Adeno-Associated Virus Vectors and Hematology

Gene therapists have a special fondness for hematology. Initially, this was due to the many inherited diseases that could potentially be cured by ex vivo genetic modification of hematopoietic stem cells. Once the early murine leukemia virus (MLV) vectors were found to transduce only a small percentage of primate stem cells, the search began for other types of vectors that might work more efficiently, and adeno-associated virus (AAV) vectors were among the first alternatives to be investigated. Although a substantial amount of work with AAV vectors has occurred over the last several years, the bulk of evidence suggests that they transduce hematopoietic stem cells poorly, and that they will not replace retroviral vectors in stem cell gene transfer applications. Instead, it is applications involving the production of secreted proteins that now appear especially promising for AAV vectors, where they are delivered by direct injection rather than ex vivo cell manipulation. Recent animal studies have shown that AAV vectors encoding factor IX can lead to long-term, therapeutic clotting factor levels after in vivo administration, setting the stage for what may well be a cure for hemophilia B in the near future. Animal experiments have also shown that AAV vectors expressing erythropoietin can produce dramatic increases in hematocrit levels. Here we will review the role of AAV vectors in the field of hematology, including the basic biology of AAV, vector transduction properties, their use in hematopoietic cells, and their potential for delivering secreted proteins such as clotting factors and cytokines.