HbS-Savaria: the anti-polymerization effect of a single mutation in human alpha-chains

An α-chain modification rivals the effect of fetal hemoglobin in retarding the rate of sickle cell fiber formation

Adults with sickle cell disease bear a mutation in the β-globin gene, leading to the expression of sickle hemoglobin (HbS; α2βS2). Adults also possess the gene for γ-globin, which is a component of fetal hemoglobin (HbF, α2γ2); however, γ-chain expression normally ceases after birth. As HbF does not form the fibers that cause the disease, pharmacological and gene-modifying interventions have attempted to either reactivate expression of the γ chain or introduce a gene encoding a modified β chain having γ-like character. Here, we show that a single-site modification on the α chain, αPro114Arg, retards fiber formation as effectively as HbF. Because this addition to the repertoire of anti-sickling approaches acts independently of other modifications, it could be coupled with other therapies to significantly enhance their effectiveness.

Hb Bakersfield (HBA1: c.151_152insGGAGCC): The Insertion of Arg-His Between Codons 49 and 50 of the α1-Globin Chain Leads to Increased Oxygen Affinity

We describe an insertion variant on the α1-globin gene (HBA1) identified in a 49-year-old woman of Jurassian ancestry presenting with macrocytosis and erythrocytosis. The variant resulted in a peak of 15.5% of the total hemoglobin (Hb) on high performance liquid chromatography (HPLC). Stability and oxygen affinity testing revealed that the variant was stable and had an increased oxygen affinity. Molecular genetic testing detected the heterozygous sequence variant Hb Bakersfield [α50(CE8)His→0; Arg-Ser-His- inserted between 49(CE7) and 51(CE9) of α1; HBA1: c.151_152insGGAGCC (p.Ser50_His51insArgSer)] in the index patient, one of her sons, as well as in two of her grandchildren, who showed a similar hematological pattern.

Hb Savaria [α49(CE7)Ser→Arg; HBA2: c.150C > A]: A New Case and Complete Description

Hb Savaria [α49(CE7)Ser→Arg; HBA2: c.150C > A] is a rare hemoglobin (Hb) variant, initially described in Eastern Europe but present worldwide. It belongs to that class of variants which can be confused with Hb S [β6(A3)Glu→Val; HBB: c.20A > T] by automated protein analysis and thus needs special tests for proper identification. Because it could arise from different nucleotide substitutions and according to the rules of the Human Genome Variation Society (HGVS) nomenclature, three 'Hb Savaria' variants are possible. In the case reported here it resulted from the HBA2: c.148A > C change.

Structural basis for the antipolymer activity of Hb ζ2βs2 trapped in a tense conformation

The phenotypical severity of sickle-cell disease (SCD) can be mitigated by modifying mutant hemoglobin S (Hb S, Hb α₂β^s₂) to contain embryonic ζ-globin in place of adult α-globin subunits (Hb ζ₂β^s₂). Crystallographical analyses of liganded Hb ζζ₂β^s₂, though, demonstrate a tense (T-state) quaternary structure that paradoxically predicts its participation in--rather than its exclusion from--pathological deoxyHb S polymers. We resolved this structure-function conundrum by examining the effects of α→ζ exchange on the characteristics of specific amino acids that mediate sickle polymer assembly. Superposition analyses of the β^s subunits of T-state deoxyHb α₂β^s₂ and T-state CO-liganded Hb ζ₂β^s₂ reveal significant displacements of both mutant β^sVal6 and conserved β-chain contact residues, predicting weakening of corresponding polymer-stabilizing interactions. Similar comparisons of the α- and ζ-globin subunits implicate four amino acids that are either repositioned or undergo non-conservative substitution, abrogating critical polymer contacts. CO-Hb ζ₂β^s₂ additionally exhibits a unique trimer-of-heterotetramers crystal packing that is sustained by novel intermolecular interactions involving the pathological β^sVal6, contrasting sharply with the classical double-stranded packing of deoxyHb S. Finally, the unusually large buried solvent-accessible surface area for CO-Hb ζ₂β^s₂ suggests that it does not co-assemble with deoxyHb S in vivo. In sum, the antipolymer activities of Hb ζ₂β^s₂ appear to arise from both repositioning and replacement of specific α- and β^s-chain residues, favoring an alternate T-state solution structure that is excluded from pathological deoxyHb S polymers. These data account for the antipolymer activity of Hb ζ₂β^s₂, and recommend the utility of SCD therapeutics that capitalize on α-globin exchange strategies.

Sickle Cell Hemoglobin with Mutation at αHis-50 Has Improved Solubility

The unliganded tetrameric Hb S has axial and lateral contacts with neighbors and can polymerize in solution. Novel recombinants of Hb S with single amino acid substitutions at the putative axial (recombinant Hb (rHb) (βE6V/αH20R) and rHb (βE6V/αH20Q)) or lateral (rHb (βE6V/αH50Q)) or double amino acid substitutions at both the putative axial and lateral (rHb (βE6V/αH20R/αH50Q) and rHb (βE6V/αH20Q/αH50Q)) contact sites were expressed in Escherichia coli and purified for structural and functional studies. The (1)H NMR spectra of the CO and deoxy forms of these mutants indicate that substitutions at either αHis-20 or αHis-50 do not change the subunit interfaces or the heme pockets of the proteins. The double mutants show only slight structural alteration in the β-heme pockets. All mutants have similar cooperativity (n50), alkaline Bohr effect, and autoxidation rate as Hb S. The oxygen binding affinity (P50) of the single mutants is comparable with that of Hb S. The double mutants bind oxygen with slightly higher affinity than Hb S under the acidic conditions. In high salt, rHb (βE6V/αH20R) is the only mutant that has a shorter delay time of polymerization and forms polymers more readily than Hb S with a dextran-Csat value of 1.86 ± 0.20 g/dl. Hb S, rHb (βE6V/αH20Q), rHb (βE6V/αH50Q), rHb (βE6V/αH20R/αH50Q), and rHb (βE6V/αH20Q/αH50Q) have dextran-Csat values of 2.95 ± 0.10, 3.04 ± 0.17, 11.78 ± 0.59, 7.11 ± 0.66, and 10.89 ± 0.83 g/dl, respectively. rHb (βE6V/αH20Q/αH50Q) is even more stable than Hb S under elevated temperature (60 °C).