Oncogenic <i>NRAS</i> Mutation in Incipient Sarcomatous Transformation of Cystic Nephroma From a Patient With <i>DICER1</i> ‐Related Tumor Predisposition Syndrome

Anaplastic sarcoma of the kidney (ASK) is a DICER1-associated malignant tumor presumed to arise in a benign precursor, pediatric cystic nephroma (PCN). However, the initial oncogenic alteration(s) associated with malignant transformation are unknown. Here, we report genomic characterization of a PCN harboring focal anaplasia consistent with early sarcomatous transformation. Multi-region sequencing of PCN and anaplastic areas revealed an NRAS Q61R mutation exclusively in the latter. Germline and somatic DICER1 alterations were common to both. This is the first report of an oncogenic alteration in incipient sarcomatous transformation in a PCN and supports the histogenetic association between PCN and ASK.