Epigenetic Inheritance Mediated By Coupling Of RNAi And Histone H3K9 Methylation

RNAs can trigger a multigenerational epigenetic memory in the Germline of C. elegansIn this paper, the main question is the mechanism of the inheritance of epigenetic information of C. elegans for multiple generations through RNAi and chromatin modifications by piRNA. C. elegans was used as model organism in the discovery of small RNA pathways in regulating genes. It is known that the small RNA binds to proteins for regulating genomes in eukaryotes. Previous studies had look into piRNA, a small sequence of RNA which silences the activity of endogenous materials and transposons in germlines.

Evidence also suggested that small RNAs could be inherited through multiple generations. However, the mechanisms of the inheritance involving small RNAi and chromatin modifications has not been clear. To look into how piRNA triggers multigenerational gene silencing, the researchers crossed between C. elegans, one homozygous for piRNA sensor, where the GFP was silenced, with homozygous for piRNA with mutated prg, which represses endogenous piRNA function. It was shown that the offspring for several generations shown silenced GFP and the piRNA sensor has reactivated the mutants. This implies piRNA is needed for triggering transgenerational silencing as the piRNA had all been eliminated by the prg mutant. In order to see the relationship between multigeneration gene silencing and piRNA silencing, they would like to look at the nuclear factors associated with these two pathways.

Through forward genetic screening, the genes that are related to small RNA or chromatin modification pathways were identified. Immunostaining was then done, factors required in nuclear small RNA and chromatin modification produced were shown during both multigeneration gene silencing and piRNA silencing pathways. The researchers suggested that both piRNA and multigeneration gene silencing were associated with the nuclear RNAi and chromatin modification pathways for germlines. These pathways would silence the genes and could be inherited through multiple generations. These pathways do not require dsRNAi. Further studies could investigate on how multicopy transgene silencing works through this pathway and whether it could be triggered by endogenous piRNA.

piRNAs can trigger a multigenerational epigenetic memory in the germline of C. elegans. Epigenetic inheritance mediated by coupling RNAi and histone H3K9 methylationThe main question of the paper is how coupling of RNAi and histone methylation maintains epigenetic materials (intergenerationally) of fission yeast. Histone modifications were known to regulate gene expression. Epigenetic inheritance could be observed but it was suggested that a mediator is needed to have strong positive feedback. It is known that RNAi induced H3K9could lead to epigenetic inheritance through positive feedback. However, it is not known if this inheritance was through regulation on the DNA sequence or whole chromatin structure. To look into how the silencing is inherited, centromeric transgenes were inserted to the yeast attached to adenine. The progeny with adenine silenced and not silenced were crossed and PCR has been done on these specific alleles. It shows that the inheritance is through cis, meaning the inheritance is regulates specific DNA sequence.

In order to see how the coupling of siRNA and H3K9me silences genome, Chip sequence was used to see the level of H3K9 methylation and sequencing of sRNA has been also done on the same clones including when centromeric transgene was or was not connected to adenine and if the adenine has been silenced or not. It is found out that there is a correlation between the methylation level of the targeted gene as well as the genes beside it. Also, when the amount of methylation increases, there is also an accumulation in secondary siRNA produced from the adjacent genes. This shows these pathways are dependent of each other.

The researchers suggested that RNAi and H3K9 methylation could assist the inheritance of epigenetic cell of the cell through binding at specific sites of the DNA. The coupling could also protect the epigenetic state of the genome through demethylation of H3K9 or pairing with homologous chromosomes during erasure pathways.