title: Molecular diagnosis of synovial sarcoma and characterization of a variant SYT-SSX2 fusion transcript.
abstract: The translocation t(X;18)(p11;q11) is seen in > 80% of synovial sarcomas (SS) with informative karyotypes. The breakpoints of the t(X;18) have 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 result of the translocation. Recently, SSX has been shown to be duplicated, with both copies, SSX1 and SSX2, located within distinct subregions of Xp11. We performed a reverse transcriptase polymerase chain reaction (RT-PCR) assay for both chimeric SYT-SSX transcripts in a series of 35 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 cases that had poor quality RNA, the proportion of positives rose to 91% (29/32). The 29 positive cases were further studied using primers specific for either SSX1 or SSX2; 19 cases were positive for SYT-SSX1 and 10 for SYT-SSX2. The relationship of histological subtype (monophasic versus biphasic) to SSX1 or SSX2 involvement was not statistically significant. In a single histologically unremarkable monophasic SS, a slightly larger SYT-SSX2 RT-PCR product was observed. Sequencing of this novel variant showed a 129-bp segment inserted between the usual SYT and SSX2 fusion points, of which 126 bp were derived from a more proximal (5') portion of SSX2 The 3 bp immediately 5' to the fusion point could not be assigned to either SYT or SSX2 and may represent an insertion-deletion or a cryptic splicing event. This fragment maintains the reading frame of the chimeric product and encodes a predicted protein larger by 43 amino acids, which nevertheless replaces the region homologous to the transcriptional repression domain Kruppel-associated box, recently recognized in the 5' portion of the SSX genes, with all but the 3' end of the SYT transcript. Thus, a diagnosis of SS may be confirmed in > 90% of cases using RT-PCR detection of the chimeric transcript resulting from the t(X;18), and the incidence of molecular variants appears low.
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000595:Amino Acid Sequence; D001483:Base Sequence; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008969:Molecular Sequence Data; D009363:Neoplasm Proteins / Q000032:analysis*; D009372:Neoplasms, Connective Tissue / Q000737:chemistry* / Q000175:diagnosis; D016133:Polymerase Chain Reaction; D011506:Proteins / Q000032:analysis; D011518:Proto-Oncogene Proteins; D012334:RNA, Neoplasm / Q000032:analysis* / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000032:analysis*; D012097:Repressor Proteins / Q000032:analysis; D013584:Sarcoma, Synovial / Q000737:chemistry* / Q000175:diagnosis; D014158:Transcription, Genetic; D014178:Translocation, Genetic / Q000235:genetics
chemical_list: D009363:Neoplasm Proteins; D011506:Proteins; D011518:Proto-Oncogene Proteins; D012334:RNA, Neoplasm; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; C093961:SS18 protein, human; C093962:synovial sarcoma X breakpoint proteins