MutSβ and histone deacetylase complexes promote expansions of trinucleotide repeats in human cells

Abstract

Trinucleotide repeat (TNR) expansions cause at least 17 heritable neurological diseases, including Huntington’s disease. Expansions are thought to arise from abnormal processing of TNR DNA by specific trans-acting proteins. For example, the DNA repair complex MutSβ (MSH2–MSH3 heterodimer) is required in mice for on-going expansions of long, disease-causing alleles. A distinctive feature of TNR expansions is a threshold effect, a narrow range of repeat units (∼30–40 in humans) at which mutation frequency rises dramatically and disease can initiate. The goal of this study was to identify factors that promote expansion of threshold-length CTG•CAG repeats in a human astrocytic cell line. siRNA knockdown of the MutSβ subunits MSH2 or MSH3 impeded expansions of threshold-length repeats, while knockdown of the MutSα subunit MSH6 had no effect. Chromatin immunoprecipitation experiments indicated that MutSβ, but not MutSα, was enriched at the TNR. These findings imply a direct role for MutSβ in promoting expansion of threshold-length CTG•CAG tracts. We identified the class II deacetylase HDAC5 as a novel promoting factor for expansions, joining the class I deacetylase HDAC3 that was previously identified. Double knockdowns were consistent with the possibility that MutSβ, HDAC3 and HDAC5 act through a common pathway to promote expansions of threshold-length TNRs.

MSH3 Promotes Dynamic Behavior of Trinucleotide Repeat Tracts In Vivo

“…Nonetheless, loss of MSH2 or MSH3 leads to a significant decrease in expansion events in mouse models of HD (Manley et al 1999; Owen et al 2005) and MD1 (van den Broek et al 2002;Foiry et al 2006). Similarly, Msh3 promotes expansions in human cells (Gannon et al 2012;Halabi et al 2012). We recently demonstrated a significant decrease in expansion of both CAG and CTG repeat tracts in Saccharomyces cerevisiae in an msh3D background (Kantartzis et al 2012).…”

“…However, through a mechanism(s) that remains unclear, a TNR tract can expand, increasing the number of repeats within the tract. Initially, this brings the tract into a threshold-length range (Gannon et al 2012;Concannon and Lahue 2014) (Figure 1 and Figure 2), in which these somewhat longer tracts are not pathogenic but are increasingly susceptible to expansion; individuals with this phenomenon are carriers for disease. Once a tract has expanded sufficiently, it crosses a threshold; tracts above this threshold (which is disease specific) are pathogenic and cause disease ( Figure 1).…”

Replication stalling and heteroduplex formation within CAG/CTG trinucleotide repeats by mismatch repair

“…Chromatin structures of DNA in these two eukaryotic cell types share similarities as well as differences, and could certainly be one of the factors involved in this discrepancy. In human cells, the HDAC3 and HDAC5 histone deacetylase complexes are involved in CAG/CTG repeat expansions, and in yeast cells the HDAC3 homologue has the same effect [30,70]. However, it was shown that chromatin organization at trinucleotide repeats embedded in a yeast natural chromosome exhibit a noncanonical structure, involving binding of Hmo1p, a high mobility group protein without obvious orthologue in the human genome [71].…”

“…More recently, it was shown that short CAG/CTG slip-out structures were efficiently repaired by human MutL and MutS complexes [27,28], and to a lesser extent by MutS [27]. In mice in which Msh2, Msh3, Pms2, Mlh1 or Mlh3 were reduced or abolished, CAG/CTG repeat expansions were decreased [29][30][31][32][33][34][35][36][37]. This is also true when Msh2 was partially or totally depleted in a mouse model for fragile X premutation [38].…”

The CAA Repeat Polymorphism in the <i>ZFHX3</i> Gene Is Associated with Risk of Coronary Heart Disease in a Chinese Population

“…These properties of trinucleotide repeats may include their ability to form higher order structures in single-stranded DNA and transcripts (Liu et al 2012). Higher order structures may play an important regulatory role in numerous cellular processes, such as DNA replication repair and at various steps of gene expression ranging from transcription to mRNA decay (Gannon et al 2012). The ZFHX3 gene, also called AT motif-binding factor 1 (ATBF1), was first described as a transcription factor that inhibits the human AFP gene expression in the liver (Sun et al 2012).…”