The phenotype of an unstable Hb may be modified by epistatic factors that could be genetic or environmental. For example, an unstable Hb may be better tolerated when associated with a high level of Hb F, resulting from a he­reditary persistence of Hb F. Carbon monoxide (CO) is known to stabilize Hb Zürich [β63(E7)His→Arg], which is thus better tolerated by smokers than by non smokers, as a result of the 65-fold increase in the affinity of the abnor­mal β-globin chain of this variant for this ligand [26]. Conversely, hereditary hyperbilirubinemias, found in the Gilbert syndrome, increase the risk for gall bladder lithi­asis, and mutations such as the factor V Leiden mutation or some mutations in the prothrombin gene may facilitate thromboembolic complications. Interaction with other Hb variants is always possible: Hb Saki [β14(A11)Leu→Pro] was found to be associated to a β-thal trait leading to thal­assemia intermedia [27], and in another observation to Hb S, with a phenotype expressing a higher level of Hb S than Hb Saki and resulting in a hemolytic sickle cell syn­drome [28].

Figure 1.  Pedigree of family with Hb Duino. In this family several members including the father carried Hb Camperdown [b104(G6)Arg®Ser], a very slightly unstable variant causing no hematological symptoms. In one of the siblings who suffered from a marked hemolytic anemia, the Hb study showed that the abnormal chain of this patient carried, together with the mutation of Hb Camperdown, a second mutation identified as being that of Hb Newcastle [b92(F8)His®Pro], another mutation known to cause instability and hemolytic anemia.

Figure 2. Schematic differences between the two categories of unstable b chain variants.

Figure 3. Electrospray ionization mass spectrometry analysis of the hemolysate from the carrier of Hb Esch. This patient had borderline a⁺-thalassemic RBC parameters, and IEF revealed a faint band between the positions of Hb F and Hb A. The ESI-MS showed a small signal, 400 mass units above the normal value for a chains, which corresponded to no known adduct but rather to some complex variant. This Hb was found, by DNA sequencing, to result from a duplica­tion of the sequence Ala⁶⁵-Leu-Thr-Asn⁶⁸, which is in good agreement with the difference in mass.