Mutation of a highly conserved residue of betaI spectrin associated with fatal and near-fatal neonatal hemolytic anemia

Molecular characteristics of hereditary red blood cell membrane disorders in Thailand: a multi-center registry

Red blood cell (RBC) membrane disorders represent a significant category of hereditary hemolytic anemia; however, information from Southeast Asia is limited. We established a national registry aiming to characterize RBC membrane disorders and their molecular features in Thailand. A total of 100 patients (99 kindreds) diagnosed with RBC membrane disorders between 2011 and 2020 from seven university hospitals were enrolled. The most prevalent disorders observed were hereditary elliptocytosis (HE; n=33), hereditary pyropoikilocytosis (HPP; n=28), hereditary spherocytosis (HS; n=19), Southeast Asian ovalocytosis (SAO; n=10 of 9 kindreds), and two cases of homozygous SAO. The remaining cases were grouped as unclassified membrane disorder. Seventy-six patients (76%) were molecularly confirmed by PCR, direct DNA sequencing, or hi-throughput sequencing. The primary causative gene for HE and HPP was SPTB, accounting for 28 out of 29 studied alleles for HE and 56 of 56 studied alleles for HPP. In the case of HS, dominant sporadic mutations in the ANK1 gene (n=4) and SPTB gene (n=3) were identified as the underlying cause. Notably, the four most common variants causing HE and HPP were SPTB Providence (c.6055 T>C), SPTB Buffalo (c.6074 T>G), SPTB Chiang Mai (c.6224 A>G), and SPTB c.6171__82delins TGCCCAGCT. These recurrent SPTB mutations accounted for 79 out of 84 mutated SPTB alleles (94%). In summary, HE and hereditary HPP associated with recurrent SPTB mutations are the predominant types of RBC membrane disorders observed in Thailand. These findings have significant implications for the clinical management and future research of RBC membrane disorders in the region.

Whole-exome sequencing uncovered genetic diagnosis of severe inherited haemolytic anaemia: Correlation with clinical phenotypes

Next-generation sequencing has shed light on the diagnosis of previously unsolved cases of inherited haemolytic anaemia (IHA). We employed whole-exome sequencing to explore the molecular diagnostic spectrum of 21 unrelated Thai paediatric patients with non-thalassemic IHA, presenting hydrops fetalis and/or becoming transfusion-dependent for 1 year or more or throughout their lifespan. Anaemia was detected prenatally, within the first month and the fifth year of life in three, 12 and six patients respectively. Molecular diagnosis obtained from all patients revealed SPTB as the most frequently mutated gene (four reported, three novel), found in 31 of 42 studied alleles. The other two mutated genes identified were ANK1 (three novel) and KLF1 (two reported). Four recurring mutations within exon 29/30 (NM_001024858.2) accounted for the vast majority (90%) of mutated SPTB alleles, biallelic inheritance of which resulted in the most severe phenotypes: hydrops fetalis and life-long transfusion dependency. Dominant ANK1 (n = 3) and SPTB (n = 2) mutations and biallelic class 2 KLF1 mutations (n = 1) led to a shorter period of transfusion dependency. Our study demonstrated that mutated SPTB causing red-cell membranopathy is likely the most common cause of severe non-thalassemic IHA among Thai patients. This urges carrier screening in the population to prevent subsequent, severely affected births.

Heterozygous variants in SPTBN1 cause intellectual disability and autism

Spectrins are common components of cytoskeletons, binding to cytoskeletal elements and the plasma membrane, allowing proper localization of essential membrane proteins, signal transduction, and cellular scaffolding. Spectrins are assembled from α and β subunits, encoded by SPTA1 and SPTAN1 (α) and SPTB, SPTBN1, SPTBN2, SPTBN4, and SPTBN5 (β). Pathogenic variants in various spectrin genes are associated with erythroid cell disorders (SPTA1, SPTB) and neurologic disorders (SPTAN1, SPTBN2, and SPTBN4), but no phenotypes have been definitively associated with variants in SPTBN1 or SPTBN5. Through exome sequencing and case matching, we identified seven unrelated individuals with heterozygous SPTBN1 variants: two with de novo missense variants and five with predicted loss-of-function variants (found to be de novo in two, while one was inherited from a mother with a history of learning disabilities). Common features include global developmental delays, intellectual disability, and behavioral disturbances. Autistic features (4/6) and epilepsy (2/7) or abnormal electroencephalogram without overt seizures (1/7) were present in a subset. Identification of loss-of-function variants suggests a haploinsufficiency mechanism, but additional functional studies are required to fully elucidate disease pathogenesis. Our findings support the essential roles of SPTBN1 in human neurodevelopment and expand the knowledge of human spectrinopathy disorders.

