β-Spectrin Promissão: A Translation Initiation Codon Mutation of the β-Spectrin Gene (ATG → GTG) Associated With Hereditary Spherocytosis and Spectrin Deficiency in a Brazilian Family

To the Editor:

Hereditary spherocytosis (HS) is a common inherited anemia characterized by the presence of spheroidal red cells and increased osmotic fragility of erythrocytes.1 This disorder is heterogeneous in terms of its clinical presentation, molecular basis, and inheritance.2 HS mutations have been ascribed to several genes,1 including the β-spectrin gene. So far 13 β-spectrin mutations have been described associated with HS.3-6 

We have studied a Brazilian family with HS diagnosed in eight subjects from two generations and inherited in an autosomal dominant fashion (Fig 1). The propositus was a 28-year-old black man, who presented compensated hemolytic disease with splenomegaly, hyperbilirubinemia, increased osmotic fragility, and a regular number of spherocytes and acanthocytes in the blood smear (Fig1). His recent hematological profile was: hemoglobin (Hb) 15.0 g/dL, red blood cell 4.49 × 10¹²/L, mean corpuscular volume 88 fL, mean corpuscular hemoglobin concentration 38.0 g/dL, reticulocyte count 530 × 10⁹/L (11.8%). His mother, uncle, and two cousins were splenectomized. Densitometric scans of Coomassie blue–stained sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the propositus membrane proteins showed an 18% reduction in spectrin content (Fig 1). This pointed to the β-spectrin gene as the most likely candidate for bearing the primary defect. Therefore, we started screening for mutations in the β-spectrin gene. This was performed through the amplification of the individual exons of the β-spectrin gene with intronic primers. The amplification products were submitted to nonradioactive single-strand conformation polymorphism (SSCP) technique in a PhastSystem apparatus (Pharmacia, Uppsalla, Sweden) to detect sequence abnormalities. The DNA amplification products of exon 2 of the patient and his mother showed an identical band shift in two independent experiments (Fig2). No such band pattern was observed in 2 independent controls, nor was it observed in 12 other HS patients with spectrin deficiency and acanthocytes in the blood smear, suggesting that this patient bore a unique or at least a rare sequence alteration in this region of the gene. Sequencing revealed an heterozygous A → G nucleotide substitution at the translation initiation codon of the β-spectrin gene (ATG → GTG) (Fig3). This is the first report of an initiation codon mutation in this gene, but such mutations have already been reported for other genes, including the β-globin gene.7 In this case, the mutations led to β⁰-thalassemia phenotypes, suggesting that alternative triplets for ATG are nonfunctional as initiation codons for this gene. Therefore, we suggest that in our case translation from the mutated allele is also impaired. Based on this supposition, the ATG → GTG mutation could be held responsible for the reduced spectrin content observed in the patient. The propositus would have only one functional allele and, as β-spectrin quantities are considered limiting for membrane assembly, this could account for the picture of spherocytosis observed in this patient.