Telomeres reforged with non-telomeric sequences in mouse embryonic stem cells (WGS)

Telomere maintenance is indispensable for perpetuated cell division, but telomeres are not necessarily composed of a fixed sequence. Here we report the establishment of a model for alternative lengthening of telomeres (ALT) in mouse embryonic stem cells (mESCs) in which telomeres are reconstructed with an internal ALT template. Longitudinal whole-genome analyses of ALT mESCs showed that pre-amplification of the template into a telomeric region was a prerequisite for ALT activation, and that extensive copy number variations became concentrated in subtelomeric regions. Epigenomic analysis revealed the heterochromatic structure of the ALT telomeres, except for an insulator region within the ALT template. Quantitative proteomics followed by single-cell RNA-sequencing and functional assays revealed that HMGN1 protected new telomeres by regulating telomere repeat-containing RNA transcription and R loop formation in ALT mESCs. These findings implicate an evolutionarily conserved ALT mechanism driven by an internal template and provide a molecular basis underlying telomere evolution. Overall design: Genome profiling of Terc knock-out mouse Embryonic Stem Cell (mESC) by passage doubling