Neonatal hereditary spherocytosis caused by a de novo frameshift mutation of the SPTB gene characterized by hydrops fetalis: A case report

RATIONALE

The etiology of non-immune hydrops fetalis is complex, and its prognosis is poor. One of its main causes is anemia. There are few reports on hydrops fetalis due to anemia caused by hereditary spherocytosis (HS), especially regarding its occurrence in the neonatal period. Thus, we report on a case of neonatal HS caused by a new SPTB gene mutation that was characterized by hydrops fetalis.

PATIENT CONCERNS

A neonate with intrauterine hydrops fetalis showed severe hyperbilirubinemia and anemia, reticulocytosis, and hepatosplenomegaly. Laboratory examination findings were normal.

DIAGNOSES

Gene sequencing of the patient and his parents showed a de novo frameshift mutation in the patient's SPTB gene. Ultimately, the patient was diagnosed with HS.

INTERVENTIONS

Exchange and red blood cell transfusions were performed in the neonatal period.

OUTCOMES

The child was discharged from the hospital 14 days postnatal because his hemoglobin and bilirubin levels were stable. Red blood cell transfusion was performed once in infancy; however, no further red blood cell transfusions were required within 2 years of age.

LESSONS

Hydrops fetalis can be a manifestation of HS. Genetic detection can help confirm the diagnosis of suspected neonatal HS undocumented by other laboratory examinations.

Rapid Identification of Biallelic SPTB Mutation in a Neonate with Severe Congenital Hemolytic Anemia and Liver Failure

Heterozygous pathogenic variants in SPTB cause autosomal dominant hereditary spherocytosis, an important cause of neonatal nonimmune hemolytic anemia. Biallelic mutations are rarely reported, all with severe neonatal presentation. We describe rapid (68 h) genomic diagnosis of homozygous β-spectrin deficiency in a newborn with severe transfusion-dependent hemolytic anemia, conjugated hyperbilirubinemia, and progressive liver failure. Trio whole-exome sequencing identified a novel biallelic SPTB variant (c.6119C>T; p.Thr2040Ile) located in the critical spectrin repeat region. Pretransfusion blood film showed marked spherocytosis including microspherocytes and nucleated erythrocytes, and eosin-5-maleimide (E5M) staining was markedly reduced, supporting pathogenicity. Both asymptomatic heterozygous parents demonstrated mildly reduced E5M staining, with occasional spherocytes and elliptocytes. Early molecular diagnosis facilitated hypertransfusion to suppress ineffective erythropoiesis and reverse hepatic dysfunction. This report broadens the genotypic and phenotypic spectrum of spectrin deficiency and highlights the utility of rapid genomic testing in facilitating early diagnosis and informing targeted therapy in critically ill patients.

Spectrin tetramer formation is not required for viable development in Drosophila

The dominant paradigm for spectrin function is that (αβ)2-spectrin tetramers or higher order oligomers form membrane-associated two-dimensional networks in association with F-actin to reinforce the plasma membrane. Tetramerization is an essential event in such structures. We characterize the tetramerization interaction between α-spectrin and β-spectrins in Drosophila. Wild-type α-spectrin binds to both β- and βH-chains with high affinity, resembling other non-erythroid spectrins. However, α-spec(R22S), a tetramerization site mutant homologous to the pathological α-spec(R28S) allele in humans, eliminates detectable binding to β-spectrin and reduces binding to βH-spectrin ∼1000-fold. Even though spectrins are essential proteins, α-spectrin(R22S) rescues α-spectrin mutants to adulthood with only minor phenotypes indicating that tetramerization, and thus conventional network formation, is not the essential function of non-erythroid spectrin. Our data provide the first rigorous test for the general requirement for tetramer-based non-erythroid spectrin networks throughout an organism and find that they have very limited roles, in direct contrast to the current paradigm.

