applications with human ES cells are limited. Ethical issues regarding the source of ES cells and rejection responses are resolved by the establishment of patient-derived induced pluripotent stem (iPS) cells. Xu et al. [20] reported successful iPS-cell therapy using a hemophilia A mouse model. These authors differentiated iPS cells into endothe- lial cells through EBs in the presence of a vascular endothe- lial growth factor. Injection of iPS cell-derived endothelial cells into the livers of X-ray-irradiated hemophilia A mice resulted in successful engraftment into the sinusoids and increased FVIII:C. However, this strategy cannot be directly applied for treating human hemophilia A, because immuno- logically mismatched iPS cells from other donors will be rejected, and iPS cells from hemophilia A patients are unsuitable for cell therapy due to F8 gene aberrations. For human applications, regardless of the type of stem cells used, introduction of the F8 gene into stem cells is, there- fore, essential, unless an iPS cell bank possessing sufficient iPS-cell variations to match all patients immunologically is established.
The genetic risks of cell therapy have been reduced by genome-editing technology, including transcription activator-like effector nucleases and the CRISPR/Cas system [21,22], which facilitates in situ correction of mutant genes, reduced risks for random genome insertions, and ex vivo selection of genetically safe cells. The survival rate of transplanted cells remains unsatisfactory; therefore, methods were developed to promote hepatic regeneration, such as CCl4 treatment, X-ray irradiation, and partial hepa- tectomy. As shown following CCl4-induced liver damage, coagulation-factor activities decreased, except for FVIII and VWF, which likely increased due to excessive release from storage granules in damaged endothelial cells. Because hemophilia A mice are intrinsically deficient in FVIII, CCl4 treatment resulted in reduced activities in most key coagulation factors, resulting in a tendency for severe or sometimes lethal bleeding. Instead of using these inva- sive methods clinically, novel methods for the effective engraftment of transplanted cells should be explored.