The new results, reported in the September 2002 issue of the Journal of Clinical Investigation, come from a patient who suffers from two serious genetic diseases.
When he was six months old, doctors at Children's Hospital Los Angeles diagnosed this patient with X-linked severe combined immune deficiency (SCID). Often known as "bubble boy disease" after David Vetter, the patient who lived in a germ-free plastic bubble for 12 years, SCID results from a genetic mutation that greatly reduces the body's ability to fight off disease. Patients have little resistance to infections such as pneumonia and meningitis, which can therefore be life threatening.
Roughly one in 100,000 newborns has one of the several forms of SCID. The X-linked type, which affects only males, occurs most frequently. The most common treatment for X-SCID is a bone marrow transplant, which in effect gives the patient the donor's resistance system. The Los Angeles team, headed by bone marrow transplant specialist Dr. Kenneth Weinberg, used bone marrow from the boy's father. The team followed up the treatment with immunosuppressive drugs to prevent rejection of the transplanted bone marrow.
As he grew, the boy had problems with mobility. He didn't walk until he was two years old. He showed no interest in running or other forms of physical activity. He fell frequently at school. His doctors attributed the problem to the after-effects of the transplant and the immunosuppressive drugs given for it.
During a regular check-up when the boy was 12, however, Weinberg's team realized that the difficulties with mobility arose from an entirely different cause. In addition to SCID, the boy had another disease restricted to males: Duchenne muscular dystrophy (DMD), which also passed on through the X-chromosome from mothers to sons.
The most common and most severe form of muscular dystrophy, DMD affects about one in 4,000 males. Caused by mutation of a gene called the dystrophin gene, it shows up as general weakening of the muscles, starting with those in the limbs and the trunk. Most patients lose their ability to walk by the age of 10. Few survive beyond their late twenties, commonly due to weakening muscle fibers of vital organs such as the lungs which causing fatal pneumonia or other complications.
Intriguingly, the patient in Los Angeles had a very mild form of DMD. Once they diagnosed the disease, Weinberg and his colleague Dr. Jan Nolta speculated whether the bone marrow transplant might have reduced the severity of the disease by allowing normal muscle cells to have been produced from a type of stem cell in the donor's marrow called "mesenchymal stem cells."
At that point Weinberg got in touch with Louis Kunkel, Ph.D., chief of the Division of Genetics at Children's Hospital Boston and professor of pediatrics and genetics at Harvard Medical School. Over the past five years, Kunkel's group has shown that stem cells in bone marrow transplanted into mice with DMD can restore the function of the dystrophin gene in the mice.
Weinberg's patient offered an unexpected opportunity to check whether the same effect occurred in a human patient with DMD. "It's a beautiful experiment of nature," Kunkel said.
The two scientific teams needed evidence that the father's cells donated in the bone marrow transplant remained in the patient's muscle. They set out to answer the group's speculative questions: "Was it possible that the bone marrow transplant ameliorated the symptoms of the disease? Were the transplanted cells the reason for the mild nature of the boy's disease?"
To answer these questions, a biopsy of the boy's muscle tissue was needed. In November 2000, Weinberg's team biopsied two large muscles samples from separate parts of the boy's body. Analysis of the samples by geneticist Emanuela Gussoni , Ph.D., first author of the paper and assistant professor of pediatrics at Children's and Harvard Medical School, showed that they did indeed contain a small number of the donated cells.
Disappointingly, subsequent analysis showed that those cells did not appear to have any effect on the severity of the boy's DMD. "The percentage of donated cells was not sufficient to explain the patient's clinical course," says Gussoni. However, the discovery of the donated cells in the patient's muscle tissue 13 years after the transplant indicated that bone marrow transplants may play a role in developing a treatment based on bone marrow transplantation for patients with DMD.
What the investigators did not have was evidence of what those transplanted cells were doing at different points during the boy's growth over the past 13 years. Gussoni set out to figure out if the transplanted cells were doing what they were supposed to do and if they were increasing in number.
"It would have been nice to see at earlier time points whether the transplanted muscle cells were growing throughout those years," says Gussoni. "It could be that the cells decreased or increased over time. We cannot tell with the cells from the biopsy we took at one point in time at age 14." However, Nolta and Gussoni were able to show that the patient still has mesenchymal stem cells in the bone marrow that had survived in the marrow for 13 years. This finding indicates that the patient may be continuing to make muscle cells from the original transplant.
To continue this research, the Los Angeles and Boston teams, in collaboration with Dr. Edwin Horwitz at St. Jude's Childrens Research Hospital in Memphis, plan to give the boy a second bone marrow transplant from his father. This will introduce more donated mesenchymal stem cells than the first transplant. The newly transplanted cells will be biologically tagged to differentiate them from cells introduced during the first transplant. The operation may take place by the end of this year.
"We will continue to collaborate on evaluating the tissue samples," says Weinberg. "Time will tell whether this therapy may be effective. Our hope is that bone marrow transplantation can eventually become a valuable therapy for patients with Duchenne muscular dystrophy and possibly other muscle diseases."
Kunkel's team is continuing to perform bone marrow transplants on mice with DMD. The team hopes to learn more about the biological signals that send the donor cells into muscle. Weinberg's team is studying how the immune system affects the success of the transplantation in making new muscle cells. Together, these studies will help investigators to improve the impact of bone marrow transplants on the disease.
Plenty of research remains to be done. But these findings hold hope for huge medical dividends. "If it proves out," Kunkel says, "any patient with any form of muscular disease could be helped - even someone who had muscle tissue damaged in an accident."
Children's Hospital Boston is the primary pediatric teaching affiliate of Harvard Medical School, home to the world's leading pediatric research enterprise, and the largest provider of health care to the children of Massachusetts. For more information about the hospital visit: www.childrenshospital.org.
Journal
Journal of Clinical Investigation