Methods to Study Repeat Fragility and Instability in Saccharomyces cerevisiae

The RNA export and RNA decay complexes THO and TRAMP prevent transcription-replication conflicts, DNA breaks, and CAG repeat contractions

Expansion of structure-forming CAG/CTG repetitive sequences is the cause of several neurodegenerative disorders and deletion of repeats is a potential therapeutic strategy. Transcription-associated mechanisms are known to cause CAG repeat instability. In this study, we discovered that Thp2, an RNA export factor and member of the THO (suppressors of transcriptional defects of hpr1Δ by overexpression) complex, and Trf4, a key component of the TRAMP (Trf4/5-Air1/2-Mtr4 polyadenylation) complex involved in nuclear RNA polyadenylation and degradation, are necessary to prevent CAG fragility and repeat contractions in a Saccharomyces cerevisiae model system. Depletion of both Thp2 and Trf4 proteins causes a highly synergistic increase in CAG repeat fragility, indicating a complementary role of the THO and TRAMP complexes in preventing genome instability. Loss of either Thp2 or Trf4 causes an increase in RNA polymerase stalling at the CAG repeats and other genomic loci, as well as genome-wide transcription-replication conflicts (TRCs), implicating TRCs as a cause of CAG fragility and instability in their absence. Analysis of the effect of RNase H1 overexpression on CAG fragility, RNAPII stalling, and TRCs suggests that RNAPII stalling with associated R-loops are the main cause of CAG fragility in the thp2Δ mutants. In contrast, CAG fragility and TRCs in the trf4Δ mutant can be compensated for by RPA overexpression, suggesting that excess unprocessed RNA in TRAMP4 mutants leads to reduced RPA availability and high levels of TRCs. Our results show the importance of RNA surveillance pathways in preventing RNAPII stalling, TRCs, and DNA breaks, and show that RNA export and RNA decay factors work collaboratively to maintain genome stability.

Relocation of Collapsed Forks to the Nuclear Pore Complex Depends on Sumoylation of DNA Repair Proteins and Permits Rad51 Association

Expanded CAG repeats form stem-loop secondary structures that lead to fork stalling and collapse. Previous work has shown that these collapsed forks relocalize to nuclear pore complexes (NPCs) in late S phase in a manner dependent on replication, the nucleoporin Nup84, and the Slx5 protein, which prevents repeat fragility and instability. Here, we show that binding of the Smc5/6 complex to the collapsed fork triggers Mms21-dependent sumoylation of fork-associated DNA repair proteins, and that RPA, Rad52, and Rad59 are the key sumoylation targets that mediate relocation. The SUMO interacting motifs of Slx5 target collapsed forks to the NPC. Notably, Rad51 foci only co-localize with the repeat after it is anchored to the nuclear periphery and Rad51 exclusion from the early collapsed fork is dependent on RPA sumoylation. This pathway may provide a mechanism to constrain recombination at stalled or collapsed forks until it is required for fork restart.

Sequence and Nuclease Requirements for Breakage and Healing of a Structure-Forming (AT)n Sequence within Fragile Site FRA16D

Common fragile sites (CFSs) are genomic regions that display gaps and breaks in human metaphase chromosomes under replication stress and are often deleted in cancer cells. We studied an ∼300-bp subregion (Flex1) of human CFS FRA16D in yeast and found that it recapitulates characteristics of CFS fragility in human cells. Flex1 fragility is dependent on the ability of a variable-length AT repeat to form a cruciform structure that stalls replication. Fragility at Flex1 is initiated by structure-specific endonuclease Mus81-Mms4 acting together with the Slx1-4/Rad1-10 complex, whereas Yen1 protects Flex1 against breakage. Sae2 is required for healing of Flex1 after breakage. Our study shows that breakage within a CFS can be initiated by nuclease cleavage at forks stalled at DNA structures. Furthermore, our results suggest that CFSs are not just prone to breakage but also are impaired in their ability to heal, and this deleterious combination accounts for their fragility.

The nuclear pore complex prevents sister chromatid recombination during replicative senescence

The Nuclear Pore Complex (NPC) has emerged as an important hub for processing various types of DNA damage. Here, we uncover that fusing a DNA binding domain to the NPC basket protein Nup1 reduces telomere relocalization to nuclear pores early after telomerase inactivation. This Nup1 modification also impairs the relocalization to the NPC of expanded CAG/CTG triplet repeats. Strikingly, telomerase negative cells bypass senescence when expressing this Nup1 modification by maintaining a minimal telomere length compatible with proliferation through rampant unequal exchanges between sister chromatids. We further report that a Nup1 mutant lacking 36 C-terminal residues recapitulates the phenotypes of the Nup1-LexA fusion indicating a direct role of Nup1 in the relocation of stalled forks to NPCs and restriction of error-prone recombination between repeated sequences. Our results reveal a new mode of telomere maintenance that could shed light on how 20% of cancer cells are maintained without telomerase or ALT.

