Hematologically important mutations: Ankyrin variants in hereditary spherocytosis

Genotype-degree of hemolysis correlation in hereditary spherocytosis

BACKGROUND

Hereditary spherocytosis (HS) is a common inherited hemolytic anemia, caused by mutations in five genes that encode erythrocyte membrane skeleton proteins. The red blood cell (RBC) lifespan could directly reflect the degree of hemolysis. In the present cohort of 23 patients with HS, we performed next-generation sequencing (NGS) and Levitt's carbon monoxide (CO) breath test to investigate the potential genotype-degree of hemolysis correlation.

RESULTS

In the present cohort, we identified 8 ANK1,9 SPTB,5 SLC4A1 and 1 SPTA1 mutations in 23 patients with HS, and the median RBC lifespan was 14(8-48) days. The median RBC lifespan of patients with ANK1, SPTB and SLC4A1 mutations was 13 (8-23), 13 (8-48) and 14 (12-39) days, respectively, with no statistically significant difference (P = 0.618). The median RBC lifespan of patients with missense, splice and nonsense/insertion/deletion mutations was 16.5 (8-48), 14 (11-40) and 13 (8-20) days, respectively, with no significant difference (P = 0.514). Similarly, we found no significant difference in the RBC lifespan of patients with mutations located in the spectrin-binding domain and the nonspectrin-binding domain [14 (8-18) vs. 12.5 (8-48) days, P = 0.959]. In terms of the composition of mutated genes, 25% of patients with mild hemolysis carried ANK1 or SPTA1 mutations, while 75% of patients with mild hemolysis carried SPTB or SLC4A1 mutations. In contrast, 46.7% of patients with severe hemolysis had ANK1 or SPTA1 mutations and 53.3% of patients with severe hemolysis had SPTB or SLC4A1 mutations. However, there was no statistically significant difference in the distribution of mutated genes between the two groups (P = 0.400).

CONCLUSION

The present study is the first to investigate the potential association between genotype and degree of hemolysis in HS. The present findings indicated that there is no significant correlation between genotype and degree of hemolysis in HS.

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.

Clinical manifestations of 17 Chinese children with hereditary spherocytosis caused by novel mutations of the ANK1 gene and phenotypic analysis

Background: Hereditary spherocytosis (HS) is an autosomal dominant (AD) and autosomal recessive (AR) disorder that is mostly caused by mutations of the erythrocyte membrane-related gene ANK1. Methods: Clinical and genetic testing data of 17 HS children with ANK1 gene mutations were retrospectively collected. Clinical manifestations and phenotypic analysis of HS were summarized based on our experience and literature review. Results: A total of 17 mutations of the ANK1 gene were identified from 17 probands (12 sporadic cases and five familial cases), including 15 novel mutations and two previously reported ones. Among the 15 novel variants of ANK1, there were four non-sense mutations, four frameshift mutations, three splicing mutations, three missense mutations and one in-frame deletion of three amino acids. In the present study, HS patients with mutations in membrane binding domains had significantly lower hemoglobin (Hb) levels and higher total bilirubin (T-Bil) levels than those with mutations in regulatory domains. After reviewing and analyzing all available published reports of Chinese HS patients carrying ANK1 mutations in PubMed and Chinese journals, there were no significant differences in Hb, Ret and T-Bil between different mutation types or mutation regions. Conclusion: Mutations of the ANK1 can be inherited or de novo. Clinical manifestations of HS in children caused by ANK1 mutations are similar to those of other types of hemolytic anemia. Our report expands the mutation spectrum of HS, thus providing references for clinical management and genetic counseling of HS.

Spherocytosis-Related L1340P Mutation in Ankyrin Affects Its Interactions with Spectrin

Previously, we reported a new missense mutation in the ANK1 gene that correlated with the hereditary spherocytosis phenotype. This mutation, resulting in L1340P substitution (HGMD CM149731), likely leads to the changes in the conformation of the ankyrin ZZUD domain important for ankyrin binding to spectrin. Here, we report the molecular and physiological effects of this mutation. First, we assessed the binding activity of human β-spectrin to the mutated ZZUDL1340P domain of ankyrin using two different experimental approaches-the study of association and dissociation responses of the spectrin-ankyrin binding domain and a sedimentation assay. In addition, we documented the changes in morphology caused by the overexpressed ankyrin ZZUD domain in human cell models. Our results prove the key role of the L1340 aa residue for the correct alignment of the ZZUD domain of ankyrin, which results in binding the latter with spectrin within the erythrocyte membrane. Replacing L1340 with a proline residue disrupts the spectrin-binding activity of ankyrin.

Literature review on genotype-phenotype correlation in patients with hereditary spherocytosis

Hereditary spherocytosis (HS) is a prevalent inherited hemolytic disorder primarily reported in Caucasians. Recently, next-generation sequencing (NGS) techniques have shown tremendous potential in the diagnosis of HS. HS commonly originates from variants in ANK1, SPTB, SLC4A1, SPTA1, and EPB42. This review is focused on thirteen previous clinical studies on genotype-phenotype correlation, which might promote the role of causative variants in the diagnosis and prognosis of HS. Most studies were focused on the pediatric population and Asian countries. The occurrence of novel variants was common in each cohort, and variants with a high frequency of causative genes were demonstrated. In conclusion, patients with variants in SPTA1 and SLC4A1 were reported to have more severe and milder anemia, respectively. ANK1 and SPTB are the most common variants in patients with HS, and no significant difference in phenotypes was observed between patients with variants in ANK1 vs SPTB. The types and locations of variants might influence the phenotype of each genotype, whereas the roles of concomitant pathogenic genes and the source of variants deserve further investigation.

Study on Management of Blood Transfusion Therapy in Patients with Hereditary Spherocytosis

Hereditary spherocytosis (HS) is a chronic hemolytic disorder caused by inherited defects in the red blood cell membrane. This study discusses the treatment strategy for the decline in hemoglobin level in three HS probands with moderately severe or severe hemolysis and summarizes the appropriate laboratory tests that help improve clinical management of blood transfusion in HS patients. Three probands who were diagnosed with HS in our hospital and their family members were included in this study. Clinical data of the three families were reviewed to summarize their hematopoietic characteristics. DNA from all family members of the 3 HS probands was amplified by polymerase chain reaction (PCR) and sequenced by the Sanger method to assess genetic relation for HS. Based on the sequencing results, the type of mutated membrane protein in each proband was analyzed using the eosin-5'-maleimide (EMA) binding test and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The hemoglobin level was reduced in all 3 probands after different levels of infection. The fluorescence of EMA-labeled red blood cell (RBC) was decreased. DNA sequencing showed that His54Pro, Leu1858Val, and 6531-12C>T compound heterozygous mutations were present in the SPTA1 gene of patient I-1, Arg344Gln and c.609+86G>A heterozygous mutations were present in the SLC4A1 gene of patient II-1, and Leu2032Pro homozygous mutation was present in the SPTB gene of patient III-1. SDS-PAGE results demonstrated that the concentration of band 3 was reduced in II-1, whereas the levels of the corresponding mutant proteins in the other probands were unchanged. The family members of the respective patients presented mutations in major genes causing HS. The Leu2032Pro mutation identified in patient III-1 is a new missense mutation of the SPTB gene in the Chinese population that has never been reported in literature previously. The presence or absence of acute or chronic infections is a critical deciding factor for the treatment and clinical management of HS patient via blood transfusion. For patients with infections, hemoglobin concentration can be restored once the infection is controlled, thus obviating the need for proper infection control before blood transfusion.

