Mutations in the murine erythroid alpha-spectrin gene alter spectrin mRNA and protein levels and spectrin incorporation into the red blood cell membrane skeleton

Tetramers of alpha- and beta-spectrin heterodimers, linked by intermediary proteins to transmembrane proteins, stabilize the red blood cell cytoskeleton. Deficiencies of either alpha- or beta-spectrin can result in severe hereditary spherocytosis (HS) or hereditary elliptocytosis (HE) in mice and humans. Four mouse mutations, sph, sph(Dem), sph(2BC), and sph(J), affect the erythroid alpha-spectrin gene, Spna1, on chromosome 1 and cause severe HS and HE. Here we describe the molecular alterations in alpha-spectrin and their consequences in sph(2BC)/sph(2BC) and sph(J)/sph(J) erythrocytes. A splicing mutation, sph(2BC) initiates the skipping of exon 41 and premature protein termination before the site required for dimerization of alpha-spectrin with beta-spectrin. A nonsense mutation in exon 52, sph(J) eliminates the COOH-terminal 13 amino acids. Both defects result in instability of the red cell membrane and loss of membrane surface area. In sph(2BC)/sph(2BC), barely perceptible levels of messenger RNA and consequent decreased synthesis of alpha-spectrin protein are primarily responsible for the resultant hemolysis. By contrast, sph(J)/sph(J) mice synthesize the truncated alpha-spectrin in which the 13-terminal amino acids are deleted at higher levels than normal, but they cannot retain this mutant protein in the cytoskeleton. The sph(J) deletion is near the 4.1/actin-binding region at the junctional complex providing new evidence that this 13-amino acid segment at the COOH-terminus of alpha-spectrin is crucial to the stability of the junctional complex.     

Defective spectrin integrity and neonatal thrombosis in the first mouse model for severe hereditary elliptocytosis

Mutations affecting the conversion of spectrin dimers to tetramers result in hereditary elliptocytosis (HE), whereas a deficiency of human erythroid alpha- or beta-spectrin results in hereditary spherocytosis (HS). All spontaneous mutant mice with cytoskeletal deficiencies of spectrin reported to date have HS. Here, the first spontaneous mouse mutant, sph(Dem)/ sph(Dem), with severe HE is described. The sph(Dem) mutation is the insertion of an intracisternal A particle element in intron 10 of the erythroid alpha-spectrin gene. This causes exon skipping, the in-frame deletion of 46 amino acids from repeat 5 of alpha-spectrin and alters spectrin dimer/tetramer stability and osmotic fragility. The disease is more severe in sph(Dem)/sph(Dem) neonates than in alpha-spectrin-deficient mice with HS. Thrombosis and infarction are not, as in the HS mice, limited to adults but occur soon after birth. Genetic background differences that exist between HE and HS mice are suspect, along with red blood cell morphology differences, as modifiers of thrombosis timing. sph(Dem)/sph(Dem) mice provide a unique model for analyzing spectrin dimer- to-tetramer conversion and identifying factors that influence thrombosis.     

Stabilities of folding of clustered, two-repeat fragments of spectrin reveal a potential hinge in the human erythroid spectrin tetramer

The large size of spectrin, the flexible protein promoting reversible deformation of red cells, has been an obstacle to elucidating the molecular mechanism of its function. By studying cloned fragments of the repeating unit domain, we have found a correspondence between positions of selected spectrin repeats in a tetramer with their stabilities of folding. Six fragments consisting of two spectrin repeats were selected for study primarily on the basis of the predicted secondary structures of their linker regions. Fragments with a putatively helical linker were more stable to urea- and heat-induced unfolding than those with a putatively nonhelical linker. Two of the less stably folded fragments, human erythroid alpha-spectrin repeats 13 and 14 (HEalpha13,14) and human erythroid beta-spectrin repeats 8 and 9 (HEbeta8,9), are located opposite each other on antiparallel spectrin dimers. At least partial unfolding of these repeats under physiological conditions indicates that they may serve as a hinge. Also less stably folded, the fragment of human erythroid alpha-spectrin repeats 4 and 5 (HEalpha4,5) lies opposite the site of interaction between the partial repeats at the C- and N-terminal ends of beta- and alpha-spectrin, respectively, on the opposing dimer. More stably folded fragments, human erythroid alpha-spectrin repeats 1 and 2 (HEalpha1,2) and human erythroid alpha-spectrin repeats 2 and 3 (HEalpha2,3), lie nearly opposite each other on antiparallel spectrin dimers of a tetramer. These clusterings along the spectrin tetramer of repeats with similar stabilities of folding may have relevance for spectrin function, particularly for its well known flexibility.     

