Sequence and exon-intron organization of the DNA encoding the alpha I domain of human spectrin. Application to the study of mutations causing hereditary elliptocytosis

Molecular insights into hereditary elliptocytosis and pyropoikilocytosis: NGS uncovers multiple potential candidate genes

Hereditary elliptocytosis (HE) and pyropoikilocytosis (HPP) are considered a group of hemolytic anemias (HE/HPP) due to inherited abnormalities of erythrocyte membrane proteins with a worldwide distribution. Most cases are associated with molecular abnormalities linked to spectrin, band 4.1, and ankyrin. The present study aimed to identify significant molecular signatures on a target panel of 8 genes using whole exome sequencing (WES) in 9 Bahraini patients with elliptocytosis. Case selection was based on presence of anemia not associated with iron deficiency or hemoglobinopathy and demonstrating > 50% elliptocytes in blood smears. The c.779 T > C mutation of SPTA1 (Spectrin alpha), which is a known deleterious missense mutation that inhibits normal association of spectrin molecules to form tetramers, was seen in 4 patients in homozygous (n = 1) and heterozygous (n = 3) states. The αLELY abnormality in association with compound heterozygous mutations in SPTA1 was present in 5 patients (2 associated with the SPTA1 c.779 T > C variant; 3 with c.3487 T > G and various other SPTA1 mutations of uncertain/unknown significance). Seven patients had SPTB (Spectrin beta) mutations, predicted as likely benign by in silico analysis. A novel EPB41 (Erythrocyte Membrane Protein Band 4.1) mutation with potential deleterious impact was also seen. Finally, 2 cases showed an InDel (insertion-deletion mutations) abnormality in the gene that codes for the mechanosensitive ion-channel PIEZO (Piezo Type Mechanosensitive Ion Channel Component 1). PIEZO mutations are reported to cause red cell dehydration but have not been previously described in HE/HPP. Results of this study confirm the involvement of previously reported abnormalities in SPTA1 and suggest possible involvement of other candidate genes in a disorder involving polygenic interactions.

Identifying interpretable gene-biomarker associations with functionally informed kernel-based tests in 190,000 exomes

Here we present an exome-wide rare genetic variant association study for 30 blood biomarkers in 191,971 individuals in the UK Biobank. We compare gene-based association tests for separate functional variant categories to increase interpretability and identify 193 significant gene-biomarker associations. Genes associated with biomarkers were ~ 4.5-fold enriched for conferring Mendelian disorders. In addition to performing weighted gene-based variant collapsing tests, we design and apply variant-category-specific kernel-based tests that integrate quantitative functional variant effect predictions for missense variants, splicing and the binding of RNA-binding proteins. For these tests, we present a computationally efficient combination of the likelihood-ratio and score tests that found 36% more associations than the score test alone while also controlling the type-1 error. Kernel-based tests identified 13% more associations than their gene-based collapsing counterparts and had advantages in the presence of gain of function missense variants. We introduce local collapsing by amino acid position for missense variants and use it to interpret associations and identify potential novel gain of function variants in PIEZO1. Our results show the benefits of investigating different functional mechanisms when performing rare-variant association tests, and demonstrate pervasive rare-variant contribution to biomarker variability.

Aberrant Membrane Composition and Biophysical Properties Impair Erythrocyte Morphology and Functionality in Elliptocytosis

Red blood cell (RBC) deformability is altered in inherited RBC disorders but the mechanism behind this is poorly understood. Here, we explored the molecular, biophysical, morphological, and functional consequences of α-spectrin mutations in a patient with hereditary elliptocytosis (pEl) almost exclusively expressing the Pro260 variant of SPTA1 and her mother (pElm), heterozygous for this mutation. At the molecular level, the pEI RBC proteome was globally preserved but spectrin density at cell edges was increased. Decreased phosphatidylserine vs. increased lysophosphatidylserine species, and enhanced lipid peroxidation, methemoglobin, and plasma acid sphingomyelinase (aSMase) activity were observed. At the biophysical level, although membrane transversal asymmetry was preserved, curvature at RBC edges and rigidity were increased. Lipid domains were altered for membrane:cytoskeleton anchorage, cholesterol content and response to Ca²⁺ exchange stimulation. At the morphological and functional levels, pEl RBCs exhibited reduced size and circularity, increased fragility and impaired membrane Ca²⁺ exchanges. The contribution of increased membrane curvature to the pEl phenotype was shown by mechanistic experiments in healthy RBCs upon lysophosphatidylserine membrane insertion. The role of lipid domain defects was proved by cholesterol depletion and aSMase inhibition in pEl. The data indicate that aberrant membrane content and biophysical properties alter pEl RBC morphology and functionality.

