Instability of CAG and CTG Trinucleotide Repeats in <i>Saccharomyces cerevisiae</i>

Miret, Juan J.; Pessoa-Brandao, Luis; Lahue, Robert S.

doi:10.1128/mcb.17.6.3382

Cited by 84 publications

(74 citation statements)

References 37 publications

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“…In the wild type strain, the percentage of CTG tract expansion was very low (0.8% for CTG-85 and 1.3% for CTG-155) as compared with contractions, which were much higher than expansions and increased from a tract length of 85 to 155 (5.3% for CTG-85 and 24.1% for CTG-155) ( Table 2). This result is consistent with the previous reports that show a bias toward contraction in yeast cells (15,53). As has been reported earlier, in the rad27⌬ strain, the rate of both expansion and contraction were higher as compared with the wild type, with a strong bias toward expansion (6,25,51).…”

Section: Concerted Action Of Exo and Gen Activities Resolves (Gaa) N supporting

confidence: 82%

Concerted Action of Exonuclease and Gap-dependent Endonuclease Activities of FEN-1 Contributes to the Resolution of Triplet Repeat Sequences (CTG) - and (GAA) -derived Secondary Structures Formed during Maturation of Okazaki Fragments

Singh

Zheng

Chavez

et al. 2007

Journal of Biological Chemistry

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There is much evidence to indicate that FEN-1 efficiently cleaves single-stranded DNA flaps but is unable to process double-stranded flaps or flaps adopting secondary structures. However, the absence of Fen1 in yeast results in a significant increase in trinucleotide repeat (TNR) expansion. There are then two possibilities. One is that TNRs do not always form stable secondary structures or that FEN-1 has an alternative approach to resolve the secondary structures. In the present study, we test the hypothesis that concerted action of exonuclease and gap-dependent endonuclease activities of FEN-1 play a role in the resolution of secondary structures formed by (CTG) n and (GAA) n repeats. Employing a yeast FEN-1 mutant, E176A, which is deficient in exonuclease (EXO) and gap endonuclease (GEN) activities but retains almost all of its flap endonuclease (FEN) activity, we show severe defects in the cleavage of various TNR intermediate substrates. Precise knock-in of this point mutation causes an increase in both the expansion and fragility of a (CTG) n tract in vivo. Taken together, our biochemical and genetic analyses suggest that although FEN activity is important for singlestranded flap processing, EXO and GEN activities may contribute to the resolution of structured flaps. A model is presented to explain how the concerted action of EXO and GEN activities may contribute to resolving structured flaps, thereby preventing their expansion in the genome.

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Section: Concerted Action Of Exo and Gen Activities Resolves (Gaa) N supporting

confidence: 82%

Concerted Action of Exonuclease and Gap-dependent Endonuclease Activities of FEN-1 Contributes to the Resolution of Triplet Repeat Sequences (CTG) - and (GAA) -derived Secondary Structures Formed during Maturation of Okazaki Fragments

Singh

Zheng

Chavez

et al. 2007

Journal of Biological Chemistry

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“…Several studies using model organisms indicate that replication slippage plays a major role in the repeat tract instability (7)(8)(9)(10)(11)(12)(13)(14). The latter model is also supported by the in vitro observations that CAG repeats can form hairpin structures, with the CTG hairpin being more stable than the CAG hairpin (15)(16)(17)(18).…”

supporting

confidence: 54%

Meiotic instability of CAG repeat tracts occurs by double-strand break repair in yeast

Jankowski

Nasar

Nag

2000

Proc. Natl. Acad. Sci. U.S.A.