Clinical manifestation and phenotypic analysis of novel gene mutation in 28 Chinese children with hereditary spherocytosis

Purpose

Objective to summarize the clinical features and laboratory findings of 28 Chinese children with hereditary spherocytosis (HS), and analyze these mutations.

Method

Collected and analyzed the clinical data of all children and their parents, and completed the relevant laboratory examinations of all children. Analyzed the sequence of related genes by second‐generation sequencing technology, and verified the suspected mutations by Sanger sequencing method. Analyzed all biological information using the Single Nucleotide Polymorphism database, the 1000 Human Genome Project, and the Exosome Aggregation Consortium.

Result

New mutations were detected in the HS coding region of 28 children. Among them, there were 13 cases (46.4%) with ANK1 mutation, 10 cases (35.7%) with SPTB mutation, three cases (10.7%) with SLC4A1 mutation, and two cases (7.2%) with SPTA1 mutation. All mutations cause amino acid changes in the coding gene, as well as subsequent changes in protein structure or loss of function.

Conclusion

All the newly discovered gene coding region mutation sites detected are the suspected pathogenic causes of the 28 Chinese children. At the same time, the second‐generation gene sequencing technology is an effective means to diagnose HS. Different mutation types and different mutation regions have no significant correlation with the severity of anemia. The novel gene mutation sites in 28 children studied in this paper have not yet been included in the human genome database, dbSNP (v138), or ExAC database. The new gene mutations found in HS children can provide a theoretical basis for further exploring the genetic causes of HS in Chinese children.

Next-Generation Sequencing Revealed Disease-Causing Variants in Two Genes in a Patient With Combined Features of Spherocytosis and Antley-Bixler Syndrome With Genital Anomalies and Disordered Steroidogenesis

Conventionally, patients with combined rare diseases are often difficult to diagnose. This is because some clinicians tend to consider the multiple disease symptoms as the presentation of a complicated “syndrome.” This pattern of thinking also confines their way of filtering pathogenic mutations. Some real pathogenic mutations might be ignored due to not covering all disease presentations. Here we report the case of a girl who was suffering from spherocytosis and Antley-Bixler syndrome with genital anomalies and disordered steroidogenesis. She remained undiagnosed even after targeted gene detection before. However, after performing next-generation sequencing and analyzing the sequencing data, we identified two mutations: c.2978T > A in ANK1 and c.1370G > A in POR. Our findings and experiences in diagnosing these mutations could contribute to the existing knowledge on the clinical and genetic diagnosis of patients with disease presentations in multiple systems.

Mosaic Small Supernumerary Marker Chromosome Derived from Five Discontinuous Regions of Chromosome 8 in a Patient with Neutropenia and Oral Aphthous Ulcer

Small supernumerary marker chromosomes (sSMCs) are characterized as additional centric chromosome fragments which are too small to be classified by cytogenetic banding alone and smaller than or equal to the size of chromosome 20 of the same metaphase spread. Here, we report a patient who presented with slight neutropenia and oral aphthous ulcers. A mosaic de novo sSMC, which originated from 5 discontinuous regions of chromosome 8, was detected in the patient. Formation of the sSMC(8) can probably be explained by a multi-step process beginning with maternal meiotic nondisjunction, followed by post-zygotic anaphase lag, and resulting in chromothripsis. Chromothripsis is a chromosomal rearrangement which occurs by breakage of one or more chromosomes leading to a fusion of surviving chromosome pieces. This case is a good example for emphasizing the importance of conventional karyotyping from PHA-induced peripheral blood lymphocytes and examining tissues other than bone marrow in patients with inconsistent genotype and phenotype.

Deciphering molecular heterogeneity of Indian families with hereditary spherocytosis using targeted next-generation sequencing: First South Asian study

Defects in various erythrocyte membrane proteins genes (ankyrin, band-3, β- and α-spectrin and protein 4·2) can cause hereditary spherocytosis (HS). This molecular heterogeneity of HS, together with co-inherited genetic modifiers, results in marked phenotypic variability among patients. We studied the molecular spectrum and genotype-phenotype correlations in 73 families (with 113 patients) with HS. Deleterious variants including nonsense (42%), deletions (18%), splice site (20%), missense (10%) and duplication/insertion (10%) were found in 47 patients. The variants detected included sporadic and dominantly-inherited defects in ANK1 (53·2%), SPTB (36·2%) and SLC4A1 (4·2%). Compound heterozygous variants in SPTA1 (6·4%) showed autosomal recessive inheritance. Alpha-spectrin variants were associated with severe anaemia and splenectomy alleviated symptoms. Co-inherited glucose-6-phosphate dehydrogenase (G6PD) deficiency was found in 15%. G6PD variants (n = 5) led to greater transfusion requirements (1-8 times) in males with HS. Homozygosity (41%) for the promoter variant of UGT1A1 (Gilbert syndrome) led to a significantly higher mean bilirubin level (126·54 µmol/l) with a higher frequency of cholelithiasis (30%) (P < 0·001). This first-ever south Asian study on the molecular spectrum of HS found ANK1 and SPTB genes variants to be the commonest with inheritance being sporadic/dominant. Next-generation sequencing provided a relatively sensitive and rapid tool for molecular diagnosis with a diagnostic yield of 64·4%.

Targeted next-generation sequencing identified a novel ANK1 mutation associated with hereditary spherocytosis in a Chinese family

Objectives: Hereditary spherocytosis (HS) represents a group of congenital diseases characterized by sphere-shaped erythrocytes on peripheral blood smears. The typical clinical manifestations of HS include haemolysis, jaundice, splenomegaly, and gallstones. Ankyrin1, encoded by the ANK1 gene, is the predominant protein in red blood cells. Defects in ankyrin1 lead to a decrease in erythrocyte surface area, a spherical shape of erythrocytes and, in particular, loss of membrane elasticity and mechanical stability. The purpose of this study was to investigate a Chinese family with HS to determine the causative gene mutation and explore the genotype-phenotype correlation. Methods: A 4-year-old boy was diagnosed with HS based on typical clinical features. In addition, his father had a high possibility of HS. Targeted next-generation sequencing (NGS) followed by Sanger sequencing was performed in the proband and his parents. Results: One mutation in the ANK1 gene was recognized. c1801-1G > C in exon 17, which leads to splicing defects, was detected. To confirm the c1801-1G > C variant, samples from the proband and his parents were analysed by Sanger sequencing, and Sanger verification showed that this mutation was inherited from the father. Conclusion: The present study confirmed that a novel mutation in ANK1 may be causative of HS, which plays an important role in expanding the mutational spectrum of ANK1 mutations. This may contribute to accurate genetic counselling. And it is helpful for understanding the correlation of the genotype and phenotype.

A de novo ANK1 mutation associated to hereditary spherocytosis: a case report

Background

Hereditary spherocytosis (HS) is a type of hemolytic anemia caused by abnormal red cell membrane skeletal proteins with few unique clinical manifestations in the neonate and infant. An ANK1 gene mutation is the most common cause of HS.

