Telomere maintenance in telomerase-deficient mouse embryonic stem cells: Characterization of an amplified telomeric DNA

Telomere dynamics, chromosomal instability, and cellular viability were stu died in serial passages of mouse embryonic stem (ES) cells in which the tel omerase RNA (mTER) gene was deleted. These cells lack detectable telomerase activity, and their growth rate was reduced after more than 300 divisions and almost zero after 450 cell divisions. After this growth crisis, survivo r cells with a rapid growth rate did emerge. Such survivors were found to m aintain functional telomeres in a telomerase-independent fashion. Although telomerase-independent telomere maintenance has been reported for some immo rtalized mammalian cells, its molecular mechanism has not been elucidated. Characterization of the telomeric structures in one of the survivor mTER(-/ -) cell lines showed amplification of the same tandem arrays of telomeric a nd nontelomeric sequences at most of the chromosome ends. This evidence imp licates cis/trans amplification as one mechanism for the telomerase-indepen dent maintenance of telomeres in mammalian cells.