Systematic generation and imaging of tandem repeats reveal base-pairing properties that promote RNA aggregation.

A common pathological feature of RNAs containing expanded repeat sequences is their propensity to aggregate in cells. While some repeat RNA aggregates have been shown to cause toxicity by sequestering RNA-binding proteins, the molecular mechanism of aggregation remains unclear. Here, we devised an efficient method to generate long tandem repeat DNAs and applied it to systematically determine the sequence features underlying RNA aggregation. Live-cell imaging of repeat RNAs indicated that aggregation was promoted by multivalent RNA-RNA interactions via either canonical or noncanonical base pairs. While multiple runs of two consecutive base pairs were sufficient, longer runs of base pairs such as those formed by GGGGCC hexanucleotide repeats further enhanced aggregation. In summary, our study provides a unifying model for the molecular basis of repeat RNA aggregation and a generalizable approach for identifying the sequence and structural determinants underlying the distinct properties of repeat DNAs and RNAs.