SEATTLE — Dec. 19, 2001 — Using the latest DNA analysis techniques, Fred Hutchinson Cancer Research Center scientists have discovered that some tissue-type mismatches are permissible in bone-marrow or stem-cell transplantation to treat leukemia. This knowledge opens doors for many people who are unable to find fully-matched donors.
The study, led by Dr. Effie Petersdorf, associate member at Fred Hutchinson and associate professor of medicine at the University of Washington, provides a means to improve the effectiveness of transplantation as a cure for malignancies of the blood and immune systems. Results from the study will appear in the Dec. 20 issue of the New England Journal of Medicine.
Petersdorf and colleagues initiated the research to determine whether subtle differences — only detectable by highly sensitive DNA analysis — between tissue-type genes from a healthy donor and a leukemia patient impact the success of transplantation. Their findings show that single mismatches detectable only by DNA typing methods are well tolerated by patients.
"This work demonstrates that transplant success can be enhanced by applying state-of-the-art technology for genetic matching of stem cell donors," says Petersdorf. "By looking carefully at the precise genetic mismatches between donor and recipient, we've learned that some mismatches are well-tolerated — and others are less so. This is information we can begin to implement immediately to help patients find appropriate donors."
According to Petersdorf, without the high-precision DNA analytical tools available today the study would not have been possible. The DNA typing technology is founded on the same principles used in forensics.
Bone-marrow and stem-cell transplantation are therapies in which donated healthy progenitor cells that form the blood and immune system are infused into a patient with leukemia or another blood or immune system disorder. The tissue-type compatibility between donor and recipient is a critical factor in the outcome of a transplant, as mismatches can lead to transplant rejection.
The goal is for a perfect or near-perfect match, a prospect that can be difficult for individuals with rare tissue types and no closely matched relatives. Patients without matched relatives may identify potential donors through volunteer donor registries worldwide. The National Marrow Donor Program, a non-profit organization based in Minneapolis, Minn., is the largest donor registry and facilitates marrow and blood stem cell transplants for patients who do not have a matched donor in their family.
"The matching process has changed over time," says Petersdorf. "The definition of a perfect match is an evolving concept. As we learn about new genes important for a successful transplant and as more sophisticated technology for analyzing them becomes available, we continually refine the matching process."
Tissue type is determined by the information specified in six genes known as HLA (human leukocyte antigen) genes. Since every individual inherits two sets of chromosomes (and thus two versions, known as alleles, of each HLA gene), every person can have up to 12 different tissue type proteins. Ideally, donor and recipient possess identical information in all twelve genes, but a small degree of disparity between individuals may be tolerated.
Historically, HLA typing has been performed with serological tests, in which blood from the patient or donor is tested with a panel of antibodies that react against HLA proteins. More recently, DNA analysis, including DNA sequencing, has been used in addition to serological testing. This kind of sensitive molecular analysis can detect subtle differences that may cause no discernable serological reaction.
In the study, DNA sequencing was used to determine the exact tissue type of 471 Fred Hutchinson patients who received unrelated donor transplants for chronic myeloid leukemia between 1985 and 2000. For each patient, sequence information was obtained for each of the six HLA genes. With this DNA sequence information researchers correlated mismatch status with transplant outcome. They found no difference in graft failure between those with perfect matches and those with a single allelic mismatch — a mismatch in the DNA sequence that has no serological difference.
"Patients and donors are always typed and donors are always selected using the best technology and information available at the time," says Petersdorf. Most of the patients in this study underwent transplantation before DNA methods were even available."
Petersdorf and colleagues believe this means that the overall success of transplantation is not compromised by a single allelic mismatch. In contrast, a single so-called antigenic mismatch — a sequence difference that does cause a serological reaction — was associated with increased risk of graft rejection. Because the three-dimensional structure of HLA proteins has been determined, the researchers could identify which portions of the protein are affected by differences in HLA DNA sequence.
Not surprisingly, mismatches that affect the portion of the HLA protein that is recognized by T-cells-and thus provoke an immune response when donor and recipient are not identical — contribute most significantly to graft rejection.
In addition, the researchers discovered that whether the recipient has two identical copies (alleles) of the same HLA gene — a situation known as homozygous — plays a critical role in the matching process. Individuals with two different alleles for the same gene are said to be heterozygous.
"If the recipient has two identical HLA alleles and is mismatched with a heterozygous donor, the risk of graft failure increases. We hypothesize that there is HLA disparity in only one direction, that of the patient against the donor," says Petersdorf. "In contrast, if both the recipient and the donor are heterozygous and mismatched for a single allele, the graft is not rejected. This is another piece of information we can use to help patients find the best match."
According to Petersdorf, the Clinical Immunogenetics Laboratory at the Seattle Cancer Care Alliance, which performs tissue-type matching for Fred Hutchinson patients and donors, already performs their typing with a higher degree of discrimination than is routinely available.
"But we're always refining the process," says Petersdorf. "Retrospective studies on transplant outcome provide invaluable information which can be applied to make transplantation a safe and effective therapy."
# # #
Fred Hutchinson Cancer Research Center
The Fred Hutchinson Cancer Research Center, home of two Nobel Prize laureates, is an independent, nonprofit research institution dedicated to the development and advancement of biomedical technology to eliminate cancer and other potentially fatal diseases. Fred Hutchinson receives more funding from the National Institutes of Health than any other independent U.S. research center. Recognized internationally for its pioneering work in bone-marrow transplantation, the center's four scientific divisions collaborate to form a unique environment for conducting basic and applied science. Fred Hutchinson, in collaboration with its clinical and research partners, the University of Washington Academic Medical Center and Children's Hospital and Regional Medical Center, is the only National Cancer Institute-designated comprehensive cancer center in the Pacific Northwest and is one of 38 nationwide. For more information, visit the center's Web site at www.fhcrc.org.