A New Fast-Moving Variant Causing Erythrocytosis and Mild Hemolysis: HB Gàmbara [β82(EF6)Lys→glu]

Abstract

Hb Gámbara is a new hemoglobin variant with abnormal β chains that has been found in three out of four members of a family of Lombardy origin (Gàmbara, Brescia, Northern Italy). The affected subjects led a normal life, but they had modest erythrocytosis and mild (compensated) hemolysis with slight splenomegaly. Their abnormal hemoglobin was about 52% of the total hemoglobin, and was shown to be stable by the isopropanol test. Whole blood P₅₀ of the proband was 19.3 Torr, Bohr effect was decreased (-0.25), as well as the molar ratio between the 2,3diphosphoglycerate level and total hemoglobin of erythrocytes (0.68). The purified abnormal hemoglobin was characterized by an altered oxygen affinity, low n-factor, chloride, and 2,3-diphosphoglycerate effects. The Bohr effect was about 40% of the normal control. The abnormal hemoglobin moved faster than Hb A at alkaline electrophoresis, and split into two fractions, probably due to the formation of hybrid tetramers (α₂β^Aβ^X). The reversed phase high performance liquid chromatogram from the tryptic digest of the aminoethylated abnormal β chain subunits indicated the presence of an extra peptide, βT-9,10, replacing the individual peptides βT-9 and βT-10. Finally, the proband's DNA, drawn from a suitable segment of the β structural gene (exon 2), revealed a nucleotide sequence carrying the heterozygous mutation AAG→GAG at codon 82. This led to a Lys→Glu substitution at position 82(EF6) of the β chain.