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 (α2βAβX). 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.