The congenital dyserythropoieitic anemias: genetics and pathophysiology

PURPOSE OF REVIEW

The congenital dyserythropoietic anemias (CDA) are hereditary disorders characterized by ineffective erythropoiesis. This review evaluates newly developed CDA disease models, the latest advances in understanding the pathogenesis of the CDAs, and recently identified CDA genes.

RECENT FINDINGS

Mice exhibiting features of CDAI were recently generated, demonstrating that Codanin-1 (encoded by Cdan1) is essential for primitive erythropoiesis. Additionally, Codanin-1 was found to physically interact with CDIN1, suggesting that mutations in CDAN1 and CDIN1 result in CDAI via a common mechanism. Recent advances in CDAII (which results from SEC23B mutations) have also been made. SEC23B was found to functionally overlap with its paralogous protein, SEC23A, likely explaining the absence of CDAII in SEC23B-deficient mice. In contrast, mice with erythroid-specific deletion of 3 or 4 of the Sec23 alleles exhibited features of CDAII. Increased SEC23A expression rescued the CDAII erythroid defect, suggesting a novel therapeutic strategy for the disease. Additional recent advances included the identification of new CDA genes, RACGAP1 and VPS4A, in CDAIII and a syndromic CDA type, respectively.

SUMMARY

Establishing cellular and animal models of CDA is expected to result in improved understanding of the pathogenesis of these disorders, which may ultimately lead to the development of new therapies.

Deficiency of Lipin2 Results in Enhanced NF-κB Signaling and Osteoclast Formation in RAW-D Murine Macrophages

Lipin2 is a phosphatidate phosphatase that plays critical roles in fat homeostasis. Alterations in Lpin2, which encodes lipin2, cause the autoinflammatory bone disorder Majeed syndrome. Lipin2 limits lipopolysaccharide (LPS)-induced inflammatory responses in macrophages. However, little is known about the precise molecular mechanisms underlying its anti-inflammatory function. In this study, we attempted to elucidate the molecular link between the loss of lipin2 function and autoinflammatory bone disorder. Using a Lpin2 knockout murine macrophage cell line, we showed that lipin2 deficiency enhances innate immune responses to LPS stimulation through excessive activation of the NF-κB signaling pathway, partly because of TAK1 signaling upregulation. Lipin2 depletion also enhanced RANKL-mediated osteoclastogenesis and osteoclastic resorption activity accompanied by NFATc1 dephosphorylation and increased nuclear accumulation. These results suggest that lipin2 suppresses the development of autoinflammatory bone disorder by fine-tuning proinflammatory responses and osteoclastogenesis in macrophages. Therefore, this study provides insights into the molecular pathogenesis of monogenic autoinflammatory bone disorders and presents a potential therapeutic intervention.

The pathogenesis, diagnosis and management of congenital dyserythropoietic anaemia type I

Congenital dyserythropoietic anaemia type I (CDA-I) is one of a heterogeneous group of inherited anaemias characterised by ineffective erythropoiesis. CDA-I is caused by bi-allelic mutations in either CDAN1 or C15orf41 and, to date, 56 causative mutations have been documented. The diagnostic pathway is reviewed and the utility of genetic testing in reducing the time taken to reach an accurate molecular diagnosis and avoiding bone marrow aspiration, where possible, is described. The management of CDA-I patients is discussed, highlighting both general and specific measures which impact on disease progression. The use of interferon alpha and careful management of iron overload are reviewed and suggest the most favourable outcomes are achieved when CDA-I patients are managed with a holistic and multidisciplinary approach. Finally, the current understanding of the molecular and cellular pathogenesis of CDA-I is presented, highlighting critical questions likely to lead to improved therapy for this disease.

