Photo by Clay Eals
Cancer patients seek good matches in a variety of ways: the best doctor, a comfortable hospital, and the appropriate treatment.
But for leukemia patients hoping for a life-saving transplant, the most important match lies in the microscopic makeup of their own genes with those of a donor.
The success or failure of a transplant hinges on matching the tissue types of patient and donor, said Anajane Smith, manager of the Clinical Immunogenetics Laboratory at the Seattle Cancer Care Alliance. "Recent studies at the Hutch and elsewhere have shown clearly that patients can tolerate only a limited mismatch for a successful transplant," she said.
Determining the extent of that match is the responsibility of Smith and her 40 colleagues on the seventh floor of the Alliance clinic building, one of the largest such labs in the world, and one that frequently discovers new tissue-type variants.
The lab staff analyze the tissue type - consisting of HLA (human leukocyte antigen) molecules - for every prospective bone-marrow or stem-cell transplant recipient treated at the Alliance and, if possible, close family members.
In addition to providing a crucial role in patient care, the lab is a major collaborator in research in the Hutch's Immunogenetics Program to understand genetic variation in the human immune system.
HLA molecules are proteins that sit on the surfaces of all cells in the body. These proteins provide a molecular identification badge that enables the immune system to distinguish "self" from "non-self." This is the basis for the body's ability to react to and destroy foreign cells such as pathogenic bacteria and viruses or incompatible transplanted tissues.
Genes that specify a person's HLA type are part of a large region of DNA on chromosome 6 known as the major histocompatibility complex (MHC), which contains the genetic information for an enormous array of immune system functions. MHC DNA contains more variation than any other part of the genome, resulting in a near-unique fingerprint for every person.
Scientists know that five of the HLA genes - A, B, C, DR and DQ - are most critical when technicians try to match a patient with an appropriate donor. Each human being inherits one form of each of the five genes from his/her mother and father. This means that a total of 10 alleles, orvariants, must be analyzed during the matching process.
A near-identical match is crucial to minimizing the risks of two potential barriers to a successful transplant. Lois Regen, Clinical Immunogenetics Laboratory supervisor, likens the twin barriers to the Greek mythological perils of Scylla and Charybdis.
"The two complications we want to avoid are graft rejection and graft-vs.-host-disease, and each tends to be caused by mismatches at HLA genes," she said. "Both conditions can cause major complications or death."
Mismatches at HLA-A, -B, or -C, the so-called Class I HLA genes, increase the probability of graft rejection. This condition that results when residual immune T cells in the patient recognize donor cells and mount an immune response against donor cells, preventing the graft from taking hold.
Graft-vs.-host-disease is a complication unique to bone marrow and stem cell transplantation. Because both procedures involve transfer of hematopoietic stem cells, which give rise to the immune and blood cells, mismatched donor cells can reject the patient's tissue, causing symptoms ranging from skin rashes to devastating gastrointestinal damage. HLA mismatches increase the chance and severity of graft-vs.-host disease.
Evaluating a prospective transplant recipient's HLA type and identifying a matched donor are the first steps patients encounter in the Alliance transplant program, a process typically beginning before a patient sets foot in the clinic.
"We're often the first point of contact that patients have here," Smith said. "We typically get blood samples sent to us for typing by the referring physician before the patient even gets to the Alliance."
Initially, testing is performed on the patient and his or her siblings. Based on laws of heredity, there is about a one in four chance that any two siblings will have an identical HLA type.
"When a patient first contacts us, we do what is called level-one testing to see if there is an HLA identical or matched sibling," Smith said. "There are two ways to do this: by serology, an older method, and by DNA analysis. We're moving to a mostly DNA-based system, but there will probably always be a small serological component of our laboratory."
Serological typing methods use antibodies that recognize the set of HLA proteins on the surface of the patient's - and potential donors' - cells.
Level-one DNA typing uses a method called polymerase chain reaction and sequence-specific oligonucleotide probes and primers to analyze the genes that direct the synthesis of the HLA proteins.
Additional sensitive tests
If a patient and a sibling appear to be identical by level-one testing, additional sensitive tests are performed to confirm complete identity, a process overseen by Sue McKinney, a Clinical Immunogenetics Laboratory lab supervisor.
"One essential part of the HLA work-up is cross-matching, which tells us whether the patient has antibodies in the blood that react against donor antigens," she said. "If they do, this is a strong indication of graft rejection. It means they can mount an immune reaction against the graft."
About 70 percent of patients will not have a related donor. In that case, an unrelated donor search may be initiated, said clinical case coordinator Sandy Warnock. The National Marrow Donor Program, developed in 1986, has access to a registry of six million donors worldwide.
"When a search initiates, the Alliance unrelated-donor program forwards the patient's HLA typing results to the marrow-donor program's national coordinating center in Minneapolis," Warnock said. "That center then contacts the appropriate donor centers. There is no contact between patient and donor. The process is confidential, with the donor-marrow program acting as the interface."
Outcomes for HLA-identical patients and donors are about the same whether the donor is related or unrelated.
How much mismatch can be tolerated?
Very little, according to a study published in 1998 by Dr. Effie Petersdorf, an investigator in the Hutch's Clinical Research Division.
Her study indicated that single mismatches at any of the five important HLA genes result in about the same survival rate as a perfect match of 10 out of 10 alleles. But patients and donors mismatched for multiple alleles, especially mismatches at both a class I and class II allele, have significantly lower survival rates.
In addition to HLA typing, the Clinical Immunogenetics Laboratory performs chimerism testing, a technique similar to DNA fingerprinting that evaluates post-transplant patients to determine the degree of engraftment.
The lab does this by testing whether patients' marrow and blood consist solely of donor cells, with little or no trace of residual diseased cells.
Overall, Smith and her colleagues find their work enjoyable and challenging.
"That's because we encounter new and interesting HLA alleles regularly," she said, "but even more important is the knowledge that what we do directly benefits patients."