In utero hematopoietic stem cell transplants for inherited diseases

The treatment of choice for many inherited diseases is bone marrow transplantation (BMT). Limitations to using marrow transplants for inherited diseases include (a) the toxicity associated with high doses of chemotherapy necessary to obtain engraftment; (b) the complications associated with graft-versus-host disease (GVHD); (c) the fact that only 20-25% of children will have a human leukocyte antigen (HLA)-matched donor; and (d) the concern that, at least for some inherited diseases, significant organ damage, especially to the nervous system, has occurred by the time the child is diagnosed and evaluated for possible BMT. In utero transplantation of hematopoietic stem cells (HSCs) offers the possibility of overcoming many of these limitations.

One of the biggest hurdles to a successful transplant is the ability of the recipient to reject the donor marrow. Except in patients with severe combined immunodeficiency disease (SCID), overcoming this hurdle requires high doses of chemotherapy. Early in gestation, the fetus is significantly immunoincompetent. Before 14-15 weeks of gestation, the human fetus appears to be similar to a child with SCID in its inability to reject allogeneic cells. Potential sources for HSCs are HLA-matched sibling marrow, fetal liver, parental bone marrow, and cord blood.

With fetal liver, only cells from fetuses < 10-12 weeks are acceptable because of the high risk of GVHD. With parental marrow, the cells must be T cell depleted in order to minimize the risk for GVHD. Problems in using fetal liver include the inability to obtain sufficient numbers of cells and inadequate supplies of donor tissue. The source and supply of parental bone marrow is almost unlimited, but, because of the need for T-cell depletion, bone marrow from a parent may have a lower engraftment rate in the child.

Studies in fetal murine and Rhesus models using fetal liver or T cell-depleted bone marrow from adult animals suggest that engraftment can be successfully obtained, providing the transplant is performed sufficiently early in gestation. To date, at least a dozen in utero human transplants have been attempted worldwide in fetuses diagnosed with a variety of inherited diseases. Because of the small number of transplanted fetuses and the variety of diseases and differing transplant conditions, it is difficult to draw any firm conclusions regarding ultimate efficacy of the procedure and its risk. However, it does appear that the age of gestation of the recipient, the dose of cells infused, and possibly the route of administration of the HSCs will be critical factors in determining success rates for this approach. The successful application of in utero transplantation would allow treatment of a variety of inherited diseases early in gestation while eliminating many of the risks associated with conventional BMT.