Fig. 1

Distinct loss-of-function and gain-of-function mechanisms of disease for various repeat expansion disorders. Repeat expansions can occur in 5’ or 3’ UTRs, exons, or introns. Expanded tandem repeat-containing RNA (xtrRNA) may not be transcribed due to epigenetic silencing, thereby causing loss of gene function. If transcribed, xtrRNA may become trapped in the cell nucleus where it can form focal aggregates and functionally deplete important RNA binding proteins. The xtrRNA may also be exported to the cytoplasm where it can undergo translation to produce repeat-containing polypeptides that disrupt cellular processes. In some cases, xtrRNA can form focal nuclear aggregates and also be translated into repeat-containing polypeptides. Repeat-containing polypeptides can be toxic in multiple ways, including insoluble aggregation, blocking normal host protein function, inhibiting nucleocytoplasmic transport, and disrupting other critical cellular functions