KLF1 mutation E325K induces cell cycle arrest in erythroid cells differentiated from congenital dyserythropoietic anemia patient-specific induced pluripotent stem cells

Krüppel-like factor 1 (KLF1), a transcription factor controlling definitive erythropoiesis, is involved in sequential control of terminal cell division and enucleation via fine regulation of key cell cycle regulator gene expression in erythroid lineage cells. Type IV congenital dyserythropoietic anemia (CDA) is caused by a monoallelic mutation at the second zinc finger of KLF1 (c.973G>A, p.E325K). We recently diagnosed a female patient with type IV CDA with the identical missense mutation. To understand the mechanism underlying the dyserythropoiesis caused by the mutation, we generated induced pluripotent stem cells (iPSCs) from the CDA patient (CDA-iPSCs). The erythroid cells that differentiated from CDA-iPSCs (CDA-erythroid cells) displayed multinucleated morphology, absence of CD44, and dysregulation of the KLF1 target gene expression. In addition, uptake of bromodeoxyuridine by CDA-erythroid cells was significantly decreased at the CD235a⁺/CD71⁺ stage, and microarray analysis revealed that cell cycle regulator genes were dysregulated, with increased expression of negative regulators such as CDKN2C and CDKN2A. Furthermore, inducible expression of the KLF1 E325K, but not the wild-type KLF1, caused a cell cycle arrest at the G1 phase in CDA-erythroid cells. Microarray analysis of CDA-erythroid cells and real-time polymerase chain reaction analysis of the KLF1 E325K inducible expression system also revealed altered expression of several KLF1 target genes including erythrocyte membrane protein band 4.1 (EPB41), EPB42, glutathione disulfide reductase (GSR), glucose phosphate isomerase (GPI), and ATPase phospholipid transporting 8A1 (ATP8A1). Our data indicate that the E325K mutation in KLF1 is associated with disruption of transcriptional control of cell cycle regulators in association with erythroid membrane or enzyme abnormalities, leading to dyserythropoiesis.

R2* imaging of transfusional iron burden at 3T and comparison with 1.5T

PURPOSE

To determine normative R2* values in the liver and heart at 3T, and establish the relationship between R2* at 3T and 1.5T over a range of tissue iron concentrations.

MATERIALS AND METHODS

A total of 20 healthy control subjects and 14 transfusion-dependent patients were scanned at 1.5T and 3T. At each field strength R2* imaging was performed in the liver and heart.

RESULTS

Normative R2* values in the liver were estimated from the control group to be 39.2 +/- 9.0 second(-1) at 1.5T and 69.1 +/- 21.9 second(-1) at 3T. Normative cardiac values were estimated as 23.4 +/- 2.2 second(-1) at 1.5T and 30.0 +/- 3.7 second(-1) at 3T. The combined R2* data from patients and control subjects exhibited a linear relationship between 3T and 1.5T. In the liver, the line of best fit to the 3T vs. 1.5T data had a slope of 2.00 +/- 0.06 and an intercept of -11 +/- 4 second(-1). In the heart, it had a slope of 1.88 +/- 0.14 and an intercept of -15 +/- 4 second(-1).

CONCLUSION

These preliminary data suggest that the iron-dependent component of R2* scales linearly with field strength over a wide range of tissue iron concentrations. The incidence of susceptibility artifacts may, however, also increase with field strength.

Congenital disorders of glycosylation: review of their molecular bases, clinical presentations and specific therapies

Congenital disorders of glycosylation (CDG, formerly named carbohydrate-deficient glycoprotein syndromes) are a rapidly growing family of inherited disorders affecting the assembly or processing of glycans on glycoconjugates. The clinical spectrum of the different types of CDG discovered so far is variable, ranging from severe multisystemic disorders to disorders restricted to specific organs. This review deals with clinical, diagnostic, and biochemical aspects of all characterized CDGs, including a disorder affecting the N-glycosylation of erythrocytes, congenital dyserythropoietic anemia type II (CDA II/HEMPAS), and the first disorders affecting O-glycosylation. Since the clinical spectrum of symptoms in CDG is variable and may be unspecific, a generous selective screening for the presence of CDG is recommended.

Possible oxidative stress involvement in congenital dyserythropoietic anemia type 1

INTRODUCTION

Congenital dyserythropoietic anemia type 1 (CDA1) patients may suffer from iron overload, associated with oxidative damage. The aim of this study was to evaluate possible involvement of oxidative stress in the pathogenesis of CDA1.