Identification of quantitative trait loci that modify the severity of hereditary spherocytosis in wan, a new mouse model of band-3 deficiency

Defects in red blood cell (RBC) membrane skeleton components cause hereditary spherocytosis (HS). Clinically, HS varies significantly even among individuals with identical gene defects, illustrating the profound effects of genetic background on disease severity. We exploited a new spontaneous mouse model, wan, which arose on the inbred C3H/HeJ strain, to identify quantitative trait loci (QTL) that modify the HS phenotype. Homozygous wan mice have severe HS due to a complete deficiency of erythroid band 3. A QTL analysis of RBC count, hemoglobin, hematocrit, mean corpuscular volume (MCV), and mean corpuscular hemoglobin content (MCHC) was performed in wan/wan mice from an F2 intercross between C3H/HeJ(+/wan) and CAST/Ei(+/+) F1 hybrids. Hematologic and survival data from C3H, CAST/Ei F2 wan homozygotes support the hypothesis that genetic modifiers significantly influence the band-3 null HS phenotype. Significant QTL were identified for the MCV trait only, suggesting that RBC membrane characteristics are a target for modifier gene action. The most significant quantitative trait locus, Hsm1 (hereditary spherocytosis modifier 1), localizes to mouse Chromosome 12 and is dominant. The peak LOD score was obtained with a marker for Spnb1 encoding erythroid beta-spectrin, an obvious candidate gene.     

Ankyrin and the hemolytic anemia mutation, nb, map to mouse chromosome 8: presence of the nb allele is associated with a truncated erythrocyte ankyrin.

Mice with normoblastosis, nb/nb, have a severe hemolytic anemia. The extreme fragility and shortened lifespan of the mutant erythrocytes result from a defective membrane skeleton. Previous studies in our laboratory indicated a 50% deficiency of spectrin and an absence of normal ankyrin in erythrocyte membranes of nb/nb mice. We now report genetic mapping data that localize both the nb and erythroid ankyrin (Ank-1) loci to the centromeric end of mouse chromosome 8. Using immunological and biochemical methods, we have further characterized the nature of the ankyrin defect in mutant erythrocytes. We do not detect normal sized (210 kDa) erythroid ankyrin by immunoblot analysis in nb/nb reticulocytes. However, nb/nb reticulocytes do contain a 150-kDa ankyrin immunoreactive protein. The 150-kDa protein is present with normal-sized ankyrin in nb/+ reticulocytes but is not found in +/+ reticulocytes. Our genetic and biochemical data indicate that the nb mutation results from a defect in the erythroid ankyrin gene. A human hereditary spherocytosis putatively resulting from an ankyrin defect maps to a segment of human chromosome 8 that is homologous to the nb-ankyrin region of mouse chromosome 8. The linkage data suggest that the mouse and human diseases result from mutations in homologous loci.     

