SS18-SSX2 fusion transcript in the diagnosis of a poorly differentiated synovial sarcoma

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Fig. 3. Gel electrophoresis following reverse transcriptase polymerase chain reaction showing the absence of the 151 bp band in the patient lane designated P1 in the left one-third of the image, and the presence of a 109 bp band in the lane designated P1 in the mid-third of the image, in the presence of appropriate positive and negative controls (lanes designated “pos” and “neg” respectively), with amplification of the reference PGK1 gene (right one-third of the image), confirming the presence of the SS18-SSX2 fusion transcript in the analyzed tissue.

To identify gene fusions, FISH is performed using locus-specific probes, which are gene-specific complementary sequences of DNA that hybridize with the specific gene targets in the analyzed tissue.^8,13 The translocation in SS involves the SS18 gene on chromosome 18 and one of several genes (usually SSX1 or SSX2 and, much less commonly, SSX4) on the X chromosome and results in formation of the SS18-SSX oncogenes.^1,2,4 RT-PCR is a rapid, highly specific, and sensitive technique requiring extraction of tumor RNA, preferably from snap-frozen tissue to yield better RNA integrity, followed by reverse transcription to DNA, and finally PCR for DNA amplification utilizing primers flanking the chimeric gene to be detected.^6,7 In fixed tissues, RNA integrity depends on the fixative and the time interval between the surgery and the tissue fixation, thus requiring an assay control (housekeeping gene to be amplified). In our case, primers specific for SS18 and SSX were used, and the amplified product was analyzed via electrophoresis and compared with appropriate positive and negative controls (Fig. 3).⁷ Table 1 shows a comparison of RT-PCR and FISH assays in detecting the SS18-SSX fusion transcripts in SS.

Approximately two-thirds of SS cases have the SS18-SSX1 fusion, and one-third carry the SS18-SSX2 fusion. Most biphasic SS have the SS18-SSX1, and monophasic tumors may have either fusion. Earlier studies showed significantly improved prognosis with the SS18-SSX2 fusion; however, more recent studies have shown that the SS18-SSX fusion type is not a significant factor in prognosis.^4,14

To summarize, the use of RT-PCR and FISH assays for the detection of the SS18-SSX fusion is the gold standard for the diagnosis of SS. These assays can be used with limited available tissue, as with fine needle core biopsies, and can be applied to formalin-fixed, paraffin-embedded tissues, in contrast to cytogenetic analysis that requires fresh, viable tissue.

The authors would like to thank André M. Oliveira, MD, PhD, of Mayo Clinic, Rochester, Minn., for kindly providing the image for the RT-PCR assay and the FISH and RT-PCR procedural references.

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Dr. Washburn is a PGY 2 resident in anatomic and clinical pathology, Dr. Frauenhoffer is a professor of pathology and orthopedics and unit director of bone and soft tissue pathology, and Dr. Kansal is an associate professor of pathology and medical director of the diagnostic molecular laboratory—all in the Department of Pathology and Laboratory Medicine at Penn State Milton S. Hershey Medical Center, Penn State College of Medicine, Hershey, Pa.