Procoagulant Platelet Potential Is Inversely Correlated with Bleeding and Joint Disease in Severe Hemophilia A

The variability of clinical phenotype in patients with severe hemophilia A (factor VIII activity < 1%) has been well established, currently and in the era prior to wide-spread prophylaxis. The introduction of prophylaxis as a treatment modifier of the natural history of severe hemophilia A has led to the introduction of composite scores to assist in the assessment of disease severity. Perhaps the most prominent of these is that proposed by Schulman et al (J Thromb Haem 2008;6(7):1113-21) which utilizes three domains of phenotypic presentation to characterize a patient's clinical phenotype, acute (annualized bleeding rate), chronic (joint status), and treatment intensity (normalized factor usage). However, the biologic mechanisms driving this variability in clinical phenotype continue to remain largely uncertain. Procoagulant platelet potential is an individual's ability to generate procoagulant platelets in response to a standard concentration of agonist. This response, which generates platelets capable of intensely supporting coagulation, tends to remain relatively constant and varies substantially between individuals. A few small case-control studies have investigated the role of procoagulant platelet potential or platelet-supported thrombin generation in mediating the phenotypic variability with discordant results.

The relationship of procoagulant platelet potential and platelet-supported thrombin generation to phenotypic severity was investigated in a single-center cohort of 92 pediatric and adult severe hemophilia A patients without active inhibitors. Procoagulant platelet potential (defined as the ratio of procoagulant platelets / all activated platelets following stimulation with thrombin (0.5 units/mL) and convulxin (250 ng/mL)) and the expression of other platelet activation markers, including P-selectin (granule release) and PAC-1 (integrin activation) were investigated in washed platelets. Platelet-dependent thrombin generation was investigated in a limited cohort (45) of these patients. Phenotypic data was collected in regard to reported bleeding events over the past 5 years, joint health using the hemophilia joint health score, intensity of factor therapy, and therapeutic intent (prophylaxis or on-demand). Correlations were assessed as continuous variables using regression analysis with p <0.05 deemed significant.

Procoagulant platelet potential was inversely correlated with both acute (annualized bleeding rate) (r = -0.47, p = 0.0006) and chronic measures (joint score) (r = -0.38, p = 0.0001) of phenotypic severity. In contrast, when platelet-dependent thrombin generation or other markers of thrombin or GPVI-dependent platelet activation potential were assessed i.e. P-selectin expression or PAC-1, no correlation was identified with either acute or chronic measures of phenotypic severity or with the HSS composite score. Surprisingly, when the composite score of HSS as defined by Schulman (acute + chronic + factor utilization) was utilized to assess a potential relationship this correlation was not observed. Further analysis revealed that this lack of correlation was due to the fact that therapeutic intensity (factor utilization) was directlycorrelated with procoagulant platelet potential (r = 0.54, p = 0.0006).

Here we have identified a potentially critical role for procoagulant platelets as biologic modifiers of hemophilia A. Furthermore, we have identified a risk associated with the use of a composite score in assessing disease severity in hemophilia. Utilization of a composite score as the sole measure would have missed the significant effects noted in each of the individual domains. Procoagulant platelets both support thrombin generation and inflammatory cell recruitment. Thus, one potential hypothesis for these opposing effects on the domains of the composite score is that increased procoagulant platelet formation supports stable hemostatic plug formation (reduced bleeding rate) or increases inflammatory presentation resulting in therapy intensification or both. Finally, our results here point towards new tools and pathways for risk stratification and therapeutic modulation in patients with hemophilia A based on assessment of procoagulant platelet potential.