The utility of fluorescence in situ hybridization (FISH) in the diagnosis of myxoid soft tissue neoplasms.

Diagnosing myxoid soft tissue neoplasms can be challenging because of overlapping histologic features. Distinct chromosomal translocations have been identified in several myxoid sarcomas, including t(12;16)(q13;p11) FUS-DDIT3 in myxoid liposarcoma, t(7;16)(q34;p11) FUS-CREB3L2 in low-grade fibromyxoid sarcoma, and t(9;22)(q31;q12) EWSR1-NR4A3 in extraskeletal myxoid chondrosarcoma. These recurrent chromosomal alterations are attractive targets for diagnostic studies. To that end, dual-color, break-apart fluorescence in situ hybridization (FISH) probes spanning the genomic regions of EWSR1 (22q12), DDIT3 (12q13), and FUS (16p11) (Vysis, Downer's Grove, IL) were evaluated in formalin-fixed, paraffin-embedded tissues from myxoid neoplasms, including intramuscular myxoma (n=10), myxoid liposarcoma (n=18), low-grade fibromyxoid sarcoma (n=10), extraskeletal myxoid chondrosarcoma (n=13), and myxofibrosarcoma (n=8). Of the myxoid liposarcomas, 18/18 cases had a rearrangement of the DDIT3 gene, with 17/18 (94.4%) showing both DDIT3 and FUS gene rearrangements. A FUS gene rearrangement was identified in 7/10 (70%) of low-grade fibromyxoid sarcomas, with no changes involving EWSR1 or DDIT3. An EWSR1 translocation was seen in 6/13 (46.2%) of extraskeletal myxoid chondrosarcomas, without changes in DDIT3 or FUS genes. The remaining neoplasms studied showed no rearrangements involving DDIT3, FUS, or EWSR1 genes. In conclusion, interphase FISH using DDIT3 and FUS probes identifies the characteristic translocation in myxoid liposarcoma. FUS and EWSR1 probes are useful in confirming the diagnosis of low-grade fibromyxoid sarcoma and extraskeletal myxoid chondrosarcoma, respectively. The specificity of the probes is documented as none of the non-translocation-associated myxoid tumors showed genomic abnormalities with the probes tested. FISH is capable of providing specific ancillary information useful in this often difficult differential diagnosis.