Germline immortality in C. elegans depends on epigenetic inheritance
Inheriting a trait from a grandparent doesn’t always involve their DNA sequences. In many organisms, some traits can be passed down for multiple generations via non-sequence based mechanisms, a phenomenon called transgenerational epigenetic inheritance. The most familiar example is that human disease risk might be influenced by the lifestyle of a person’s grandparents. But by far, the most unambiguous and robustly-studied cases occur in tiny laboratory workhorses like yeast and the nematode worm C. elegans. In the July issue of GENETICS, Spracklin et al. identify the genetic components required for RNA interference (RNAi) inheritance—a type of transgenerational epigenetic inheritance that occurs in C. elegans. They also show that this machinery is required for maintenance of the germline, suggesting an important natural role for this intriguing mode of inheritance.
Normally, small double-stranded RNAs (dsRNAs) are produced by the organism to regulate mRNA expression and transposable element activity. This regulatory machinery intercepts particular transcripts, marks them for degradation, and sometimes triggers epigenetic changes that suppress further expression. RNAi is a powerful genetic tool that allows researchers to easily manipulate gene expression levels by taking advantage of the worm’s natural regulatory systems.
In the lab, treatment with specially designed dsRNAs co-opts this regulation to dampen expression of a target gene. But the RNAi-mediated epigenetic alterations affect not only the worm initially treated with dsRNA, they persist in up to five generations of its offspring. That is, RNAi knockdown is heritable in worms. Some of the genetic components required for RNAi inheritance have been identified already, but much about this process is still unknown.
In this study, Spracklin et al. used a straightforward genetic screen to identify the RNAi inheritance machinery. They took worms expressing green fluorescent protein (GFP) in their germline cells and exposed them to mutagens to randomly break genes across the genome. Then they inhibited the expression of GFP with RNAi and let the worms reproduce. They chose offspring that didn’t inherit the RNAi and still expressed GFP for further study. After screening an astounding 6 million haploid germline genomes, they found six unique, newly identified genes needed for RNAi inheritance.
Not only do these mutant alleles disrupt RNAi inheritance, they doom their carrier’s descendants to extinction. In all eukaryotes, germline cells retain the ability to divide forever—they are immortal. Previous work found mutations that interfered with RNAi inheritance also resulted in germline mortality: in three to five generations the individual’s offspring become sterile. The new mutations identified by Spracklin et al. also cause germline mortality.
Critically, they also showed that germline mortality is likely caused by deregulation of an epigenetic process and not DNA changes from transposable elements gone awry, as previously hypothesized. Indeed, one of the genes they identified as part of the RNAi machinery is a histone tail methyltransferase responsible for making epigenetic marks. This work shows that epigenetic changes that persist across generations are a fundamental factor necessary for one of the most important processes in a living organism—preserving germ cells critical for reproduction.
The RNAi Inheritance Machinery of Caenorhabditis elegans
George Spracklin, Brandon Fields, Gang Wan, Diveena Becker, Ashley Wallig, Aditi Shukla and Scott Kennedy
GENETICS July 1, 2017 vol. 206 no. 3 1403-1416