Endogenous Genes for HSC Expansion

Gene products that contribute to stem cell self renewal and proliferation are highly sought after as tools for overcoming the limited cell numbers associated with stem cell therapies. Considerable progress has been made in identifying growth factors that can maintain or expand the stem cell pool ex vivo (41). However, growth factors act on natural receptors, and thus cannot be used to specifically enrich transduced stem cell population in vivo. Therefore, genes that provide an intrinsic proliferative advantage to stem cells are being evaluated as tools for enriching transduced populations. Endogenous genes, which have a natural role in stem cell self renewal, as well as recombinant genes engineered for this activity, have been assessed as candidates for this approach.

Although gene products that have a natural role in stem cell self renewal and proliferation will likely have the most potential for this type of application, many have also been implicated in leukemogenesis. The homeobox transcription factors are a good example of this. This gene family was initially discovered for its role in embryogenesis, but many of the hox genes have an ongoing role in hematopoiesis. Several members from the hoxA, B, and C gene clusters have distinct expression patterns that are restricted to different lineages or stages of hematopoietic differentiation (42-44). Ectopic expression of these genes, either with retroviral vectors, or by naturally occurring translocation events, has also been linked to major perturbations in hematopoiesis (45). The HoxB4 transcription factor appears to be an exception. Transduction of murine or human HSCs with HoxB4 has been shown to induce expansion in vitro and

Fig. 1. Enrichment of transduced HSCs in vivo. Transduced HSCs can be enriched in vivo using genes that provide a proliferative advantage (left) or a survival advantage (right) over untransduced cells.

in vivo (46-51). Murine HSCs that overexpress HoxB4 have up to a 50-fold competitive repopulation advantage over untransduced cells. In these HoxB4 studies stem cell expansion did not progress beyond normal stem cell levels, which suggests the existence of an environmental sensor of stem cell density (47). The level of HoxB4 expression also seems to determine its biological activity. Beslu et al. demonstrated that increased HoxB4 expression levels correlated with increased repopulating potential (52). HoxB4 applied to cells in protein form has also been shown to allow ex vivo stem cell expansion (49,50). This strategy offers the potential for preloading stem cells prior to transplant, allowing transient expansion without a requirement for stable expression. Delivery of other hox members with this strategy may allow for transient expansion of specific lineages without the associated risk of leukemogenesis. HoxB4 expression in human CD34+ cord blood cells has been reported to impair lymphomyeloid differentiation (51). Thus, the necessary feedback signals that appear to be present in the all-murine model may not be recognized by human cells when expanded ex vivo or transplanted into NOD/SCID mice.

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