Hematopoiesis separated by transplant: Characterization of clonal dynamics using duplex sequencing in donor-recipient pairs decades after hematopoietic cell transplantation

After allogeneic hematopoietic cell transplantation (HCT), a very small number of donor stem cells reconstitutes the recipient hematopoietic system, whereas the donor is left with a near normal pool of stem cells. We hypothesized that the increased replicative stress on transplanted donor cells in the recipient could lead to the disproportionate proliferation of clonal hematopoiesis (CH) variants. We obtained blood samples from 16 related donor-recipient pairs at a median of 33.8 years (range 6.6-45.7) after HCT, including the longest surviving HCT recipients in the world. For 11/16 pairs, a donor sample from time of HCT was available for comparison. We performed ultra-sensitive duplex sequencing of genes recurrently mutated in myeloid malignancies and CH, as well as a set of functionally neutral genomic regions representative of human genomic content at large. CH variants were observed in all donors, as young as 12 years old. The average mutation rate (change in mutation frequency/year) in donors compared to recipients post-HCT where baseline donor pre-HCT sample was available was similar (2.0% vs. 2.6% per year, respectively) within genes recurrently mutated in myeloid malignancies. Twenty-two (5.6%) of the 393 variants shared between paired donors and recipients post-HCT showed >= 10-fold higher variant allele frequency (VAF) in the recipient. Longer time since HCT showed a modest positive association with expansion of shared variant VAFs in the recipient. In conclusion, even many decades after HCT, there does not appear to be widespread accelerated clonal expansion in the transplanted cells, highlighting the immense regenerative capacity of the human hematopoietic system.