Functional properties of the newly observed Gγ-chain fetal hemoglobin variant Hb F-Monserrato-Sassari (HBG2:c.280T > C) or [ Gγ93 (F9) Cys → Arg]

Background: HbF-Monserrato-Sassari is a newly discovered abnormal fetal hemoglobin observed in an apparently normal newborn baby during a hemoglobinopathies survey at birth in North Sardinian population. Methods: Electrophoretic analysis of the cord blood lysate evidenced for an abnormal tetramer due to a mutated fetal globin chain. Electrospray ionisation-mass spectrometry and gene sequencing were used to identify the mutation. Oxygen binding ability of the variant Hb was determined. Results: Sequencing of the gamma globin genes revealed the TGT -> CGT transition at codon 93 in one of the two (G)gamma genes, which leads to the Arg for Cys amino acid replacement at position 9 of the F alpha-helix. The amino acid substitution was confirmed by mass spectrometric analysis of the globin chains. Since modifications or substitutions at position beta 93 are known to affect the arrangement of a salt bridge at the alpha 1 beta 2 sliding contacts that are crucial for subunit cooperativity, the functional properties of the variant were studied to evaluate the effect of the replacement at the same position in the gamma globin chain. With respect to normal HbF, the variant showed a significant increase in oxygen affinity and a slight decrease of both Bohr effect and cooperativity. General significance: Result indicates a key role of the Cys gamma 93 residue for subunit cooperativity in the T -> R transition of the HbF tetramer. Substitutions at the F9 position of the (G)gamma globin may result in stabilization of the high affinity R-state of the Hb tetramer. Because of the loss of Cys gamma 93 residue, this variant is considered to be potentially compromised in nitric oxide transport.