hypoglycosylation of AE1 protein is the outcome of the expression of the mutated gene SEC23B.39
Sequencing analysis in 33 patients from 28 unrelated families showed heterogeneous mutations in the SEC23B gene, either in compound heterozygous or homozygous states.2,8,39 An in vitro model of gene silencing demonstrated that suppression of SEC23B expression recapitulates the cellular defects in SEC23B-silenced cells.39 Knockdown of zebrafish SEC23B also leads to aberrant erythrocyte development.39
SEC23B is a cytoplasmic coat protein (COP) II component involved in the secretory pathway of eukaryotic cells. COPII is a multisubunit complex essential for transport of correctly folded proteins from the ER toward the Golgi apparatus.40 This pathway is critical for membrane homeostasis, localization of proteins within cells and secretion of extracellular factors.40,41
CDA II belongs to COPII-related human genetic disorders.42 Alterations in SAR1B, a paralogue of SEC23B, are identified as the cause of chylomicron retention disease (Anderson disease),43 while a specific mutation in the SEC23A gene causes craniolenticulosutural dysplasia (Boyadjiev-Jabs syndrome).44 The specificity of the CDA II phenotype seems to be determined by tissue-specific expression of SEC23B versus SEC23A during erythroid differentiation.39 Alternatively, this specificity could be explained by the presence of tissue-specific proteins (such as band 3 in red blood cells) which might require high levels and full function of a specific COPII component to be correctly transported.42,45
So far, more than 60 different causative mutations have been described worldwide.2,8 A genotype–phenotype correlation seems to exist. Particularly, compound heterozygosity for missense and nonsense mutations tends to produce more severe clinical presentations than homozygosity or compound heterozygosity for two missense mutations. Homozygosity or compound heterozygosity for two nonsense mutations has not been reported, suggesting it may be lethal.46 Sec23b-deficient mice (Sec23b gt/gt) have been generated and are born without anemia but die shortly after birth, with degeneration of secretory organs, including the pancreas and salivary glands.47
The disparate phenotypes in mouse and human could result from residual SEC23B function associated with the hypomorphic mutations observed in humans, or, alternatively, might be explained by species-specific functional differences.48
TREATMENT, COURSE, AND PROGNOSIS
The clinical course of this condition is quite heterogeneous. Treatment approaches depend on age, severity of phenotype and comorbidity. Most patients have only mild or moderate anemia and do not require medical intervention. Approximately 10 percent of neonates need at least one erythrocyte transfusion, and some remain transfusion-dependent.8,31 In most adolescents and adults, transfusional needs are limited to aplastic crises, pregnancy, coexistent infections, or major operations.
The more common, moderate forms may only be diagnosed in adult life because of iron overload (Chap. 43) that is consistently observed even in the absence of transfusions.2,8,49 Patients with severe forms of CDA II may be transfusion-dependent. In some cases, severe phenotypes could be the result of additional genetic abnormalities, such as coinheritance of glucose-6-phosphate dehydrogenase (G6PD) deficiency or thalassemic trait.50
The iron overload is associated with high levels of growth differentiation factor 15 (GDF15).51 However, GDF15 concentrations are significantly lower in CDA II compared to CDA I patients, despite a similar degree of iron overload in both patient groups. It can be speculated that additional signals may determine hepatic hepcidin expression and the degree of iron overload in CDA II.51
Ferritin levels should be controlled at least annually, even in patients with only mild anemia. Achievement of normal ferritin concentrations is desirable.52 Iron chelation should be instituted when ferritin level exceeds 500 to 1000 cg/L (Chap. 43). If phlebotomies are tolerated, this is the preferred treatment. In instances where the patient cannot tolerate phlebotomy, chelating agents may be used.
Cholelithiasis and splenomegaly are common complications. Coinheritance of the UGT1A (TA)7/(TA)7 genotype could account for the increased rate of gallstones.53 Cholelithiasis, which is frequent in all types of CDA, may require cholecystectomy; decision making should follow therapy guidelines for cholelithiasis.54 Splenectomy is not universally recommended for CDA II or CDA I; individual decisions should be influenced by transfusion dependency and the presence of a massively enlarged spleen. Generally accepted criteria for splenectomy have not been defined. Splenectomy leads to a moderate, sustained increase in hemoglobin concentrations and decrease of serum bilirubin levels, but it does not prevent iron overload, and hemoglobin levels postsplenectomy generally do not reach normal values.5,6 In non–transfusion-dependent patients, it is advisable to follow the guidelines for mild cases of HS.54
Allogeneic marrow transplantation from an human leukocyte antigen (HLA)-identical sibling has been successful in transfusion-dependent children with very severe CDA II and in one adult with CDA II and β-thalassemia trait.32,33,55,56