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

Fragile 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.

To 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.

This 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.