After years of insufficiency, we are in the process of overcoming major hurdles to retroviral (including lentiviral) gene delivery into human hematopoietic stem cells. But with the increasing potency of the methods used, dose- finding is becoming more important. This is underlined by recent observations of serious side effects of gene therapeutic interventions which occurred not only in a mouse model (Li et al, Science. 2002;296:497) but also in the first really successful clinical trial (Check, Nature. 2002;420:116-118).

As genotoxicity by random transgene uptake is a default issue of any integrating vector technology, a thorough characterization of insertion site frequency and locations is urgently required. The study by Woods and colleagues (page 1284) is an important step in this direction. They show that a subset of cultured CD34+ cells is at increased susceptibility to lentiviral gene transfer, potentially resulting in multiple (more than 5) vector insertions per single cell. Some insertions may result in oncogene activation or disruption of a tumor suppressor allele, as documented for BRCA1 in this study. It is puzzling to note that this event was observed in an animal with seemingly monoclonal human hematopoiesis.

These findings may catalyze a new phase in the improvement of transgene delivery methods that does not simply focus on improving overall efficiency. In fact, a number of preclinical studies in both conventional retroviral and lentiviral vector development may have been conducted under conditions of high copy number that do not appear desirable for clinical use. To achieve a fair assessment of both potency and side effects of genetic interventions, a new discipline is arising that could be addressed as transgene toxicology. Now that we have learned to open the door, it will be a challenge to define the boundaries.

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