Page 703 - Williams Hematology ( PDFDrive )

678 Part VI: The Erythrocyte Chapter 46: Erythrocyte Membrane Disorders 679

in helix C of α0 has been identified as a mutation “hotspot” since four duplications result in mutant proteins that are smaller or larger than
different point mutations occur in this position, resulting in different normal. 122
amino acid substitutions, and the mutations have also been found in sev- Glycophorin C GPC and GPD carry the Gerbich antigens and rare
eral unrelated kindred. Arginine 28 is a highly conserved amino acid patients with the Leach phenotype are Gerbich-negative and lack both
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and any changes in this position are typically associated with severe HE GPs. The underlying mutations are either a 7-kb deletion of genomic
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or HPP. 133,134 An interesting case of HE Spα involving an intragenic DNA or a frameshift mutation. Heterozygous carriers are asymptom-
crossover in the α-spectrin gene and uniparental disomy, together with atic, with normal red blood cell morphology, whereas homozygous sub-
an underlying R34P mutation, was recently described in a Utah family. 126 jects exhibit elliptocytes on the blood film and present with mild HE,
Spα defects are also caused by mutations in β-spectrin, which presumably as a result of the concomitant partial deficiency of 4.1R. 13,145
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presumably expose the αI domain of spectrin to increased tryptic diges-
tion. These abnormalities are all located in partial repeat 17. Missense Molecular Determinants of Clinical Severity
mutations are found in both helices A and B of the β17 repeat, but some HE patients exhibit marked clinical heterogeneity ranging from asymp-
in helix A are particularly severe, including spectrin Providence , spectrin Cagliari , tomatic carrier to severe, transfusion-dependent anemia. In patients
and spectrin Buffalo , which cause severe fetal or neonatal anemia and non- with spectrin heterodimer self-association defects, the resultant increase
immune hydrops fetalis when inherited in the homozygous state. 105,106,135 in spectrin dimers and concomitant decrease in spectrin tetramers,
Frame-shift mutations and splicing defects predominate in helix B, result- weakens the membrane skeleton and facilitates the formation of ellipto-
ing in truncated spectrin molecules lacking the self-association site. 13,122,136 cytes under circulatory shear stress. The most important determinants
Spα is a mild defect, even in the homozygous state, because of a of the severity of hemolysis in these patients are the percentage of spec-
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duplication of leucine 154 in helix C of the α1 repeat. It is very com- trin dimers and the spectrin content of the membrane skeleton. These
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mon in blacks from West and Central Africa, as well as Arabs in North parameters are influenced by the degree of dysfunction of the mutant
Africa, suggesting genetic selection, possibly by protecting carriers spectrin, and the gene dose (heterozygote versus homozygote or com-
against P. falciparum malaria. 13,122,123 pound heterozygote). Genotype–phenotype correlations indicate that
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Spα I/46 or 50a mutations are distal from the self-association site the order of clinical severity of αI domain defects is Spα > Spα I/46–50a
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and usually occur close to the helical linker regions between individ- > Spα and it depends on the position of the mutations within the
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ual repeats and often involve the substitution of an amino acid with a proteins, as well as the type of mutation. Defects in the spectrin dimer
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proline residue, which is a helix breaker. 13,122 In vitro studies on Q471P self-association contact site leading to Spα mutants are the most
between repeats 4 and 5 of α-spectrin showed that the mutation uncou- severe and, for example, codon 28 mutations, which affect a highly
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pled the repeats and caused cooperative unfolding, which abolished the conserved and critical arginine residue, are generally associated with
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stabilizing influence of the helical linker on adjacent repeats. Because phenotypically severe HE or HPP. A more distal mutation such as the
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β-spectrin has fewer repeats than α-spectrin, the alignment of the het- duplication of leucine 154, which causes Spα , is phenotypically very
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erodimers places α4 and α5 in contact with β16 and β17, suggesting mild, even in the homozygous state. Proline or glycine helix-breaking
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that unfolding of the mutant spectrin repeats interferes with the self- mutations resulting in Spα I/46 or 50a are more severe even though they are
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association site and prevents tetramer formation. The L260P muta- further away from the self-association site. 138
tion is in a similar position to Q471P, but is between repeats α2 and α3 The clinical expression of HE often varies within the same kin-
of spectrin. When heterodimers are aligned, repeats α03 are not in con- dred, despite all the affected individuals carrying the same causative
tact with β-spectrin and they represent an open dimer configuration, mutation. This heterogeneity is a result of the inheritance of modifier
which facilitates tetramer formation. Open dimers are in equilibrium alleles or additional defects. The low-expression α LELY is the most com-
with closed dimers whereby α0 to α3 are folded onto β16 and β17 of mon polymorphism affecting spectrin content and clinical severity. The
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the same dimer, thus preventing bivalent tetramer formation. In vitro allele is characterized by an L1857V amino acid substitution, and partial
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experiments on the L260P mutation revealed a conformational change, skipping of exon 46 in 50 percent of the α-spectrin mRNA. The six
which stabilized the mutant spectrin in the closed dimer configuration amino acids encoded by exon 46 are essential for spectrin heterodimer
and reduced tetramer assembly. 140 assembly and therefore Spα LELY results in a reduced amount of spectrin,
Mutations in the αII domain of spectrin implicated in HE are as monomers are rapidly degraded. The Spα LELY allele is clinically
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rare. Spectrin St Claude is caused by a single point mutation in intron 19 of silent, even when homozygous, because α-spectrin is normally synthe-
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α-spectrin, 141,142 which creates complex splicing events that ultimately sized in three- to fourfold excess. Inheritance of Spα LELY in cis to an
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impair the function of both α- and β-spectrin, resulting in decreased elliptocytogenic α-spectrin mutation ameliorates symptoms, whereas
binding to ankyrin, defective spectrin self-association and spectrin defi- inheritance in trans causes a relative increase in the mutant spectrin and
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ciency. These membrane abnormalities have profound effects on red therefore exacerbates the disease. 94
blood cell morphology and survival, manifesting as severe HE. Coinheritance of other molecular defects also plays a role in mod-
Protein 4.1R Defects in the erythrocyte isoform of protein 4.1 ifying the clinical expression. HPP patients are very severely affected
associated with HE are relatively common in some Arab and Euro- because they are homozygous or doubly heterozygous for spectrin
pean populations. Heterozygotes exhibit partial deficiency of 4.1R, self-association mutations and are also deficient in spectrin. Several
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13
manifesting as mild or asymptomatic HE, whereas homozygotes lack molecular mechanisms have been identified that underlie the spectrin
4.1R and p55, have a reduced content of GPC, and present with severe deficiency, including an RNA processing defect ; reduced α-spectrin
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HE. These red blood cells are mechanically unstable and fragment at mRNA and protein synthesis ; abnormal splicing resulting in a pre-
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moderate shear stress, but the stability can be restored by reconstitut- mature stop codon ; and degradation of α-spectrin. A recent study
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ing the deficient red cells with 4.1R or the 4.1R spectrin–actin binding revealed the complexity of genotype–phenotype interactions in two
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domain. The 4.1R null erythrocytes demonstrate decreased invasion large Utah families of northern European descent in whom a novel
and growth of P. falciparum parasites in vitro. 144 R34P mutation in α-spectrin was associated with three morphologic
Mutations in the 4.1R gene often affect the erythroid-specific phenotypes. This heterogeneity was caused by an intricate interplay
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initiation codon, which abolishes transcription, or else they tend to and coinheritance of other factors, including Spα LELY in trans, reduced
cluster in the spectrin-actin binding domain where exon deletions or transcription from the α-spectrin gene and intragenic crossover. 126
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