Transfer of therapeutic genes into haematopoietic stem cells can potentially cure blood disorders such as X-linked severe combined immunodeficiency. But human stem cells are relatively intractable to the available viral vectors. In August Blood, George Vassilopoulos and colleagues from University of Washington, Seattle, describe a new vector system based on foamy viruses from the spumavirus family that can be used for gene transfer into murine haematopoietic stem cells.

Foamy viruses are nonpathogenic retroviruses with a wide tissue tropism that are commonly found in mammalian species. Vassilopoulos et al. have developed replication-defective foamy virus vectors and demonstrate that these vector particles efficiently transferred a marker gene into repopulating mouse haematopoietic stem cells and into human CD34⁺ cells in vitro (Blood 2001, 98: 604-609).

Vassilopoulos et al. speculate that the high and persistent levels of marker-gene expression obtained are probably due to improved transduction of quiescent cells and to the novel envelope/receptor system used for stem cell entry. If proven efficient for human stem cell transfer in vivo, this vector may open new possibilities for gene therapy.