HB Osler [β145(HC2)TYRàASP] Results from Posttranslational Modification

We studied two members of an African American family with erythrocytosis. An abnormal hemoglobin variant with an electrophoretic pattern on cellulose acetate similar to Hb J was identified. The oxygen dissociation curve using whole blood was biphasic, dramatically left-shifted, and hyperbolic. Sequence analysis of DNA from the proband showed heterozygosity for a T-->A change at the first position of codon 145 in the beta-globin gene which results in the substitution of an asparagine residue for normal tyrosine. The second cycle of C-terminal amino acid sequence analysis of a mixture of alpha- and beta-globin chains showed tyrosine, aspartic acid, and small amounts of asparagine. Collectively, these results indicate the existence of a mutation at codon 145 of the beta-globin gene which encodes for asparagine instead of tyrosine, and that asparagine then undergoes a partial posttranslational deamidation to aspartic acid. This amino acid substitution corresponds to Hb Osler, which is a high oxygen affinity hemoglobin variant, initially described to be caused by a substitution of Tyr-->Asp at beta 145. Posttranslational amino acid modification may constitute an important component in the pathophysiology of hemoglobinopathies.