Summary
Ectopic delivery of factor VIII (FVIII) to megakaryocytes (Mk) represents a viable
approach for localized tenase generation by concentrating the FVIIIa/FIXa enzymecofactor
complex onto activated platelet membranes. We utilized a core rat platelet factor
4 (PF4) promoter for Mk/platelet-restricted expression of human B-domain-deleted (hBDD)
FVIII within the background of a haemophilia A mouse (rPF4/hBDD/FVIII-/-). Platelets from rPF4/hBDD/FVIII-/- mice contained ∼122 mU FVIII:C/1x109 platelets/ml with no detectable plasmatic FVIII:C,and with no effect on α-granule-derived
platelet factorV/Va function.Paired tenase assays (± thrombin) confirmed that platelet
(pt) FVIII (unlike platelet FV) required thrombin cleavage for complete activation.
rPF4/hBDD/FVIII-/- mice exposed to a thrombocytotic stimulus (thrombopoietin, TPO) demonstrated a statistically-significant
66% reduction in molar ptFVIII activity with a non-significant reduction in total
ptFVIII biomass. Decreased molar ptFVIII concentration correlated with loss of phenotypic
correction as evaluated using a haemostatic tail-snip assay. Comparative studies using
a transgenic mouse expressing human amyloid-β-precursor protein (hAβPP) from the rPF4
promoter confirmed diminished hAβPP expression without affecting endogenous α-granule
PF4, establishing generalizability of these observations.While Mk/plateletreleased
ptFVIII (unlike pFV) is proteolytically inactive, we also conclude that thrombocytotic
stimuli negatively affect ptFVIII bioavailability and phenotypic efficacy, results
which correlate best with molar ptFVIII concentration, and not systemically available
ptFVIII.
Keywords
Animal models - gene targeting - coagulation factors