Molecular basis of spectrin deficiency in hereditary pyropoikilocytosis

Hereditary pyropoikilocytosis (HPP) is a recessively inherited hemolytic anemia characterized by severe poikilocytosis and red blood cell fragmentation. HPP red blood cells are partially deficient in spectrin and contain a mutant alpha or beta-spectrin that is defective in terms of spectrin self-association. Although the nature of the latter defect has been studied in considerable detail and many mutations of alpha-spectrin and beta spectrin have been identified, the molecular basis of spectrin deficiency is unknown. Here we report two mechanisms underlying spectrin deficiency in HPP. The first mechanism involves a thalassemia-like defect characterized by a reduced synthesis of alpha-spectrin as shown by studies involving synthesis of spectrin in two unrelated HPP probands and their parents: One parent carries the elliptocytogenic spectrin mutation, whereas the other parent is fully asymptomatic. Peripheral blood mononuclear cells as a source of erythroid burst-forming unit (BFUe) were cultured in a two-phase liquid culture system that gives rise to terminally differentiated erythroblasts. Pulse-labeling studies of an equal number of erythroblasts or morphologically identical maturity showed that the synthesis of alpha-spectrin as well as the mRNA levels as measured by the competitive polymerase chain reaction (PCR) method are markedly reduced in the presumed asymptomatic carriers and the HPP probands. In contrast, the synthesis and mRNA levels of beta-spectrin were normal. These results constitute a direct demonstration of an alpha-spectrin synthetic defect in a subset of asymptomatic carriers of HPP and HPP probands. The second mechanism underlying spectrin deficiency involves increased degradation of mutant spectrin before its assembly on the membrane. This is evidenced by pulse labeling studies of erythroblasts from a patient with HPP associated with a homozygous state for spectrin alpha I/46 mutation (leu-pro mutation at AA 207 of alpha-spectrin). These studies showed that although spectrin is synthesized in the cytosol in normal amounts, the rate of turnover of alpha-spectrin is faster resulting in about 40% to 50% reduced assembly of alpha-spectrin and beta-spectrin on the membrane. Thus, spectrin deficiency in this case is at least in part caused by increased susceptibility of the mutant spectrin to degradation before its assembly on the membrane. We conclude that at least two separate mechanisms underlie the molecular basis of spectrin deficiency in HPP.     

Remarkable homology among the internal repeats of erythroid and nonerythroid spectrin.

A cDNA clone for nonerythroid alpha-spectrin was identified by direct immunological screening of a chicken smooth muscle cDNA library. A library prepared in the expression plasmids pUC8 and pUC9 was screened with an antiserum specific for chicken alpha-spectrin. Blots of poly(A)+ RNA from various tissues of chicken and mouse show that the cDNA hybridizes to an 8-kilobase mRNA. The cDNA hybridizes to a single-copy sequence on Southern blots of chicken genomic DNA. The complete nucleic acid sequence of the clone has a single 1419-base open reading frame. The derived amino acid sequence is organized into two partial and three complete 106-amino-acid repeats that show homology to the repeats described for human erythroid alpha- and beta-spectrin. Immunological and biochemical data indicate that chicken nonerythroid and human erythroid alpha-spectrin are two of the more widely diverged members of the spectrin family of proteins. In this respect, the degree of homology found between them was unexpected. Our data suggest a common evolutionary origin for these two alpha-spectrins and allow some predictions concerning spectrin gene structure.     

Mouse bone marrow and peripheral blood erythroid cell counts are regulated by different autosomal genetic loci

Erythropoiesis is under fine control and genetic loci that affect it are likely to be important in a range of conditions. To assess the relative contributions of different genetic loci to parameters of erythropoiesis, we have measured RBC counts in the peripheral circulation and committed erythroid cells (RBC and small normoblasts) in the bone marrow in a cohort of (CBA/H x C57BL/6) F2 mice to map quantitative trait loci (QTL). Candidate genes were assessed using bioinformatics and DNA sequencing. Different autosomal loci affect bone marrow (chromosomes 5, 11 and 19) and peripheral blood (chromosome 4) erythroid cell counts but there may be a common chromosome X locus. Spleen weight QTL were found on chromosomes 3, 15 and 17. Surprisingly, erythropoietin (Epo) is the best candidate quantitative trait gene (QTG) in the chromosome 5 locus that affects bone marrow but not peripheral blood erythroid cell counts. Epo gene expression is known to be genetically regulated in mice, but our data suggest a tissue-specific role for epo in mouse erythropoiesis that is also genetically determined. The identity of the other QTG will be important both to further knowledge of the control of erythropoiesis and as potential modifier genes for haematological disorders.     

