UC Berkeley

Alternative lengthening of telomeres (ALT) is a DNA recombination pathway which grants replicative immortality in approximately 10% of all cancers. Despite the high prevalence of ALT in cancer, the genetics and mechanism by which cells activate this pathway are largely not understood. A major challenge in dissecting ALT is the extremely low frequency of ALT induction in genetically tractable human cell systems. Guided by the genetic lesions that have been associated with ALT from cancer sequencing studies, we genetically engineered primary human cells to deterministically induce ALT. Using this system, we demonstrate that disruption of the p53 and Rb pathways in combination with ATRX loss-of-function is, in a cell type-specific manner, sufficient to induce all hallmarks of ALT and results in functional immortalization. We demonstrate that ALT can be induced in the presence of telomerase, is neither dependent on telomere shortening nor crisis, but is rather the consequence of epigenetic changes induced by differentiation of ATRX-deficient cells.