Analysis of CHOP rearrangement in pleomorphic liposarcomas using fluorescence in situ hybridization

Pleomorphic liposarcoma (PLS) is an aggressive subtype of liposarcoma composed of high-grade sarcoma with pleomorphic lipoblasts. PLS usually exhibits a heterogeneous histology and sometimes has a myxoid or round cell area similar to myxoid/round cell liposarcomas (MLS/RCs). Using fluorescence in situ hybridization (FISH) analysis, we investigated the existence of CHOP split signals in various histological areas of PLS including the MLS/RC-like feature and also estimated the distribution of various signals with polyploidy and amplification. Moreover, to detect CHOP fusion transcripts we performed nested reverse transcription-polymerase chain reaction (RT-PCR). Seven PLSs and three MLS/RCs were selected for FISH analysis using the locus-specific indicator CHOP (12q13) dual color, break apart probe (Vysis, USA). The FISH analysis was applied to formalin-fixed, paraffin-embedded tissue sections of representative areas in all cases. Six of seven PLS cases showed the CHOP split signal ranging from 0.5% to 3% of counted nuclei, while all cases of MLS/RC exhibited CHOP rearrangement in more than 50% of counted nuclei. All cases of PLS showed a varied distribution of extra signals with polyploidy and amplification in each histological area. No CHOP fusion transcript was found in any case of PLS by nested RT-PCR. A CHOP rearrangement in PLS should be recognized only as a representative part of complex karyotypes, because the number of cells with split signals was minute compared with that of MLS/RC, and the signals were found in any area despite their histological differences. The cytogenetic background of PLS and that of MLS/RC are obviously different despite histological similarity.