The common hereditary elliptocytosis-associated α-spectrin L260P mutation perturbs erythrocyte membranes by stabilizing spectrin in the closed dimer conformation

Hereditary elliptocytosis (HE) and hereditary pyropoikilocytosis (HPP) are common disorders of erythrocyte shape primarily because of mutations in spectrin. The most common HE/HPP mutations are located distant from the critical αβ-spectrin tetramerization site, yet still interfere with formation of spectrin tetramers and destabilize the membrane by unknown mechanisms. To address this question, we studied the common HE-associated mutation, αL260P, in the context of a fully functional mini-spectrin. The mutation exhibited wild-type tetramer binding in univalent binding assays, but reduced binding affinity in bivalent-binding assays. Biophysical analyses demonstrated the mutation-containing domain was only modestly structurally destabilized and helical content was not significantly changed. Gel filtration analysis of the αL260P mini-spectrin indicated more compact structures for dimers and tetramers compared with wild-type. Chemical crosslinking showed structural changes in the mutant mini-spectrin dimer were primarily restricted to the vicinity of the αL260P mutation and indicated large conformational rearrangements of this region. These data indicate the mutation increased the stability of the closed dimer state, thereby reducing tetramer assembly and resulting in membrane destabilization. These results reveal a novel mechanism of erythrocyte membrane destabilization that could contribute to development of therapeutic interventions for mutations in membrane proteins containing spectrin-type domains associated with inherited disease.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

Canine elliptocytosis due to a mutant beta-spectrin

A 5-year-old, spayed female, mixed-breed dog with persistent elliptocytosis was evaluated at the Veterinary Medical Teaching Hospital at Kansas State University. The elliptocytosis was asymptomatic and was detected during the evaluation of lameness. When subjected to shear stress in an ektacytometer, the dog's erythrocytes had reduced cellular deformability and erythrocyte membranes had decreased mechanical stability. Analysis of erythrocyte membrane spectrin by nondenaturing gel electrophoresis revealed an increased amount of spectrin dimers, indicating a defect in spectrin self-association. DNA analysis detected a beta-spectrin mutation in codon 2110 in which threonine was replaced by methionine. This mutation likely altered the molecular structure of the erythrocyte membrane, leading to impaired spectrin self-association and elliptocyte formation.

Sequences downstream of the erythroid promoter are required for high level expression of the human alpha-spectrin gene

Alpha-spectrin is a membrane protein critical for the flexibility and stability of the erythrocyte. We are attempting to identify and characterize the molecular mechanisms controlling the erythroid-specific expression of the alpha-spectrin gene. Previously, we demonstrated that the core promoter of the human alpha-spectrin gene directed low levels of erythroid-specific expression only in the early stages of erythroid differentiation. We have now identified a region 3' of the core promoter that contains a DNase I hypersensitive site and directs high level, erythroid-specific expression in reporter gene/transfection assays. In vitro DNase I footprinting and electrophoretic mobility shift assays identified two functional GATA-1 sites in this region. Both GATA-1 sites were required for full activity, suggesting that elements binding to each site interact in a combinatorial manner. This region did not demonstrate enhancer activity in any orientation or position relative to either the alpha-spectrin core promoter or the thymidine kinase promoter in reporter gene assays. In vivo studies using chromatin immunoprecipitation assays demonstrated hyperacetylation of this region and occupancy by GATA-1 and CBP (cAMP-response element-binding protein (CREB)-binding protein). These results demonstrate that a region 3' of the alpha-spectrin core promoter contains a GATA-1-dependent positive regulatory element that is required in its proper genomic orientation. This is an excellent candidate region for mutations associated with decreased alpha-spectrin gene expression in patients with hereditary spherocytosis and hereditary pyropoikilocytosis.

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.

Erythroid expression of the human alpha-spectrin gene promoter is mediated by GATA-1- and NF-E2-binding proteins

alpha-Spectrin is a highly expressed membrane protein critical for the flexibility and stability of the erythrocyte. Qualitative and quantitative defects of alpha-spectrin are present in the erythrocytes of many patients with abnormalities of red blood cell shape including hereditary spherocytosis and elliptocytosis. We wished to determine the regulatory elements that determine the erythroid-specific expression of the alpha-spectrin gene. We mapped the 5' end of the alpha-spectrin erythroid cDNA and cloned the 5' flanking genomic DNA containing the putative alpha-spectrin gene promoter. Using transfection of promoter/reporter plasmids in human tissue culture cell lines, in vitro DNase I footprinting analyses, and gel mobility shift assays, an alpha-spectrin gene erythroid promoter with binding sites for GATA-1- and NF-E2-related proteins was identified. Both binding sites were required for full promoter activity. In transgenic mice, a reporter gene directed by the alpha-spectrin promoter was expressed in yolk sac, fetal liver, and erythroid cells of bone marrow but not adult reticulocytes. No expression of the reporter gene was detected in nonerythroid tissues. We conclude that this alpha-spectrin gene promoter contains the sequences necessary for low level expression in erythroid progenitor cells.

Spectrin oligomerization is cooperatively coupled to membrane assembly: a linkage targeted by many hereditary hemolytic anemias?

