Mechanisms of telomere instability in the Drosophila female germline

Abstract

Telomeres are nucleoprotein complexes that protect the ends of eukaryotic linear chromosomes from degradation and fusion. Telomere dysfunction leads to developmental disorders, oncogenesis and aging. A telomere consists of DNA repeats and their bound proteins, as well as a telomeric RNA that is transcribed from telomeric repeats. Transcription of telomeric repeats is a conserved feature of telomeres in all studied species. Telomeric transcripts are maintained at a low level but their significant fraction is retained near telomeres. The telomeres of Drosophila are maintained in the absence of telomerase, by the transpositions of the specialized telomeric non-LTR retrotransposons; the HeT-A element being the most abundant. Here, we address a question on the factors of telomere instability in the Drosophila female germline. The level of HeT-A RNA in the germline is regulated by Piwi-interacting (pi) RNAs, the nuclear RNA surveillance system and transcription factors. Depletions of any of these components leading to the upregulation of HeT-A expression cause early embryonic lethality. It is suggested that accumulation of chromatin-bound telomeric RNA can destabilize telomeric DNA, leading to DNA lesions and recombination events. To find out more about a link between telomeric RNA abundance and telomere instability, we performed a study of the telomere integrity when the telomeric RNA biogenesis was impaired, which led to HeT-A overexpression. Moreover, abundant telomeric transcripts were revealed in a specific RNA fraction associated with chromatin. We detected the presence of H2Av, the main marker of DNA breaks, on telomeres after HeT-A derepressing, suggesting that HeT-A overexpression could cause DNA breaks in telomeres. Moreover, the presence of DNA breaks in telomeres was accompanied by the appearance of R-loops, the DNA-RNA hybrid structures associated with DNA damage. Chromatin immunoprecipitation was done to prove the accumulation of R-loops in telomeres. The formation of R-loops is most likely caused by retention of HeT-A transcripts in chromatin. Thus, telomere-associated RNA is an essential factor of telomere stability during normal oogenesis and early development. Funding: Program for Molecular and Cellular Biology of Presidium of Russian Academy of Sciences