THE T(X18)(P11.2Q11.2) TRANSLOCATION FOUND IN HUMAN SYNOVIAL SARCOMASINVOLVES 2 DISTINCT LOCI ON THE X-CHROMOSOME

A high proportion of synovial sarcomas contain the reciprocal transloc ation t(X;18)(p11.2;q11.2). We have previously localized the breakpoin t on the X chromosome between the X chromosome marker DXS255 and an or nithine aminotransferase (OAT) pseudogene region designated OATL2. Sub sequently by fluorescence in situ hybridization (FISH) we provided evi dence that YACs corresponding to the OATL2 locus spanned the breakpoin t. In order to confirm the position of this breakpoint cosmids corresp onding to the OATL2 region were isolated. Most of these cosmids mapped to four cosmid contigs designated C1-C4. Analysis of two contigs, C1- and C4, using FISH established that in four of six synovial sarcomas e xamined the breakpoint occurs between these two contigs: C1 lies dista l to the breakpoint while C4 is proximal. In contrast we provide evide nce that the breakpoint in the remaining two tumours mapped to a secon d pseudogene region called OATL1 that is telomeric to the OATL2 locus. This heterogeneity of the breakpoint position on the X chromosome exp lains why in previous mapping studies there have been discrepancies be tween the results obtained by different laboratories.