H3K9me2 protects lifespan against the transgenerational burden of germline transcription in C. elegans

Chromatin state influences lifespan and may allow for the epigenetic inheritance of this complex trait. At sites of active transcription, the COMPASS complex methylates histone H3 at lysine 4 (H3K4me). In Caenorhabditis elegans, reductions in COMPASS extend lifespan, and wild-type descendants of COMPASS mutants inherit longevity for four generations. Here we show that the longevity of COMPASS mutants is itself a transgenerational trait caused by gradual changes in the repressive chromatin factor H3K9me2. H3K9me2 is required for longevity in COMPASS mutants and can confer longevity when increased in other chromatin modifier mutants. H3K9me2 levels also correlate with a transgenerational decline in wild-type lifespan after freezing or starvation. We propose that germline transcription-coupled H3K4me encroaches on H3K9me2 to limit lifespan. Loss of COMPASS complex alleviates the burden of H3K4me and therefore extends lifespan. This study suggests a causal role for a single heterochromatin factor in the establishment and inheritance of longevity.