104

100

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Expansion of trinucleotide repeats is associated with a growing number of human diseases. The mechanism and timing of expansion of the repeat tract are poorly understood. In humans, trinucleotide repeats show extreme meiotic instability, and expansion of the repeat tract has been suggested to occur in the germ-line mitotic divisions or postmeiotically during early divisions of the embryo. Studies in model organisms have indicated that polymerase slippage plays a major role in the repeat tract instability and meiotic instability is severalfold higher than the mitotic instability. We show here that meiotic instability of the CAG͞CTG repeat tract in yeast is associated with double-strand break (DSB) formation within the repeated sequences, and that the DSB formation is dependent on the meiotic recombination machinery. The DSB repair results in both expansions and contractions of the CAG repeat tract. E xpansion of trinucleotide repeats has been shown to be associated with a growing number of human diseases (1). Several of these genetic diseases are associated with alterations of the CAG͞CTG (hereafter CAG) repeat tract length. The expansion mutation is highly dependent on the repeat tract length: the longer the repeat tract, the greater the possibility for expansion (2-4). Such mutational changes are dynamic, since the mutated region can undergo further changes in subsequent generations and during the lifespan of an individual.Two mechanisms have been used to explain the expansion of trinucleotide repeats: unequal genetic recombination and error in DNA replication caused by polymerase slippage (5, 6). In the recombination model, unequal crossing-over or gene conversion between triplet repeats either on sister chromatids or on homologs results in an altered tract length. In the slippage model, the replicating strand becomes dissociated, and then it misaligns during reassociation. This misalignment causes the formation of an unpaired sequence either on the template strand or on the nascent strand. A failure to repair the unpaired sequence will result in a DNA molecule containing a larger or smaller number of repeats after the next round of DNA replication. Several studies using model organisms indicate that replication slippage plays a major role in the repeat tract instability (7-14). The latter model is also supported by the in vitro observations that CAG repeats can form hairpin structures, with the CTG hairpin being more stable than the CAG hairpin (15-18). Recent studies with diploid yeast strains containing heterozygous trinucleotide repeat-insertion mutations suggest that trinucleotide repeats in single-stranded DNA are likely to form hairpin structures in vivo (19). During DNA replication, the hairpin formation on the template strand or on the newly synthesized strand would produce a deletion or an expansion of the repeat tract, respectively. This hypothesis is consistent with the observation that CAG-repeat instability depends on the direction in which the replication fork proceeds through the repeat tract (...

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“…More complex models envisage repeat-mediated breakage, leading to recombinational repair, single-strand annealing or inability to restart a stalled replication fork at the TNR. These break repair activities typically require recombinational proteins such as Rad52 but previous data in our system showed no contraction phenotype in a rad52 mutant [41]. We favor the simpler model where Mrc1, Tof1 and Csm3 help prevent contractions by limiting the accumulation of single-strand template DNA prior to the formation of aberrant secondary structure.…”

Section: Discussionmentioning

confidence: 97%

Mrc1, Tof1 and Csm3 inhibit CAG·CTG repeat instability by at least two mechanisms

Razidlo

Lahue

2008

DNA Repair

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Trinucleotide repeats frequently expand and contract in humans and model organisms. Protein factors that modulate this process have been found by candidate gene approaches or mutant screens for increased expansion rates. To extend this effort, Saccharomyces cerevisiae mutants with higher CAG CTG repeat contraction rates were sought using a disruption library. This screen identified Mrc1, the homolog of human Claspin, which mediates the replication and DNA damage checkpoints, and also couples the replicative helicase and polymerase. Genetic analysis showed that Mrc1, along with Tof1 and Csm3, inhibits instability in two distinct ways. Contraction rates of (CAG) 20 tracts are elevated by loss of Mrc1, Tof1 or Csm3, but not by defects in most replication checkpoint or DNA damage checkpoint proteins. The three proteins likely inhibit contractions primarily through their coupling activity, which would prevent accumulation of single-strand template DNA prior to the formation of aberrant secondary structure. In contrast, expansion rates of (CTG) 13 are elevated in strains defective for Mrc1, Tof1, Csm3, Mec1, Ddc2, Rad24, Ddc1, Mec3, Rad17, Rad9, Rad53 or Chk1, suggesting that the DNA damage checkpoint inhibits expansions after formation of repeat-dependent structures. Together, these results indicate that at least two Mrc1-dependent mechanisms function to reduce CAG•CTG repeat instability.

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Instability of CAG and CTG Trinucleotide Repeats in Saccharomyces cerevisiae

Cited by 84 publications

References 37 publications

Concerted Action of Exonuclease and Gap-dependent Endonuclease Activities of FEN-1 Contributes to the Resolution of Triplet Repeat Sequences (CTG) - and (GAA) -derived Secondary Structures Formed during Maturation of Okazaki Fragments

Concerted Action of Exonuclease and Gap-dependent Endonuclease Activities of FEN-1 Contributes to the Resolution of Triplet Repeat Sequences (CTG) - and (GAA) -derived Secondary Structures Formed during Maturation of Okazaki Fragments

Meiotic instability of CAG repeat tracts occurs by double-strand break repair in yeast

Mrc1, Tof1 and Csm3 inhibit CAG·CTG repeat instability by at least two mechanisms

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