Stem-Cell Gene Therapy

The important take-home messages from the first Conference on Stem Cell Gene Therapy: Biology and Technology, held in Rockville, Md., on Sept. 28-Oct. 1, is that procedures that increase the transduction efficiency in the colony forming units (CFUs) assay are not predictive-and even may be misleading-for determining transduction efficiency of repopulating cells in vivo and that, at this time, there is no surrogate assay to predict in vivo reconstitution.

For years, researchers have been examining techniques for transducing normally quiescent hematopoietic stem cells. Peter Quesenberry of the University of Massachusetts Medical Center in Worcester demonstrated that when stem cells are stimulated to divide by the addition of cytokines and 5-fluorouracil (5-FU), they may lose the ability to engraft in bone marrow. Use of cytokines and 5-FU may stress the cells and cause differentiation. Peggy Goodell of the Whitehead Institute for Biomedical Research in Cambridge, Mass., described the isolation of a unique population of bone marrow cells that, when mixed with nonproliferating marrow cells, were found to be enriched 1,000-fold for reconstitution activity, as shown by the competitive repopulation assay. These cells could be identified in several species-including humans (in marrow and cord blood)-by sorting after Hoescht 33342 staining.

On another front, there was considerable controversy concerning data from studies using adeno-associated virus (AAV) vectors for transduction of quiescent stem cells. Several researchers reported long-term expression of marker genes in cells bearing the CD34+ surface marker, which enriches for early progenitor cells. For example, Saswati Chatterjee of the City of Hope National Medical Center in Duarte, Calif., reported six-month expression of a marker gene transplanted into mice. Chatterjee's lab also found expression in animals that received cell transplants from the primary transplanted animals. However, other teams attempting to reproduce the results with crude AAV lysates like those used by Chatterjee detected contaminants in the lysate that may have simulated AAV transduction, leading to what is being dubbed "pseudotransduction."

- Catherine McKeon and David Badman, NIDDK

- Pamela Gehron Robey, NIDR

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