STUDY DESIGN

: Blood samples from 10 children diagnosed as CDA1 patients from five Bedouin families, were studied. In this study, activities of superoxide dismutase and catalase were evaluated as well as methemoglobin, plasma total thiols, plasma total antioxidant capacity and glycerol lysis time.

RESULTS

Normal values were found for superoxide dismutase, methemoglobin, trolox equivalent antioxidant capacity and total plasma thiols in CDA1 patients. However average catalase levels were significantly reduced (P<0.001) and glycerol lysis test was significantly prolonged (P<0.001). Ferritin levels, which were slightly increased in all patients, positively correlated with catalase values (r = 0.74, P = 0.022).

CONCLUSION

Oxidative stress has not been proven in CDA1 pediatric patients. Some indications of oxidative damage exist, but it may not be directly related to the mechanism of anemia.

Congenital disorders involving defective N-glycosylation of proteins

This review deals with several of the main autosomal recessive congenital disorders involving defective N-glycosylation of proteins (the addition of glycans linked to the polypeptide chain by a beta-linkage between the anomeric carbon of N-acetylglucosamine and the amido group of L-asparagine). These congenital disorders of glycosylation (CDG, previously known as carbohydrate-deficient glycoprotein syndromes) are a group of multisystemic diseases often involving severe psychomotor retardation. Six distinct variants of CDG in group I (types Ia-If) have been described to date and the defects have been localized to deficiencies in the assembly of the dolichylpyrophosphate-linked oligosaccharide N-glycan precursor and its transfer to asparagine residues on the nascent polypeptides. Two variants of CDG group II (types IIa and IIb) have been identified as defects in the processing of protein-bound N-glycans. Hereditary erythroblastic multinuclearity with a positive acidified-serum lysis test (HEMPAS; congenital dyserythropoietic anemia type II) presents as a relatively mild dyserythropoietic anemia. The genetic defect in most cases of HEMPAS is not known, but alpha-3/6-mannosidase II is involved in at least some patients. Leukocyte adhesion deficiency type II (LAD II) is a rare disorder characterized by recurrent infections, persistent leukocytosis and severe mental and growth retardation. LAD II is due to lack of availability of GDP-fucose. The study of these diseases and of relevant animal models has provided strong evidence that N-glycans are essential for normal mammalian development.

Short report: erythrocyte membranes from a patient with congenital dyserythropoietic anaemia type I (CDA-I) show identical, although less pronounced, glycoconjugate abnormalities to those from patients with CDA-II (HEMPAS)

Congenital dyserythropoietic anaemias (CDAs) are rare hereditary disorders characterized by ineffective erythropoiesis and multinuclearity of erythroblasts. Three main types of the disease have been described. Glycoconjugate abnormalities in erythrocyte membrane glycoconjugates, consisting of hypoglycosylation of band 3 and accumulation of certain glycosphingolipids including lactotriaosylceramide, neolactotriaosylceramide and polyglycosylceramides, have been described only in patients with CDA type II (CDA-II). We report on identical, although less pronounced, abnormalities in erythrocyte glycoconjugates from a patient with CDA-I. A low degree of hypoglycosylation of band 3 in our patient with CDA-I suggests that hypoglycosylation is not a cause, but, most probably, a consequence of dyserythropoiesis.

HEMPAS. Hereditary erythroblastic multinuclearity with positive acidified serum lysis test

Congenital dyserythropoietic anemia type II or HEMPAS (hereditary erythroblastic multinuclearity with positive acidified serum lysis test) is a genetic anemia in humans caused by a glycosylation deficiency. Erythrocyte membrane glycoproteins, such as band 3 and band 4.5, which are normally glycosylated with polylactosamines lack these carbohydrates in HEMPAS. Polylactosamines accumulate as glycolipids in HEMPAS erythrocytes. Analysis of N-glycans from HEMPAS erythrocyte membranes revealed a series of incompletely processed N-glycan structures, indicating defective glycosylation at N-acetylglucosaminyltransferase II (GnT-II) and/or alpha-mannosidase II (MII) steps. Genetic analysis has identified two cases from England in which the MII gene is defective. Mutant mice in which the MII gene was inactivated by homologous recombination resulted in a HEMPAS-like phenotype. On the other hand, linkage analysis of HEMPAS cases from southern Italy excluded MII and GnT-II as the causative gene, but identified a gene on chromosome 20q11. HEMPAS is therefore genetically heterogeneous. Regardless of which gene is defective, HEMPAS is characterized by incomplete processing of N-glycans. The study of HEMPAS will identify hitherto unknown factors affecting N-glycan synthesis.

