Coinheritance of α- and β-Spectrin Gene Mutations in a Case of Hereditary Elliptocytosis

To the Editor:

Many mutations in the α- and β-spectrin genes are known to be associated with hereditary elliptocytosis (HE). Spectrin abnormalities are detected as abnormal peptides after limited trypsin digestion of spectrin (Sp).1 We had an opportunity to study a Filipino family living in Kuwait. The proband first presented with hemolysis at the age of 4 months and he was transfusion dependent until he was 12 months old. This family was referred to us when the proband was 16 months old. The parents are unrelated and there is no hemolytic history on either side (Table 1). The proband’s blood film showed obvious elliptocytes with few poikilocytes. His mother’s red blood cells were elliptocytic whereas the father’s red blood cell morphology was apparently normal. In osmotic gradient ektacytometry, red blood cells from all three members produced trapezoidal deformability curves, which are typical of HE with reduced DI_iso values2 (Table 1). Spectrin analysis showed that all three individuals are heterozygous for the HE Spα^I/74 variant and have defective Sp dimer self-association process (shown as increased percent of spectrin dimer content in the cold low-ionic-strength membrane extracts) (Table 1). The Sp^V41 polymorphism3 was also identified in the proband and his mother. This polymorphism is associated with the low-expressed α-Sp^Lely allele.4 The high percentage of the α^V41 peptide in the mother is related to homozygosity for α-Sp^Lely allele (Table 1).

Direct sequencing of exon 2 in α-Sp gene and exon 30 in β-Sp gene5 has shown that the mother is heterozygous for a previously described mutation in codon 28 of α-Sp gene (CGT → TGT; R28C).6 The father is heterozygous for a new β-Sp gene mutation in codon 2018 (GCC → GAC; A2018D), which we designate as β-Sp^Kuwaitino. Another point mutation previously reported in this codon was β-Sp^Cagliari(GCC → GGC; A2018G).7 The proband was found to be heterozygous for both mutations (α-Sp R28C and β-Sp A2018D). The homozygosity for α-Sp^Lely allele in the mother has nullified the expected reduced expression of Spα^I/74variant when this HE allele is inherited in cis to an α-Sp^Lely allele.8 In analogy to another α-Sp HE mutation,9 the low percentage of the α^V41 peptide in the proband’s tryptic digest can be accounted for by the cis inheritance of the α-Sp^Lely in respect to the deleterious HE α-Sp allele.

The appearance of HE Spα^I/74 variants can be due to impairment of either spectrin genes, and more precisely exon 2 in α-Sp gene and exon 30 in β-Sp gene.1,10 Although the two point mutations detected in the heterozygous HE parents involve the codons of the Sp genes already known to be mutated in other HE cases, the present study is the first report describing an HE patient who has co-inherited both α- and β-gene mutations. The molecular combinations of these two mutations, including the presence of the α-spectrin mutation (R28C) on a low-expressed α-Sp^Lely, have produced in the proband a hematological and biochemical picture of an apparently heterozygous HE phenotype, which is similar to that of his heterozygous HE parents, without any Sp deficiency (Table 1). The absence of typical elliptocytic cells in the father, despite his typical HE trapezoidal deformability curve, has already been observed in another HE family with a point mutation in β-spectrin.11 Because no more transfusions were needed 1 year after birth, the severity of the proband’s disease in the first year of life appears consistent with a transient poikilocytosis in infancy.10 However, such diagnosis has to be confirmed by an accurate clinical follow-up of the proband, especially as the percentage of αI/74 peptide being incorporated into his red blood cell membranes was found to be slightly higher than those observed in both parents (Table 1).