Genes encoding alpha and beta subunits of Na,K-ATPase are located on three different chromosomes in the mouse.

We have made use of a panel of mouse-hamster somatic cell hybrids and restriction fragment length polymorphisms between two mouse species (Mus musculus and Mus spretus) to determine the chromosomal localization of genes encoding the alpha and beta subunits of the Na,K-ATPase (Na+,K+-activated ATP phosphohydrolase, EC 3.6.1.3). DNA probes for three distinct isoforms of the Na,K-ATPase alpha subunit mapped to three different mouse chromosomes: the alpha 1 gene (Atpa-1) cosegregated with the Egf gene on chromosome 3; alpha 2 (Atpa-2) with the cytochrome P-450PB gene family/coumarin hydroxylase locus on chromosome 7; alpha 3 (Atpa-3) with the alpha-spectrin gene on chromosome 1. The Na,K-ATPase beta-subunit gene (Atpb) mapped to the same region of chromosome 1, but it was not tightly linked to the Atpa-3 gene. These results indicate that three isoforms of the Na,K-ATPase alpha subunit are encoded by three distinct genes. The dispersion of Na,K-ATPase genes suggests that their expression is not likely to be controlled by a common cis-acting regulatory element.     

Expression of the beta subunit of spectrin in nonerythroid cells.

Antibodies raised against electrophoretically purified chicken erythrocyte beta subunit of spectrin, called "beta-spectrin," have been used to demonstrate the presence of an immunoreactive form of this polypeptide in nonerythroid tissues. Immunoautoradiography shows that, in chicken erythrocytes, this antiserum reacts with beta-spectrin (Mr 220,000) and another polypeptide (Mr 230,000) that, by two-dimensional tryptic peptide analysis, shows extensive homology with beta-spectrin but not with the alpha subunit of spectrin, called "alpha-spectrin." Immunoautoradiography and immunoprecipitation of various chicken tissues with this antiserum shows that either one variant or both variants of beta-spectrin are expressed. Indirect immunofluorescence reveals that the antiserum reacts with a plasma membrane-associated component of erythroid and some nonerythroid cells. Particularly strong fluorescence is observed in skeletal and cardiac muscle cells where beta-spectrin appears to form a grid-like network along the inner surface of the sarcolemma. The noncoordinated distribution of alpha- and beta-spectrin variants indicates that their expression may be tailored to the functional requirements of the plasma membrane in different cells.     

Molecular basis of spectrin and ankyrin deficiencies in severe hereditary spherocytosis: evidence implicating a primary defect of ankyrin.

While varying degrees of spectrin deficiency have been found in the majority of patients with hereditary spherocytosis (HS), a combined severe deficiency of both spectrin and the spectrin-binding protein, ankyrin, has been reported only in two patients with severe HS. To elucidate the molecular basis of these protein deficiencies, we have studied the synthesis, assembly, and the mRNA levels of spectrin and ankyrin in peripheral blood reticulocytes in one of the previously reported probands. Pulse-labeling studies showed that in HS reticulocytes, the synthesis of alpha-spectrin was comparable with control reticulocytes while that of beta-spectrin was increased about fourfold, presumably reflecting increased erythropoietic drive. On the HS reticulocyte membrane, the amount of newly assembled spectrin was reduced to about half of the control values, presumably reflecting a decrease in the synthesis of the spectrin binding protein, ankyrin: the ankyrin synthesis was nearly absent in the cytosol and the amounts of membrane-associated ankyrin were reduced to about half of the normal values. The changes in the amounts of spectrin and ankyrin mRNAs quantitated by slot blot and Northern blot analyses were comparable with changes in the synthesis of these proteins: The alpha spectrin mRNA was within a control range and the beta-spectrin mRNA was slightly increased, while the amounts of ankyrin mRNA were reduced to about 50% of control values. We conclude that the primary defect underlying the combined spectrin and ankyrin deficiency is a deficiency of ankyrin mRNA leading to a reduced synthesis of ankyrin which, in turn, underlies the decreased assembly of spectrin on the membrane.     

