Telomere Length Determines TERRA and R-Loop Regulation through the Cell Cycle

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• Telomere length maintenance is vital for preventing cellular senescence.
• Homology-directed repair (HDR) repairs critically short telomeres in the absence of telomerase, preventing premature senescence.
• The specific targeting of shortest telomeres for HDR remains unclear.

Purpose of the Study:

• To investigate the mechanism by which shortest telomeres are preferentially targeted for homology-directed repair (HDR).
• To elucidate the role of the non-coding RNA TERRA and RNA-DNA hybrids (R-loops) in this process.
• To understand the regulation of TERRA degradation and its impact on telomere replication and senescence.

Main Methods:

• Analysis of TERRA accumulation at telomeres of varying lengths.
• Investigation of RNA degradation pathways involving Rat1 and RNase H2 nucleases.
• Assessment of DNA damage response (DDR) activation and Rad51 recombinase recruitment.

Main Results:

• TERRA accumulates as HDR-promoting RNA-DNA hybrids (R-loops) specifically at very short telomeres.
• A local defect in RNA degradation by Rat1 and RNase H2 nucleases causes increased TERRA and R-loops at shortened telomeres.
• Altered coordination of TERRA degradation with telomere replication at shortened telomeres leads to R-loop persistence.

Conclusions:

• Telomere length-dependent regulation of TERRA and its R-loops is critical for HDR targeting.
• Persistent R-loops at short telomeres activate the DNA damage response (DDR) and promote Rad51 recruitment.
• This mechanism critically determines the rate of replicative senescence.