The Lingner lab could shed further light on the functions of long noncoding RNA Telomeric repeat containing RNAs (TERRA). At telomeres, TERRA basepairs with DNA to form R-Loops, whose formation could now be shown depend on the DNA recombinase RAD51 and its enhancer RAD51AP. The loops hinder semiconservative DNA replication and trigger two repair pathways break-induced replication (BIR) and PRIMPOL-dependent repair. The elimination of both repair systems is lethal for U2OS ALT cancer cells, which can elongate telomeres without telomerase through Alternative Lengthening of Telomeres (ALT). The induction of telomeric R-loops is enough to trigger known ALT telomere repair without other ALT associated chromatin changes in telomeric regions.
Synopsis
ALT cancer cells maintain telomeres via homology-directed repair instead of telomerase. This article demonstrates that experimental induction of TERRA R-loops at telomeres in non-ALT cells induces ALT cancer-cell-specific telomere repair mechanisms.
- TERRA R-loop formation at telomeres depends on both, RAD51 and RAD51AP1.
- TERRA R-loop-induced replication stress is repaired through break-induced replication (BIR) and PRIMPOL-dependent repriming.
- PRIMPOL depletion is synthetic-lethal with BIR deficiency in ALT cancer cells.
Abstract
TERRA long noncoding RNAs associate with telomeres post transcription through base-pairing with telomeric DNA forming R-loop structures. TERRA regulates telomere maintenance but its exact modes of action remain unknown. Here, we induce TERRA transcription and R-loop formation in telomerase-expressing cells and determine that TERRA R-loop formation requires non-redundant functions of the RAD51 DNA recombinase and its enhancer RAD51AP1. TERRA R-loops interfere with semiconservative DNA replication, promoting telomere maintenance by a homology-directed repair (HDR) mechanism known as break-induced replication (BIR), which ensures telomere maintenance in ALT cancer cells. In addition, TERRA induces PRIMPOL-dependent repair, which can initiate DNA synthesis de novo downstream of replication obstacles. PRIMPOL acts in parallel to BIR for telomere maintenance of TERRA-overexpressing cells, promoting their survival. Similarly, we find that PRIMPOL depletion is synthetic-lethal with BIR deficiency in U2OS ALT cancer cells. Therefore, TERRA R-loops by themselves are sufficient to induce ALT-typical telomere repair mechanisms, in the absence of other ALT-typical telomeric chromatin changes.
Read the Publication in the EMBO Journal (Open Access)
Abstract, figure, highlights, synopsis and title from In et al (2025) EMBO J published under a CC BY 4.0 license.