In the erythrocyte, ankyrin is the major adapter protein linking tetramers of band 3 to the spectrin-actin cytoskeleton. This linkage involves a direct interaction between ankyrin and the 14th-15th repeat unit of beta-spectrin. The spectrin cytoskeleton itself is stabilized by the self-association of spectrin heterodimers into tetramers and larger oligomers, a process mediated by the 17th repeat unit of beta-spectrin and a short NH(2) -terminal sequence in alpha-spectrin. The self-association of spectrin and its ankyrin-mediated membrane binding have generally been considered independent events. We now demonstrate that spectrin self-association, the binding of spectrin to ankyrin, and the binding of ankyrin to the 43-kDa cytoplasmic domain of band 3 (cdb3) are coupled in a positively cooperative way. In solution, [(125)I]-labeled ankyrin was found by ND-PAGE3 to enhance the affinity of spectrin self-association by 10-fold. The reciprocal process was also true, in that spectrin tetramers and oligomers bound ankyrin with enhanced affinity relative to dimer spectrin. Saturation of the beta-spectrin self-association site by an NH(2) -terminal 80-kDa alpha-spectrin peptide enhanced the affinity of spectrin dimer for ankyrin, indicating a direct relationship between ankyrin binding and the occupancy of the beta-spectrin self-association site. cdb3 accentuated these cooperative interactions. Several inherited spectrin mutations that cause hemolytic disease but that do not directly destabilize the self-association or ankyrin-binding sites can be explained by these results. Three classes of mutations appear to disrupt cooperative coupling between self-association and ankyrin binding: (i) mutation of the linker sequences that join helices C and A in repeat units that intervene between the two functional sites, mutations that presumably block repeat-to-repeat transfer of conformational information; (ii) mutations in alpha-spectrin repeats 4 to 6 that disrupt the ability of this region to trans-regulate ankyrin binding by the adjacent beta-spectrin repeats 14-15; and (iii) exon-skipping mutations that shorten alpha-spectrin and force repeats 4 to 6 to fall out-of-register with the ankyrin-binding motif in beta-spectrin. Collectively, these results demonstrate a molecular mechanism whereby a membrane receptor can directly promote cytoskeletal assembly.

Genetic basis of the polymorphisms of the alphaI domain of spectrin

Defects of alpha spectrin have been identified in many cases of hereditary elliptocytosis (HE) and hereditary pyropoikilocytosis (HPP). To aid in the genetic analysis of families with these disorders, the locations of three alpha-spectrin gene polymorphisms were mapped, the genetic basis of these polymorphisms identified, and PCR-based assays designed for their identification. The frequencies of these polymorphisms were determined in two populations and in patients with alphaI/50a HE and HPP. These studies identified two distinct haplotypes and provided evidence that two HE/HPP mutations associated with the alphaI/50a protein phenotype, L207P and L260P, arose on separate chromosomal backgrounds.

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

We studied an infant with severe nonimmune hemolytic anemia and hydrops fetalis at birth. His neonatal course was marked by ongoing hemolysis of undetermined etiology requiring repeated erythrocyte transfusions. He has remained transfusion-dependent for more than 2 yr. A previous sibling born with hemolytic anemia and hydrops fetalis died on the second day of life. Peripheral blood smears from the parents revealed rare elliptocytes. Examination of their erythrocyte membranes revealed abnormal mechanical stability as well as structural and functional abnormalities in spectrin. Genetic studies revealed that the proband and his deceased sister were homozygous for a mutation of betaIsigma1 spectrin, L2025R, in a region of spectrin that is critical for normal function. The importance of leucine in this position of the proposed triple helical model of spectrin repeats is highlighted by its evolutionary conservation in all beta spectrins from Drosophila to humans. Molecular modeling demonstrated the disruption of hydrophobic interactions in the interior of the triple helix critical for spectrin function caused by the replacement of the hydrophobic, uncharged leucine by a hydrophilic, positively charged arginine. This mutation must also be expressed in the betaIsigma2 spectrin found in muscle, yet pathologic and immunohistochemical examination of skeletal muscle from the deceased sibling was unremarkable.

Combination of two mutant alpha spectrin alleles underlies a severe spherocytic hemolytic anemia

We studied a patient with a severe spherocytic hemolytic anemia without family history of spherocytosis. Analysis of patient's erythrocyte membrane proteins revealed spectrin deficiency and a truncated alpha spectrin protein. We determined that the patient is a compound heterozygote with two mutations in alpha spectrin gene. Mutation in the paternal allele, designated alpha spectrin(PRAGUE), is a transition A to G in the penultimate position of intron 36 that leads to skipping of exon 37, frameshift, and production of the truncated alpha spectrin protein. The maternal allele, designated alpha spectrin(LEPRA), contains transition C-->T in position -99 of intron 30. This mutation enhances an alternative acceptor splice site 70 nucleotides upstream from the regular site. The alternative splicing causes a frameshift and premature termination of translation leading to a significant decrease in alpha spectrin production. The alpha(LEPRA) mutation is linked to a spectrin alphaIIa marker that was found to be associated with recessive or nondominant spectrin-deficient hereditary spherocytosis in approximately 50% of studied families. We conclude that the alpha(LEPRA) mutation combined in trans with the alpha(PRAGUE) mutation underlie the severe hemolytic anemia in the proband. We suggest that allele alpha spectrin(LEPRA) may be frequently involved in pathogenesis of recessive or nondominant spectrin-deficient hereditary spherocytosis.

