Mechanism of sickling in deer erythrocytes

The genetic basis and evolution of red blood cell sickling in deer

Crescent-shaped red blood cells, the hallmark of sickle-cell disease, present a striking departure from the biconcave disc shape normally found in mammals. Characterized by increased mechanical fragility, sickled cells promote haemolytic anaemia and vaso-occlusions and contribute directly to disease in humans. Remarkably, a similar sickle-shaped morphology has been observed in erythrocytes from several deer species, without obvious pathological consequences. The genetic basis of erythrocyte sickling in deer, however, remains unknown. Here, we determine the sequences of human β-globin orthologues in 15 deer species and use protein structural modelling to identify a sickling mechanism distinct from the human disease, coordinated by a derived valine (E22V) that is unique to sickling deer. Evidence for long-term maintenance of a trans-species sickling/non-sickling polymorphism suggests that sickling in deer is adaptive. Our results have implications for understanding the ecological regimes and molecular architectures that have promoted convergent evolution of sickling erythrocytes across vertebrates.

Kinetics of the in vitro gelation of a sickling haemoglobin from Hog deer (Axis porcinus)

In order to distinguish the molecular mechanisms involved in the polymerisation of animal and human sickling haemoglobins, the gelation properties of hog deer (Axis porcinus) haemoglobin have been studied. Continuous monitoring of viscosity and minimum gelling concentration measurements of hog deer haemolysates were made over a range of pH, temperature, ionic strengths and in the presence of urea and tris(hydroxymethyl)methylamine. The inhibition of gelling caused by lowering the pH or increasing ionic strength and the abolition of the reversible endothermic nature of gelation by urea suggest that electrostatic interactions predominate in polymerisation but that weak hydrogen and hydrophobic bonds may also be present. Kinetic viscosity data demonstrated a pre-gelation lag phase, dependent on ahigh power of the haemoglobin concentration, similar to the nucleation of human haemoglobin S (HbS). The results indicate similarities in the kinetics of hog deer polymerisation with those of HbS, but major differences in the type of intermolecular attractions involved.