COL1A1-PDGFB fusion in a ring chromosome 4 found in a dermatofibrosarcoma protuberans

Dermatofibrosarcoma protuberans (DP), an infiltrative skin tumor of intermediate malignancy, presents specific cytogenetic features such as reciprocal translocations t(17;22)(q22;q13.1) or, more often, supernumerary ring chromosomes derived from t(17;22). Different translocations, including t(2;17) and t(X;7), have also been described. We have shown previously that both r(17;22) and t(17;22) present the same molecular rearrangement fusing the COL1A1 gene on chromosome 17 and the PDGFB gene on chromosome 22. Out of our series of 16 DPs, we detected an extra ring chromosome in tumor T96-1175, which juxtaposed sequences from chromosomes 4 and 17. As shown by fluorescence in situ hybridization (FISH) using chromosome painting and alpha-satellite probes, T96-1175 apparently lacked chromosome 22 material in the ring. However, involvement of chromosome 22 through a rearrangement of PDGFB was shown by Southern blotting, reverse transcriptase-polymerase chain reaction (RT-PCR), and FISH. This study demonstrates that a cryptic molecular rearrangement between chromosomes 17 and 22 occurred in addition to the recombination of chromosomes 4 and 17 initially identified by FISH. Assessment for cryptic molecular events should be performed in other variant DP rearrangements.