Soluble transferrin receptor as a potential determinant of iron loading in congenital anaemias due to ineffective erythropoiesis

Congenital anaemias due to ineffective erythropoiesis may be associated with excessive iron absorption and progressive iron loading. We investigated whether the soluble transferrin receptor (TfR) level was related to the degree of iron overload in 20 patients with thalassaemia intermedia, six patients with congenital dyserythropoietic anaemia type II (CDA II) and four patients with X-linked congenital sideroblastic anaemia (XLSA). All but two patients had increased serum ferritin levels (median 601 microgram/l, range 105-2855 microgram/l). Multiple regression analysis showed that 62% (P < 0.0001) of the variation in serum ferritin was explained by age and by changes in soluble TfR.

Congenital dyserythropoietic anaemias: clinical features, haematological morphology and new biochemical data

Three types of congenital dyserythropoietic anaemia (CDA) were originally identified on the basis of the pattern of dysplastic changes in the erythroblasts and the results of the acidified serum lysis test (Ham test). These were designated CDA types I, II and III. Several other types have been described subsequently and new forms continue to be reported. Some patients with CDA develop iron overload even without repeated blood transfusion and may present with the complications of severe iron overload. Dysmorphic features are seen in some cases, especially of CDA type I. In CDA type II, incomplete processing of N-linked oligosaccharides leads to a marked reduction of polylactosamines associated with band 3 of the red cell membrane. A few cases of CDA type III develop lymphoid neoplasms. Some of the Swedish cases of CDA type III have developed a retinal abnormality characterized by angioid streaks and macular degeneration. The chromosomal localizations of the disease gene in CDA types I and II and in the Swedish family with CDA type III are now known, but the identities of the mutant genes are still unknown. Cases of CDA type I have shown a partial haematological response to interferon-alpha, however the biochemical basis of this response is unclear. An important step in the diagnosis of sporadic cases of CDA is the exclusion of known causes of acquired dyserythropoiesis.

Reduced interferon-alpha production by Epstein-Barr virus transformed B-lymphoblastoid cell lines and lectin-stimulated lymphocytes in congenital dyserythropoietic anaemia type I

The concentrations of interferon-alpha (IFN-alpha) in supernatants from cultures of Epstein-Barr virus (EBV) transformed B-lymphoblastoid cell lines derived from seven patients with congenital dyserythropoietic anaemia (CDA) type I were below the 95% confidence limits for those derived from six healthy subjects. In contrast, the concentrations of IFN-alpha in supernatants from cultures of EBV-transformed lymphoblastoid cell lines derived from four patients with other types of CDA and four patients with hereditary sideroblastic anaemia were normal. Supernatants from cultures of peripheral blood lymphocytes stimulated with phytohaemagglutinin or pokeweed mitogen contained less IFN-alpha when the cells were derived from patients with CDA type I than when derived from healthy subjects. Since patients with CDA type I show a substantial haematological response to treatment with IFN-alpha, the data suggest that impaired IFN-alpha production may be an important pathogenetic mechanism in CDA type I.