Molecular basis and haplotyping of the alphaII domain polymorphisms of spectrin: application to the study of hereditary elliptocytosis and pyropoikilocytosis

Hereditary elliptocytosis (HE) and hereditary pyropoikilocytosis (HPP) are inherited disorders of erythrocyte shape that are frequently associated with abnormalities in alpha-spectrin, one of the principal structural proteins of the erythrocyte membrane skeleton. Five polymorphisms of the alpha-spectrin gene, located in a 6-kb interval of genomic DNA, were identified and analyzed in normal and mutant alpha-spectrin alleles. Three of these polymorphisms are due to single nucleotide substitutions in the alpha-spectrin gene coding region that lead to changes in the amino acid sequence. In combination, these three polymorphisms are responsible for the different peptide phenotypes of the alphaII domain previously observed following limited tryptic digestion of spectrin protein. The most common haplotype, type 1, was found predominantly in Caucasians and was the only haplotype identified in Asians. Haplotypes 2, 3, and 4 were identified predominantly in individuals of African ancestry and were commonly found in patients with HE or HPP. Analysis of coinheritance of alphaII domain polymorphisms with alpha-spectrin gene mutations causing HE or HPP in African-American patients with HE and HPP suggests that, with one exception, a given HE/HPP mutation is present in an alpha-spectrin gene of only one haplotype, indicating a founder effect. The other two polymorphisms located in this region of the alpha-spectrin gene do not change the amino acid sequence of the encoded alpha-spectrin chain and are not in linkage disequilibrium with three of the four alphaII domain haplotypes. A model is proposed for the evolutionary origin of the different haplotypes.

Spectrin mutations in hereditary elliptocytosis and hereditary spherocytosis

Hereditary elliptocytosis (HE), its aggravated form hereditary pyropoikilocytosis (HPP), and hereditary spherocytosis (HS) designate a set of congenital hemolytic syndromes. The responsible mutations lie in several genes encoding proteins of the red cell membrane. In particular, they involve the SPTA1 and SPTB genes that encode erythroid spectrin alpha- and beta-chains, respectively. In situ, spectrin is a alpha 2 beta 2 fibrillar tetramer resulting from the head-to-head self-association of two alpha beta dimers. In HE, the 24 known alpha-chain mutations lie in the self-association site or its vicinity, whereas the 17 beta-chain mutations occur in the self-association site itself (record of November 30, 1995). Allele alpha LELY (LELY: Low Expression LYon) is found in ethnic groups remote from one another with a uniform frequency (20-30% of all alpha-alleles). It allows an expanded expression of any HE alpha-allele located in trans and results in severe HE or in HPP. In HS, a number of spectrin mutations have been recorded recently. Allele alpha LEPRA (LEPRA: Low Expression PRAgue) would occur in a recurrent fashion.

Ethnic distribution of allele alpha LELY, a low-expression allele of red-cell spectrin alpha-gene

Allele alpha LELY is a low-expression allele of erythroid spectrin alpha-chain. It carries mutations both in exon 40 and intron 45 and is associated with partial skipping of exon 46. Allele alpha LELY remains asymptomatic by itself. In contrast, it enhances the expression level of deleterious alpha-alleles occurring in trans, and as such has clinical importance. The aim of this study was to evaluate the incidence of allele alpha LELY in various ethnic groups, i.e. Caucasians, African Blacks, Japanese and Chinese. Allele alpha LELY occurred in all groups investigated with a fairly uniform frequency: 31%, 21%, 20% and 22%, respectively. Mutations in exon 40 and intron 45 appeared linked to one another without exception. Partial skipping of exon 46 or the low-expression feature, whenever they could be assessed, were invariably observed. Allele alpha LELY appears to be an ancient and stable allele.

Recurrent fatal hydrops fetalis associated with a nucleotide substitution in the erythrocyte beta-spectrin gene

We studied a kindred in which four third-trimester fetal losses occurred, associated with severe Coombs-negative hemolytic anemia and hydrops fetalis. Postmortem examination of two infants revealed extensive extramedullary erythropoiesis. Studies of erythrocytes and erythrocyte membranes from the parents revealed abnormal erythrocyte membrane mechanical stability as well as structural and functional abnormalities in spectrin, the principal structural protein of the erythrocyte membrane. Genetic studies identified a point mutation of the beta-spectrin gene, S2019P, in a region of beta spectrin that is critical for normal spectrin function. Both parents and two living children were heterozygous for this mutation; three infants dying of hydrops fetalis were homozygous for this mutation. In an in vitro assay using recombinant peptides, the mutant beta-spectrin peptide demonstrated a significant abnormality in its ability to interact with alpha spectrin. This is the first description of a molecular defect of the erythrocyte membrane associated with hydrops fetalis.

Molecular basis of clinical and morphological heterogeneity in hereditary elliptocytosis (HE) with spectrin alpha I variants

The impaired ability of spectrin dimers to self-associate into tetramers is one of the most frequent defects associated with hereditary elliptocytosis (HE) and its more serious form, hereditary pyropoikylocytosis (HPP). We previously described four proteic variants of the spectrin (Sp) alpha I tryptic domain associated with the Sp dimer self-association defect (Sp alpha I/78, Sp alpha I/74, Sp alpha I/65, Sp alpha I/46 variants). Following the characterization of proteic variants, genomic molecular defects were identified and most of the mutations appeared to lie either in or near the self-association site, i.e. in the alpha I tryptic domain or in the beta I tryptic domain. The clinical severity of these different mutations varies considerably and ranges from asymptomatic to severe haemolytic disease such as in heterozygous HPP patients and in some homozygous HE patients. Studies of 113 patients from 61 HE families showed a correlation among parameters and showed which factors modulate the clinical expression of the molecular defect. Our analysis indicated that the clinical expression was directly correlated with the severity of the spectrin dimer self-association defect as evaluated by the increase in the Sp dimer percentage found in the 4 degrees C extract. A critical threshold of 40-50% of unassembled Sp dimer was determined; above that, patients exhibited severe haemolysis requiring splenectomy. The percentage of Sp dimer depends, in turn, on two factors: (i) the nature of the variant in relation to the position of the mutation versus the tetramerization site; (ii) the relative amount of mutant spectrin present in the membrane (ranging from 15% to 80% in heterozygous patients). As for the severity of haemolysis, the ghost mechanical stability to shear stress, as measured by ektacyometer, was also found to depend on the Sp dimer self-association defect. In contrast, the decrease in erythrocyte deformability was not related to the amount of unassembled Sp dimer but appeared to be correlated with the amount of mutant spectrin whatever the variant. Concerning erythrocyte morphology and the number of elliptocytes, the Sp alpha I/65 variant appears to be the most 'elliptocytogenic' variant, indicating that erythrocyte shape abnormality is not linked to the Sp dimer self-association defect.