Binding of polarity-sensitive hydrophobic ligands to erythroid and nonerythroid spectrin: fluorescence and molecular modeling studies

We have used three polarity-sensitive fluorescence probes, 6-propionyl 2-(N,N-dimethyl-amino) naphthalene (Prodan), pyrene and 8-anilino 1-naphthalene sulphonic acid, to study their binding with erythroid and nonerythroid spectrin, using fluorescence spectroscopy. We have found that both bind to prodan and pyrene with high affinities with apparent dissociation constants (Kd) of .50 and .17 μM, for prodan, and .04 and .02 μM, for pyrene, respectively. The most striking aspect of these bindings have been that the binding stoichiometry have been equal to 1 in erythroid spectrin, both in dimeric and tetrameric form, and in tetrameric nonerythroid spectrin. From an estimate of apparent dielectric constants, the polarity of the binding site in both erythroid and nonerythroid forms have been found to be extremely hydrophobic. Thermodynamic parameters associated with such binding revealed that the binding is favored by positive change in entropy. Molecular docking studies alone indicate that both prodan and pyrene bind to the four major structural domains, following the order in the strength of binding to the Ankyrin binding domain > SH3 domain > Self-association domain > N-terminal domain of α-spectrin of both forms of spectrin. The binding experiments, particularly with the tetrameric nonerythroid spectrin, however, indicate more toward the self association domain in offering the unique binding site, since the binding stoichiometry have been 1 in all forms of dimeric and tetrameric spectrin, so far studied by us. Further studies are needed to characterize the hydrophobic binding sites in both forms of spectrin.

Hereditary spherocytosis, elliptocytosis, and other red cell membrane disorders

Hereditary spherocytosis and elliptocytosis are the two most common inherited red cell membrane disorders resulting from mutations in genes encoding various red cell membrane and skeletal proteins. Red cell membrane, a composite structure composed of lipid bilayer linked to spectrin-based membrane skeleton is responsible for the unique features of flexibility and mechanical stability of the cell. Defects in various proteins involved in linking the lipid bilayer to membrane skeleton result in loss in membrane cohesion leading to surface area loss and hereditary spherocytosis while defects in proteins involved in lateral interactions of the spectrin-based skeleton lead to decreased mechanical stability, membrane fragmentation and hereditary elliptocytosis. The disease severity is primarily dependent on the extent of membrane surface area loss. Both these diseases can be readily diagnosed by various laboratory approaches that include red blood cell cytology, flow cytometry, ektacytometry, electrophoresis of the red cell membrane proteins, and mutational analysis of gene encoding red cell membrane proteins.

Spectrin Breakdown Products (SBDPs) as Potential Biomarkers for Neurodegenerative Diseases

The world's human population ages rapidly thanks to the great advance in modern medicine. While more and more body system diseases become treatable and curable, age-related neurodegenerative diseases remain poorly understood mechanistically, and are desperately in need of preventive and therapeutic interventions. Biomarker development consists of a key part of concerted effort in combating neurodegenerative diseases. In many chronic neurodegenerative conditions, neuronal damage/death occurs long before the onset of disease symptoms, and abnormal proteolysis may either play an active role or be a companying event of neuronal injury. Increased spectrin cleavage yielding elevated spectrin breakdown products (SBDPs) by calcium-sensitive proteases such as calpain and caspases has been established in conditions associated with acute neuronal damage such as traumatic brain injury (TBI). Here we review literature regarding spectrin expression and metabolism in the brain, and propose a potential use of SBDPs as biomarkers for neurodegenerative diseases such as Alzheimer's diseases.

Oxidative crosslinking, spectrin and membrane interactions of hemoglobin mixtures in HbEbeta-thalassemia

The authors have studied the interactions of intact hemoglobin mixtures of HbE and HbA, with the major erythroid membrane skeletal protein, spectrin and tailor-made phospholipids membranes containing aminophospholipids to understand the role of spectrin and phospholipids of erythrocytes in the overall pathophysiology of the hemoglobin disorders. Hemoglobin mixtures were isolated and purified from the peripheral blood samples of HbE carriers and different HbEbeta thalassemia patients, taken for diagnosis. Spectrin binding was studied by fluorescence and oxidative crosslinking, by SDS-PAGE. Membrane perturbation experiments were carried out to study the leakage of the self-quenching fluorophore, carboxyfluorescein, entrapped in the phospholipid vesicles. Hemoglobin mixtures with elevated levels of HbE showed stronger interactions with spectrin reflected in the decrease in binding dissociation constant from 17 to 5 muM upon increase in HbE% from about 30 to 90% in the hemolysates. The yield of the spectrin crosslinked complexes of such hemoglobin mixtures also increased with increase in HbE levels. Presence of ATP/Mg and DPG were found to decrease the overall yield of such complexes and the binding affinity of hemoglobins to spectrin. HbE rich hemolysates also induced greater leakage of entrapped carboxyfluorescein (CF) from phospholipid membranes containing aminophospholipids. Results from this study indicate the roles of skeletal proteins and aminophospholipids, particularly under oxidative stress conditions to be important in the premature destruction of erythrocytes in hemoglobin disorders, e.g. HbEbeta-thalassaemia.

