Rescue of fatal neonatal hemorrhage in factor V deficient mice by low level transgene expression

Factor V (FV is a critical component of the coagulation cascade. FV-deficie nt patients suffer moderate to severe bleeding, though residual FV activity is detectable in nearly all cases. In contrast, FV-deficient mice die eith er during mid-embryogenesis, or of massive perinatal hemorrhage. In order t o examine the requirements for FV in murine embryogenesis and hemostasis, w e generated transgenic mouse lines expressing a Fv minigene under control o f either the tissue-specific albumin (Malb) or rat platelet factor 4 (Rpf4) promoter. A total of 12 Malb and 3 Rpf4 lines were analyzed. Though expres sion in the target tissue was detectable in most lines by RT-PCR, only low levels of transgene expression were achieved (<3% of endogenous Fv in all l ines). Despite a low level of Fv transgene expression, rescue of the lethal Fv-/- phenotype was observed with one of the Malb transgenic (Tg+) lines. However, rescue appeared to be incomplete with continued loss of >1/2 of ex pected Tg+,Fv-/- mice in early embryogenesis. Rescued Tg+,Fv-/- mice have u ndetectable Ri (<0.1%) in both plasma and platelet compartments, but surviv e the perinatal period and mature to adulthood without spontaneous hemorrha ge. We conclude that FV present at <0.1% is sufficient to support postnatal survival. Failure of the Malb transgene to rescue the midembryonic block s uggests that FV expression is required during mammalian development at high er levels or with a different tissue-specific or temporal pattern. Taken to gether, these data may explain the observation of residual Ri activity in m ost human FV-deficient patients due to early embryonic lethality in those a bsolutely deficient and suggest that minimal levels of FV expression, below the level of detection, also may be sufficient to support survival in huma ns.