AGG/CCT interruptions affect nucleosome formation and positioning of healthy-length CGG/CCG triplet repeats

Abstract

BackgroundFragile X Syndrome (FXS), the most common inherited form of mental retardation, is caused by expansion of a CGG/CCG repeat tract in the 5′-untranslated region of the fragile X mental retardation (FMR1) gene, which changes the functional organization of the gene from euchromatin to heterochromatin. Interestingly, healthy-length repeat tracts possess AGG/CCT interruptions every 9–10 repeats, and clinical data shows that loss of these interruptions is linked to expansion of the repeat tract to disease-length. Thus, it is important to understand how these interruptions alter the behavior of the repeat tract in the packaged gene.ResultsTo investigate how uninterrupted and interrupted CGG/CCG repeat tracts interact with the histone core, we designed experiments using the nucleosome core particle, the most basic unit of chromatin packaging. Using DNA containing 19 CGG/CCG repeats, flanked by either a nucleosome positioning sequence or the FMR1 gene sequence, we determined that the addition of a single AGG/CCT interruption modulates both the ability of the CGG/CCG repeat DNA to incorporate into a nucleosome and the rotational and translational position of the repeat DNA around the histone core when flanked by the nucleosome positioning sequence. The presence of these interruptions also alters the periodicity of the DNA in the nucleosome; interrupted repeat tracts have a greater periodicity than uninterrupted repeats.ConclusionsThis work defines the ability of AGG/CCT interruptions to modulate the behavior of the repeat tract in the packaged gene and contributes to our understanding of the role that AGG/CCT interruptions play in suppressing expansion and maintaining the correct functional organization of the FMR1 gene, highlighting a protective role played by the interruptions in genomic packaging.

Of Men and Mice: Modeling the Fragile X Syndrome

“…It has been noticed that the number of repeats, the content of interspersed AGG and the haplotype are able to influence the stability of FMR1 (Oberlé et al, 1991 ; Eichler et al, 1994 ; Gunter et al, 1998 ; Hirst and White, 1998 ; Taylor et al, 1999 ; Larsen et al, 2000 ; Dombrowski et al, 2002 ; Nolin et al, 2003 , 2013 , 2015 ; Yrigollen et al, 2012 , 2013 , 2014 ; Avitzour et al, 2014 ; Weiss et al, 2014 ). AGG interruptions, for instance, have been indicated to support the stability of FMR1 by reducing secondary structure formation (Weisman-Shomer et al, 2000 ; Jarem et al, 2010 ) and promoting appropriate DNA conformations (Jarem et al, 2010 ) as well as adequate DNA packing (Mulvihill et al, 2005 ; Volle and Delaney, 2013 ). Furthermore, the number of repeats was shown to directly correlate with the instability of the tandem tracts (Oberlé et al, 1991 ; Eichler et al, 1994 ; Taylor et al, 1999 ; Avitzour et al, 2014 ): it is assumed that the size of G-rich tracts directly correlates with the formation of secondary structures and polymerase slippage during replication (Mornet et al, 1996 ; Freudenreich et al, 1997 ; Weitzmann et al, 1997 ; Hirst and White, 1998 ).…”

Unusual association of a unique CAG interruption in 5′ of DM1 CTG repeats with intergenerational contractions and low somatic mosaicism

“…() showed that the degree of somatic mosaicism is low in an atypical DM1/Charcot Dutch family compared with the expected somatic mosaicism for the corresponding age of sampling and progenitor allele length (Higham & Monckton, ). Different studies have shown that formation of secondary DNA structures, nucleosome positioning, methylation status and transcription level can be affected by the AGG, CAT, GGA, and CCG interruptions described in SCA1, FXS, FRDA, and DM1 (Jarem, Huckaby, & Delaney, ; Mulvihill, Nichol Edamura, Hagerman, Pearson, & Wang, ; Pearson et al., ; Sakamoto et al., ; Santoro et al., ; Volle & Delaney, ). However, the role of interruptions in the mechanisms of TNR stabilization/contraction remains elusive, especially for DM1.…”

Unique AGG Interruption in the CGG Repeats of the FMR1 Gene Exclusively Found in Asians Linked to a Specific SNP Haplotype

“…Simplified pathways of the hypothesis are shown in Figure 3 . In addition, perhaps the 6A pattern enhances the stability of CGG repeat tracts [ 22 , 23 ]. Thus, chromosomes with 36 CGG repeats linked to the 6A pattern have become the third most common allele in only Asian populations.…”