Structural and functional effects of hereditary hemolytic anemia-associated point mutations in the alpha spectrin tetramer site

The most common hereditary elliptocytosis (HE) and hereditary pyropoikilocytosis (HPP) mutations are alpha-spectrin missense mutations in the dimer-tetramer self-association site. In this study, we systematically compared structural and functional properties of the 14 known HE/HPP mutations located in the alpha-spectrin tetramer binding site. All mutant alpha-spectrin recombinant peptides were well folded, stable structures, with only the R34W mutant exhibiting a slight structural destabilization. In contrast, binding affinities measured by isothermal titration calorimetry were greatly variable, ranging from no detectable binding observed for I24S, R28C, R28H, R28S, and R45S to approximately wild-type binding for R34W and K48R. Binding affinities for the other 7 mutants were reduced by approximately 10- to 100-fold relative to wild-type binding. Some sites, such as R28, were hot spots that were very sensitive to even relatively conservative substitutions, whereas other sites were only moderately perturbed by nonconservative substitutions. The R34W and K48R mutations were particularly intriguing mutations that apparently either destabilize tetramers through mechanisms not probed by the univalent tetramer binding assay or represent polymorphisms rather than the pathogenic mutations responsible for observed clinical symptoms. All alpha0 HE/HPP mutations studied here appear to exert their destabilizing effects through molecular recognition rather than structural mechanisms.

Erythrocyte disorders in the perinatal period

Anemia is a commonly encountered problem in the fetal and neonatal period, and can lead to significant morbidity and mortality. Intrinsic disorders of the erythrocyte, such as the hemoglobinopathies, enzyme deficiencies, and membrane defects are common causes of neonatal anemia. Genetic diseases that lead to decreased erythrocyte production, such as Diamond-Blackfan anemia, Schwachman-Diamond syndrome, and Congential Dyserythropoietic Anemia, are rare causes of perinatal anemia, but are important to recognize as they are often associated with other congenital abnormalities and require specialized treatment. This review focuses on the perinatal presentation and management of intrinsic erythrocyte disorders, as well as on the diagnosis and management of genetic conditions leading to erythrocyte underproduction.

Nonimmune hydrops fetalis in two cases of consanguineous parents and associated with hereditary spherocytosis and hemophagocytic hystiocytosis

Nonimmune hydrops fetalis may occur as a result of different etiological conditions and in about one-third of cases no cause could be identified. Here, we report two cases of nonimmune hydrops fetalis associated with hereditary spherocytosis and hemophagocytic hystiocytosis. We think that babies with hydrops fetalis born of consanguineous parents should be examined for hereditary diseases, and that these rare causes should be taken into account in problematic cases.

Spectrin organization and dynamics: new insights

Spectrin is the major constituent protein of the erythrocyte cytoskeleton which forms a filamentous network on the cytoplasmic face of the membrane by providing a scaffold for a variety of proteins. In this review, several aspects of spectrin organization are highlighted, particularly with respect to its ability to bind hydrophobic ligands and its interaction with membrane surfaces. The characteristic binding of the fluorescent hydrophobic probes Prodan and pyrene to spectrin, which allows an estimation of the polarity of the hydrophobic probe binding site, is illustrated. In addition, the contribution of uniquely localized and conserved tryptophan residues in the 'spectrin repeats' in these processes is discussed. A functional implication of the presence of hydrophobic binding sites in spectrin is its recently discovered chaperone-like activity. Interestingly, spectrin exhibits residual structural integrity even after denaturation which could be considered as a hallmark of cytoskeletal proteins. Future research could provide useful information about the possible role played by spectrin in cellular physiology in healthy and diseased states.

