No significant increase in spontaneous and ethyl methane sulfonate-induced sister chromatid exchanges at the Xq27.3 fragile site.

The present study was performed to determine if the fra(X)(q27.3) site is inherently a high sister chromatid exchange (SCE) site independent of fragility. Therefore, we studied baseline and ethyl methane sulfonate (EMS)-induced SCEs in peripheral blood lymphocytes from 10 retarded fragile-X male patients and eight retarded nonfragile-X male controls. The distributions of SCE scores per chromosome within each experimental condition showed significant interindividual variability in response to EMS treatment in the fragile-X group. Each fragile-X patient was therefore compared with a matched control. No significant differences were found in the distribution of SCE scores per chromosome. In addition, at the Xq27 site, whatever the degree of expressed fragility, no significant differences between fragile-X and control groups were evident in the spontaneous or EMS-induced SCEs. The fra(X)(q27.3) site therefore is not a hot spot for spontaneous or EMS-induced SCEs. Because fluorodeoxyuridine (FUdR) treatment has been shown to induce SCE at this site, our results indicate that the Xq27.3 site must be structurally different from other nonfragile SCE sites.