Subclinical extramedullary leukemia in the skin is a source of relapsed leukemia

Summary: Abstract Body: Acute Myeloid Leukemia (AML) is the most common adult leukemia with a 5-year survival rate of 32% and a relapse rate of greater than 50%. Extramedullary leukemia, where leukemic cells seed organs outside the blood and bone marrow, occurs in ~10% of AML patients. Leukemia cutis, or clinically apparent leukemic cell skin infiltrates, is the most common extramedullary site. We observed the pervasive presence of AML-associated mutations in clinically normal patient skin. Event-free survival was 14.3 months versus 8.1 months (p=0.002) for those who had below median variant allele frequency (<2.5% VAF) of AML mutations in the skin (N=94) compared to those who had >2.5% VAF (N=96). A multivariate Cox proportional hazards analysis including the percentage of blasts in marrow and blood, and WBC count revealed that >2.5% AML VAF in the skin was an independent predictor of worse event-free survival (HR=1.49, p=0.051). Some patients retain AML variants in the skin despite clearing marrow after chemotherapy, suggesting that subclinical skin disease may be a source of relapse. Indeed, we observed sporadic leukemic cell infiltration of normal skin at the dermal subcutaneous junction in two murine AML models, one driven by FLT3-ITD and DNMT3A deficiency and another by MYC overexpression. We demonstrate that subclinical skin resident AML is sufficient to cause medullary relapse using a skin transplant model. Single-cell RNA sequencing of marrow, blood, lung, and skin tissue revealed heterogeneity in AML cells across the tissues, including distinct clusters in extramedullary tissues which exhibited downregulation of Il2ra, a T-cell activating gene, and upregulation of the cellular motility and invasion genes S100a4 and Ahnak. We demonstrate that subclinical leukemia is common, and high levels are associated with poor outcomes. In an experimental model, we demonstrate that normal skin can adoptively transfer AML, suggesting that skin-directed therapy and surveillance may be important in AML patients. Tucker Hansen¹, Clement Collignon², Francesca Ferraro¹, David Chen¹ 1. Medicine, Washington University in St Louis School of Medicine, St. Louis, MO, United States. 2. Hematologie, Centre Hospitalier Regional Universitaire de Nancy, Nancy, Grand Est, France. Translational Studies: Preclinical