Early embryonic death-associated changes in genome-wide gene expression profiles in the fetal placenta of the cow carrying somatic nuclear-derived cloned embryo

Successful somatic nuclear transfer-derived cloning has been reported in cattle; however, the cloned embryo is highly susceptible to death around day 60 of gestation leading to early embryonic loss. The early embryonic death is postulated to possibly arise in part from an atypical placentation. We have performed cDNA macroarray analysis using 3,353 of the previously cataloged 4,165 genes, in order to characterize the early embryonic death-associated changes in genome-wide gene expression profiles in the fetal placenta of the cow carrying somatic nuclear transfer-derived cloned embryo. A more marked difference in the expression profiles was observed between the fetal placentas of the cows with the cloned immotile embryo (CD) and with the cloned motile embryo (CL) or artificial insemination-derived motile embryo (AI), as compared to between the CL and AI placentas, suggesting an aberration of the expression profile in the CD placenta among the three placentas. Further, 291 and 77 genes showed more than twofold elevation and less than 50% reduction, respectively, in either or both of two CD (CD1 and CD2) placentas in comparison with the CL placenta, but no differential expression between the CL and AI placentas. The expression patterns of six genes in the AI, CL, and CD placentas were confirmed in an experiment with an additional sample for each of the three placentas. Among the placental genes showing the early embryonic death-associated changes of expression in the cow with the cloned embryo, IGF2 (elevated gene), and HBA1, HBA2, SPTB, and SPTBN1 genes (reduced gene) are intriguing in that the changes of expression in these genes were observed in an additional sample of CD placenta as well as the CD1 and CD2 placentas, and in that overexpression (for IGF2) and dysfunction or deficiency (for HBA1, HBA2, SPTB, and SPTBN1) result in embryonic lethality.

Effect of ionic strength on the organization and dynamics of tryptophan residues in erythroid spectrin: A fluorescence approach

The ionic strength of the medium plays an important role in the structure and conformation of erythroid spectrin. The spectrin dimer is a flexible rod at physiological ionic strength. However, lower ionic strength results in elongation and rigidification (stiffening) of spectrin as shown earlier by electron microscopy and hydrodynamic studies. The ionic strength induced structural transition does not involve any specific secondary structural changes. In this article, we have used a combination of fluorescence spectroscopic approaches that include red edge excitation shift (REES), fluorescence quenching, time-resolved fluorescence measurements, and chemical modification of the spectrin tryptophans to assess the environment and dynamics of tryptophan residues of spectrin under different ionic strength conditions. Our results show that while REES, fluorescence anisotropy, lifetime, and chemical modification of spectrin tryptophans remain unaltered in low and high ionic strength conditions, quenching of tryptophan fluorescence by the aqueous quencher acrylamide (but not the hydrophobic quencher trichloroethanol) and resonance energy transfer to a dansyl-labeled fatty acid show differences in tryptophan environment. These results, which report tertiary structural changes in spectrin upon change in ionic strength, are relevant in understanding the molecular details underlying the conformational flexibility of spectrin.

Chaperone activity and prodan binding at the self-associating domain of erythroid spectrin

Spectrin, the major constituent protein of the erythrocyte membrane skeleton, exhibits chaperone activity by preventing the irreversible aggregation of insulin at 25 degrees C and that of alcohol dehydrogenase at 50 degrees C. The dimeric spectrin and the two subunits, alpha-spectrin and beta-spectrin prevent such aggregation appreciably better, 70% in presence of dimeric spectrin at an insulin:spectrin ratio of 1:1, than that in presence of the tetramer of 25%. Our results also show that spectrin binds to denatured enzymes alpha-glucosidase and alkaline phosphatase during refolding and the reactivation yields are increased in the presence of the spectrin derivatives when compared with those refolded in their absence. The unique hydrophobic binding site on spectrin for the fluorescence probe, 6-propionyl-2-(dimethylamino)naphthalene (Prodan) has been established to localize at the self-associating domain with the binding stoichiometry of one Prodan/both dimeric and tetrameric spectrin. The other fluorescence probe, 1-anilinonaphthalene-8-sulfonic acid, does not show such specificity for spectrin, and the binding stoichiometry is between 3 and 5 1-anilinonaphthalene-8-sulfonic acid/dimeric and tetrameric spectrin, respectively. Regions in alpha- and beta-spectrins have been found to have sequence homology with known chaperone proteins. More than 50% similarities in alpha-spectrin near the N terminus with human Hsp90 and in beta-spectrin near the C terminus with human Hsp90 and Escherichia coli DnaJ have been found, indicating a potential chaperone-like sequence to be present near the self-associating domain that is formed by portions of alpha-spectrin near the N terminus and the beta-spectrin near the C terminus. There are other patches of sequences also in both the spectrin polypeptides, at the other termini as well as in the middle of the rod domain having significant homology with well known chaperone proteins.