Inherited disorders of glycoprotein synthesis: cell biological insights

Disorders of glycoprotein synthesis have been described only recently, and few have been studied extensively at both the clinical and biochemical level. The identification and characterization of these rare diseases are important, not only for the patients and their families, but because they offer enormous insight into biological processes. For example, the targeting of acid hydrolases to lysosomes by mannose-6-phosphate was discovered as a direct result of the elucidation of the defect in I-cell disease. The notion of carbohydrates as targeting agents continues to have ramifications today, with the success of macrophage-targeted enzyme replacement therapy for Gaucher disease. Likewise, confirmation of the in vivo role of fucose-containing glycans and selectins in neutrophil function came from studies using specimens from patients with leucocyte adhesion deficiency type II due to reduced availability of GDP-fucose. Identification of the in vivo ligands of selectins also has implications for anti-inflammatory therapies. Macular corneal dystrophy and spondyloepiphyseal dysplasia tarda offer an opportunity to investigate the number of different sulfotransferases in cells, their substrates, and their tissue expression. The Ehlers-Danlos progeroid variant offers insight into the function and regulation of the proteoglycan decorin, and suggests that several of the enzymes involved in proteoglycan synthesis may function as a multienzyme complex. The common occurrence of hypergonadotropic hypogonadism in patients with galactosemia or carbohydrate-deficient glycoprotein protein syndrome, due to defective N-linked glycosylation, suggests that ovarian function is particularly dependent on proper glycan-synthesis. A host of other concepts await discovery as a fuller contingent of human disorders of glycan synthesis achieves recognition.

Incompletely processed N-glycans of serum glycoproteins in congenital dyserythropoietic anaemia type II (HEMPAS)

Congenital dyserythropoietic anaemia type II, or HEMPAS (hereditary erythroblastic multinuclearity with positive acidified serum lysis test) is a genetic disease caused by membrane disorganization of erythroid cells. The primary defect of this disease lies in the gene encoding enzyme(s) which is responsible for the biosynthesis of Asn-linked oligosaccharides chains of glycoproteins (Fukuda et al, 1990). In order to know whether this gene defect affects the glycosylation in the cells other than the erythroid cells, the carbohydrate structures of the transferrin isolated from the sera of HEMPAS patients were analysed. Fast atom bombardment mass spectrometry analysis showed the presence of high mannose type and hybrid type oligosaccharides in the HEMPAS transferrin which is in contrast to the complex-type oligosaccharides found in the normal transferrin. The results strongly suggest that biosynthesis of Asn-linked oligosaccharide chains in HEMPAS hepatocytes is disturbed. As a result, the serum glycoproteins with incompletely processed carbohydrates are circulating in the plasma in HEMPAS patients, but they must have been absorbed by the cells in the liver and the reticuloendothelial cells. Upon intravenous infusion into rats, as much as 30% of the HEMPAS transferrin was cleared from the plasma circulation. The majority of the HEMPAS transferrins was taken up by the liver, and transferrin was distributed both in the hepatocytes and the Kupffer cells. The presence of enormous amounts of aberrantly glycosylated serum glycoproteins may lead to the liver cirrhosis and secondary tissue siderosis seen in HEMPAS patients.

A new congenital dyserythropoietic anaemia

A new dominantly inherited dyserythropoietic anaemia is described. In the bone marrow, many of the nonspecific morphological characteristics described in congenital dyserythropoietic anaemias were seen; however, dysmorphic cells were rare. The acidified serum test was positive with one out of 17 sera tested; the negative sera included two that had haemolysed HEMPAS erythrocytes in the acid Ham test. Anti-i-agglutination was negative. No aberrations of red cell membrane protein glycosylation were observed. Serum cholesterol was low. Bilirubin conjugation was deficient but icterus was resolved by treatment with phenobarbital.

Anomalous clustering of underglycosylated band 3 in erythrocytes and their precursor cells in congenital dyserythropoietic anemia type II

Congenital dyserythropoietic anemia type II (CDA II or HEMPAS) is a genetic anemia caused by membrane abnormality. Our previous studies indicated that in HEMPAS, erythrocytes band 3 and band 4.5 are not glycosylated by polylactosaminoglycans. The present study was aimed at determining how such underglycosylated band 3 behaves in erythrocyte membranes. By using anti-band 3 antibodies, immunogold electron microscopy revealed that band 3s are clustered in HEMPAS erythrocyte membranes. By freeze-fracture electron microscopy, band 3s were also seen as lightly clumped intramembrane particles on a protoplasmic fracture face. Erythrocyte precursor cells stained by anti-band 3 antibodies showed that band 3s are present in the cytoplasmic area of the reticulocytes as scattered single particles. However, in young erythrocytes in which intracellular membranes are almost degenerated, band 3s were clustered in the cytoplasmic area of the cell. These observations suggest that band 3s cluster before they are incorporated into the plasma membranes of HEMPAS erythrocytes. In contrast to band 3, glycophorin A detected by anti-glycophorin A antibodies did not show a noticeable difference between normal and HEMPAS. Such a clustering of band 3 may cause abnormal localization of band 3-associated proteins and may thus result in the macroscopic membrane abnormality seen in HEMPAS erythrocytes.

