KMT2A-rearranged sarcoma with unusual fusion gene CBX6::KMT2A::PYGO1.

Recently, rare sarcomas harboring KMT2A rearrangements have been reported. They occur in relatively young individuals, exhibit a sclerosing epithelioid fibrosarcoma-like morphology, and often have an aggressive prognosis. YAP1::KMT2A::YAP1 is the most common fusion gene, followed by VIM::KMT2A. We report the case of a 47-year-old man with a spindle cell tumor arising from the subcutaneous tissue of the right anterior chest. The tumor harbored an unusual novel fusion gene, CBX6::KMT2A::PYGO1. Histologically, the tumor consisted of proliferating spindle-shaped cells with uniform nuclei, which varied in cell density and the amount of intervening collagen fibers. After 2 years and 8 months without postoperative treatment, the patient showed no recurrence or metastasis. Although highly likely irreproducible, tumors with the CBX6::KMT2A::PYGO1 fusion gene were morphologically somewhat different from those containing the YAP1::KMT2A::YAP1. This suggests that KMT2A rearrangements with fusion gene partners different from YAP1 result in purely spindle-shaped cell tumors that produce collagen fibers.