Novel SMARCA2::DDIT3 Fusion in Primary Subcutaneous Myxoid Liposarcoma due to an Unusual Unbalanced Chromosomal Translocation.

Myxoid liposarcoma (MLS) accounts for 20%-30% of all liposarcomas, with most cases harboring the fusion gene FUS::DDIT3, while approximately 5% exhibit the EWSR1::DDIT3 fusion. We report the case of a 26-year-old male patient with a right upper arm mass. The tumor displayed the classic histological features of MLS, including small spindle/ovoid cells, variable univacuolated lipoblasts, and a prominent myxoid stroma with delicate arborizing vasculature. Despite these characteristic features, fluorescence in situ hybridization (FISH) revealed no apparent rearrangement of the DDIT3 locus. Next-generation sequencing (NGS) identified a novel fusion transcript in which SMARCA2 exon 4 was fused in-frame with DDIT3 exon 2. Chromosomal microarray analysis demonstrated the unbalanced nature of the rearrangement, with partial deletions of 0.243 and 0.176 Mb flanking the centromeric end of the DDIT3 locus on 12q13.3 (which also included GLI) and disrupting the SMARCA2 locus on 9p24, respectively. The resultant chimeric fusion protein is predicted to lack the SMARCA2 DNA-binding domains while retaining the DDIT3 leucine zipper dimerization domain. These findings indicate an unusual and complex rearrangement, leading to the recruitment of a novel DDIT3 partner gene. Moreover, they emphasize that the functional aspects of myxoid liposarcoma fusion genes depend on the retention of the key DDIT3 domain. Finally, this case illustrates how classic morphology can appropriately trigger reflex molecular analyses, which may, in turn, uncover novel fusion genes or other molecular alterations.