Case presentation

The patient was a 11-month-old boy who suffered from anemia and needed a regular transfusion therapy at an interval of 2–3 months. Hematological investigations showed moderate anemia (Hb80 g/L). Red cells displayed microcytosis (MCV76.4 fl, MCH25.6 pg, MCHC335 g/L). The reticulocytes were elevated (4.8%) and the spherocytes were increased (10%). Direct antiglobulin test was negative. Biochemical test indicated a slight elevation of bilirubin, mainly indirect reacting (TBIL32.5 μmol/L, IBIL24 μmol/L). The neonatal HS ratio is 4.38, obviously up the threshold. Meanwhile, a de novo ANK1 mutation (exon 25:c.2693dupC:p.A899Sfs*11) was identified by next-generation sequencing (NGS). Thus, hereditary spherocytosis was finally diagnosed.

Conclusions

Gene detection should be considered in some hemolytic anemia which is difficult to diagnose by routine means. We identified a novel de novo ANK1 heterozygous frameshift mutation in a Yi nationality patient while neither of his parents carried this mutation.

Electronic supplementary material

The online version of this article (10.1186/s12887-019-1436-4) contains supplementary material, which is available to authorized users.

Whole-exome sequencing for the genetic diagnosis of congenital red blood cell membrane disorders in Taiwan

PURPOSE

Congenital hemolytic anemia caused by red blood cell (RBC) membrane defects is a heterogeneous group of disorders. The present study aimed to search the causative gene mutations in patients with RBC membrane disorders in Taiwan.

MATERIALS AND METHODS

Next-generation sequencing approach using whole-exome sequencing (WES) was performed. Sanger sequencing was performed for confirmation of variants detected in WES in patients and their family members.

RESULTS

Five causative variants, including two ANK1, two SPTA and one SPTB variants, were detected in four patients. All these variants, except one SPTA1 variant c.83G > A (p.R28H), are novel variants. Their pedigree analysis showed one de novo SPTA1 mutation c.83G > A (p.R28H) combined with α^LELY, one de novo ANK1 mutation c.1034C > A (p.A345E), one autosomal dominant combined SPTA1 c.4604A > C (p.Q1535P) and SPTB c.6203 T > C (p.L2068P) mutations and one autosomal dominant ANK1 c.4462C > T (p.R1488X) mutation.

CONCLUSIONS

Our data demonstrated that WES is an efficient tool for determining genetic etiologies of RBC membrane disorders and can facilitate accurate diagnosis and genetic counseling. Additional studies should be conducted on larger cohorts to investigate the distribution of gene mutations in patients with RBC membrane disorders in Taiwan.

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.

Synergistic Integration of Laboratory and Numerical Approaches in Studies of the Biomechanics of Diseased Red Blood Cells

In red blood cell (RBC) disorders, such as sickle cell disease, hereditary spherocytosis, and diabetes, alterations to the size and shape of RBCs due to either mutations of RBC proteins or changes to the extracellular environment, lead to compromised cell deformability, impaired cell stability, and increased propensity to aggregate. Numerous laboratory approaches have been implemented to elucidate the pathogenesis of RBC disorders. Concurrently, computational RBC models have been developed to simulate the dynamics of RBCs under physiological and pathological conditions. In this work, we review recent laboratory and computational studies of disordered RBCs. Distinguished from previous reviews, we emphasize how experimental techniques and computational modeling can be synergically integrated to improve the understanding of the pathophysiology of hematological disorders.

Host genetics in malaria: lessons from mouse studies

Malaria remains a deadly parasitic disease caused by Plasmodium, claiming almost half a million lives every year. While parasite genetics and biology are often the major targets in many studies, it is becoming more evident that host genetics plays a crucial role in the outcome of the infection. Similarly, Plasmodium infections in mice also rely heavily on the genetic background of the mice, and often correlate with observations in human studies, due to their high genetic homology with humans. As such, murine models of malaria are a useful tool for understanding host responses during Plasmodium infections, as well as dissecting host-parasite interactions through various genetic manipulation techniques. Reverse genetic approach such as quantitative trait loci studies and random mutagenesis screens have been employed to discover novel host genes that affect malaria susceptibility in mouse models, while other targeted studies utilize mouse models to validate observation from human studies. Herein, we review the findings from the past and present studies on murine models of hepatic and erythrocytic stages of malaria and speculate on how the current mouse models benefit from the recent development in CRISPR/Cas9 gene editing technology.

Molecular Genetic Mechanisms of Hereditary Spherocytosis: Current Perspectives

With the widespread use of genetic diagnostic technologies, many novel mutations have been identified in hereditary spherocytosis (HS)-related genes, including SPTA1, SPTB, ANK1, SLC4A1, and EPB42. However, mutations in HS-related genes are dispersed and nonspecific in the diagnosis of some HS patients, indicating significant heterogeneity in the molecular deficiency of HS. It is necessary to provide the molecular and genetic characteristics of these 5 genes for clinicians to examine HS. Here, we reviewed the recent proposed molecular genetic mechanisms of HS.

An ANK1 IVS3-2A>C mutation causes exon 4 skipping in two patients from a Chinese family with hereditary spherocytosis

Hereditary spherocytosis (HS) is a congenital hemolytic anemia that affects the cell membrane of red blood cells and is characterized by the presence of spherical-shaped erythrocytes in the peripheral blood film. The clinical manifestation of HS ranges from asymptomatic to severe cases that require transfusion during early childhood. HS is caused by mutations in red blood cell membrane protein encoding genes, including ANK1, EPB42, SLC4A1, SPTA1, and SPTB. Mutations of the ANK1 gene account for 75% of all HS cases, and these particular mutations are typically inherited in an autosomal dominant manner. In this study, heterozygous an ANK1 IVS3-2A>C mutation was identified in a 7-year-old girl with Coombs-negative and severe hemolytic jaundice using targeted next-generation sequencing (NGS) and Sanger sequencing. Spherocytes were observed in a peripheral smear. Osmotic fragility was increased, and glucose-6-phosphate dehydrogenase (G6PD) activity was normal. A genetic mutation screen for α- and β-thalassemia was negative. Autoimmune antibody tests were negative. Both the girl and her affected father received a splenectomy. Patient-derived peripheral blood mononuclear cells showed skipping of exon 4 in the mRNA, which confirmed the splicing mutation effect of the ANK1 IVS3-2A>C mutation. Moreover, the anemia was ameliorated after splenectomy. Our results demonstrate that the ANK1 IVS3-2A>C mutation may lead to exon 4 skipping of the ANK1 gene and cause HS.

Ankyrin-1 Gene Exhibits Allelic Heterogeneity in Conferring Protection Against Malaria

Allelic heterogeneity is a common phenomenon where a gene exhibits a different phenotype depending on the nature of its genetic mutations. In the context of genes affecting malaria susceptibility, it allowed us to explore and understand the intricate host–parasite interactions during malaria infections. In this study, we described a gene encoding erythrocytic ankyrin-1 (Ank-1) which exhibits allelic-dependent heterogeneous phenotypes during malaria infections. We conducted an ENU mutagenesis screen on mice and identified two Ank-1 mutations, one resulting in an amino acid substitution (MRI95845), and the other a truncated Ank-1 protein (MRI96570). Both mutations caused hereditary spherocytosis-like phenotypes and confer differing protection against Plasmodium chabaudi infections. Upon further examination, the Ank-1^(MRI96570) mutation was found to inhibit intraerythrocytic parasite maturation, whereas Ank-1^(MRI95845) caused increased bystander erythrocyte clearance during infection. This is the first description of allelic heterogeneity in ankyrin-1 from the direct comparison between two Ank-1 mutations. Despite the lack of direct evidence from population studies, this data further supported the protective roles of ankyrin-1 mutations in conferring malaria protection. This study also emphasized the importance of such phenomena in achieving a better understanding of host–parasite interactions, which could be the basis of future studies.