Organization and dynamics of tryptophan residues in erythroid spectrin: novel structural features of denatured spectrin revealed by the wavelength-selective fluorescence approach

We have investigated the organization and dynamics of the functionally important tryptophan residues of erythroid spectrin in native and denatured conditions utilizing the wavelength-selective fluorescence approach. We observed a red edge excitation shift (REES) of 4 nm for the tryptophans in the case of spectrin in its native state. This indicates that tryptophans in spectrin are localized in a microenvironment of restricted mobility, and that the regions surrounding the spectrin tryptophans offer considerable restriction to the reorientational motion of the water dipoles around the excited state tryptophans. Interestingly, spectrin exhibits a REES of 3 nm even when denatured in 8 M urea. This represents the first report of a denatured protein displaying REES. Observation of REES in the denatured state implies that some of the structural and dynamic features of this microenvironment around the spectrin tryptophans are retained even when the protein is denatured. Fluorescence quenching data of denatured spectrin support this conclusion. In addition, we have deduced the organization and dynamics of the hydrophobic binding site of the polarity-sensitive fluorescent probe PRODAN that binds erythroid spectrin with high affinity. When bound to spectrin, PRODAN exhibits a REES of 9 nm. Because PRODAN binds to a hydrophobic site in spectrin, such a result would directly imply that this region of spectrin offers considerable restriction to the reorientational motion of the solvent dipoles around the excited state fluorophore. The results of our study could provide vital insight into the role of tryptophans in the stability and folding of spectrin.

Hereditary elliptocytosis: spectrin and protein 4.1R

Hereditary elliptocytosis (HE) is a common disorder of erythrocyte shape, occurring especially in individuals of African and Mediterranean ancestry, presumably because elliptocytes confer some resistance to malaria. The principle lesion in HE is mechanical weakness or fragility of the erythrocyte membrane skeleton due to defects in alpha-spectrin, beta-spectrin, or protein 4.1. Numerous mutations have been described in the genes encoding these proteins, including point mutations, gene deletions and insertions, and mRNA processing defects. Several mutations have been identified in a number of individuals on the same genetic background, suggesting a "founder effect." The majority of HE patients are asymptomatic, but some may experience hemolytic anemia, splenomegaly, and intermittent jaundice.

Mutation of a highly conserved isoleucine disrupts hydrophobic interactions in the alpha beta spectrin self-association binding site

We studied an infant with severe neonatal hemolytic anemia and hyperbilirubinemia that evolved into a partially compensated ellipto-poikilocytic anemia. His father had typical elliptocytosis. Their erythrocyte membranes demonstrated structural and functional defects in spectrin. Genetic studies revealed that the proband and his father were heterozygous for an alpha-spectrin mutation, Ile24Thr, in the alpha beta spectrin self-association binding site. The proband also carried the low expression allele alpha(LELY) in trans, influencing the clinical phenotype. The importance of isoleucine in this position of the proposed triple helical model of spectrin repeats is highlighted by its evolutionary conservation in all alpha spectrins from Drosophila to humans. Molecular modeling demonstrated that replacement of a hydrophobic isoleucine with a hydrophilic threonine disrupts highly conserved hydrophobic interactions in the interior of the spectrin triple helix critical for spectrin function.

Dynamic molecular modeling of pathogenic mutations in the spectrin self-association domain

Disruption of spectrin self-association underlies many inherited hemolytic disorders. Using dynamic modeling and energy minimization, the 3-dimensional structure of the self-association domain has been estimated in human erythrocyte spectrin and the structural consequences of 17 elliptogenic mutations determined. The predicted structure of the normal self-association domain was remarkably similar to the crystal structure of the Drosophila alpha-spectrin 14th repeat unit, despite replacement in the human sequence of over 70% of the amino acids relative to fly spectrin, including 2 prolines in the human sequence that appear in helical regions of the fly structure. The predicted structure placed all hydrophilic residues at the surface and identified 4 salt bridges, 9 hydrophobic interactions, and 4 H-bonds that stabilize the native self-association unit. Remarkably, every pathologic point mutation, including seemingly conservative substitutions such as G for A, A for V, or K for R (single-letter amino acid codes), led to conformational rearrangements in the predicted structure. The degree of structural disruption, as measured by root-mean-square deviation of the predicted backbone structure from the Drosophila structure, correlated strongly with the severity of clinical disease associated with each mutation. This approach thus enables an accurate prediction, from the primary sequence, of the clinical consequences of specific point mutations in spectrin. The 3-dimensional structure of the self-association domain derived here is likely to be accurate. It provides a powerful heuristic model for understanding how point mutations disrupt cytoskeletal function in a variety of hemolytic disorders.

