Highly aggressive undifferentiated small round blue cell tumor of foot with unique SMARCA1, KAT6A and NAV3 mutations

Malignancies characterized histologically by high-grade monotonous small round blue cells (SRBCs) belong to a heterogeneous group of neoplasms often referred to as Ewing family of tumors. The most common molecular confirmation of these neoplasms is by fusions between EWSR1 gene on chromosome 22 and the ETS family of transcription factors, including FLI1 gene (11q24) and the ERG (21q22), that are implicated in the development of different tissues as well as cancer progression. In this article, we present a case of highly aggressive extraskeletal SRBC tumor involving the foot of a 24-year-old male with sole molecular findings of mutations in KAT6A, NAV3 and SMARCA1 genes with high expression of soft tissue markers (COL1A1, COL1A2, COL3A1) and MYC mRNA. To our knowledge, this unique mutational pattern has not previously been described in SRBCs.