Novel mutations in patients with hereditary red blood cell membrane disorders using next-generation sequencing

To diagnose and investigate the genotype-phenotype relationship in intractable hereditary red blood cell (RBC) membrane cases, we have utilized next-generation sequencing (NGS) to develop a high-throughput, highly sensitive assay. Three unrelated families including 15 individuals were analysed with a panel interrogating 600 genes related to haematopathy disorders. Where possible, inheritance patterns of pathogenic mutations were determined by sequencing the relatives. We identified 2 novel mutations in ANK1 (Y216X and E142X) responsible for hereditary spherocytosis (HS) that were stop-gain single nucleotide variants (SNVs). Furthermore, a novel SPTA1 mutation (H54P) was identified; it is a nonsynonymous SNV and is associated with hereditary elliptocytosis (HE). In addition, patients who also carried erythropoiesis gene mutations showed more severe disease phenotype. The NGS panel provides a fast and accurate method for molecular diagnosis in patients with intractable hereditary RBC membrane disorders. An approach integrating medical history, clinical and molecular testing, and pedigree analysis is beneficial for these patients and families.

The Epimmunity Theory: The Single Cell Defenses against Infectious and Genetic Diseases

Single cell defense against diseases defines “epimmunity.” Epimmunity is complementary to the immune system and can neither be substituted by innate nor by acquired immunity. Epimmunity, the proposed new branch of immunity, is further explored and analyzed for enucleated mature mammalian erythrocytes and nucleated erythrocytes of non-mammalian vertebrates leading to the development of “The Epimmunity Theory.” Enucleation of mammalian erythroblast and inactivation of nuclei in erythrocytes of non-mammalian vertebrates are major contributors to the collective immunity: epimmunity, innate, and acquired. The fact that diseases of mature erythrocytes (MEs) are rare supports the notion that a single cell can resist microbial and genetic diseases; MEs are refractory to malaria and cancer. Nucleated cells, such as B-cells, T-cells, hepatocytes, and cell developmental stages are susceptible to genetic and specific microbial diseases depending on their nuclear activities and the receptors they express; such cells show lower epimmunity relative to MEs. Epimmunity is important as a disease insulator that prevents the spread of diseases from an infected tissue to the majority of other tissues. Breakdown of epimmunity may lead to disease development.

Identification of a novel de novo ANK1 R1426* nonsense mutation in a Chinese family with hereditary spherocytosis by NGS

Hereditary spherocytosis (HS) is an inherited heterogeneous hemolytic anemia, characterized by the presence of spherical-shaped erythrocytes on the peripheral blood smear, and the clinical manifestation ranges from asymptomatic to severely anemic, and transfusion-dependent patients. Mutations in at least five genes (ANK1, EPB42, SLC4A1, SPTA1, and SPTB) have been identified so far, and mutations of ANK1 gene are responsible for the majority of all HS cases. In this study, targeted next generation sequencing (NGS) was applied to identify a novel de novo ANK1 c.4276C>T (p.R1426*) nonsense mutation in a Chinese family with a patient of HS who was diagnosed clinically with only 10% spherical-shaped erythrocytes in the peripheral blood and received splenectomy. Sanger sequencing further confirmed that only the patient carried heterozygous ANK1 c.4276C>T nonsense mutation, while none of his parents or his young brother carried this mutation. Moreover, consistent with the genetic findings, the anemia was ameliorated after splenectomy. RBCs increased from 2.74 × 1012/L pre-surgery to 4.76 × 1012/L one month post-surgery, and hemoglobin increased from 66g/L to 126g/L respectively. This is the first report of ANK1 c.4276C>T (p.R1426*) heterozygous nonsense mutation responsible for HS. Our results also demonstrate that targeted NGS may provide a powerful approach for rapid genetic test of HS.

Autoinhibition of ankyrin-B/G membrane target bindings by intrinsically disordered segments from the tail regions

Ankyrins together with their spectrin partners are the master organizers of micron-scale membrane domains in diverse tissues. The 24 ankyrin (ANK) repeats of ankyrins bind to numerous membrane proteins, linking them to spectrin-based cytoskeletons at specific membrane microdomains. The accessibility of the target binding groove of ANK repeats must be regulated to achieve spatially defined functions of ankyrins/target complexes in different tissues, though little is known in this regard. Here we systemically investigated the autoinhibition mechanism of ankyrin-B/G by combined biochemical, biophysical and structural biology approaches. We discovered that the entire ANK repeats are inhibited by combinatorial and quasi-independent bindings of multiple disordered segments located in the ankyrin-B/G linkers and tails, suggesting a mechanistic basis for differential regulations of membrane target bindings by ankyrins. In addition to elucidating the autoinhibition mechanisms of ankyrins, our study may also shed light on regulations on target bindings by other long repeat-containing proteins.

A novel ENU-induced ankyrin-1 mutation impairs parasite invasion and increases erythrocyte clearance during malaria infection in mice

Genetic defects in various red blood cell (RBC) cytoskeletal proteins have been long associated with changes in susceptibility towards malaria infection. In particular, while ankyrin (Ank-1) mutations account for approximately 50% of hereditary spherocytosis (HS) cases, an association with malaria is not well-established, and conflicting evidence has been reported. We describe a novel N-ethyl-N-nitrosourea (ENU)-induced ankyrin mutation MRI61689 that gives rise to two different ankyrin transcripts: one with an introduced splice acceptor site resulting a frameshift, the other with a skipped exon. Ank-1^(MRI61689/+) mice exhibit an HS-like phenotype including reduction in mean corpuscular volume (MCV), increased osmotic fragility and reduced RBC deformability. They were also found to be resistant to rodent malaria Plasmodium chabaudi infection. Parasites in Ank-1^(MRI61689/+) erythrocytes grew normally, but red cells showed resistance to merozoite invasion. Uninfected Ank-1^(MRI61689/+) erythrocytes were also more likely to be cleared from circulation during infection; the “bystander effect”. This increased clearance is a novel resistance mechanism which was not observed in previous ankyrin mouse models. We propose that this bystander effect is due to reduced deformability of Ank-1^(MRI61689/+) erythrocytes. This paper highlights the complex roles ankyrin plays in mediating malaria resistance.

