Genome-wide Screen Identifies Pathways that Govern GAA/TTC Repeat Fragility and Expansions in Dividing and Nondividing Yeast Cells

School of Biology and Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, GA 30332, USA.
Molecular cell (Impact Factor: 14.02). 09/2012; 48(2). DOI: 10.1016/j.molcel.2012.08.002
Source: PubMed


Triplex structure-forming GAA/TTC repeats pose a dual threat to the eukaryotic genome integrity. Their potential to expand can lead to gene inactivation, the cause of Friedreich's ataxia disease in humans. In model systems, long GAA/TTC tracts also act as chromosomal fragile sites that can trigger gross chromosomal rearrangements. The mechanisms that regulate the metabolism of GAA/TTC repeats are poorly understood. We have developed an experimental system in the yeast Saccharomyces cerevisiae that allows us to systematically identify genes crucial for maintaining the repeat stability. Two major groups of mutants defective in DNA replication or transcription initiation are found to be prone to fragility and large-scale expansions. We demonstrate that problems imposed by the repeats during DNA replication in actively dividing cells and during transcription initiation in nondividing cells can culminate in genome instability. We propose that similar mechanisms can mediate detrimental metabolism of GAA/TTC tracts in human cells.

Download full-text


Available from: Sergei M Mirkin
  • Source
    • "Additionally, breakage of longer CAG tracts (79–250 repeats) are visible as physical chromosome breaks in mitosis (Freudenreich et al., 1998) and meiosis (Jankowski et al., 2000). GAA/TCC tracts and other AT-rich elements predicted to form hairpins or cruciforms also cause chromosome fragility (Kim et al., 2008; Lobachev et al., 2002; Zhang et al., 2012b), as does the ATTCT repeat unwinding element (Cherng et al., 2011 "
    [Show abstract] [Hide abstract]
    ABSTRACT: Abstract The expansion of repeated sequences is the cause of over 30 inherited genetic diseases, including Huntington disease, myotonic dystrophy (types 1 and 2), fragile X syndrome, many spinocerebellar ataxias, and some cases of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Repeat expansions are dynamic, and disease inheritance and progression are influenced by the size and the rate of expansion. Thus, an understanding of the various cellular mechanisms that cooperate to control or promote repeat expansions is of interest to human health. In addition, the study of repeat expansion and contraction mechanisms has provided insight into how repair pathways operate in the context of structure-forming DNA, as well as insights into non-canonical roles for repair proteins. Here we review the mechanisms of repeat instability, with a special emphasis on the knowledge gained from the various model systems that have been developed to study this topic. We cover the repair pathways and proteins that operate to maintain genome stability, or in some cases cause instability, and the cross-talk and interactions between them.
    Full-text · Article · Jan 2015 · Critical Reviews in Biochemistry and Molecular Biology
  • [Show abstract] [Hide abstract]
    ABSTRACT: Many proteins comprised of tandem repeats (a linear motif, directly repeated within the sequence) are substrates for post-translational modifications (PTMs). Tandem repeats are also dynamic in number, presumably due to instability in the underlying DNA sequence. These observations lead to a hypothesis that cells use a combination of PTMs and variability in repeat number to mediate protein function. Evidence of these processes co-regulating diverse aspects of cellular function can be found in all organisms from bacteria to humans suggesting a common, but poorly described, mechanism for regulating and diversifying protein function. This review highlights several examples whereby protein modifications and repetitive protein domains impart diversity. Lastly, it speculates on the possibility of using chemically-modified repetitive amino acid sequences to develop peptide-based biomolecules with novel functions.
    No preview · Article · Nov 2012 · ACS Chemical Biology
  • [Show abstract] [Hide abstract]
    ABSTRACT: Transcription blockage can strongly affect gene expression and trigger other important biological phenomena like transcription-coupled repair (Hanawalt & Spivak, 2008). Thus, it is of interest to study the various factors that can cause transcription blockage and to elucidate mechanisms of their action. We studied T7 RNA polymerase (T7 RNAP) transcription blockage caused by single-stranded breaks localize either in the template or the nontemplate DNA strand (Belotserkovskii et al., 2013; Neil, Belotserkovskii, & Hanawalt, 2012). Partial T7 RNAP blockage was observed in both cases, but the patterns of blockage signals differed dramatically for these two types of lesions. A break in the template strand produces a sharp predominant blockage signal corresponding to the position of the break, as expected for an interruption in the DNA strand that is continuously tracked by RNAP during transcription. In contrast, a break in the nontemplate strand produces an irregular ladder of weak blockage signals that begins approximately at the position of the break and then extends far downstream from the break position, without either a predominant signal at the break position, or a pronounced downstream “end” of the ladder. The blockages produced by the break in the nontemplate strand increase dramatically when they are closely adjacent to G-rich homopurine sequences. These sequences cause partial transcription blockage, as we have previously established (Belotserkovskii et al., 2010); and in the presence of the nearby nontemplate strand break, the resulting blockage is greatly enhanced (Belotserkovskii et al., 2013). Based upon these and other observations, we suggest that transcription blockage by breaks in the nontemplate strand is due to their propensity to induce R-loop formation which destabilizes the transcription complex and renders it prone to spontaneous premature blockage/termination.
    No preview · Article · Jan 2013 · Journal of biomolecular Structure & Dynamics
Show more