Break-induced replication and recombinational telomere elongation in yeast

Telomere dysfunction remains a poorly understood contributor to genomic instability. While telomerase is known to maintain telomere length, alternative repair mechanisms become active in its absence. Prior research has shown that telomere shortening can lead to chromosomal abnormalities. However, the specific pathways that resolve these abnormalities remain unclear. No prior work had resolved how cells lacking telomerase manage to elongate telomeres. This gap motivated investigations into recombination-based DNA repair processes. The role of Rad51 recombinase in telomere repair is established, but its necessity in all contexts is uncertain. Disruptions in telomere-binding proteins may trigger alternative repair mechanisms. The mechanisms by which telomeres elongate through recombination remain underexplored.

This study aimed to clarify the mechanisms by which yeast cells lacking telomerase repair chromosomal damage. The specific problem addressed is the activation of break-induced replication (BIR) when telomeres become dysfunctional. The motivation stems from observations that telomere shortening leads to genomic instability. Researchers sought to determine how BIR contributes to telomere elongation. They focused on two distinct BIR pathways in budding yeasts. The study examined whether these pathways produce different repair outcomes. The goal was to identify the template sources for BIR events. The researchers also wanted to understand how elongated sequences spread to other telomeres.

The investigation utilized budding yeast strains deficient in telomerase activity. Researchers monitored telomere length changes using Southern blot analysis. They compared outcomes between strains with and without Rad51 recombinase. Fluorescence in situ hybridization (FISH) was used to track telomere elongation patterns. The study examined chromosomal stability through karyotyping. Researchers analyzed the role of telomere-binding proteins in triggering BIR events. They tested whether recombination at dysfunctional telomeres produces circular DNA. The experiments focused on the 'roll and spread' mechanism of telomere elongation.

The study found that Rad51-dependent and Rad51-independent BIR pathways produce distinct survivor cell types. Telomere elongation occurred through a rolling circle mechanism using a telomeric circle as a template. Southern blot analysis showed elongated telomeres in telomerase-deficient strains. FISH confirmed that elongated sequences spread to all telomeres. The rolling circle mechanism was observed in 70% of BIR events. Telomere-binding protein disruptions consistently triggered recombination events. The study identified that elongated sequences originated from a single dysfunctional telomere. The process of spreading elongated sequences required additional BIR events.

The authors propose that BIR serves as an alternative telomere elongation mechanism in telomerase-deficient yeast. They suggest that Rad51-dependent and Rad51-independent pathways produce different survivor cell types. The study supports the 'roll and spread' model for telomere elongation. The findings indicate that telomeric circles act as templates for rolling circle BIR events. The researchers suggest that telomere elongation spreads to all telomeres through subsequent BIR events. The study confirms that telomere-binding protein disruptions trigger recombination. The authors propose that the rolling circle mechanism is a common pathway for telomere elongation. They suggest that this mechanism may explain chromosomal stability in telomerase-deficient cells.