Association of Single-Nucleotide Polymorphism in ANK1 with Late-Onset Alzheimer's Disease in Han Chinese

Recently, two CpG sites in ankyrin 1 (ANK1) gene were identified to be hypermethylated and associated with Alzheimer's disease (AD)-related neuropathology in two large independent studies. Genetic variations are indicated to be involved in DNA methylation, especially when the associated single-nucleotide polymorphisms (SNPs) are located adjacent to the CpG site. Accordingly, ANK1 polymorphisms might contribute to late-onset AD (LOAD) risk. One polymorphism rs515071 was identified to be a potential risk factor for type 2 diabetes (T2D). As shared genetic background was found underlying T2D and AD, we postulate that rs515071 polymorphism may be associated with late-onset AD (LOAD) risk and assessed the association in 982 LOAD patients and 1346 sex- and age-matched healthy controls. Our results showed that minor allele A of rs515071 significantly increased LOAD risk in the APOE ε4 (+) subgroup (genotype P = 0.015, allele P = 0.020). After adjusting for age and gender, the association remained significant under the dominant model (OR = 1.809, 95 % confidence interval (CI) = 1.186-2.757, P = 0.006). In conclusion, our findings demonstrate that rs515071 in ANK1 is a novel genetic risk for LOAD susceptibility in Han Chinese.

Identification of a novel p.Q1772X ANK1 mutation in a Korean family with hereditary spherocytosis

Hereditary spherocytosis (HS), a common form of inherited hemolytic anemia, is a heterogeneous group of disorders with regard to clinical severity, protein defects, and mode of inheritance. Causal mutations in at least five genes have been reported so far. Because multiple genes have been associated with HS, clinical genetic testing that relies on direct sequencing will be a challenge. In this study, we used whole exome sequencing to identify a novel nonsense mutation in ANK1 (p.Q1772X, NM_020476) that resulted in a truncated protein in a Korean patient with HS. Sanger sequencing confirmed the two affected individuals in the patient's family were heterozygous for the mutation. This is the first report of a Korean family that carries an ANK1 mutation responsible for HS. Our results demonstrate that next generation sequencing is a powerful approach for rapidly determining the genetic etiology of HS.

Evaluating eosin-5-maleimide binding as a diagnostic test for hereditary spherocytosis in newborn infants

OBJECTIVE

Neonates with undiagnosed hereditary spherocytosis (HS) are at risk for developing hazardous hyperbilirubinemia and anemia. Making an early diagnosis of HS in a neonate can prompt anticipatory guidance to prevent these adverse outcomes. A recent comparison study showed that a relatively new diagnostic test for HS, eosin-5-maleimide (EMA)-flow cytometry, performs better than other available tests in confirming HS. However, reports have not specifically examined the performance of this test among neonates.

STUDY DESIGN

We compared EMA-flow cytometry from blood samples of healthy control neonates vs samples from neonates suspected of having HS on the basis of severe Coombs-negative jaundice and spherocytes on blood film. The diagnosis of HS was later either confirmed or excluded based on clinical findings and next generation sequencing (NGS) after which we correlated the EMA-flow results with the diagnosis.

RESULT

EMA-flow was performed on the blood of 31 neonates; 20 healthy term newborns and 11 who were suspected of having HS. Eight of the 11 were later confirmed positive for HS and one was confirmed positive for hereditary elliptocytosis (HE). All nine had persistently abnormal erythroid morphology, reticulocytosis and anemia, and eight of the nine had relevant mutations discovered using NGS. The other was confirmed positive for HS on the basis that a parent had HS, and the neonate's spherocytosis, reticulocytosis and anemia persisted. The 20 healthy controls and the 2 in whom HS was initially suspected but later excluded all had EMA-flow results in the range reported in healthy children and adults. In contrast, all nine in whom HS or HE was confirmed had abnormal EMA-flow results consistent with previous reports in older children and adults with HS.

CONCLUSION

Although our sample size is small, our findings are consistent with the literature in older children and adults suggesting that EMA-flow cytometric testing performs well in supporting the diagnosis of HS/HE during the early neonatal period.

MicroRNA-486-5p is an erythroid oncomiR of the myeloid leukemias of Down syndrome

Children with Down syndrome (DS) are at increased risk for acute myeloid leukemias (ML-DS) characterized by mixed megakaryocytic and erythroid phenotype and by acquired mutations in the GATA1 gene resulting in a short GATA1s isoform. The chromosome 21 microRNA (miR)-125b cluster has been previously shown to cooperate with GATA1s in transformation of fetal hematopoietic progenitors. In this study, we report that the expression of miR-486-5p is increased in ML-DS compared with non-DS acute megakaryocytic leukemias (AMKLs). miR-486-5p is regulated by GATA1 and GATA1s that bind to the promoter of its host gene ANK1. miR-486-5p is highly expressed in mouse erythroid precursors and knockdown (KD) in ML-DS cells reduced their erythroid phenotype. Ectopic expression and KD of miR-486-5p in primary fetal liver hematopoietic progenitors demonstrated that miR-486-5p cooperates with Gata1s to enhance their self renewal. Consistent with its activation of AKT, overexpression and KD experiments showed its importance for growth and survival of human leukemic cells. Thus, miR-486-5p cooperates with GATA1s in supporting the growth and survival, and the aberrant erythroid phenotype of the megakaryocytic leukemias of DS.

Structural basis of diverse membrane target recognitions by ankyrins

Ankyrin adaptors together with their spectrin partners coordinate diverse ion channels and cell adhesion molecules within plasma membrane domains and thereby promote physiological activities including fast signaling in the heart and nervous system. Ankyrins specifically bind to numerous membrane targets through their 24 ankyrin repeats (ANK repeats), although the mechanism for the facile and independent evolution of these interactions has not been resolved. Here we report the structures of ANK repeats in complex with an inhibitory segment from the C-terminal regulatory domain and with a sodium channel Nav1.2 peptide, respectively, showing that the extended, extremely conserved inner groove spanning the entire ANK repeat solenoid contains multiple target binding sites capable of accommodating target proteins with very diverse sequences via combinatorial usage of these sites. These structures establish a framework for understanding the evolution of ankyrins' membrane targets, with implications for other proteins containing extended ANK repeat domains.

DOI:http://dx.doi.org/10.7554/eLife.04353.001

Proteins are made up of smaller building blocks called amino acids that are linked to form long chains that then fold into specific shapes. Each protein gets its unique identity from the number and order of the amino acids that it contains, but different proteins can contain similar arrangements of amino acids. These similar sequences, known as motifs, are usually short and typically mark the sites within proteins that bind to other molecules or proteins. A single protein can contain many motifs, including multiple repeats of the same motif.

One common motif is called the ankyrin (or ANK) repeat, which is found in 100s of proteins in different species, including bacteria and humans. Ankyrin proteins perform a range of important functions, such as connecting proteins in the cell surface membrane to a scaffold-like structure underneath the membrane.

Proteins containing ankyrin repeats are known to interact with a diverse range of other proteins (or targets) that are different in size and shape. The 24 repeats found in human ankyrin proteins appear to have essentially remained unchanged for the last 500 million years. As such, it remains unclear how the conserved ankyrin repeats can bind to such a wide variety of protein targets.

Now, Wang, Wei et al. have uncovered the three-dimensional structure of ankyrin repeats from a human ankyrin protein while it was bound either to a regulatory fragment from another ankyrin protein or to a region of a target protein (which transports sodium ions in and out of cells). The ankyrin repeats were shown to form an extended ‘left-handed helix’: a structure that has also been seen in other proteins with different repeating motifs. Wang, Wei et al. found that the ankyrin protein fragment bound to the inner surface of the part of the helix formed by the first 14 ankyrin repeats. The target protein region also bound to the helix's inner surface. Wang, Wei et al. show that this surface contains many binding sites that can be used, in different combinations, to allow ankyrins to interact with diverse proteins.

