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672 Part VI: The Erythrocyte Chapter 46: Erythrocyte Membrane Disorders 673

Missense mutations have been documented in all the ankyrin domains α LEPRA (low-expression Prague), which produces less than 20 percent of
and are thought to disrupt normal ankyrin–protein interactions. A few the normal amount of α-spectrin transcripts as a result of a splicing and
splicing mutations have been identified, including a mutation in intron mRNA processing defect, but does not cause any symptoms even in the
16, which created a new splice acceptor site and a complex pattern of homozygous state. However, in combination with another mutation on
aberrant splicing. Both parents were heterozygous for this mutation the other α-spectrin allele, which produces a nonfunctional truncated
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and the proband was homozygous, indicating that homozygosity for an protein, it causes severe spectrin deficiency and anemia. A polymor-
ankyrin mutation is compatible with life. phic missense mutation in the αII domain in spectrin Bug Hill has
Mutations in the erythroid-specific promoter of the ANK1 gene are been identified in several patients with spectrin-deficient, recessive HS
common in recessive HS. A dinucleotide deletion impairs the binding who carry another uncharacterized α-spectrin gene defect that causes
of a transcription factor complex, which leads to a reduced number of the disease. Extensive analysis of the α-spectrin gene in a proband
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ankyrin transcripts. Point mutations in a barrier insulator element of with severe nondominant HS revealed a partial maternal isodisomy
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the promoter also decrease transcription of the gene. 73 of chromosome 1, resulting in homozygosity of the 1q23 region con-
Cytogenetic studies have identified a few ankyrin-deficient HS taining the maternal SPTA1 gene, which carried an R891X nonsense
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patients with a contiguous gene syndrome that includes deletion of the mutation. Uniparental disomy therefore unmasked a recessive muta-
ankyrin gene locus at 8p11.2. These patients additionally suffer from tion in the mother, which caused severe clinical symptoms in the
dysmorphic features, psychomotor retardation, and hypogonadism. 74 child.
Band 3 A subset of HS patients present with a band 3 deficiency, β-Spectrin The production of β-spectrin polypeptides is the lim-
typically accompanied by a secondary decrease in protein 4.2, a result iting factor in spectrin heterodimer formation and one mutant allele is
of the reduction in protein 4.2 binding sites in the cytoplasmic domain sufficient to cause spectrin deficiency in autosomal dominant HS. The
of band 3. The extent of band 3 deficiency in heterozygous patients blood films of these patients typically show a subpopulation of spiculated
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ranges between 20 and 50 percent, depending on the severity of the cells (acanthocytes and echinocytes) in addition to spherocytes. Muta-
mutation, and the compensatory effect of the in trans normal allele. tions in β-spectrin are found throughout the gene and are mainly null
Mushroom-shaped “pincered” cells are commonly seen on the blood mutations caused by frameshift, nonsense, splicing, and initiator codon
film of HS patients with a band 3 abnormality. defects, which silence the mutant allele. With a few exceptions the
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More than 55 underlying mutations have been described; they mutations are all kindred-specific. Truncated β-spectrin chains have also
are variable and occur throughout the band 3 gene. 13,66 Null mutations been described and are caused by frameshift mutations, in-frame dele-
are typically family-specific and are caused by frameshift or nonsense tions, or exon skipping. These mutations lead to, for example, reduced
mutations, or, in a few cases, by abnormal splicing, all of which result synthesis of an unstable protein, or they impair the interaction with
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in truncated nonfunctional proteins or unstable transcripts that are ankyrin and thereby the insertion of spectrin into the membrane. A few
not translated into protein. Missense mutations are common and often missense mutations have been identified, including β-spectrin Kissimmee ,
occur in several kindred. Highly conserved arginine residues at the which is caused by a mutation in the 4.1R/actin–binding domain of the
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internal boundaries of the transmembrane segments of the protein (see protein. The mutant protein is unstable and does not bind to 4.1R, and
Fig. 46–2) are frequently mutated, including residues 490, 518, 760, 808, thus it only interacts weakly with actin, which may explain why these
and 870. 66,75 The mutations probably interfere with the cotranslational red cells are deficient in spectrin. 87
insertion of band 3 into the endoplasmic reticulum and ultimately into Protein 4.2 Protein 4.2 deficiency is common in Japanese patients
the red cell membrane. Short in-frame insertions or deletions have been with recessively inherited HS who exhibit almost a complete absence of
documented and presumably also impair insertion of the mutant pro- the protein. Defects in this protein also occur in whites and other pop-
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tein into the lipid bilayer. ulation groups, and 13 mutations have been described in the 4.2 gene
Mutations in the cytoplasmic domain of band 3 impact on the of individual kindred, including missense mutations and in-frame dele-
interaction of band 3 with proteins in the membrane skeleton, or may tion and insertion of nucleotides. Nonsense, frameshift, and splicing
alter the conformation of the protein rendering it unstable and prone to defects result in premature termination of translation and these mutant
degradation prior to insertion into the membrane. Some cytoplasmic truncated proteins are not detected on the membrane, indicating that
mutations, such as band 3 Cape Town and band 3 Mondega, are silent they are unstable and presumably degraded. Amino acids 306 to 320
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in the heterozygous state, but exacerbate the clinical presentation when are highly conserved and five of the known mutations (three missense
inherited in trans to another mutation. 76,77 and two nonsense) occur in this region, which is adjacent to the hairpin
Spectrin Erythrocytes from HS patients with defects in spectrin that binds to band 3 in the predicted tertiary structure of protein 4.2.
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or ankyrin are deficient in spectrin. The degree of deficiency correlates The only recurrent and most common mutation, protein 4.2 Nippon,
with the severity of hemolysis, the response to splenectomy and the is caused by a point mutation that affects mRNA processing. Patients
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ability to withstand mechanical shear stress. 78,79 Visualization of the are either homozygous for this mutation or heterozygous for a second
membrane skeleton of these red cells revealed a decreased density of mutation on the other allele. Mutations have also been identified in
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the spectrin filaments connecting the junctional complexes. The caus- individual patients with recessive HS from Europe, Tunisia, and Paki-
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ative mutations occur in either α- or β-spectrin genes. stan. South African kindred with autosomal dominant HS from a defi-
α-Spectrin Defects in α-spectrin are rare and are associated with ciency of protein 4.2 have been noted, but the underlying mutations
severe recessive HS. During erythropoiesis α-spectrin is synthesized have not been investigated.
in a two- to fourfold excess over β-spectrin and heterozygotes thus
still produce sufficient α-spectrin to form heterodimers with all the β- Secondary Membrane Defects
spectrin molecules, which will not result in spectrin deficiency. The The decreased membrane surface area in hereditary spherocytes
defect will only be manifested in individuals who are homozygous involves a symmetrical loss of each species of membrane lipid. The rel-
or doubly heterozygous for mutations in α-spectrin. The mechanism ative proportions of cholesterol and phospholipids are therefore normal
underlying spectrin deficiency has not been fully elucidated, but a low- and the asymmetrical distribution of phospholipids is maintained.
expression allele or a polymorphism inherited in trans to a causative HS red cells exhibit increased cation permeability, presumably
null mutation plays a role. An example of a low-expression allele is secondary to the underlying membrane defect. The excessive sodium
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