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Sarcomas are rare cancers thought to arise from aberrant mesenchymal stem cell (MSC) differentiation. Liposarcoma (LPS) is among the most commonly diagnosed sarcomas and provides insights into dysfunctional differentiation through its well- and dedifferentiated subtypes (WDLPS and DDLPS). Despite differences in histology and clinical behavior, the molecular pathways underlying each subtype remain poorly defined, leaving patients with DDLPS reliant on empiric chemotherapies. We applied single-nucleus multiome sequencing and spatial profiling to human normal adipose, WDLPS, and DDLPS tissues and identified lineage-specific blocks in differentiation within LPS. We found that DDLPS is characterized by loss of insulin-like growth factor 1 (IGF1) signaling and activation of early mesenchymal and glucagon-like peptide-1 (GLP-1)-associated programs. IGF1 signaling loss was restricted to the DDLPS component within mixed histology tumors and correlated with poor survival in patients with LPS. In normal adipocytes, IGF1 drives differentiation through peroxisome proliferator-activated receptor gamma 2 (PPARG2). We found that DDLPS cells lack PPARG2, causing a barrier to differentiation. This defect rendered DDLPS cells unresponsive to exogenous proadipogenic signals. Restoration of PPARG2 expression alone was sufficient to reenable adipogenesis, pinpointing PPARG2 as the key molecular determinant of lineage fate. Last, IGF1 deficiency in DDLPS was associated with up-regulation of the IGF1 receptor (IGF1R), creating a selective vulnerability to IGF1R-targeted antibody-drug conjugates. In summary, we identified lineage-specific defects in DDLPS, with PPARG2 as the molecular mediator of differentiation state in LPS. More broadly, our findings demonstrate how defining lineage-specific mechanisms of tumor state can inform the development of nonchemotherapeutic treatment approaches.