Photo by Michelle Hruby
Some tissue-type mismatches are permissible in bone-marrow or stem-cell transplantation to treat leukemia, according to a new study led by Dr. Effie Petersdorf of the Clinical Research Division.
The study provides a way to improve the effectiveness of transplantation as a cure for malignancies of the blood and immune systems.
The findings appear in today's New England Journal of Medicine and involved collaborators at the Hutch, the University of Washington and the Seattle Cancer Care Alliance.
Petersdorf and colleagues initiated the research to determine whether subtle differences 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.
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. Doctors strive for a perfect or near-perfect match, a prospect that can be difficult for individuals with rare tissue types and no closely matched relatives.
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 12 genes, but subtle disparity between individuals may be tolerated.
"Our work demonstrates that transplant success can be enhanced by applying state-of-the-art technology for genetic matching of stem-cell donors," Petersdorf said. "By looking carefully at precise mismatches between donor and recipient, we've learned that some mismatches are well-tolerated, and others are less so. This is information that we can implement immediately to help patients find appropriate donors."
In bone-marrow and stem-cell transplantation, donated healthy cells that form the blood and immune system are infused into a patient with leukemia or another blood or immune-system disorder. The process for matching patient and donor has changed over time, Petersdorf said.
"The definition of a perfect match is an evolving concept," she said. "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."
Without high-precision DNA analytical tools available today, including gene chips and array robots available at the Hutch, Petersdorf said the study would not have been possible.
Historically, HLA typing has been performed with serological tests, in which blood serum from the patient or donor is tested with a panel of antibodies that react against HLA proteins. More recently, DNA sequencing and serological testing have been used. This sensitive molecular analysis can detect subtle differences that may cause no discernable serological reaction.
Petersdorf and colleagues used DNA sequencing to determine the exact tissue type of 471 Hutch patients who received unrelated donor transplants for chronic myeloid leukemia between 1985 and 2000. Sequences were obtained for each of the six HLA genes.
"Patients and donors are always typed, and donors are always selected using the best technology and information available at the time," she said. "Some of the patients in this study underwent transplantation before DNA methods were even available."
With this information, Petersdorf correlated mismatch status with transplant outcome.
"We 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. What this means for patients is 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 contribute most significantly to graft rejection by provoking an immune response when donor and recipient are not identical.
In addition, the researchers discovered that whether the recipient is homozygous - has two identical copies (alleles) of the same HLA gene - 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," Petersdorf said. "We hypothesize that there is HLA disparity in only one direction, that of the patient against the donor."
"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."
Petersdorf said the Clinical Immunogenetics Laboratory at the Alliance, which performs tissue-type matching for Hutch patients and donors, already performs typing with a higher degree of discrimination than is routinely available.
"But we're always refining the process," she said. "Retrospective studies on transplant outcome provide invaluable information that can be applied to make transplantation a safe and effective therapy."