Two distinct truncated variants of ankyrin associated with hereditary spherocytosis

Targeted next-generation sequencing identifies novel deleterious variants in ANK1 gene causing severe hereditary spherocytosis in Indian patients: expanding the molecular and clinical spectrum

Hereditary Spherocytosis (HS) is a common cause of hemolytic anemia varying from mild to severe hemolysis due to defects in red cell membrane protein genes, namely ANK1, SPTB, SPTA1, SLC4A1, and EPB42. These genes are considerably very large spaning 40-50 exons making gene-by-gene analysis costly and laborious by conventional methods. In this study, we explored 26 HS patients harboring 21 ANK1 variants identified by next-generation sequencing (NGS), characteristics and spectrum of the detected ANK1variants were analyzed in this study. Clinically, all the HS patients showed moderate to severe transfusion-dependent hemolytic anemia, some requiring splenectomy. We identified 13 novel and 8 reported variants, mainly 9 frameshifts, 2 missense, 6 nonsense, and 4 splice site ANK1 variants, using NGS technology. Frameshifts were remarkably the most common variant type seen in Indian HS patients with ANK1 gene defects. We have also explored expression levels of red cell membrane ankyrin protein by flow cytometry in 14 HS patients with ANK1 gene defects and a significant reduction in ankyrin protein expression has been found. This report mainly illustrates the molecular and phenotypic heterogeneity of ANK1 variants causing HS in Indian patients. Ankyrin-1 mutations are a significant cause of loss of function in dominant HS in the Indian population. Comprehensive genetic and phenotypic evaluation assists in implementing the knowledge of genetic patterns and spectrum of ANK1 gene variants, providing molecular support for HS diagnosis.

Spectrum of Ankyrin Mutations in Hereditary Spherocytosis: A Case Report and Review of the Literature

BACKGROUND/AIMS

Hereditary spherocytosis (HS) is a common pediatric hemolytic anemia caused by congenital red blood cell defects. HS due to ankyrin 1 (ANK1) mutations is the most common type. We explored an ANK1 mutation from an HS patient and reviewed the literature.

METHODS

We detected the mutation in a Chinese family in which 2 members were diagnosed with HS by next-generation sequencing. The proband was diagnosed with HS in the newborn period, based on clinical manifestations, laboratory data, and family history. The mutation spectrum of the ANK1 gene was summarized based on 85 patients diagnosed with HS carrying ANK1 mutations, and the ANK1 mutation spectrum was summarized and analyzed.

RESULTS

We identified a novel mutation affecting ANK1 gene splicing (a splicing mutation) in both the patient and her mother, which is a substitution of T>G 2 nt after exon 25 in intron 26. The study expands our knowledge of the ANK1 gene mutation spectrum, providing a molecular basis for HS.

CONCLUSION

A novel ANK1 mutation (NM_000037.3, c.2960+2T>G, intron 26) that is potentially associated with HS was identified. To date, 80 ANK1 mutations have been reported to be associated with HS in humans.

Mutational characteristics of ANK1 and SPTB genes in hereditary spherocytosis

The aim of this study was to describe the mutational characteristics in Korean hereditary spherocytosis (HS) patients. Relevant literatures including genetically confirmed cases with well-documented clinical summaries and relevant information were also reviewed to investigate the mutational gene- or domain-specific laboratory and clinical association. Twenty-five HS patients carried one heterozygous mutation of ANK1 (n = 13) or SPTB (n = 12) but not in SPTA1, SLC4A1, or EPB42. Deleterious mutations including frameshift, nonsense, and splice site mutations were identified in 91% (21/23), and non-hotspot mutations were dispersed across multiple exons. Genotype-phenotype correlation was clarified after combined analysis of the cases and the literature review; anemia was most severe in HS patients with mutations on the ANK1 spectrin-binding domain (p < 0.05), and SPTB mutations in HS patients spared the tetramerization domain in which mutations of hereditary elliptocytosis and pyropoikilocytosis are located. Splenectomy (17/75) was more frequent in ANK1 mutant HS (32%) than in HS with SPTB mutation (10%) (p = 0.028). Aplastic crisis occurred in 32.0% of the patients (8/25; 3 ANK1 and 5 SPTB), and parvovirus B19 was detected in 88%. The study clarifies ANK1 or SPTB mutational characteristics in HS Korean patients. The genetic association of laboratory and clinical aspects suggests comprehensive considerations for genetic-based management of HS.

