PLAG1-Rearranged Fibromyxoid and Lipomatous Neoplasms in Children and Adults: Separate Entities or a Morphological...

Fusions involving the PLAG1 gene are associated with multiple cancers and benign tumors, including lipoblastoma and the more recently described pediatric fibromyxoid soft tissue tumor. We report two PLAG1-rearranged mesenchymal tumors arising in adults which, although largely similar histologically to the fibromyxoid tumors reported in infants, display limited adipocytic differentiation. In both cases, the novel fusion partner was DLEU2. Whole genome sequencing of one of the tumors also showed loss of 13p including the RB1 locus. Expression of PLAG1 was demonstrated by extensive immunoreactivity in both cases. We discuss the similarities of our cases to the previously described fibroblastic variants of lipoblastomas and the recently reported cases of PLAG1-rearranged fibromyxoid soft tissue tumors, highlighting the overlapping morphological and molecular features. We consider that there is growing evidence that these histological entities are related to conventional lipoblastoma and represent tumors of adipocytic lineage exhibiting different stages of cellular maturation.