The polymerase chain reaction. Applications in dermatology

Within the space of the last 5 years, application of the revolutionary in vitro method of deoxyribonucleic acid (DNA) amplification known as the polymerase chain reaction (PCR), has become ubiquitous. The rapidly increasing number of clinical and research articles utilizing this technology, both in the dermatologic and general medical literature, requires one to have at least a basic understanding of how the PCR is conducted, what it has to offer, and the potential shortcomings. Such knowledge will hopefully allow a more critical appraisal of an increasingly complex literature. This review aims to describe the methodology and medical applications of this powerful technique with special consideration to the increasing role PCR may have on dermatologic research and practice.

Low expression allele alpha LELY of red cell spectrin is associated with mutations in exon 40 (alpha V/41 polymorphism) and intron 45 and with partial skipping of exon 46

The alpha V/41 polymorphism of erythroid alpha-spectrin has been characterized initially by an increased susceptibility to proteolysis of the alpha IV-alpha V domain junction (Alloisio N., L. Morlé, J. Maréchal, A.-F. Roux, M.-T. Ducluzeau, D. Guetarni, B. Pothier, F. Baklouti, A. Ghanem, R. Kastally, et al. 1991. J. Clin. Invest. 87:2169-2177). Until now, it has been found associated invariably with a low expression level of the corresponding alpha chain. Among 61 chromosomes investigated in French and North African individuals or kindreds, we observed 19 chromosomes with the alpha V/41 polymorphism. With no single exception, the latter displayed a point mutation in exon 40 (Leu-->Val; CTA-->GTA) at position alpha 1857. According to the triple helical model of spectrin structure, this change accounts for the peptide maps' abnormalities. Sequencing the entire alpha V domain cDNA disclosed, in addition, a partial skipping of exon 46. At the gene level, a substitution (C-->T) was evidenced at nucleotide -12 of intron 45. This mutation appeared linked to the exon 40 mutation in 17 chromosomes, again with no single exception, among 53 examined chromosomes. We hypothesized that the lack of exon 46 would hamper the nucleation process and eventually account for the low expression feature. The present doubly mutated allele was renamed allele alpha LELY (low expression, Lyon).

An alpha-spectrin mutation responsible for hereditary elliptocytosis associated in cis with the alpha v/41 polymorphism

The alpha 207 Leu-->Pro mutation in spectrin has recently been identified as a cause of alpha I/50-46a hereditary elliptocytosis (HE) or pyropoikilocytosis among Black people. We have found this mutation in a Moroccan family in both the heterozygous and homozygous states. The mutated alpha-spectrin allele carried, in cis, the alpha V/41 polymorphism, a common polymorphism altering the peptide maps and associated with a low-expression level. This is the first report of the cis combination of an HE mutation and the alpha V/41 polymorphism. Presumably, such a combination accounts for the very low expression of the abnormal allele in the heterozygous state.

Severe poikilocytosis associated with a de novo alpha 28 Arg-->Cys mutation in spectrin

Severe poikilocytosis was observed in an Italian child. The mutation responsible was a de novo alpha 28 Arg-->Cys substitution (CGT-->TGT) in spectrin, a mutation known to cause hereditary elliptocytosis or hereditary pyropoikilocytosis. In this particular case the severity of the manifestations were accounted for by the occurrence, in trans to the alpha 28 mutation, of the alpha V/41 polymorphism. The latter has been shown previously to be associated with structural abnormalities at the alpha IV-alpha V domain junction and with a low expression level. The pronounced alteration of the dimer self association process was also explained by the location of the alpha 28 mutation. This mutation occurs in helix 3 of repeating segment alpha 1, e.g. precisely in the head-to-head contact between the spectrin alpha and beta chains. The present phenotype was compared to that yielded by another alpha 28 mutation (Arg-->His) also combined, in trans, with the alpha V/41 polymorphism. The pictures were very much alike, stressing the functional importance of residue alpha 28. The de novo character of the present mutation strengthens the view that codon alpha 28 is a 'hot spot' for mutations.

