Unlike a solid organ transplantation, which requires moving an intact organ from donor to recipient, a hematopoietic cell transplant (HCT) involves the transferal of just some of the donor’s blood stem cells into a recipient. A small number of these stem cells are responsible for re-populating the entire hematopoietic system almost entirely from scratch.
With this high demand for fast replication, there is a real possibility that some cells might outgrow the rest—either by already possessing a cancer-like mutation or getting one through sloppy genome replication. Over decades of blood cell division post-transplant, even a small advantage could lead to one population dominating or even causing malignancies down the line.
This situation is called clonal hematopoiesis, and it can be associated with negative transplantation outcomes. For example, expansion of a T-cell clone that reacts poorly to the microbiome of its new host can exacerbate severity of graft-versus-host-disease (GVHD), a damaging and sometimes deadly immune reaction caused by the donor immune cells attacking the recipient. (Don’t worry, though: GVHD is on the decline for transplant recipients at Fred Hutch). Clonal imbalance doesn’t require complete dominance for problems to arise: even having one clone making up as little as 2% of blood cells is associated with an increased risk of blood cancers, heart disease, or death.
When bone marrow and other stem cell transplantation treatments were pioneered at Fred Hutch over 50 years ago, the technologies to detect mutations at this low level did not exist. It has therefore been difficult to study clonal or sub-clonal variants at anything lower than a 2% allelic frequency, and even more challenging to understand how clonal expansion can impact transplantation outcomes.
In a study recently published in Science Translational Medicine, Dr. Masumi Ueda Oshima and colleagues working with the group of Dr. Rainer Storb, a transplantation pioneer, sought to address this important gap in the field. They were well poised to do so, as Fred Hutch has treated some of the longest living HCT patients in the world and has maintained close relationships with patients for over 50 years.
For keeping in touch with patients, the authors credit Judy Campbell, who was one of four nurses brought on board in 1969 when Dr. Storb and colleagues were converting an abandoned obstetrics ward at the USPHS Hospital into a 6-bed transplant ward. Although Judy had retired after 45 years of service, she knew all of the early patients. “When she called them, literally all donors and recipients agreed to donate a blood sample,” Dr. Storb said.
The researchers acquired contemporary blood draws from 16 donor-recipient pairs, and set out to compare the sequences in today’s blood to samples banked from donors pre-transplantation—with amazing foresight from Hutch researchers of yesteryear, some pre-transplantation samples were obtained over 35 years ago!
The authors identified clonal variants using duplex sequencing, a highly sensitive and specific method that virtually eliminates errors introduced by PCR. They specifically narrowed in on a panel of genes that are often mutated in bone marrow tumors and found that all donors—from the youngest at 12 years old to the oldest at 84—had detectable mutations in one or several genes often associated with cancer. When they compared the contemporary samples to the pre-donation time point, they found that mutations correlated with age of the donor, which we expect as older people tend to accumulate mutations after decades of their cells replicating.
A welcome surprise, however, was that these rare clones did not seem to have a growth advantage in recipients, even though they were present in the donor’s blood prior to donation and presumably transferred during the transplant. “Mutations passed on from a donor into the recipient … do not expand to a significant degree in the recipient,” Ueda Oshima says, ”even after creating an entirely new hematopoietic system from a small number of transplanted cells, and even after many decades of forming blood cells.”
When the authors calculated the rate of mutation accumulation over time, they found that there was no significant difference between donors and recipients. “In short, with respect to clonal hematopoiesis, aging in the blood cells in the recipient after allogeneic transplantation does not differ greatly from aging of the blood cells in the donor,” Ueda Oshima explains.
Although these findings will need to be confirmed with follow-up studies and larger cohorts, there may be important implications if they hold true. In particular, Ueda Oshimi wants to investigate the “fitness of older donors carrying clonal hematopoiesis mutations to serve as a stem cell donor when younger donor options are not available”, which could hopefully increase donor pools to include older individuals.
The spotlighted research was funded by the National Institutes of Health, the Laura Landro/Richard Salomon Family Foundation, and Gabrielle’s Angel Foundation.
Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium members Drs. Masumi Ueda Oshima, Timothy Randolph, Cecilia Yeung, Paul Carpenter, Stephanie Lee, Mary Flowers, Jerry Radich and Rainer Storb contributed to this research.
Ueda Oshima, M., Higgins, J., Jenkins, I., Randolph, T., Smith, T., Valentine, C., Salk, J., Yeung, C., Beppu, L., Campbell, J., Carpenter, P. A., Lee, S. J., Flowers, M. E., Radich, J. P., & Storb, R. (2024). Characterization of clonal dynamics using duplex sequencing in donor-recipient pairs decades after hematopoietic cell transplantation. Science Translational Medicine, 16(770).
Hannah Lewis is a postdoctoral research fellow with Jim Boonyaratanakornkit’s group in the Vaccine and Infectious Disease Division (VIDD). She is developing screens to find rare B cells that produce protective antibodies against human herpesviruses. She obtained her PhD in molecular and cellular biology from the University of Washington.
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