Introduction
Hemophilia A is an inherited X-linked, recessive hemorrhagic disorder caused by a deficiency of clotting factor VIII (FVIII) and is commonly treated by FVIII replacement. Concentrated FVIII products are derived from human plasma or produced recombinantly. Advances in viral-screening and -inactivation methods have improved the safety of plasma-derived products. Furthermore, the infectious risk of recombinant FVIII products is lowered by excluding human- or animal-derived proteins from cell culture solutions [1]. However, FVIII concen- trates remain extraordinarily expensive, and frequent injections are required for FVIII replenishment, especially with severe hemophilia A patients.
Since hemophilia A is a single-gene disorder and small increases in FVIII levels exert a curative influence, the dis- ease is a good candidate for gene therapy. However, prob- lems are associated with gene therapy, such as the development of leukemia and decreased target-protein expression following cytotoxic CD8 T-cell induction against viral vector-derived capsid epitopes [2]. Nevertheless, a sin- gle injection of an adeno-associated virus serotype 8 vector in patients with severe hemophilia B caused prolonged factor IX (FIX) overexpression and clinical improvement [3]. How- ever, viral vectors employed in gene therapy cannot deliver the F8 gene due to its large size. Therefore, truncated F8 gene variants, such as a B-domain-deleted variant, have been used, but successful hemophilia A gene therapy using viral vectors has not been achieved clinically. A recent study using a hemophilia A mouse model demonstrated that trans- ducing the entire F8 gene via the piggyBac vector improved clotting activity [4]. However, as the vector preferentially integrates near transcriptional start sites [5], insertional mutagenesis and genotoxicity remain significant concerns.
As the liver is the major site of FVIII synthesis, liver transplantation is effective in hemophilia A patients [6]; however, the lack of donor organs hinders clinical applica- tions. Recent studies revealed that the main source of FVIII is liver sinusoidal endothelial cells (LSECs), and transplanta- tion experiments using murine or human LSECs in hemophi- lia A mice demonstrated symptomatic improvement [7]. However, such transplantation requires a donor liver from which LSECs can be isolated, and contamination with different donor cells remains a potential problem. Several therapies have been investigated using cells capable of differentiating into LSECs [8–11].
Considering their potential for multilineage development and illimitable proliferation potential, embryonic stem (ES) cells may enable cell transplantation therapy. Fair et al. [12] reported that injecting mouse ES cells into the livers of hemophilia B mice corrected FIX deficiency. However, there have been few reports describing cell therapy for hemophilia A using ES cells. Previously, we established
mouse Ainv18 ES cells (tet-226aa/N6-Ainv18), which secrete human FVIII (hFVIII) by introducing the human F8 gene [13]. Here, we investigated ES-cell transplantation therapy against hemophilia A.