Alpha I/65 hereditary elliptocytosis in southern Italy: evidence for an African origin

alpha I/65 Hereditary elliptocytosis (HE) is due to the duplication of TTG codon 154 (leucine) of alpha-spectrin and is associated with a constant haplotype. It was encountered exclusively in African and American Blacks, and in North Africans. We assumed that it diffused from the Benin-Togo area to Northern Africa. We now report two South Italian families with alpha I/65 HE. The phenotype fully conformed to previous descriptions. The mode of transmission was dominant; however, the manifestations were more pronounced when the common, low expression level alpha V/41 allele occurred in trans to the alpha I/65 allele, also conforming to previous records. The mutation underlying alpha I/65 HE turned out to be, again, the duplication of TTG codon 154 and the associated haplotype was the same as that encountered previously (+-+; XbaI, PvuII, MspI). Thus, the alpha I/65 allele found in Italy must have been introduced from North Africa across the Sicilian channel and would ultimately have originated from the Benin-Togo area. It would witness the same migratory stream as that followed by the Benin type haemoglobin S allele, which is also present in Southern Italy.

Fast screening methods to detect mutations of spectrin in subjects with hereditary elliptocytosis

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A common type of the spectrin alpha I 46-50a-kD peptide abnormality in hereditary elliptocytosis and pyropoikilocytosis is associated with a mutation distant from the proteolytic cleavage site. Evidence for the functional importance of the triple helical model of spectrin

We studied nine individuals from five unrelated families with alpha I/46-50a hereditary elliptocytosis (HE) or hereditary pyropoikilocytosis (HPP), including one of the original HHP probands first reported by Zarkowsky and colleagues (1975. Br. J. Haematol. 29:537-543). Biochemical analysis of erythrocyte membrane proteins from these patients revealed, as a common abnormality, the presence of the alpha I/46-50a peptide after limited tryptic digestion of spectrin. The polymerase chain reaction was utilized to study the structure of the DNA encoding the alpha I domain of spectrin in the affected individuals. The DNA sequence of the alpha-spectrin gene encoding the region of the alpha-spectrin chain surrounding the abnormal proteolytic cleavage site was normal. We identified a point mutation causing the replacement of a highly conserved leucine residue by proline at position 207 in the alpha-spectrin chain, a site 51 residues to the amino-terminal side of the abnormal proteolytic cleavage site. Analysis of the proposed triple helical model of spectrin repeats reveals that the mutation occurs in helix 2 at a position directly opposite the abnormal proteolytic cleavage site in helix 3, making this the first report of a mutation occurring in helix 2 of a repeat in the alpha I domain of spectrin. These results add to the molecular heterogeneity of mutations associated with HE/HPP and provide further support for the proposed triple helical model of spectrin. Disruption of this proposed alpha-helical structure by helix-breaking proline substitutions may result in a functionally defective spectrin chain.

Four different mutations in codon 28 of alpha spectrin are associated with structurally and functionally abnormal spectrin alpha I/74 in hereditary elliptocytosis

Hereditary elliptocytosis (HE) Sp alpha I/74 is a disorder associated with defective spectrin (Sp) heterodimer self-association and an abnormal tryptic cleavage of the 80-kD alpha I domain of Sp resulting in increased amounts of a 74-kD peptide. The molecular basis of this disorder is heterogeneous and mutations in codons 28, 46, 48, and 49 (codons 22, 40, 42, and 43 in the previous nomenclature which did not include the six NH2-terminal amino acids) have been reported. In this study we present data on seven unrelated HE Sp alpha I/74 kindred from diverse racial backgrounds in whom we identified four different mutations all occurring in exon 2 of alpha Sp at codon 28. Utilizing the polymerase chain reaction we established a CGT----CTT; Arg----Leu 28 mutation in one kindred of Arab/Druze origin. In two unrelated white kindred of English/European origin the substitution is CGT----AGT; Arg----Ser 28 and in two apparently unrelated white kindred from New Zealand, the mutation is CGT----TGT; Arg----Cys 28. Finally, in one American black kindred and in a black kindred from Ghana the mutation involves CGT----CAT; Arg----His 28. Allele specific oligonucleotide hybridization confirmed that the probands are heterozygous for the respective mutant alleles. All four point mutations abolished an Aha II restriction enzyme site which allowed verification of linkage of the mutation with HE Sp alpha I/74. Our results imply that codon 28 of alpha Sp is a "hot spot" for mutations and also indicate that Arg 28 is critical for the conformational stability and functional self association of Sp heterodimers.

Sp alpha V/41: a common spectrin polymorphism at the alpha IV-alpha V domain junction. Relevance to the expression level of hereditary elliptocytosis due to alpha-spectrin variants located in trans

Spectrin alpha-chain mutants associated with hereditary elliptocytosis are highly variable in their level of expression. It has been assumed that the degree of elliptocytosis can be increased when the spectrin alpha chain, encoded by the alpha gene in trans to the variant, is expressed at a low level. We now provide strong evidence for the existence of low-level expression of spectrin alpha chains. This condition is referred to as the alpha V/41 polymorphism. It has been observed in 15 different families or individuals of French, North African, and African ancestry in which seven distinct elliptocytogenic alpha-spectrin variants were co-inherited. Whenever the alpha V/41 polymorphism was present, the severity of the biochemical, morphological, and, sometimes, the clinical phenotype of elliptocytosis was increased. The alpha V/41 polymorphism was also frequently encountered among 36 unrelated control subjects in the heterozygous or homozygous states, and was entirely asymptomatic in both cases. The main biochemical feature was an increased susceptibility to proteolysis of the alpha IV-alpha V domain junction. Alteration of the facing beta IV domain of spectrin was demonstrated by in vitro spectrin dimer reconstitution experiments. It appears that the alpha V/41 polymorphism is often required for alpha-spectrin elliptocytogenic variants to become manifest in the heterozygous state. Thus, alpha-spectrin-related elliptocytosis may be viewed as a bifactorial condition.

