For the first time, scientists from the Whitehead Institute for Biomedical Research have used a combination of nuclear transplantation, gene therapy and embryonic stem cell differentiation to create custom-tailored cellular therapy that showed promise in mice.
The work, a collaboration between Whitehead member Rudolf Jaenisch and Whitehead Fellow George Daley, was published online as two companion papers by Cell on March 8.
POTENTIAL GENE THERAPY
The Whitehead researchers joined forces to work on a problem that until now has proven difficult to overcome.
Scientists have used nuclear transfer to create embryonic stem cells and differentiate them in culture to create many different cell types, including muscle, neurons and hematopoietic stem cells, which are the precursors to all immune and blood cells.
But scientists have never shown that the cells created in culture could be reintroduced into an animal to treat a disease.
Jaenisch and Daley used skin cells from an immune-deficient mouse to create a cellular therapy that was able to partially restore immune function.
"Though the immune system wasn't completely restored, there was enough improvement to predict that a comparable result in humans would translate into a significant clinical benefit," says Daley.
"This is a proof-of-principle experiment, which shows that nuclear transplantation therapy may be possible for human application. Furthermore, it shows that gene therapy can be incorporated into the approach to correct genetic mutations in defective cells without affecting the germ line," added Jaenisch.
Postdoctoral fellow William Rideout and graduate student Konrad Hochedlinger, both in the Jaenisch laboratory, used the nuclear transfer procedure to remove the nucleus, which contains the DNA of a cell, from an egg and replace it with the nucleus from a skin cell of an adult mouse with a genetic immune deficiency.
In this procedure, the egg resets the developmental clock of the adult nucleus and the reprogrammed cell starts developing into an embryo that is genetically identical to the donor cell.
At the stage when the embryo develops into a hollow ball of approximately a hundred cells called a blastocyst, it contains a nub composed of embryonic stem (ES) cells that have the potential to become any cell in the body. The ES cells from the blastocyst were isolated and the genetic defect causing the immune deficiency was corrected by gene therapy.
These corrected embryonic stem cells, however, cannot be transplanted into the adult mouse to treat the immune disorder because adult mice reject transplants of blood cell precursors derived from embryonic stem cells in culture.
"While embryonic stem cells could be induced to form hematopoietic cells in culture, these cells wouldn't reliably generate the blood and immune system when transplanted into mice. For the last 15 years, engrafting mice with blood derived from embryonic stem cells has been the Holy Grail of the field," Daley said.
Michael Kyba, a postdoctoral fellow in the Daley lab, found a way to achieve this goal by inserting a gene called HoxB4 that stimulates blood cell proliferation. The HoxB4- modified cells generated hematopoietic stem cell precursors that could be successfully transplanted into adult mice.
With this newfound ability, the researchers applied the same strategy to the genetically corrected embryonic stem cells made from the immunodeficient mouse. It was these genetically corrected cells that partially rescued the immune systems of mice with complete immune deficiency.
This approach may be useful for treating human patients with immune deficiency ("bubble boy disease") or be applied to a host of other genetic diseases that can be corrected by cell transplantation. Embryonic stem cells can form any tissue in the body, including neurons, muscle cells of the heart, and pancreatic beta cells, which produce insulin.
In addition, nuclear transplantation therapy to create embryonic stem cells has many benefits, such as the creation of cells that are genetically matched to the patient, the repair of genetic defects within cells to treat or cure inherited diseases and the possibility of growing embryonic stem cells in culture for continued therapy as needed.
"Before the potential of nuclear transplantation therapy can be realized, much more research about the basic biology of stem cells has to done," Daley said.
The Daley laboratory is supported by the National Institutes of Health, the National Science Foundation, the MIT Biotechnology Process Engineering Center, the Canadian Institutes of Health Research and the Alberta Heritage Foundation for Medical Research. The Jaenisch laboratory is supported by Boehringer Ingelheim Fonds and the National Cancer Institute.
A version of this article appeared in MIT Tech Talk on March 13, 2002.