The Landscape of Thrombophilia in Sickling Disorders: Unfamiliar Terrain.

Sickling disorders have long been suspected of decreasing physiologic thresholds for thrombosis, and the sickling process in tissue of being abetted by elements of the hemostatic system. The landscape of the presumed hypercoagulable state, however, has been ill-defined and differences among sickling disorders with regard to its characteristics, if any, undelineated. Learning how this landscape may reflect the pathophysiology of the sickling process may help guide further study of the interactions among the vasculature and the varieties of abnormal erythrocytes resulting from these hemoglobinopathies and define new opportunities to ameliorate the clinical manifestations of "sickle cell disease."

Methods: Using laboratory strategies developed to characterize thrombophilia in non-sickling patients with thrombosis, modified for a largely non-European population, 144 patients with sickling disorders were prospectively evaluated for predisposition to thrombosis. A standard group of hemostatic studies was performed using patient samples obtained during steady-state "non-sickling" conditions, by skilled venepuncturists working consistently with this patient group.

Genotypes included: HbSS 96/144 (67%); Hb SC 26/144 (18%;) Hb SSTh; 14/144 (9.5%); Hb S/AThal 8/144 (5.5%). 49/144 (34%) patients (pts) received regular automated erythrocytapheresis for prior stroke, recurrent acute chest syndrome or severe pain episodes > 6/yr.

Results: As expected, 2/144 (1.4%) of pts were heterozygous for the Factor V Leiden mutation, 0/144 for the Prothrombin 20210 mutation, and 20/144 (14%) heterozygous, 1/144 (<1%) homozygous for the MTHFR C677T mutation despite homocysteine levels > 12 ng/dL in 38/144 patients (26%). Protein S activity levels were <65% in 83/144 (58%) pts with commensurate free Protein S levels. Total Protein S levels varied widely with no correlation to sickling genotypes, suggesting -as with homocysteine levels -dietary variation. Hb SC genotype was strongly associated with von Willebrand antigen (vWA) levels > 175%, frequently with lesser vWFactor activity (vWF) levels, whereas Hb SSt and SThal more frequently exhibited normal vWA/vWF profiles. Factor VIII activity varied widely, exhibiting no correlation to vWF antigen or activity levels in any of the Hb genotypes. Other thrombophilic conditioners, e.g. antithrombin III activity/Ag, Factor VIII activity, plasmnogen activator inhibitor, and b2-glycoprotein I antibodies were either normal or varied as expected in a " normal," e.g European population, with no correlation to Hb genotype.

Conclusions: Thrombophilia in patients with sickling disorders appears to have varied contributors: Hb SC appears most thrombophilic with levels of vWF antigen suggestive of vascular dysfunction and supportive of intravscular platelet adhesion a aggregation. Other genotypes exhibit variable phenotypes, with Hb SThal exhibiting the least thrombogenic phenotype from a aboratory assessment perspective. vWF antigen and Protein S abnormalities emphasize the likely role of inflammation and vascular dysfunction but the dysjunction of vWF and Factor VIII activity hint at greater complexity. Further study of these phenomena and the inclusion of platelet function studies may yield additional insights as to the interaction of sickling erythrocytes, their products and vascular/hemostatic interactions. For now, routine thromboprophylaxis in patients with SC disease appears essential, and likely important in all other genotypes except Hb SThal.