The exon-intron organization of the human erythrocyte alpha-spectrin gene

The human erythrocyte alpha-spectrin gene which spans 80 kbp has been cloned from human genomic DNA as overlapping lambda recombinants. The exon-intron junctions were identified and the exons mapped. The gene is encoded by 52 exons whose sizes range from 684 bp to the smallest of 18 bp. The donor and acceptor splice site sequences match the splice site consensus sequences, with the exception of one splice site where a donor sequence begins with -GC. The size and location of exons do not correlate with the 106-amino-acid repeat, except in three locations where the surrounding codons are conserved as well. The lack of correspondence between exons and 106-amino-acid repeat is interpreted to reflect the appearance of a spectrin-like gene from a minigene early in the evolution of eukaryotes. Since current evidence indicates that introns were present in genes before the divergence of prokaryotes and eukaryotes, it is possible that the original distribution of introns within the minigene has been lost by the random deletion of introns from the spectrin gene.

Radiolabel-transfer cross-linking demonstrates that protein 4.1 binds to the N-terminal region of beta spectrin and to actin in binary interactions

Erythrocyte protein 4.1 plays a major role in stabilizing the spectrin-actin junction of the erythrocyte membrane skeleton. The particular sites on spectrin responsible for the binding of actin and protein 4.1 have not been specifically defined, although the general region of the 'tail' end, opposite the self-association site, has been deduced by electron microscopy. Using a photoactivatable, radiolabel-transfer cross-linker, 1-[N-(2-hydroxy-5-azidobenzoyl)-2-aminoethyl]-4-(N-hydroxysuccinimidyl)- succinate, we have determined that the binding site for protein 4.1 on spectrin resides in the N-terminal region of beta spectrin within a sequence homologous to the actin-binding region of alpha actinin. Moreover, this technique provided clear evidence for a direct binding interaction between actin filaments and protein 4.1 that was confirmed by rapid-sedimentation assays. In summary, use of radiolabel-transfer cross-linking has enabled assignment of the protein-4.1-binding site on erythrocyte spectrin and has identified a previously ill-defined binary interaction between protein 4.1 and F-actin.

Abnormal tryptic peptide from the spectrin alpha-chain resulting from alpha- or beta-chain mutations: two genetically distinct forms of the Sp alpha I/74 variant

Limited tryptic digestion of native spectrin (Sp) has revealed several variants in hereditary pyropoikilocytosis (HPP) and in a subset of patients with hereditary elliptocytosis (HE). In most cases, tryptic peptide corresponding to the alpha I (N-terminal) 80 kD domain is wholly or partially replaced by smaller fragments. These variants are provisionally designated according to the molecular weight of the most prominent new peptide. Partial amino acid sequences of the abnormal peptides and DNA analysis of the alpha-spectrin gene have shown that most variants result from substitution or insertion of an amino acid in the alpha I-domain. However, similar investigations did not detect any such abnormality in the spectrin alpha I-domain of an HE black kindred with one of the spectrin variants called Sp alpha I/74. In this kindred, restriction fragment length polymorphism studies and transmission of the genetic polymorphism relative to the alpha II-domain excluded the involvement of the alpha-chain in the pathological process. To ascertain whether the abnormal alpha I 74 kD peptide might be caused by a beta-chain mutation, we reconstituted hybrid dimers combining normal and HE Sp-chains. The tryptic peptide patterns of spectrin hybrid dimers containing HE alpha-chain and control beta-chain showed a normal 80 kD tryptic product. In contrast, the hybrid dimer containing normal alpha-chain and HE beta-chain gave rise to increased 74 kD peptide at the expense of the 80 kD, demonstrating that the mutation in this family resides in the beta-chain. The same method was used to show that in two other unrelated white kindreds, the elevated 74 kD peptide arose from a Sp alpha-chain defect. Thus an alteration in tryptic susceptibility within the N-terminal domain of the spectrin alpha-chain can be directed by a mutation in the beta-chain. The hybridization technique affords a definitive means of distinguishing between alpha- and beta-chain mutants.

Evidence that expression of Sp alpha I/65 hereditary elliptocytosis is compounded by a genetic factor that is linked to the homologous alpha-spectrin allele

Many cases of hereditary elliptocytosis (HE) result from mutated spectrin alpha-chains. It has repeatedly been observed that the amount of a mutant alpha-chain is different in various affected individuals, resulting in clinical pictures of variable severity. The different levels are thought to result from different percentages of the alpha-spectrin allele in trans. Such percentages, in turn, could be under genetic control. We tested this hypothesis in a large Algerian family with Sp alpha I/65 HE. In an informative sibship, we found three persons with a distinctly high level of expression of the Sp alpha I/65 variant, suggesting the existence, in trans, of a low percentage alpha-allele. The alpha-spectrin gene haplotype associated with the latter was constantly - + -, based on the XbaI, PvuII, and MspI polymorphic sites. In contrast, a basal level of expression of the Sp alpha I/65 variant in the same sibship indicated, in trans, the existence of a normal percentage alpha-allele. The haplotype corresponding to this other alpha-allele was + - +. Study of another generation of the family showed, however, that the - + - haplotype could also be linked to a normal percentage alpha-allele. These results are consistent with the view that the expression level of alpha I/65 spectrin (and of other types of alpha-variants) is compounded by a genetic factor that is linked to the normal alpha-allele in trans. The low percentage allele itself remains silent in the simple heterozygous state.