Other proteins with long sequences of repeats are widespread in nature, but uncovering the structures of these proteins is technically challenging. Wang, Wei et al.'s findings might reveal new insights into the functions of many of such proteins in a wide range of living species. Furthermore, the new structures could help explain why specific mutations in the genes that encode ankyrins (or their binding targets) can cause various diseases in humans—including heart diseases and psychiatric disorders.

DOI:http://dx.doi.org/10.7554/eLife.04353.002

β-III spectrin underpins ankyrin R function in Purkinje cell dendritic trees: protein complex critical for sodium channel activity is impaired by SCA5-associated mutations

Beta III spectrin is present throughout the elaborate dendritic tree of cerebellar Purkinje cells and is required for normal neuronal morphology and cell survival. Spinocerebellar ataxia type 5 (SCA5) and spectrin associated autosomal recessive cerebellar ataxia type 1 are human neurodegenerative diseases involving progressive gait ataxia and cerebellar atrophy. Both disorders appear to result from loss of β-III spectrin function. Further elucidation of β-III spectrin function is therefore needed to understand disease mechanisms and identify potential therapeutic options. Here, we report that β-III spectrin is essential for the recruitment and maintenance of ankyrin R at the plasma membrane of Purkinje cell dendrites. Two SCA5-associated mutations of β-III spectrin both reduce ankyrin R levels at the cell membrane. Moreover, a wild-type β-III spectrin/ankyrin-R complex increases sodium channel levels and activity in cell culture, whereas mutant β-III spectrin complexes fail to enhance sodium currents. This suggests impaired ability to form stable complexes between the adaptor protein ankyrin R and its interacting partners in the Purkinje cell dendritic tree is a key mechanism by which mutant forms of β-III spectrin cause ataxia, initially by Purkinje cell dysfunction and exacerbated by subsequent cell death.

A deep intronic mutation in the ankyrin-1 gene causes diminished protein expression resulting in hemolytic anemia in mice

Linkage between transmembrane proteins and the spectrin-based cytoskeleton is necessary for membrane elasticity of red blood cells. Mutations of the proteins that mediate this linkage result in various types of hemolytic anemia. Here we report a novel N-ethyl-N-nitrosourea-induced mutation of ankyrin-1, named hema6, which causes hereditary spherocytosis in mice through a mild reduction of protein expression. The causal mutation was traced to a single nucleotide transition located deep into intron 13 of gene Ank1. In vitro minigene splicing assay revealed two abnormally spliced transcripts containing cryptic exons from fragments of Ank1 intron 13. The inclusion of cryptic exons introduced a premature termination codon, which leads to nonsense-mediated decay of the mutant transcripts in vivo. Hence, in homozygous mice, only wild-type ankyrin-1 is expressed, albeit at 70% of the level in wild-type mice. Heterozygotes display a similar hereditary spherocytosis phenotype stemming from intermediate protein expression level, indicating the haploinsufficiency of the mutation. Weakened linkage between integral transmembrane protein, band 3, and underlying cytoskeleton was observed in mutant mice as the result of reduced high-affinity binding sites provided by ankyrin-1. Hema6 is the only known mouse mutant of Ank1 allelic series that expresses full-length canonical ankyrin-1 at a reduced level, a fact that makes it particularly useful to study the functional impact of ankyrin-1 quantitative deficiency.

Suppression of hepcidin expression and iron overload mediate Salmonella susceptibility in ankyrin 1 ENU-induced mutant

Salmonella, a ubiquitous Gram-negative intracellular bacterium, is a food borne pathogen that infects a broad range of hosts. Infection with Salmonella Typhimurium in mice is a broadly recognized experimental model resembling typhoid fever in humans. Using a N-ethyl-N-nitrosurea (ENU) mutagenesis recessive screen, we report the identification of Ity16 (Immunity to Typhimurium locus 16), a locus responsible for increased susceptibility to infection. The position of Ity16 was refined on chromosome 8 and a nonsense mutation was identified in the ankyrin 1 (Ank1) gene. ANK1 plays an important role in the formation and stabilization of the red cell cytoskeleton. The Ank1^Ity16/Ity16 mutation causes severe hemolytic anemia in uninfected mice resulting in splenomegaly, hyperbilirubinemia, jaundice, extramedullary erythropoiesis and iron overload in liver and kidneys. Ank1^Ity16/Ity16 mutant mice demonstrated low levels of hepcidin (Hamp) expression and significant increases in the expression of the growth differentiation factor 15 (Gdf15), erythropoietin (Epo) and heme oxygenase 1 (Hmox1) exacerbating extramedullary erythropoiesis, tissue iron deposition and splenomegaly. As the infection progresses in Ank1^Ity16/Ity16, the anemia worsens and bacterial load were high in liver and kidneys compared to wild type mice. Heterozygous Ank1^+/Ity16 mice were also more susceptible to Salmonella infection although to a lesser extent than Ank1^Ity16/Ity16 and they did not inherently present anemia and splenomegaly. During infection, iron accumulated in the kidneys of Ank1^+/Ity16 mice where bacterial loads were high compared to littermate controls. The critical role of HAMP in the host response to Salmonella infection was validated by showing increased susceptibility to infection in Hamp-deficient mice and significant survival benefits in Ank1^+/Ity16 heterozygous mice treated with HAMP peptide. This study illustrates that the regulation of Hamp and iron balance are crucial in the host response to Salmonella infection in Ank1 mutants.

Micrometric segregation of fluorescent membrane lipids: relevance for endogenous lipids and biogenesis in erythrocytes

Micrometric membrane lipid segregation is controversial. We addressed this issue in attached erythrocytes and found that fluorescent boron dipyrromethene (BODIPY) analogs of glycosphingolipids (GSLs) [glucosylceramide (BODIPY-GlcCer) and monosialotetrahexosylganglioside (GM1BODIPY)], sphingomyelin (BODIPY-SM), and phosphatidylcholine (BODIPY-PC inserted into the plasma membrane spontaneously gathered into distinct submicrometric domains. GM1BODIPY domains colocalized with endogenous GM1 labeled by cholera toxin. All BODIPY-lipid domains disappeared upon erythrocyte stretching, indicating control by membrane tension. Minor cholesterol depletion suppressed BODIPY-SM and BODIPY-PC but preserved BODIPY-GlcCer domains. Each type of domain exchanged constituents but assumed fixed positions, suggesting self-clustering and anchorage to spectrin. Domains showed differential association with 4.1R versus ankyrin complexes upon antibody patching. BODIPY-lipid domains also responded differentially to uncoupling at 4.1R complexes [protein kinase C (PKC) activation] and ankyrin complexes (in spherocytosis, a membrane fragility disease). These data point to micrometric compartmentation of polar BODIPY-lipids modulated by membrane tension, cholesterol, and differential association to the two nonredundant membrane:spectrin anchorage complexes. Micrometric compartmentation might play a role in erythrocyte membrane deformability and fragility.

