NEW YORK – Whole-genome sequencing of transplanted hematopoietic stem cells (HSC) has answered fundamental questions about how the cells take root and adapt to their new environment, adding to evidence that cells from younger donors are more likely to succeed than those from older ones.
Using low-coverage whole-genome sequencing of 3,399 cells from 10 matched donor and recipient pairs, researchers from the UK's Wellcome Sanger Institute and the University of Zurich were able to map their clonal dynamics up to three decades post-transplant.
The study, published Thursday in Nature, showed that transplants from younger donors result in 10 times more vital stem cells surviving the transplant process. Some cells from older donors also lose the ability to produce the range of blood cell types that are needed to repopulate the immune system. These differences could lead to reduced immunity and higher relapse risk, potentially explaining why transplants from younger donors often lead to better outcomes.
"Of the hundreds of millions of CD34+ cells infused into the recipient, only a few thousand to tens of thousands will still be contributing to hematopoiesis a decade or more later," the authors wrote. "Like the hero of a picaresque novel, a transplanted stem cell must navigate serial perils in this quest — it must mobilize from its native niche in the donor marrow, withstand direct bone marrow collection or peripheral blood apheresis, survive hours to days ex vivo, home to a new niche extensively reconditioned with chemotherapy, and then proliferate to enable multilineage blood production."
By identifying somatic mutations or epigenetic changes enriched at different stages of the transplant procedure, "it may be feasible to identify pathways that promote successful engraftment of clonally diverse transplanted stem cells," they wrote.
Stem cell transplants taken from bone marrow have been routinely used for half a century to treat blood cancers or replace a defective hematopoietic system. In the UK, over 2,000 people undergo the procedure each year.
"When you receive a transplant, it's like giving your blood system a fresh start, but what actually happens to those stem cells? Until now, we could only introduce the cells and then just monitor the blood counts for signs of recovery," Michael Chapman, a Wellcome researcher and first author of the study, said in a statement. "We've traced decades of changes in one single sample, revealing how some cell populations fall away while others dominate, shaping a patient's blood over time."
The authors noted that their study only included long-term survivors, biasing it against transplants with poor outcomes. "It would be fascinating to study whether clonal dynamics of myeloid and immune reconstitution are different in patients with poor graft function or graft-versus-host disease," they wrote. Sequencing coverage for the genomes was between 8X and 15X per colony.
In addition to the findings about donor age, the study found that individual clones often showed biases toward myeloid, B lymphoid, and T lymphoid lineages. Moreover, "recipients had lower clonal diversity than matched donors, equivalent to around 10 to 15 years of additional aging, arising from up to 25-fold greater expansion of stem cell clones," they wrote.
Despite the stress of the transplant process, stem cells gained few new genetic mutations as they rapidly divided to rebuild the patient's blood, challenging previous assumptions about high mutation rates during transplantation. These findings aligned with another study of clonality in transplanted HSCs published last week in Science Translational Medicine, which also found limited clonal expansion. That study, led by researchers at Fred Hutchinson Cancer Center, used highly sensitive duplex sequencing technology.
However, the transplant process does exaggerate some selective pressures and "forces blood and immune cells through a type of genetic bottleneck," Wellcome researcher Peter Campbell, senior author of the study, said in a statement. "We find that the bottleneck provides multiple different opportunities for some stem cells to thrive more than others in their new environment in the recipient. We believe it will be possible to find the genes responsible for enabling some stem cells to thrive better than others. These genes could then in theory be harnessed to improve the success of the transplant procedure."