CAG/CTG Repeats Alter the Affinity for the Histone Core and the Positioning of DNA in the Nucleosome

Abstract

Trinucleotide repeats (TNRs) occur throughout the genome, and their expansion has been linked to several neurodegenerative disorders, including Huntington's disease. TNRs have been studied using both oligonucleotides and plasmids; however, less is know about how repetitive DNA responds to genomic packaging. Here, we investigate the behavior of CAG/CTG repeats incorporated into nucleosome core particles, the most basic unit of chromatin packaging. To assess the general interaction between CAG/CTG repeats and the histone core, we determined the efficiency with which various TNR-containing DNA substrates form nucleosomes, revealing that even short CAG/CTG tracts are robust incorporators. However, the presence of the Huntington gene flanking sequence (htt) decreases the rate of incorporation. Enzymatic and chemical probing revealed repositioning of the DNA in the nucleosome as the number of CAG/CTG repeats increased, regardless of the flanking sequence. Notably, the periodicity of the repeat tract remained unchanged as a function of length and is consistently 10.7 bp per helical turn. In contrast, the periodicity of the nonrepetitive flanking sequence varies and is smaller than the repeat tract at ~10.0-10.5 bp per turn. Furthermore, while the CAG/CTG repeats remain as a canonical duplex in the nucleosome, nucleosome formation causes kinking in a secondary repeat tract in the htt gene, comprised of CCG/CGG repeats. This work highlights the innate ability of CAG/CTG repeats to incorporate and to position in nucleosomes and how that behavior is modulated by the htt flanking sequence. In addition, it illuminates the differences in packaging of healthy and diseased length repeat tracts within the genome.