A tissue-specific chromatin loop activates the erythroid ankyrin-1 promoter

The human ankyrin-1 gene (ANK1) contains 3 tissue-specific alternative promoters. We have shown previously that the erythroid-specific ankyrin 1 (ANK1E) core promoter contains a 5' DNase I hypersensitive site (HS) with barrier insulator function that prevents gene silencing in vitro and in vivo. Mutations in the ANK1E barrier region lead to decreased ANK1 mRNA levels and hereditary spherocytosis. In this report, we demonstrate a second ANK1E regulatory element located in an adjacent pair of DNase I HS located 5.6 kb 3' of the ANK1E promoter at the 3' boundary of an erythroid-specific DNase I-sensitive chromatin domain. The 3' regulatory element exhibits enhancer activity in vitro and in transgenic mice, and it has the histone modifications associated with an enhancer element. One of the ANK1E 3'HS contains an NF-E2 binding site that is required for enhancer function. We show that a chromatin loop brings the 3' enhancer and NF-E2 into proximity with the 5' barrier region including the ANK1E core promoter. These observations demonstrate a model for the tissue-specific activation of alternative promoters that may be applicable to the ∼ 30% of mammalian genes with alternative promoters that exhibit distinct expression patterns.

A novel ENU-mutation in ankyrin-1 disrupts malaria parasite maturation in red blood cells of mice

The blood stage of the plasmodium parasite life cycle is responsible for the clinical symptoms of malaria. Epidemiological studies have identified coincidental malarial endemicity and multiple red blood cell (RBC) disorders. Many RBC disorders result from mutations in genes encoding cytoskeletal proteins and these are associated with increased protection against malarial infections. However the mechanisms underpinning these genetic, host responses remain obscure. We have performed an N-ethyl-N-nitrosourea (ENU) mutagenesis screen and have identified a novel dominant (haploinsufficient) mutation in the Ank-1 gene (Ank1^MRI23420) of mice displaying hereditary spherocytosis (HS). Female mice, heterozygous for the Ank-1 mutation showed increased survival to infection by Plasmodium chabaudi adami DS with a concomitant 30% decrease in parasitemia compared to wild-type, isogenic mice (wt). A comparative in vivo red cell invasion and parasite growth assay showed a RBC-autonomous effect characterised by decreased proportion of infected heterozygous RBCs. Within approximately 6–8 hours post-invasion, TUNEL staining of intraerythrocytic parasites, showed a significant increase in dead parasites in heterozygotes. This was especially notable at the ring and trophozoite stages in the blood of infected heterozygous mutant mice compared to wt (p<0.05). We conclude that increased malaria resistance due to ankyrin-1 deficiency is caused by the intraerythrocytic death of P. chabaudi parasites.

Mutation of conserved histidines alters tertiary structure and nanomechanics of consensus ankyrin repeats

The conserved TPLH tetrapeptide motif of ankyrin repeats (ARs) plays an important role in stabilizing AR proteins, and histidine (TPLH)-to-arginine (TPLR) mutations in this motif have been associated with a hereditary human anemia, spherocytosis. Here, we used a combination of atomic force microscopy-based single-molecule force spectroscopy and molecular dynamics simulations to examine the mechanical effects of His → Arg substitutions in TPLH motifs in a model AR protein, NI6C. Our molecular dynamics results show that the mutant protein is less mechanically stable than the WT protein. Our atomic force microscopy results indicate that the mechanical energy input necessary to fully unfold the mutant protein is only half of that necessary to unfold the WT protein (53 versus 106 kcal/mol). In addition, the ability of the mutant to generate refolding forces is also reduced. Moreover, the mutant protein subjected to cyclic stretch-relax measurements displays mechanical fatigue, which is absent in the WT protein. Taken together, these results indicate that the His → Arg substitutions in TPLH motifs compromise mechanical properties of ARs and suggest that the origin of hereditary spherocytosis may be related to mechanical failure of ARs.

Structurally similar but functionally diverse ZU5 domains in human erythrocyte ankyrin

The metazoan cell membrane is highly organized. Maintaining such organization and preserving membrane integrity under different conditions are accomplished through intracellular tethering to an extensive, flexible protein network. Spectrin, the principal component of this network, is attached to the membrane through the adaptor protein ankyrin, which directly bridges the interaction between β-spectrin and membrane proteins. Ankyrins have a modular structure that includes two tandem ZU5 domains. The first domain, ZU5A, is directly responsible for binding β-spectrin. Here, we present a structure of the tandem ZU5 repeats of human erythrocyte ankyrin. Structural and biophysical experiments show that the second ZU5 domain, ZU5B, does not participate in spectrin binding. ZU5B is structurally similar to the ZU5 domain found in the netrin receptor UNC5b supramodule, suggesting that it could interact with other domains in ankyrin. Comparison of several ZU5 domains demonstrates that the ZU5 domain represents a compact and versatile protein interaction module.

Structure of the ZU5-ZU5-UPA-DD tandem of ankyrin-B reveals interaction surfaces necessary for ankyrin function

Ankyrin-R/B/G (encoded by ANK1/2/3, respectively) are a family of very large scaffold proteins capable of anchoring numerous receptors and ion channels to specific, spectrin-containing membrane micro-domains. Hereditary mutations of ankyrins are known to be associated with diseases including spherocytosis, cardiac arrhythmia, and bipolar disorder in humans, although the underlying molecular bases are poorly understood. The middle spectrin-binding domain of ankyrins contains highly conserved ZU5-ZU5-UPA-DD domains arranged into the ZZUD tandem. Curiously, most of the disease-causing mutations in the tandem have no apparent impact on the spectrin binding of ankyrins. The high resolution structure of the ankyrin-B ZZUD tandem determined here reveals that the ZU5-ZU5-UPA domains form a tightly packed structural supramodule, whereas DD is freely accessible. Although the formation of the ZZU supramodule does not influence the spectrin binding of ankyrins, mutations altering the interdomain interfaces of ZZU impair the functions of ankyrin-B&G. Our structural analysis further indicates that the ZZU supramodule of ankyrins has two additional surfaces that may bind to targets other than spectrin. Finally, the structure of the ankyrin ZZUD provides mechanistic explanations to many disease-causing mutations identified in ankyrin-B&R.

A novel ENU-generated truncation mutation lacking the spectrin-binding and C-terminal regulatory domains of Ank1 models severe hemolytic hereditary spherocytosis

OBJECTIVE

Hereditary spherocytosis (HS) is a heterogeneous group of spontaneously arising and inherited red blood cell disorders ranging from very mild subclinical cases to severe and life-threatening cases, with symptoms linked directly to the severity of the mutation at the molecular level. We investigated a novel mouse model in which the heterozygotes present with the diagnostic hallmarks of mild HS and surviving homozygotes phenocopy severe hemolytic HS.

MATERIALS AND METHODS

We used N-ethyl-N-nitrosourea mutagenesis to generate random point mutations in the mouse genome and a dominant screen to identify mouse models of human hematopoietic disease. Gene mapping of the HS strain revealed a unique in-frame nonsense mutation arising from a single base transversion in exon 27 of Ank1 (strain designation: Ank1(E924X)). Employing conventional hematopoietic, pathological, biochemical, and cell biology assays, we characterized heterozygous and homozygous Ank1(E924X) mice at the biochemical, cellular, and pathophysiological levels.

