Soon after the recognition of clonal hematopoiesis (CH),1 cases of donor-derived CH were rapidly identified in patients receiving allogeneic stem cell transplantation (SCT), and endless debate arose as to the clinical significance of the findings.2-5 In a landmark study by Dr. Mareike Frick and colleagues published in 2018, the multi-institutional group examined CH and outcomes in 500 donor-recipient pairs. The researchers found that donor-derived CH was associated with more rapid leukocyte engraftment, increased incidence of chronic graft-versus-host disease (cGVHD), and increased donor-cell leukemia (DCL) but decreased cumulative incidence of relapse/progression, especially if the recipient underwent SCT without achieving a complete remission and the donor-derived clone had a DNMT3A mutation.6 There was no effect of donor CH on non-relapse mortality. Despite the obviously haunting, but rare, spector of late occurrences of DCL, the lower incidence of relapse/progression suggests that while there is a significant adverse donor CH versus host effect leading to cGVHD, part of that effect is a concomitant potentially beneficial donor CH versus leukemia effect. Since this study focused on outcomes, it necessarily included patients with variable outcomes and follow-up. However, the findings that donor CH had no effect on non-relapse mortality and was compatible with long-term survival raised questions of the effects and dynamics of the donor-derived clones in long-term survivors.
In a recent study, Dr. Steffen Boettcher and colleagues took a long-term view of the clonality status of both donors and recipients with a median follow-up of 16 years (range, 10-32 years) for 42 related donor-recipient pairs. They hypothesized that the dramatic expansion of hematopoietic stem cells (HSCs) required in reconstituting the marrow following allogeneic SCT in a highly inflammatory environment would provide an appropriate setting for the expansion of donor CH that might have a survival advantage over the other HSCs. The study identified no significant difference between the rate of CH in donors and recipients (10 [23.8%] of 42 donors and 13 [31%] of 42 recipients), although there was a correlation with age of donor, as expected. Additionally, there were five examples of donor-recipient shared CH (Figure, part A). In eight cases of recipient-only CH, chimerism studies demonstrated full engraftment of the marrow (limit of detection, 0.5%), suggesting that the variants in those patients could be donor-derived, but below the limit of detection in the corresponding donors (1% variant allele fraction [VAF]). In five cases of donor-only CH, presumably the donor clones either did not engraft or remained below the limit of detection for the duration of the follow-up for the recipient. Consistent with a clonal advantage of the HSCs, VAFs of the clones were overall 2.3-fold higher in the recipients than the donors (Figure, part B). Telomere length assessment demonstrated that recipient cells were aged by approximately 20 years relative to their respective donors, as is consistent with the proliferative stress of engraftment. However, contrary to what might be expected at first blush, colony formation unit (CFU) assays did not find a significant difference in telomere length between colonies with CH and those without CH.
The CFU studies also demonstrated that in subjects with greater than one mutation (approximately 50% of subjects with CH), the mutations could be found in either separate clones or in subclones (Figure, part B, subclonal pattern shown). Since the presence of mutations in stem cells need not be equivalent to those in more differentiated cells, the researchers also examined the penetrance of the CH mutations in mature granulocytes, monocytes, B cells, and T cells, identifying high VAFs in the myeloid cells compared to the lymphoid, with quite discrepant penetrance possible in low VAF mutations.
Finally, returning to the concern for the donor cell–derived myeloid neoplasms, Dr. Boettcher and colleagues found only one example of a high-risk myelodysplastic syndrome (MDS-EB2) in a recipient 21 years after SCT while the donor also developed MDS 18 years after donation. The identical two-founder mutations were found in both the recipient and donor that clonally diverged with the additional accumulation of two additional but distinct mutations in each individual (Figure, part A).
This current study confirms that long-term survival with donor CH is not only possible, but common. The researchers largely disprove the hypothesis that an ablated marrow provides a hugely advantageous environment for donor CH to flourish. Telomere studies confirm that the initial reconstitution of the recipient marrow provides a proliferative stress on the donor stem cells, but this burden may be equally shared by HSCs with and without CH (i.e., either donor CH does not confer a significant proliferative advantage in that initial setting or some CH mutations might result in some modulation of telomere biology to prevent foreshortening of the telomeres.) Over time, although there was an ultimate 2.3-fold increase in the clone sizes in recipients compared to donors, the finding suggests that any survival advantage of the cells with donor CH is only modest, with the mean VAF achieved by the donor CH in the recipient being only approximately 10 percent.
In some ways, this study raises more questions than it answers. Do specific variants provide different fitness either for proliferation or survival in the SCT setting? For cases with multiple mutations, what are the clinical implications of separate versus subclonal patterns? Do different conditioning regimens or different degrees of residual disease modify the effects of donor CH? Can the combination of all these effects be used to reduce the risk of DCL while maximizing the effects of donor CH in decreasing the incidence of relapse/progression? In other words, while significant further study is needed, our understanding of donor CH might eventually be one additional mechanism to fine-tune the transplantation conditions for optimal outcomes in both donors and recipients.
Dr. Kim indicated no relevant conflicts of interest.