MOLECULAR DIAGNOSIS OF SYNOVIAL SARCOMA AND CHARACTERIZATION OF A VARIANT SYT-SSX2 FUSION TRANSCRIPT

The translocation t(X;18)(p11;q11) is seen in >80% of synovial sarcoma s (SS) with informative karyotypes. The breakpoints of the t(X;18) hav e been cloned and shown to involve two novel genes, SSX (at Xp11) and SYT (at 18q11), which produce a chimeric SYT-SSX transcript as a resul t of the translocation. Recently, SSX has been shown to be duplicated, with both copies, SSX1 and SSX2, located within distinct subregions o f Xp11. We performed a reverse transcriptase polymerase chain reaction (RT-PCR) assay for both chimeric SYT-SSX transcripts in a series of 3 5 SS (29 monophasic, 6 biphasic) to assess its usefulness in molecular diagnosis and to evaluate the incidence of molecular variants. Of the 35 cases, 29 (83%) showed a specific SYT-SSX RT-PCR product, using a consensus primer for SSX1 and SSX2. Upon excluding three negative case s that had poor quality RNA, the proportion of positives rose to 91% ( 29/32). The 29 positive cases were further studied using primers speci fic for either SSX1 or SSX2; 19 cases were positive for SYT-SSX1 and 1 0 for SYT-SSX2. The relationship of histological subtype (monophasic v ersus biphasic) to SSX1 or SSX2 involvement was not statistically sign ificant. In a single histologically unremarkable monophasic SS, a slig htly larger SYT-SSX2 RT-PCR product was observed. Sequencing of this n ovel variant showed a 129-bp segment inserted between the usual SYT an d SSX2 fusion points, of which 126 bp were derived from a more proxima l (5') portion of SSX2. The 3 bp immediately 5' to the fusion point co uld not be assigned to either SYT or SSX2 and may represent an inserti on-deletion or a cryptic splicing event. This fragment maintains the r eading frame of the chimeric product and encodes a predicted protein l arger by 43 amino acids, which nevertheless replaces the region homolo gous to the transcriptional repression domain Kruppel-associated box, recently recognized in the 5' portion of the SSX genes, with all but t he 3' end of the SYT transcript. Thus, a diagnosis of SS may be confir med in >90% of cases using RT-PCR detection of the chimeric transcript resulting from the t(X;18), and the incidence of molecular variants a ppears low.