Altered chromatin states drive cryptic transcription in aging mammalian stem cells

The authors show that aged mammalian stem cells produce aberrant transcripts due to profound yet characteristic changes to chromatin during aging, a phenomenon only previously known to occur in simple invertebrate models, limiting their lifespan. A repressive chromatin state featuring trimethylated lysine 36 on histone H3 (H3K36me3) and DNA methylation suppresses cryptic transcription in embryonic stem cells. Cryptic transcription is elevated with age in yeast and nematodes and reducing it extends yeast lifespan, though whether this occurs in mammals is unknown. We show that cryptic transcription is elevated in aged mammalian stem cells, including murine hematopoietic and neural stem cells and human mesenchymal stem cells. Precise mapping allowed quantification of age-associated cryptic transcription in human mesenchymal stem cells aged in vitro. Regions with significant age-associated cryptic transcription have a unique chromatin signature: decreased H3K36me3 and increased H3K4me1, H3K4me3 and H3K27ac with age. Genomic regions undergoing such changes resemble known promoter sequences and are bound by TATA-binding protein, even in young cells. Hence, the more permissive chromatin state at intragenic cryptic promoters likely underlies increased cryptic transcription in aged mammalian stem cells.