The Nuclear Pore Complex has been linked to DNA damage processing. Here the authors reveal that the Nup1 C-terminus is critical for the relocalization of eroded telomeres to nuclear pores and that modification of Nup1 promotes sister chromatid recombination and unleashes a new telomere maintenance mechanism.

Genetic Assays to Study Repeat Fragility in Saccharomyces cerevisiae

Trinucleotide repeats are common in the human genome and can undergo changes in repeat number and cause length-dependent chromosome fragility. Expanded CAG repeats have been linked to over 14 human diseases and are considered hotspots for breakage and genomic rearrangement. Here we describe two Saccharomyces cerevisiae based assays that evaluate the rate of chromosome breakage that occurs within a repeat tract (fragility), with variations that allow the role of transcription to be evaluated. The first fragility assay utilizes end-loss and subsequent telomere addition as the main mode of repair of a yeast artificial chromosome (YAC). The second fragility assay relies on the fact that a chromosomal break stimulates recombination-mediated repair. A PCR-based assay can be used to evaluate instability of the repeat in the same conditions used to measure repeat fragility. These assays have contributed to understanding the genetic mechanisms that cause chromosome breaks and tract-length changes at unstable trinucleotide repeats.

Distinct roles for S. cerevisiae H2A copies in recombination and repeat stability, with a role for H2A.1 threonine 126

CAG/CTG trinuncleotide repeats are fragile sequences that when expanded form DNA secondary structures and cause human disease. We evaluated CAG/CTG repeat stability and repair outcomes in histone H2 mutants in S. cerevisiae. Although the two copies of H2A are nearly identical in amino acid sequence, CAG repeat stability depends on H2A copy 1 (H2A.1) but not copy 2 (H2A.2). H2A.1 promotes high-fidelity homologous recombination, sister chromatid recombination (SCR), and break-induced replication whereas H2A.2 does not share these functions. Both decreased SCR and the increase in CAG expansions were due to the unique Thr126 residue in H2A.1 and hta1Δ or hta1-T126A mutants were epistatic to deletion of the Polδ subunit Pol32, suggesting a role for H2A.1 in D-loop extension. We conclude that H2A.1 plays a greater repair-specific role compared to H2A.2 and may be a first step towards evolution of a repair-specific function for H2AX compared to H2A in mammalian cells.

Mrc1 and Tof1 prevent fragility and instability at long CAG repeats by their fork stabilizing function

Fork stabilization at DNA impediments is key to maintaining replication fork integrity and preventing chromosome breaks. Mrc1 and Tof1 are two known stabilizers that travel with the replication fork. In addition to a structural role, Mrc1 has a DNA damage checkpoint function. Using a yeast model system, we analyzed the role of Mrc1 and Tof1 at expanded CAG repeats of medium and long lengths, which are known to stall replication forks and cause trinucleotide expansion diseases such as Huntington's disease and myotonic dystrophy. We demonstrate that the fork stabilizer but not the checkpoint activation function of Mrc1 is key for preventing DNA breakage and death of cells containing expanded CAG tracts. In contrast, both Mrc1 functions are important in preventing repeat length instability. Mrc1 has a general fork protector role that is evident at forks traversing both repetitive and non-repetitive DNA, though it becomes crucial at long CAG repeat lengths. In contrast, the role of Tof1 in preventing fork breakage is specific to long CAG tracts of 85 or more repeats. Our results indicate that long CAG repeats have a particular need for Tof1 and highlight the importance of fork stabilizers in maintaining fork integrity during replication of structure-forming repeats.

Guidelines for DNA recombination and repair studies: Cellular assays of DNA repair pathways

Understanding the plasticity of genomes has been greatly aided by assays for recombination, repair and mutagenesis. These assays have been developed in microbial systems that provide the advantages of genetic and molecular reporters that can readily be manipulated. Cellular assays comprise genetic, molecular, and cytological reporters. The assays are powerful tools but each comes with its particular advantages and limitations. Here the most commonly used assays are reviewed, discussed, and presented as the guidelines for future studies.