The polymerase chain reaction: miracle or mirage? A critical review of its uses and limitations in diagnosis and research

Since publication of the polymerase chain reaction (PCR) technique in 1985 (Saiki et al. Science 1985; 230: 1350-1354), there has been an explosion of reports on its use in medicine and science. We critically review its use both as a diagnostic technique and as a research tool, and show the pathologist how to evaluate PCR data and how to avoid the pitfalls of overinterpretation. We discuss the value of PCR in the characterization of genetic defects, prenatal diagnosis, carrier testing, HLA typing, detecting micro-organisms, identifying activated oncogenes, and in the characterization of leukaemias and lymphomas, and summarize the main applications in biomedical research.

Point mutation in the beta-spectrin gene associated with alpha I/74 hereditary elliptocytosis. Implications for the mechanism of spectrin dimer self-association

alpha I/74 hereditary elliptocytosis (HE) is a subgroup of HE in which patients exhibit an impaired self-association of spectrin dimers and an abnormal proteolytic cleavage of the alpha I domain of spectrin. We studied a family in which the proband presented with a severe neonatal hemolytic anemia with poikilocytosis. Biochemical analysis of erythrocytes from the proband and his family members allowed us to ascertain a diagnosis of homozygosity for alpha I/74 HE in the proband and heterozygosity in his parents and several of their offspring. Results of polymorphism linkage analysis suggested that the defect in this family was located in beta rather than alpha spectrin. We analyzed the 3' end of the beta-spectrin gene of the proband and detected a mutation that changes a codon for alanine to one for proline. Allele-specific oligomer hybridization on slot blots of DNA from other family members confirmed the presence of the mutation only in members heterozygous for the disorder. This is the first example of a point mutation in the beta-spectrin chain that is associated with defective spectrin dimer self-association and an abnormal proteolytic cleavage of the alpha chain. Based on this finding, we propose a model for the mechanism of interaction between the alpha- and beta-spectrin chains.

Two elliptocytogenic alpha I/74 variants of the spectrin alpha I domain. Spectrin Culoz (GGT----GTT; alpha I 40 Gly----Val) and spectrin Lyon (CTT----TTT; alpha I 43 Leu---Phe)

Spectrin alpha I/74 elliptocytosis results from abnormalities involving the "head" region of spectrin dimer. Increased susceptibility to trypsin enhances cleavage of the alpha spectrin chain, yielding an increased amount of the alpha I 74-kD fragment at the expense of the alpha I 80-kD parent fragment. Recently we showed that the mutations causing the Sp alpha I/74 abnormality may lie in the alpha- or the beta-chain, and that spectrin Culoz and spectrin Lyon were two (alpha I/74) alpha-variants, respectively. We now show that the spectrin Culoz alpha I domain undergoes prominent tryptic cleavage after Lys 42, whereas cleavage prevails after Arg 39 in spectrin Lyon. Applying the polymerase chain reaction (PCR) technique to exon 2 of the spectrin alpha I domain, we have established that the mutation responsible for spectrin Culoz is alpha I 40 Gly----Val; GGT----GTT. Applying the PCR technique to the cDNA derived from reticulocyte mRNA, we have shown that the mutation responsible for spectrin Lyon is alpha I 43 Leu----Phe; CTT----TTT. Studies of normal controls and of family members using dot blot hybridization with allele-specific oligonucleotide probes confirmed these results. Variants such as spectrin Culoz and spectrin Lyon should provide insight into a region that participates in spectrin dimer self-association and whose susceptibility to proteolysis must reflect subtle conformational changes.

Clinical applications of the polymerase chain reaction

The polymerase chain reaction (PCR) is a technique that allows a million-fold, or greater, amplification of defined regions of DNA or RNA. It is potentially capable of detecting a single copy of a gene, present only once in 105 eukaryotic cells. This remarkable level of sensitivity has allowed the development of many diagnostic assays for human pathogens and disease states. These include: the detection of viral, bacterial and protozoal agents; diagnosis and genetic analysis of inherited diseases such as β-thalassaemia, sickle cell disease, haemophilia, Tay-Sachs disease and many others; diagnosis and analysis of neoplastic disorders such as, chronic myelogenous leukaemia (CML), acute lymphocytic lymphoma (ALL), follicular lymphomas and various other cancers, including the detection of activated oncogenes; prenatal and pre-implantation diagnosis; and the development of genetic risk prediction.

The PCR can greatly simplify diagnostic processes that were previously difficult to perform, particularly where the initial amounts of biological material were very limited. In other cases, PCR provides the only method available for detection and diagnosis. However, although simple in theory, the PCR technique remains, for routine clinical diagnostic purposes, currently in the domain of the specialist laboratory. This is because of its sensitivity to nucleic acid contamination from other sources that can cause misleading results. Procedures and precautions are being developed to minimize this problem and there is little doubt that, in many instances, the PCR will be the diagnostic method of choice within the next few years.