Quantification of adeno-associated virus particles and empty capsids by optical density measurement.

We show here that UV absorbance of denatured adeno-associated virus (AAV) vector provides a simple, rapid, and direct method for quantifying vector genomes and capsid proteins in solution. We determined the molar extinction coefficients of capsid protein to be 3.72 x 10(6) M(-1) cm(-1) at 260 nm and 6.61 x 10(6) M(-1) cm(-1) at 280 nm. For recombinant AAV vectors, extinction coefficients can be calculated by including the predicted absorbance of the vector DNA. Since the amount of empty capsids in purified vector preparations lowers the A(260)/A(280) ratio in a predictable manner, the vector genome (vg) and capsid particle (cp) titers in purified AAV vector preparations can be calculated from the absorbance at 260 nm and the A(260)/A(280) ratio. To validate this method, the vg and cp titers calculated by UV absorbance were compared with titers determined by quantitative (Q)-PCR and capsid ELISA. The vg titers determined by absorbance agreed well with titers determined by Q-PCR. The cp/vg ratio determined by the A(260)/A(280) method also correlated well with those determined by AAV capsid ELISA in conjunction with Q-PCR. This new method provides a simple and rapid means to determine AAV vg titers and the ratio of empty to full particles in purified virus preparations.

Phenotypic correction of a mouse model of hemophilia A using AAV2 vectors encoding the heavy and light chains of FVIII.

Using separate adeno-associated viral 2 (AAV2) vectors to deliver the heavy and light chains of factor VIII (FVIII) we have overcome the packaging limitations of AAV, achieving phenotypic correction of hemophilia A in mice. AAV vectors were constructed that use a liver-specific promoter and the cDNA sequences of either the human or canine heavy and light chains of FVIII. After intraportal vein injection of these vectors in hemophilia-A mice, therapeutic to superphysiologic levels of active FVIII were achieved in plasma in a dose-dependent manner. Phenotypic correction of the bleeding diathesis was demonstrated by survival of all treated mice after tail clipping. Biochemical analysis demonstrated lower levels of heavy-chain (25- to 100-fold) compared with light-chain protein in the plasma of treated animals. Differences in gene transfer and transcription did not account for the differences in protein expression. We hypothesize that improvements in FVIII activity could be achieved by improvements in FVIII heavy-chain expression. This work demonstrates that cotransduction of liver with AAV vectors expressing the heavy and light chains of FVIII corrects hemophilia A in vivo, providing an alternative approach to the use of a single vector. This strategy may potentially be useful for other large therapeutic proteins that contain functionally distinct domains.

Sustained phenotypic correction of canine hemophilia A using an adeno-associated viral vector.

Gene therapy for hemophilia A requires efficient delivery of the factor VIII gene and sustained protein expression at circulating levels of at least 1% to 2% of normal. Adeno-associated viral type 2 (AAV2) vectors have a number of advantages over other viral vectors, including an excellent safety profile and persistent gene expression. However, a major disadvantage is their small packaging capacity, which has hampered their use in treating diseases such as hemophilia A, cystic fibrosis, and muscular dystrophy, which are caused by mutations in large genes. Here we demonstrate that this can be overcome by using small regulatory elements to drive expression of a B-domain-deleted form of FVIII. The use of this vector for hepatic gene transfer in a canine model of hemophilia A resulted in the sustained (> 14 months) expression of biologically active FVIII. FVIII activity levels of 2% to 4% were achieved. These levels correlated with a partial correction in the whole-blood clotting time and cuticle bleeding time. In addition, immunoprecipitation analysis demonstrated the expression of canine FVIII of the predicted size in the plasma of injected animals. These data support the use of AAV2 vectors in human clinical trials to treat hemophilia A patients.