Iron loading in congenital dyserythropoietic anaemias and congenital sideroblastic anaemias

The relationship between body iron status, degree of anaemia, erythroid expansion, age and sex has been studied in eight patients with congenital dyserythropoietic anaemia (CDA) and two patients with congenital sideroblastic anaemia, who had received no or very few blood transfusions and no medicinal iron during the course of their illness. All patients had increased iron stores. Iron load was mild in three women in the reproductive age and severe in two men, in middle age, who had evidence of parenchymal organ dysfunction. Iron loading, as judged by the plasma ferritin concentration, was independent of the degree of anaemia while it was closely related to the patient's age and the degree of increase in the total erythropoietic activity. It is concluded that patients with CDA or congenital sideroblastic anaemia are at high risk of developing haemochromatosis in middle age. Prophylactic phlebotomy or iron chelation therapy should be considered for such patients.

A new case of congenital dyserythropoietic anaemia, type III: studies of the cell cycle distribution and ultrastructure of erythroblasts and of nucleic acid synthesis in marrow cells

The clinical and laboratory findings in an asymptomatic 19-year-old Welshman with congenital dyserythropoietic anaemia (CDA) type III are described. The blood film showed macrocytosis and red cell fragmentation and there was biochemical evidence of intravascular haemolysis. The bone marrow showed erythroid hyperplasia, megaloblastic erythropoiesis and several giant multinucleate erythroblasts. Some mononucleate erythroblasts were large and had relative DNA contents of 4-8c and the bi- and multinucleate erythroblasts had total DNA contents of 2-16c. Some of the multinucleate erythroblasts displayed a variety of ultrastructural abnormalities, including marked differences in the appearances of the individual nuclei within the same cell. The marrow cells gave a normal deoxyuridine-suppressed value indicating that the megaloblastic changes were not caused by an impairment of the methylation of deoxyuridylate. The rates of incorporation of 14C-glycine and 14C-adenine into both the DNA and RNA of bone marrow cells were within the normal range. Furthermore, the average rate of elongation of newly-synthesised, 3H-thymidine-labelled daughter DNA strands, assessed by hydroxyapatite chromatography of alkali-denatured DNA was found to be normal. The results suggest that there is no impairment of DNA replication in the majority of the erythroblasts and that the abnormality of erythropoiesis resulted from disturbances during mitosis and the G2 phase.

Normal serum ferritin levels in a patient with HEMPAS syndrome and iron overload

Serum ferritin levels in a patient with HEMPAS syndrome (hereditary erythroblastic multinuclearity associated with positive acidified serum test) were correlated with body iron stores directly measured on spleen and liver biopsy specimens as well as by quantitative serial phlebotomy. Normal serum ferritin concentrations were found in the presence of a moderate excess in iron stores (approximately 6-12 times normal). They temporarily increased after transfusion and splenectomy with a prompt return to the normal range. As repeated phlebotomies over a period of nine months depleted the excess iron stores, the serum ferritin ultimately decreased to a subnormal concentration. The serum ferritin concentration was not a reliable index of increased body iron stores in this iron overloaded patient, but did reflect their depletion by serial phlebotomy.

Studies of ineffective erythropoiesis and peripheral haemolysis in congenital dyserythropoietic anaemia type II

Erythrokinetic parameters were estimated in six patients suffering from congenital dyserythropoietic anaemia type II (CDA II) by means of a mathematical model of iron kinetics. A wide variation in effectiveness of erythroid activity was observed, and a significant negative correlation was found between ineffective erythropoiesis and peripheral haemolysis. In four patients with prominent peripheral haemolysis, splenectomy was carried out. Marked improvement in their clinical condition and in haemoglobin level resulted.