Rhabdomyosarcoma-associated locus and MYOD1 are syntenic but separate loci on the short arm of human chromosome 11.

The MYOD1 locus is preferentially expressed in skeletal muscle and at higher levels in its related neoplasm, rhabdomyosarcoma. We have combined physical mapping of the human locus with meiotic and physical mapping in the mouse, together with synteny homologies between the two species, to compare the physical relationship between MYOD1 and the genetically ascertained human rhabdomyosarcoma-associated locus. We have determined that the myogenic differentiation gene is tightly linked to the structural gene for the M (muscle) subunit of lactate dehydrogenase in band p15.4 on human chromosome 11 and close to the p and Ldh-1 loci in the homologous region of mouse chromosome 7. Because the rhabdomyosarcoma locus maps to 11p15.5, MYOD1 is very unlikely to be the primary site of alteration in these tumors. Further, these analyses identify two syntenic clusters of muscle-associated genes on the short arm of human chromosome 11, one in the region of rhabdomyosarcoma locus that includes IGF2 and TH and the second the tightly linked MYOD1 and LDHA loci, which have been evolutionarily conserved in homologous regions of both the mouse and the rat genomes.     

Molecular cloning of the cDNA for human erythrocyte beta-spectrin

Overlapping cDNA clones, totaling 3.3 kilobases (kb) in length, which encode over 50% of the human erythrocyte beta-spectrin subunit, were isolated by antibody screening of a lambda gt11 expression library constructed from human fetal liver mRNA. The amino acid sequence of the C-terminus of beta-spectrin was derived. The size of beta-spectrin mRNA in human erythroleukemia cells was found to be 7.5 kb. Erythrocyte beta- spectrin is encoded by a gene located on human chromosome 14, as determined by cDNA hybridization to human X mouse somatic cell hybrids.     

Cloning and nucleotide sequence of a mouse erythrocyte beta-spectrin cDNA

A rabbit monospecific antibody for mouse beta-spectrin was used to screen a mouse anemic spleen cDNA expression library. A mouse beta- spectrin cDNA clone was isolated and identified by its ability to make mouse beta-spectrin-like antigens in Escherichia coli. This clone was used to probe total RNA from various mouse tissues. Anemic spleen RNA showed two strongly hybridizing RNA species of approximately 6 and 8 kb. Two very faintly hybridizing bands of about 6 kb and 10 kb could also be seen in total mouse brain RNA. All of these bands could be detected after hybridization under both stringent and nonstringent conditions. This suggests that erythroid beta-spectrin may also be expressed in the brain. No bands could be detected in kidney, liver, or spleen RNA. Southern blot analysis of mouse genomic DNA showed a single hybridizing band after digestion with several restriction endonucleases even under nonstringent conditions. Nucleotide sequencing of the cDNA insert revealed almost complete identity between the N-terminus of the deduced amino acid sequence of the cDNA clone and the C-terminal 15 amino acids of a peptide derived from the beta-8 repeat unit of human erythrocyte beta-spectrin. The deduced amino acid sequence contained most of the conserved amino acids characteristic of the 106 amino acid repeat unit first found in human alpha-spectrin and thus provides the first evidence for a complete 106 amino acid repeat unit structure in beta-spectrin.     

