A mechanism of histone exchange involved in heterochromatin

Mobility of transposons, discovered by Barbara McClintock in the middle of the 20 th century, poses a threat to genome integrity. However, most transposons are silenced and enriched in specific regions of the genome called heterochromatin. If this silencing mechanism is lost, transposons can be mobilized, with deleterious consequences, as shown in tumor formation. Maintenance of heterochromatin is relatively well investigated, but how silencing by heterochromatin is initiated in the first place is poorly understood and the main question of the project. The lab of Dr. Frederic Berger, who will host the proposed work, discovered that heterochromatin predominantly contains a distinct variant of histone H2A, one of the four histone types that constitute the building blocks of chromatin. It is hypothesized that the dynamic deposition of the specific variant, called H2A.W, is responsible for establishing heterochromatin in plants. The aim of the project is to characterize the biochemical properties of the mechanism involved in H2A.W deposition. This involves an uncharacterized protein belonging to a large class of chromatin remodelers. Studying the biochemical action of remodelers is a burgeoning field of chromatin biology. The main applicant of this proposal, Dr. Akihisa Osakabe, will provide his special expertise in chromatin biochemistry required to tackle this problem at the Vienna Biocenter. Complementary strategies will be combined to understand the properties of the H2A.W remodeler and to define its function in the establishment of heterochromatin. As a similar type of remodeler is present in animals, the findings will have a wide impact on understanding heterochromatin establishment in a broad range of species including humans and mechanisms of chromatin-related diseases.

Gene expression in multicellular eukaryotes is regulated by histones, which are proteins that directly associate with DNA. Among the five families of histones, the H2A family contains multiple variants that confer different degrees of accessibility to the DNA. In flowering plants, there are four types of H2A variant and each type confers specific properties to the associated region of the genome. Furthermore, in plants, the variant H2A.W specifically occupies Transposable Elements (TEs). TEs are selfish DNA elements that constitute up to 80% of genomes and exploit the genome and replicative machinery of host cells in order to survive and proliferate. These elements are mobile and are able to excise, multiply and reintegrate the genome. Mobility of transposons (transposition) induces mutations and result in genomic instability, which has a positive impact at the evolutionary scale (creation of genomic variations) and a negative impact on health (cancer cells). In spite of the importance of transposition, very little is known about the mechanisms involved and their regulation. Our work demonstrates that in the model plant Arabidopsis, the chromatin remodeler ASW is necessary and sufficient to silence TEs. ASW binds selectively H2A.W and delivers H2A.W to 80% of TEs in the Arabidopsis genome. We established that H2A.W deposition by ASW prevent the deposition of epigenetic marks that report the access of the transcriptional machinery that decodes the genomes into RNAs. Hence, ASW provides a camouflage that prevents TEs from being recognized by the transcriptional machinery. As a result, this novel mechanism prevents expression of genes that are carried by TEs and enable their mobility. This discovery deeply changes our conception of transposon biology and broadens our views on how genomes regulate activity of invasive elements in the context of evolution, genome stability, and disease.