β-III spectrin is critical for development of purkinje cell dendritic tree and spine morphogenesis

Mutations in the gene encoding β-III spectrin give rise to spinocerebellar ataxia type 5, a neurodegenerative disease characterized by progressive thinning of the molecular layer, loss of Purkinje cells and increasing motor deficits. A mouse lacking full-length β-III spectrin (β-III⁻/⁻) displays a similar phenotype. In vitro and in vivo analyses of Purkinje cells lacking β-III spectrin, reveal a critical role for β-III spectrin in Purkinje cell morphological development. Disruption of the normally well ordered dendritic arborization occurs in Purkinje cells from β-III⁻/⁻ mice, specifically showing a loss of monoplanar organization, smaller average dendritic diameter and reduced densities of Purkinje cell spines and synapses. Early morphological defects appear to affect distribution of dendritic, but not axonal, proteins. This study confirms that thinning of the molecular layer associated with disease pathogenesis is a consequence of Purkinje cell dendritic degeneration, as Purkinje cells from 8-month-old β-III⁻/⁻ mice have drastically reduced dendritic volumes, surface areas and total dendritic lengths compared with 5- to 6-week-old β-III⁻/⁻ mice. These findings highlight a critical role of β-III spectrin in dendritic biology and are consistent with an early developmental defect in β-III⁻/⁻ mice, with abnormal Purkinje cell dendritic morphology potentially underlying disease pathogenesis.

Novel roles for erythroid Ankyrin-1 revealed through an ENU-induced null mouse mutant

Insights into the role of ankyrin-1 (ANK-1) in the formation and stabilization of the red cell cytoskeleton have come from studies on the nb/nb mice, which carry hypomorphic alleles of Ank-1. Here, we revise several paradigms established in the nb/nb mice through analysis of an N-ethyl-N-nitrosourea (ENU)-induced Ank-1-null mouse. Mice homozygous for the Ank-1 mutation are profoundly anemic in utero and most die perinatally, indicating that Ank-1 plays a nonredundant role in erythroid development. The surviving pups exhibit features of severe hereditary spherocytosis (HS), with marked hemolysis, jaundice, compensatory extramedullary erythropoiesis, and tissue iron overload. Red cell membrane analysis reveals a complete loss of ANK-1 protein and a marked reduction in beta-spectrin. As a consequence, the red cells exhibit total disruption of cytoskeletal architecture and severely altered hemorheologic properties. Heterozygous mutant mice, which have wild-type levels of ANK-1 and spectrin in their RBC membranes and normal red cell survival and ultrastructure, exhibit profound resistance to malaria, which is not due to impaired parasite entry into RBC. These findings provide novel insights into the role of Ank-1, and define an ideal model for the study of HS and malarial resistance.

Human Red Blood Cells: Rheological Aspects, Uptake, and Release of Cytotoxic Drugs

The shape of a normal human red blood cell (RBC) is well known: under resting conditions it is that of a biconcave discocyte. However, RBCs can easily undergo transformation to other shapes with stomatocytes and echinocytes as extremes. Various anticancer agents, generally reactive and labile substances, e.g., oxazaphosphorines and fluoropyrimidines, can induce severe deformation of shape. Shape changes in erythrocytes can induce rheological disturbances, which occasionally have pathophysiological consequences. It is difficult to estimate the impact of shape changes on the in vivo behavior of agents of biological interest. However, it has been demonstrated for various anticancer agents that erythrocytes fulfill an important role in their uptake, transport, and release. Moreover, some anticancer agents are capable of influencing important transporters such as MRP and GLUT-1. Monitoring of erythrocyte concentrations of certain cytotoxic agents is therefore of interest as the data generated can have a predictive outcome for therapeutic efficacy. This is true for cyclophosphamide, ifosfamide, lometrexol, and 6-mercaptopurine, as well as MRP and GLUT-1 mediated agents.

The molecular basis of hereditary red cell membrane disorders

The red cell membrane is one of the best known membranes in terms of structure, function and genetic disorders. As any plasma membrane it mediates transport functions. It also provides the erythrocytes with their resilience and deformability. Many of the proteins and the genes performing these functions are known in great detail, although some disease-responsible genes are yet to be elucidated. Basic knowledge has shed light on important groups of genetic disorders. The latter include (i) the disorders of the red cell mechanics: hereditary spherocytosis, hereditary elliptocytosis and poikilocytosis, and (ii) the disorders of the passive flux of the monovalent cations across the membrane: the stomacytoses and allied conditions. Reciprocally, many information have come from genetics abnormalities. We will review the mutation-disease relationship. A number of points will be underscored: widespread weak alleles modulate the expression of the SPTA1 gene, encoding the alpha-chain of spectrin; mutations in the anion exchanger can give rise to an array of distinct nosological entities, including a renal condition; splenectomy is banned in the stomatocytoses; a variety of stomatocyosis is part of a pleiotropic syndrome that may includes perinatal fetal liquid effusions. The diagnosis, follow-up and treatment of the involved diseases have gradually improved.