Deficiency of alpha-spectrin synthesis in burst-forming units-erythroid in lethal hereditary spherocytosis

A child diagnosed in utero with hydrops fetalis and a hematocrit of 6.4% was studied to determine the etiology of the anemia. Fetal red blood cells (RBCs) obtained during in utero transfusion had extremely abnormal osmotic fragility. A maternal history of mild autosomal dominant hereditary spherocytosis was present, and the father, who was hematologically normal, had a slightly abnormal osmotic fragility test. The patient was transfusion dependent after birth, with circulating nucleated RBCs but less than 1% reticulocytes. The patient's anemia failed to respond to splenectomy. Because mature RBCs of the patient were not available for study, progenitor-derived erythroblasts grown in culture were investigated. Immunodot assays of the patient's progenitor-derived cells showed a total cell spectrin content 26% of normal. Immunoprecipitation of whole burst-forming units-erythroid-derived cells and solubilized membranes from cells pulse-labeled with 35S-methionine showed a severe deficiency in alpha-spectrin synthesis and a markedly reduced amount of alpha- and beta-spectrin on cell membranes. No alpha-spectrin degradation products were found within the cells or were produced during membrane preparation. Ankyrin content and band 3 synthesis were not different from control. Inheritance of two genetic defects causing severely reduced alpha-spectrin synthesis is proposed as the cause of the lethal anemia, resulting in cell fragmentation during precursor enucleation or during egress from bone marrow.     

β-Spectrin Sta Bárbara: a novel frameshift mutation in hereditary spherocytosis associated with detectable levels of mRNA and a germ cell line mosaicism

Hereditary spherocytosis (HS) is a common inherited anaemia characterized by the presence of spherocytic red cells and by a heterogeneous nature in terms of its clinical presentation, molecular basis and inheritance. Defects in several membrane protein genes have been involved in the pathogenesis of HS, including defects in the beta-spectrin gene. We detected a novel frameshift mutation in the beta-spectrin gene, a C deletion at codon 638, in a patient presenting with HS and spectrin deficiency. The mutant protein was not detected in the membrane or in other cellular compartments, but detectable levels of mutant mRNA were found in the patient. Interestingly, this mutation was not present in the patient's parents, suggesting a genetic mosaicism, especially as the patient has an affected brother with the same molecular defect. We analysed DNA from different tissues of the parents and the mutation was absent from all tissues analysed. This mutation seems to be confined to the germ cell lineage of the patient's mother and must present a mosaic pattern in these cells as the patient also has unaffected siblings.     

A large erythroid spectrin β-chain variant

A large variant of erythrocyte beta-spectrin was found in a child presenting with hereditary elliptocytosis and anaemia. This polypeptide was phosphorylated, cross-reacted with normal beta-spectrin in immunoblotting and formed a dimer with alpha-spectrin that co-purified with normal alpha beta dimer. The molecular weight was estimated to be 330 kD by SDS gel electrophoresis, which is 84 kD (35%) larger than the normal beta-chain. This variant has been tentatively named spectrin Detroit (beta Detroit). Tryptic digests demonstrated a coexisting alpha-spectrin variant Sp alpha I/65 in the propositus, his father and a paternal uncle. Anaemia and elliptocytosis was associated with Sp alpha I/65 rather than beta Detroit, since other family members with beta Detroit in whom alpha-spectrin was normal had no morphological or clinical abnormalities. Family members were identified who had normal alpha-spectrin but were heterozygotic for the large beta-spectrin. Their erythrocyte membranes were more rigid and fragile than normal. The fragility is probably a consequence of both weaker dimer association and spectrin deficiency. Variant spectrin dimers (alpha beta Detroit) had a reduced self-association constants. Binding to ankyrin was normal. Instability of beta Detroit during erythropoiesis is suggested by the fact that it comprises only 25% of the beta-spectrin in beta Detroit heterozygote erythrocytes, and total spectrin was reduced by 20%. Although beta Detroit has some functional defects, this 84 kDa insert in erythrocyte spectrin is compatible with nearly normal function.