Abnormal tryptic peptide from the spectrin α‐chain resulting from α‐ or β‐chain mutations: two genetically distinct forms of the Sp αI/74 variant

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Abstract
Summary: Limited tryptic digestion of native spectrin (Sp) has revealed several variants in hereditary pyropoikilocytosis (HPP) and in a subset of patients with hereditary elliptocytosis (HE). In most cases, tryptic peptide corresponding to the αI (N‐terminal) 80 kD domain is wholly or partially replaced by smaller fragments. These variants are provisionally designated according to the molecular weight of the most prominent new peptide. Partial amino acid sequences of the abnormal peptides and DNA analysis of the α‐spectrin gene have shown that most variants result from substitution or insertion of an amino acid in the αI‐domain. However, similar investigations did not detect any such abnormality in the spectrin αI‐domain of an HE black kindred with one of the spectrin variants called Sp αI/74. In this kindred, restriction fragment length polymorphism studies and transmission of the genetic polymorphism relative to the αII‐domain excluded the involvement of the α‐chain in the pathological process. To ascertain whether the abnormal αI 74 kD peptide might be caused by a β‐chain mutation, we reconstituted hybrid dimers combining normal and HE Sp‐chains. The tryptic peptide patterns of spectrin hybrid dimers containing HE α‐chain and control β‐chain showed a normal 80 kD tryptic product. In contrast, the hybrid dimer containing normal α‐chain and HE β‐chain gave rise to increased 74 kD peptide at the expense of the 80 kD, demonstrating that the mutation in this family resides in the β‐chain. The same method was used to show that in two other unrelated white kindreds, the elevated 74 kD peptide arised from a Sp α‐chain defect. Thus an alteration in tryptic susceptibility within the N‐terminal domain of the spectrin α‐chain can be directed by a mutation in the β‐chain. The hybridization technique affords a definitive means of distinguishing between α‐ and β‐chain mutants.
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