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.