Pair-wise interactions of polymerization inhibitory contact site mutations of hemoglobin-S.

The linkage of pair-wise interactions of contact site mutations of HbS has been studied using Le Lamentin [His-20 (alpha)-->Gln], Hoshida [Glu-43 (beta)-->Gln] and alpha(2)beta (2) (T87Q) mutations as the prototype of three distinct classes of contact sites of deoxy HbS fiber. Binary mixture experiments established that beta(A)-chain with the Thr-87 (beta)-->Gln mutation is as potent as the gamma-chain of HbF (alpha(2)gamma(2)) in inhibiting polymerization. On combining the influence of Le Lamentin mutation with that of beta (2) (T87Q) mutations; the net influence is only partial additivity. On the other hand, in binary mixture studies, combined influence of Hoshida mutation with that of beta (2) (T87Q) mutations is synergistic. Besides, a significant level of synergistic complementation is also seen when the Le Lamentin and Hoshida mutations are combined in HbS (symmetrical tetramers). Le Lamentin and Hoshida mutation introduced into the cis-dimer of the asymmetric hybrid tetramer completely neutralizes the Val-6 (beta) dependent polymerization. Accordingly, we propose that combining the perturbation of intra-double strand contact site with that of an inter-double strand contact site exhibit synergy when they are present in two different chains of the alphabeta dimer. A comparison of the present results with that of the earlier studies suggest that when the two contact site perturbations are from the same sub-unit of the alphabeta dimer only partial additivity is observed. The map of interaction linkage of the contact site mutations exposes new strategies in the design of novel anti-sickling Hbs for the gene therapy of sickle cell disease.