Hematologically important mutations: Ankyrin variants in hereditary spherocytosis

The primary defect in the hereditary spherocytosis (HS) syndromes is a qualitative or quantitative alteration in one or more erythrocyte membrane proteins. Mutation of the erythrocyte membrane protein ankyrin are the most common cause of typical, dominant HS. Ankyrin mutations also cause nondominant spherocytosis due to ankyrin gene promoter or de novo mutations. In most cases, HS-related ankyrin mutations are private. A summary of reported HS-associated ankyrin gene mutations is provided in this report.

Simultaneous (AC)n microsatellite polymorphism analysis and single-stranded conformation polymorphism screening is an efficient strategy for detecting ankyrin-1 mutations in dominant hereditary spherocytosis

Nonsense/stop mutations in the ankyrin-1 gene (ANK1) are a major cause of dominant HS (dHS) (frequency of 23% in German dHS patients). To date, no common mutation has been found and therefore a simple mutation screening is not feasible. The reduced expression of one cDNA allele in the (AC)n microsatellite polymorphism of the ankyrin-1 gene, as seen in about 20% of Czech patients with dHS, may identify candidates with a possible frameshift/nonsense mutation. In order to verify the efficiency of this screening we screened the ankyrin-1 gene of 22 Czech dHS patients for both the reduced cDNA allele expression in the frequent (AC)n and the common exonic 26/39 polymorphisms, as well as for polymerase chain reaction (PCR) single-stranded conformation polymorphisms in any one of the 42 exons of ANK1. Anomalous PCR products were sequenced. We found seven new ANK1 frameshift/nonsense mutations in nine patients with, but in none of six patients without, a reduced cDNA allele expression (efficiency of 78%). We conclude that screening of dHS patients for such a reduced allele expression in common ANK1 polymorphisms is an efficient procedure for the identification of candidates for frameshift/nonsense mutations in the ankyrin-1 gene.

Splenectomy prolongs in vivo survival of erythrocytes differently in spectrin/ankyrin- and band 3–deficient hereditary spherocytosis

Red cell (RBC) deformability and membrane-bound immunoglobulin G (IgG) were studied to better understand premature clearance of erythrocytes in hereditary spherocytosis. Averaged deformability profiles from cells having comparable cell age revealed that splenectomy was more beneficial for spectrin/ankyrin-deficient than for band 3–deficient RBCs. Splenectomy prevented an early loss of young cells in both types of deficiencies. It had an additional beneficial effect on spectrin/ankyrin-deficient but not band 3–deficient RBCs. It prolonged the survival of mature spectrin/ankyrin-deficient RBCs such that they lost their deformability more slowly than RBCs from patients who had not undergone splenectomy. Band 3–deficient RBCs lost their deformability at the same rate before and after splenectomy. In HS patients with band 3 deficiency who underwent splenectomy, RBC deformability inversely correlated with the number of RBC-bound IgG (up to 140 molecules per cell). In spectrin/ankyrin deficiency, RBC-bound IgG remained at control levels (60 IgG or less per cell). It appears that spectrin/ankyrin-deficient RBCs escaped opsonization by releasing band 3–containing vesicles because their band 3 content and deformability dropped in parallel with increasing cell age. Band 3–deficient RBCs did not lose band 3 with increasing cell age. Hence, it is possible that band 3 clusters required for bivalent binding of low-affinity–IgG, naturally occurring antibodies were retained in band 3–deficient RBCs with a relative excess of skeletal proteins but were released from spectrin/ankyrin-deficient RBCs, in which vesicle budding was facilitated by an impaired skeleton.

