Insertion of a telomere repeat sequence into a mammalian gene causes chromosome instability

Telomere repeat sequences cap the ends of eucaryotic chromosomes and help s tabilize them. At interstitial sites, however, they may destabilize chromos omes, as suggested by cytogenetic studies in mammalian cells that correlate interstitial telomere sequence with sites of spontaneous and radiation-ind uced chromosome rearrangements. In no instance is the length, purity, or or ientation of the telomere repeats at these potentially destabilizing inters titial sites known, To determine the effects of a defined interstitial telo mere sequence on chromosome instability, as well as other aspects of DNA me tabolism, we deposited 800 bp of the functional vertebrate telomere repeat, TTAGGG, in two orientations in the second intron of the adenosine phosphori bosyltransferase (APRT) gene in Chinese hamster ovary cells, In one orienta tion, the deposited telomere sequence did not interfere with expression of the APRT gene, whereas in the other it reduced mRNA levels slightly, The te lomere sequence did not induce chromosome truncation and the seeding of a n ew telomere at a frequency above the limits of detection. Similarly, the te lomere sequence did not alter the rate or distribution of homologous recomb ination events. The interstitial telomere repeat sequence in both orientati ons, however, dramatically increased gene rearrangements some 30-fold. Anal ysis of individual rearrangements confirmed the involvement of the telomere sequence. These studies define the telomere repeat sequence as a destabili zing element in the interior of chromosomes in mammalian cells.