A comparison of the in vivo kinetics of Plasmodium falciparum ring-infected erythrocyte surface antigen-positive and -negative erythrocytes

Ring-infected erythrocyte surface antigen (RESA)-positive, Plasmodium falciparum-negative red blood cells (RBCs) are cells from which the malaria parasite has been removed by the host without the destruction of the erythrocyte ("pitting"). The survival of RESA-RBCs in vivo was assessed in 14 severe and 6 uncomplicated falciparum malaria patients. The mean RESA-RBC life of 183 hours (95% confidence interval [CI], 136-246) was longer than the median parasite clearance time of 66 hours (range, 30-108 hours) but shorter than the mean red cell life of 1027 hours (95% CI, 840-1213) (P =.0004), with a median ratio of 0.2:1.0 (range, 0.1-0.7). The estimated median percentage of parasites pitted/body transit was 0.003% (range, 0.001%-0.05%). The rate of rise of the RESA-RBC count during the first 24 hours after antimalarial treatment was significantly faster (P =.036) and the subsequent RESA-RBC survival significantly shorter (P =.017) after treatment with an artemisinin derivative than after treatment with quinine. Parasitization of red cells leads to changes in the erythrocyte that shorten their survival even if the parasite is removed subsequently.

Brain and muscle express a unique alternative transcript of alphaII spectrin

Alternative splicing of pre-mRNA transcripts of alpha and beta spectrin has emerged as an important generator of diversity in this gene family, yet the functional consequences and extent of this diversity remains unknown. We have cloned and characterized full-length alphaII spectrin cDNA from human fetal brain (GenBank and ). On the basis of the predicted amino acid sequence, 11 amino acid substitutions, presumably representing polymorphisms, have been identified that distinguish this alphaII spectrin from human lung fibroblast alphaII spectrin. In addition, human fetal brain spectrin displays a novel five amino acid insertion in repeat 15 that arises from alternative mRNA splicing and that distinguishes this spectrin from lung fibroblast alphaII++ spectrin. This discovery, together with two previously identified regions of alternative mRNA splicing in alphaII spectrin suggest that as many as eight different splice forms of the mature protein might exist if all combinations (at inserts 1, 2, and 3) of alternative mRNA splicing are utilized. To assess this possibility, the tissue distribution of alternative exon usage was investigated by semiquantitative PCR with intron-jumping primer sets. Tissues examined were from mouse and included heart, kidney, lung, liver, thymus, spleen, brain, ovary, testis, and skeletal muscle, as well as mouse embryonic tissue. Transcripts both with and without insert 1, representing a 60 bp insertion within alphaII spectrin repeat 10, were identified in all tissues. In contrast, transcripts with insert 2, the novel 15 bp insertion reported here, were only expressed in brain, heart, skeletal muscle, and embryonic tissue. In all tissues examined only transcripts positive for insert 3, an 18 bp insertion in repeat 21, were amplified, even under conditions in which a 30% level of insert 3 negative transcript could be easily detected in artificially prepared control samples. All combinations of insert 1 and insert 2 were identified together in individual transcripts, verifying at least four distinct isoforms of alphaII spectrin. These have been named alphaIISigma1 through alphaIISigma4, in accord with current spectrin naming conventions. Dynamic molecular modeling of the 15th repeat unit incorporating insert 2 predicts that the spliced sequence forms a loop between helices A and B, and suggests that this insert might constitute a novel protein interaction site. The presence of this sequence in alphaIISigma3 and alphaIISigma4 spectrin suggests a specialized and heretofore unanticipated function for the 15th repeat of this molecule.

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).

Alport syndrome, mental retardation, midface hypoplasia, and elliptocytosis: a new X linked contiguous gene deletion syndrome?

