Spontaneous and radiation-induced chromosomal breakage at interstitial telomeric sites.

The Chinese hamster genome contains a total of 18 cytologically detectable arrays of interstitial telomeric sequences. A combination of G-banding and two-colour fluorescence in situ hybridization revealed that 25 out of 27 (93%) breakpoints of spontaneously occurring terminal deletions in four immortalized Chinese hamster cell lines were located in chromosomal regions containing interstitial telomeric sequences. Each of the four immortalized Chinese hamster cell lines expressed telomerase. Radiation experiments revealed the sensitivity of interstitial telomeric sequences to radiation-induced chromosomal breakage in all telomerase-positive cell lines. However, radiation-induced chromosomal breakage at interstitial telomeric sites in non-transformed, primary Chinese hamster cells was almost non-existent. Telomerase activity in primary Chinese hamster cells was not detected. These results indirectly suggest that interstitial telomeric sites represent a favourable substrate for chromosomal healing.