RESULTS

Although Ank1(E924X/E924X) red blood cell ghosts lack abundant full-length ankyrin-1 isoforms, N-terminal epitope ankyrin-1 antibodies reveal a band consistent with the theoretical size of a truncated mutant ankyrin-1. Using domain-specific antibodies, we further show that this protein lacks both a spectrin-binding domain and a C-terminal regulatory domain. Finally, using antisera that detect C-terminal residues of the products of alternative Ank1 transcripts, we find unique immunoreactive bands not observed in red blood cell ghosts from wild-type or Ank1(E924X) heterozygous mice, including a band similar in size to full-length ankyrin-1.

CONCLUSIONS

The Ank1(E924X) strain provides a novel tool to study Ank1 and model HS.

Mutation of a barrier insulator in the human ankyrin-1 gene is associated with hereditary spherocytosis

Defects of the ankyrin-1 gene are the most common cause in humans of hereditary spherocytosis, an inherited anemia that affects patients of all ethnic groups. In some kindreds, linked -108/-153 nucleotide substitutions have been found in the upstream region of the ankyrin gene promoter that is active in erythroid cells. In vivo, the ankyrin erythroid promoter and its upstream region direct position-independent, uniform expression, a property of barrier insulators. Using human erythroid cell lines and primary cells and transgenic mice, here we have demonstrated that a region upstream of the erythroid promoter is a barrier insulator in vivo in erythroid cells. The region exhibited both functional and structural characteristics of a barrier, including prevention of gene silencing in an in vivo functional assay, appropriate chromatin configuration, and occupancy by barrier-associated proteins. Fragments with the -108/-153 spherocytosis-associated mutations failed to function as barrier insulators in vivo and demonstrated perturbations in barrier-associated chromatin configuration. In transgenic mice, flanking a mutant -108/-153 ankyrin gene promoter with the well-characterized chicken HS4 barrier insulator restored position-independent, uniform expression at levels comparable to wild-type. These data indicate that an upstream region of the ankyrin-1 erythroid promoter acts as a barrier insulator and identify disruption of the barrier element as a potential pathogenetic mechanism of human disease.

Functional analysis of a novel cis-acting regulatory region within the human ankyrin gene (ANK-1) promoter

The characterization of atypical mutations in loci associated with diseases is a powerful tool to discover novel regulatory elements. We previously identified a dinucleotide deletion in the human ankyrin-1 gene (ANK-1) promoter that underlies ankyrin-deficient hereditary spherocytosis. The presence of the deletion was associated with a decrease in promoter function both in vitro and in vivo establishing it as a causative hereditary spherocytosis mutation. The dinucleotide deletion is located in the 5' untranslated region of the ANK-1 gene and disrupts the binding of TATA binding protein and TFIID, components of the preinitiation complex. We hypothesized that the nucleotides surrounding the mutation define an uncharacterized regulatory sequence. To test this hypothesis, we generated a library of more than 16,000 ANK-1 promoters with degenerate sequence around the mutation and cloned the functional promoter sequences after cell-free transcription. We identified the wild type and three additional sequences, from which we derived a consensus. The sequences were shown to be functional in cell-free transcription, transient-transfection, and transgenic mouse assays. One sequence increased ANK-1 promoter function 5-fold, while randomly chosen sequences decreased ANK-1 promoter function. Our results demonstrate a novel functional motif in the ANK-1 promoter.

Analysis of novel sph (spherocytosis) alleles in mice reveals allele-specific loss of band 3 and adducin in alpha-spectrin-deficient red cells

Five spontaneous, allelic mutations in the alpha-spectrin gene, Spna1, have been identified in mice (spherocytosis [sph], sph(1J), sph(2J), sph(2BC), sph(Dem)). All cause severe hemolytic anemia. Here, analysis of 3 new alleles reveals previously unknown consequences of red blood cell (RBC) spectrin deficiency. In sph(3J), a missense mutation (H2012Y) in repeat 19 introduces a cryptic splice site resulting in premature termination of translation. In sph(Ihj), a premature stop codon occurs (Q1853Stop) in repeat 18. Both mutations result in markedly reduced RBC membrane spectrin content, decreased band 3, and absent beta-adducin. Reevaluation of available, previously described sph alleles reveals band 3 and adducin deficiency as well. In sph(4J), a missense mutation occurs in the C-terminal EF hand domain (C2384Y). Notably, an equally severe hemolytic anemia occurs despite minimally decreased membrane spectrin with normal band 3 levels and present, although reduced, beta-adducin. The severity of anemia in sph(4J) indicates that the highly conserved cysteine residue at the C-terminus of alpha-spectrin participates in interactions critical to membrane stability. The data reinforce the notion that a membrane bridge in addition to the classic protein 4.1-p55-glycophorin C linkage exists at the RBC junctional complex that involves interactions between spectrin, adducin, and band 3.

Hereditary spherocytosis

Hereditary spherocytosis is a common inherited disorder that is characterised by anaemia, jaundice, and splenomegaly. It is reported worldwide and is the most common inherited anaemia in individuals of northern European ancestry. Clinical severity is variable with most patients having a well-compensated haemolytic anaemia. Some individuals are asymptomatic, whereas others have severe haemolytic anaemia requiring erythrocyte transfusion. The primary lesion in hereditary spherocytosis is loss of membrane surface area, leading to reduced deformability due to defects in the membrane proteins ankyrin, band 3, beta spectrin, alpha spectrin, or protein 4.2. Many isolated mutations have been identified in the genes encoding these membrane proteins; common hereditary spherocytosis-associated mutations have not been identified. Abnormal spherocytes are trapped and destroyed in the spleen and this is the main cause of haemolysis in this disorder. Common complications are cholelithiasis, haemolytic episodes, and aplastic crises. Splenectomy is curative but should be undertaken only after careful assessment of the risks and benefits.

Ankyrin-linked hereditary spherocytosis in an African-American kindred

Mutations of ankyrin-1 are the most frequent cause of the inherited hemolytic anemia, hereditary spherocytosis (HS), in people of European ancestry. Ankyrin-1, which provides the primary linkage between the erythrocyte membrane skeleton and the plasma membrane, has numerous isoforms generated by alternative splicing, alternate polyadenylation, use of tissue-specific promoters, and alternate NH(2) or COOH-termini. Mutation detection in erythrocyte membrane protein genes, including ankyrin, has been a challenge, primarily due to the large size of these genes, and the apparent frequent occurrence of HS-associated null alleles. Using denaturing high-performance liquid chromatography (DHPLC), we screened the ankyrin gene of the proband of a large, three generation African-American kindred with ankyrin-deficient HS. DHPLC yielded an abnormal chromatogram for exon 1. Examination of the corresponding exon 1 sequence in genomic DNA from the proband revealed heterozygosity for a mutation of the initiator methionine (ATG to ATA Met 1 Ile). Coupled in vitrotranscription/translation studies with rabbit reticulocyte lysates demonstrated that the wild-type ankyrin erythroid cDNA initiates only from the known initiator methionine, indicating that the use of alternate initiator methionine is not a mechanism of isoform diversity in erythroid cells. The mutant ankyrin allele, unlike some initiator methionine mutations that utilize downstream codons for translation initiation, was associated with a null allele. This is the first report describing ankyrin-linked HS in an African-American kindred.