A04 Circhtt, a circular rna from the huntington’s disease gene locus: functional characterization and possible implications for disease modulation

Background Circular RNAs (circRNAs) are a special group of non-coding RNAs formed by back-splicing. Mostly cytosolic in eukaryotic cells, circRNAs are particularly enriched and conserved in neurons and originate from protein-coding genes to function as global regulators of gene expression. Recent studies showed that circRNAs partake in brain physiology and pathology, contributing to neurological disorders, such as myotonic dystrophy type 1, Parkinson’s and Alzheimer’s Diseases. Here, we identified the first ever known brain enriched RNA circle originating from the Huntington’s Disease gene HTT (CircHTT: 484 nt, Ex 2-6, chr4:3088665-3109150) which is conserved in mouse and minipig. Aims and Methods To functionally characterize circHTT and its implication for HD pathogenesis, we first studied its expression pattern in human and mouse body districts and in iPS-derived neuronal cell lines with different CAG repeats. Then, overexpression and down-regulation of the circle were used to determine the effects on transcription of the HD gene and translation of wild-type and mutant protein. Results CircHTT expression increases significantly with increasing CAG repeats in terminally differentiated cortical neurons. Furthermore, circHtt is significantly more expressed in brain districts of Q111 and zQ175 knock-in mice. These findings indicate that the expression of circHTT/Htt occurs in a CAG repeat dependent manner in neuronal cells, typical hallmark of HD pathology. Strikingly, overexpression of the circular RNA in human HEK293T, PC3, mouse STHdh Q7/7, Q7/111 and Q111/111 cell lines consistently increase wild-type huntingtin, while decreasing mutant huntingtin protein, with no alteration of the HTT/Htt transcript level. Conclusions In conclusion, we identified a brain enriched RNA circle originating from the HD gene locus that is sensitive to the HD mutation and may modulate huntingtin expression. Our observations might pave the way to new trials of therapeutic intervention.