**5. Problems of neonatal gene therapy**

There are several problems that must be overcome before neonatal gene therapies can be used in humans. First, safety concern must be addressed, as there is the possibility of tumor devel‐ opment and of germ‐line transmission. It was reported that liver and lung cancers appeared in some mice treated using AAV‐mediated neonatal gene therapy [27, 28]. In addition, differ‐ ences in developmental stages of organs in mice and humans may be another problem. The immune system in mice is less mature at birth than that in larger animals, and the human BBB is functionally mature before birth. It is therefore not clear whether the same beneficial effect of neonatal gene therapy seen in mice would be achieved in human infants. These problems must be overcome before there can be clinical trials of neonatal gene therapy.

## **6. Summary and future developments**

We have shown that AAV‐mediated gene transfer in neonatal mice has characteristics that could potentially overcome the problems encountered with current gene therapy protocols. However, before applying neonatal gene transfer to humans, several important issues must be addressed. In particular, the safety of neonatal gene transfer must be carefully evaluated using large animal models, including nonhuman primates. Nonetheless, because of its advan‐ tages over gene therapies used to treat genetic disorders in adults, safe and effective neonatal gene therapy has the potential to be an invaluable method for treating genetic diseases.

## **Acknowledgements**

We thank Dr. James Wilson (University of Pennsylvania), Dr. R. Jude Samulski (University of North Carolina), Dr. Robert M. Kotin (National Institutes of Health) and Dr. David W. Russell (University of Washington) for kindly providing AAV packaging or vector plas‐ mids and Dr. Volkmar Gieselmann (Rheinische Friedrich‐Wilhelms‐University) and Dr. Jose Luis Millán (Stanford Children's Health Research Center) for kindly providing MLD and HPP model mice. We also thank Dr. Tae Matsumoto and Dr. Yukihiko Hirai for joint research and helpful discussions. This chapter was supported in part by grants from the Ministry of Health and Welfare of Japan and the Ministry of Education, Science and Culture of Japan.