Splenectomy prolongs in vivo survival of erythrocytes differently in spectrin/ankyrin- and band 3–deficient hereditary spherocytosis

Red cell (RBC) deformability and membrane-bound immunoglobulin G (IgG) were studied to better understand premature clearance of erythrocytes in hereditary spherocytosis. Averaged deformability profiles from cells having comparable cell age revealed that splenectomy was more beneficial for spectrin/ankyrin-deficient than for band 3–deficient RBCs. Splenectomy prevented an early loss of young cells in both types of deficiencies. It had an additional beneficial effect on spectrin/ankyrin-deficient but not band 3–deficient RBCs. It prolonged the survival of mature spectrin/ankyrin-deficient RBCs such that they lost their deformability more slowly than RBCs from patients who had not undergone splenectomy. Band 3–deficient RBCs lost their deformability at the same rate before and after splenectomy. In HS patients with band 3 deficiency who underwent splenectomy, RBC deformability inversely correlated with the number of RBC-bound IgG (up to 140 molecules per cell). In spectrin/ankyrin deficiency, RBC-bound IgG remained at control levels (60 IgG or less per cell). It appears that spectrin/ankyrin-deficient RBCs escaped opsonization by releasing band 3–containing vesicles because their band 3 content and deformability dropped in parallel with increasing cell age. Band 3–deficient RBCs did not lose band 3 with increasing cell age. Hence, it is possible that band 3 clusters required for bivalent binding of low-affinity–IgG, naturally occurring antibodies were retained in band 3–deficient RBCs with a relative excess of skeletal proteins but were released from spectrin/ankyrin-deficient RBCs, in which vesicle budding was facilitated by an impaired skeleton.

Gene transfer to ankyrin-deficient bone marrow corrects spherocytosis in vitro

OBJECTIVE

The goal of this study was to transfer by retroviral vector the cDNA for ankyrin to progenitors from normal bone marrow and from the nb/nb spherocytosis mutant deficient in expression of full-length ankyrin to achieve erythroid expression of functional ankyrin protein.

MATERIALS AND METHODS

A minigene composed of the human ankyrin promoter, murine ankyrin cDNA, and the 3' human domain corresponding to the ankyrin 2.2 isoform was assembled in the retroviral vector, pG1. Murine erythroleukemia (MEL) cells, normal murine bone marrow cells, 3T3 fibroblasts, and nb/nb mutant bone marrow and spleen cells were transduced with the retroviral supernatant. Transduced mutant cells were induced to differentiate in liquid culture. Gene transfer was assessed by colony polymerase chain reaction (PCR) and reverse transcriptase (RT)-PCR, immunofluorescence, and Southern, Northern, and Western blot analysis.

RESULTS

MEL cells, normal bone marrow progenitors, and nb/nb cells were all successfully transduced and expressed ankyrin by RT-PCR and Western blot. Transduced murine 3T3 fibroblasts and MEL cells exhibited cell membrane staining by immunofluorescence. Colony RT-PCR demonstrated dependence of expression on erythropoietin. In vitro, the transduced nb/nb cells matured to polychromatophils, whereas nontransduced nb/nb cells matured to microspherocytes.

CONCLUSION

Retroviral transfer of ankyrin corrected the defect leading to formation of microspherocytes in erythroid differentiation cultures from the nb/nb mutant. The human ankyrin promoter conferred erythropoietin-dependent expression in normal and mutant erythroid progenitors, which could have implications for the gene therapy of human hemolytic anemias.

Red blood cell membrane disorders

The recent discovery of the specific molecular defects in many patients with hereditary spherocytosis and hereditary elliptocytosis/pyropoikilocytosis partially clarifies the molecular pathology of these diseases. HE and HPP are caused by defects in the horizontal interactions that hold the membrane skeleton together, particularly the critical spectrin self-association reaction. Single gene defects cause red cells to elongate as they circulate, by a unknown mechanism, and are clinically harmless. The combination of two defective genes or one severe alpha spectrin defect and a thalassaemia-like defect in the opposite allele (alphaLELY) results in fragile cells that fragment into bizarre shapes in the circulation, with haemolysis and sometimes life-threatening anaemia. A few of the alpha spectrin defects are common, suggesting they provide an advantage against malaria or some other threat. HS, in contrast, is nearly always caused by family-specific private mutations. These involve the five proteins that link the membrane skeleton to the overlying lipid bilayer: alpha and beta spectrin, ankyrin, band 3 and protein 4.2. Somehow, perhaps through loss of the anchorage band 3 provides its lipid neighbours (Peters et al, 1996), microvesiculation of the membrane surface ensues, leading to spherocytosis, splenic sequestration and haemolysis. Future research will need to focus on how each type of defect causes its associated disease, how the spleen aggravates membrane skeleton defects (a process termed 'conditioning'), how defective red, cells are recognized and removed in the spleen, and why patients with similar or even identical defects can have different clinical severity. Emphasis also needs to be given to improving diagnostic tests, particularly for HS, and exploring new options for therapy, like partial splenectomy, which can ameliorate symptoms while better protecting patients from bacterial sepsis and red cell parasites, and perhaps from atherosclerosis (Robinette & Franmeni, 1977) and venous thrombosis (Stewart et al, 1996).