Abstract A23: Less is more: Ultra-low input sequencing allows for evaluation of stem cells as a predictor of post-transplant relapse in the myelodysplastic syndromes

Malignant hematopoietic stem cells (HSCs) initiate the myelodysplastic syndromes (MDS) and drive treatment failure through their resistance to all non-transplant therapies. As a result, allogeneic stem cell transplantation (allo-SCT) is the only curative therapy for MDS. However, relapse remains the leading cause of treatment failure after allo-SCT. Improved tools to predict relapse in this setting are necessary to allow for pre-emptive therapeutic interventions, which may be more effective when the burden of disease is still low. Given prior evidence that HSCs serve as the functional unit of resistance to non-transplant therapies, we asked whether relapse after transplant originates from MDS HSCs, and whether their persistence can predict relapse. To overcome the limiting number of HSCs available in clinical bone marrow (BM) samples, we adapted a workflow developed for assessment of cell-free DNA and utilized a hybridization capture-based targeted sequencing panel covering 260 myeloid malignancy-associated genes. Unique dual index primers (UDIs) were utilized to tag input DNA, improving PCR duplicate identification, which overcome issues with amplification bias typically faced by low-input sequencing. This ultimately allowed us to consistently capture evaluable read depths from as few as 10 sorted cells. We assembled an initial cohort of 21 patients who underwent allo-SCT for MDS or secondary acute myeloid leukemia. Inclusion criteria were follow-up time greater than 1.5 years (or relapse in this timeframe), BM sampling prior to Day+120 post-transplant and relapse, and biospecimens allowing for sorting of greater than 10 HSCs. We sequenced sorted CD34+CD38-CD45RA- HSCs and multipotent progenitors (MPPs), CD34+CD38- CD45RA+ lymphoid-primed multipotent progenitors (LMPPs), and CD34+CD38+ progenitors, as well as bulk BM. With a median follow-up of two years, 9 of the 21 patients relapsed, with an average time to relapse of 362 days. Detection of disease mutations in HSCs/MPPs in any BM at D+30-D+120 post-transplant was 100% specific and 88% sensitive for relapse, showing improvements in both metrics over bulk BM assessment (22% sensitive and 66% specific), despite far lower cell input and read depth. Average time from detection to relapse was 6.9 months. In patients destined to relapse, disease mutations are often present in HSCs at a variant allele frequency of greater than 45%, while simultaneously undetectable on high-depth sequencing of bulk cells (>400x coverage). While even higher depth error-corrected sequencing bulk BM could likely detect such mutations, it would require >20x the sequencing depth, and could not identify the involved cell populations. In conclusion, we show for the first time that relapse of MDS after allo-SCT is driven by failure to eradicate MDS HSCs. Detection of MDS HSCs early post-transplant is highly predictive for relapse and opens new doors for pre-emptive interventions to prevent relapse. These findings are being validated in an independent cohort of 46 patients and updated data will be presented at the meeting. Citation Format: Benjamin Kroger, Yi Huang, Prapti Patel, Aimaz Afrough, Gurbakhash Kaur, Larry Anderson, Farrukh Awan, Praveen Ramakrishnan, Kunal Verma, Russell Vittrup, Robert H Collins, Madhuri Vusirikala, Yazan F Madanat, David Sallman, Carmelo Gurnari, Jaroslaw Maciejewski, Roni Tamari, Stephen Chung. Less is more: Ultra-low input sequencing allows for evaluation of stem cells as a predictor of post-transplant relapse in the myelodysplastic syndromes [abstract]. In: Proceedings of the AACR Special Conference: Acute Myeloid Leukemia and Myelodysplastic Syndrome; 2023 Jan 23-25; Austin, TX. Philadelphia (PA): AACR; Blood Cancer Discov 2023;4(3_Suppl):Abstract nr A23.