We describe a family with four members, a mother, two sons, and a daughter, who show clinical features consistent with X linked Alport syndrome. The two males presented with additional features including mental retardation, dysmorphic facies with marked midface hypoplasia, and elliptocytosis. The elliptocytosis was not associated with any detectable abnormalities in red cell membrane proteins; red cell membrane stability and rigidity was normal on ektacytometry. Molecular characterisation suggests a submicroscopic X chromosome deletion encompassing the entire COL4A5 gene. We propose that the additional abnormalities found in the affected males of this family are attributable to deletion or disruption of X linked recessive genes adjacent to the COL4A5 gene and that this constellation of findings may represent a new X linked contiguous gene deletion syndrome.

Thrombosis and Secondary Hemochromatosis Play Major Roles in the Pathogenesis of Jaundiced and Spherocytic Mice, Murine Models for Hereditary Spherocytosis

Jaundiced mice, ja/ja, suffer from a severe hemolytic anemia caused by a complete deficiency of erythroid β-spectrin. We used these mice as a model to investigate the pathophysiological consequences of the deficiency, including the effects in the nonerythroid tissues where this protein is expressed. Because the ja/ja mice rarely survive beyond the fourth postnatal day, methods were assessed for extending lifespan into adulthood. Neonatal transfusion increased lifespan to a mean of 3.7 months, allowing a more complete characterization of the pathophysiology. Blood parameters and histopathology of the jaundiced mouse were compared with that from spherocytic mice, which have a hemolytic anemia caused by deficiency of erythroid α-spectrin, yet can survive the postnatal period transfusion free. The adult jaundiced and spherocytic mice present with greatly decreased hematocrit and red blood cell counts, reticulocytosis, and bilirubinemia, leading secondarily to hepatosplenomegaly and cardiomegaly. Jaundiced and spherocytic mice were analyzed histopathologically between 1.0 and 9.5 months of age. Interestingly, the complete absence of erythroid β-spectrin in jaundiced mice leads to no detectable structural defects in brain, cardiac, or skeletal muscles. However, fibrotic lesions and lymphocytic infiltration were observed in cardiac tissue from 4 of 13 jaundiced mice and 15 of 15 spherocytic mice, and thrombi were detected at either the atrioventricular valves or within the atria of 2 of 13 jaundiced mice and 15 of 15 spherocytic mice. In addition, all affected mice had a progressive renal hemosiderosis concurrent with hydronephrosis and glomerulonephritis. The severity of the renal disease and its presence in all moribund mice suggests kidney failure rather than the fibrotic heart lesions as the major cause of death in these mice.

Thrombosis and Secondary Hemochromatosis Play Major Roles in the Pathogenesis of Jaundiced and Spherocytic Mice, Murine Models for Hereditary Spherocytosis

Jaundiced mice, ja/ja, suffer from a severe hemolytic anemia caused by a complete deficiency of erythroid β-spectrin. We used these mice as a model to investigate the pathophysiological consequences of the deficiency, including the effects in the nonerythroid tissues where this protein is expressed. Because the ja/ja mice rarely survive beyond the fourth postnatal day, methods were assessed for extending lifespan into adulthood. Neonatal transfusion increased lifespan to a mean of 3.7 months, allowing a more complete characterization of the pathophysiology. Blood parameters and histopathology of the jaundiced mouse were compared with that from spherocytic mice, which have a hemolytic anemia caused by deficiency of erythroid α-spectrin, yet can survive the postnatal period transfusion free. The adult jaundiced and spherocytic mice present with greatly decreased hematocrit and red blood cell counts, reticulocytosis, and bilirubinemia, leading secondarily to hepatosplenomegaly and cardiomegaly. Jaundiced and spherocytic mice were analyzed histopathologically between 1.0 and 9.5 months of age. Interestingly, the complete absence of erythroid β-spectrin in jaundiced mice leads to no detectable structural defects in brain, cardiac, or skeletal muscles. However, fibrotic lesions and lymphocytic infiltration were observed in cardiac tissue from 4 of 13 jaundiced mice and 15 of 15 spherocytic mice, and thrombi were detected at either the atrioventricular valves or within the atria of 2 of 13 jaundiced mice and 15 of 15 spherocytic mice. In addition, all affected mice had a progressive renal hemosiderosis concurrent with hydronephrosis and glomerulonephritis. The severity of the renal disease and its presence in all moribund mice suggests kidney failure rather than the fibrotic heart lesions as the major cause of death in these mice.