A cryptic EWSR1::DDIT3 fusion in myxoid liposarcoma: Potential pitfalls with FISH and cytogenetics.

Myxoid liposarcoma (MLS) is molecularly characterized by fusions involving the DDIT3 gene in chromosome band 12q13; the fusion partner is FUS in band 16p11 in 90-95% of the cases and EWSR1 in band 22q12 in the remaining 5-10%. Hence, molecular studies, often fluorescence in situ hybridization (FISH) for DDIT3 rearrangement, are useful for establishing a correct diagnosis. Although all MLS tumors should have DDIT3 fusions, it is important to be aware of reasons for potential false-negative results. We here present a case of MLS that was negative for FISH for DDIT3, that showed an unexpected t(11;22) at G-banding, but that displayed a characteristic EWSR1::DDIT3 fusion at RNA-sequencing. The results suggest that neoplasia-associated fusions that, due to the transcriptional orientations of the two genes involved, cannot arise through only two double-strand breaks are more likely to be associated with negative FISH-findings and unexpected karyotypes.