Posted March 17, 2023

Stefan Stamm, Ph.D., University of Kentucky College of Medicine

The sad news of Bruce Willis being diagnosed with Frontotemporal Lobar Degeneration/Frontal Temporal Dementia (FTLD-FTD) came nearly a year after he had to stop acting because of aphasia. Aphasia affects the ability to speak or write and brought into spotlight the debilitating power of FTD. Pathologically characterized by aggregation of the microtubule-associated protein tau (MAPT), FTD and Alzheimer’s disease (AD) belong to the family of neurodegenerative diseases known as “tauopathies.” There is currently no cure for FTD or AD, and research efforts are beginning to reveal just how complex the causes for the underlying disease are. Two recent studies published by Dr. Stefan Stamm (an investigator supported by the Peer Reviewed Alzheimer’s Research Program) from the University of Kentucky shed new light onto the previously unrecognized role of human-specific circular RNA in tauopathy.

The central dogma, which refers to the information flow from DNA to messenger RNA (mRNA), then translation from the mRNA templates to protein, constitutes the mainstream understanding of genetics. Only 1.5% of the human genome makes up the protein-coding sequences, and much of the human genome is comprised of so-called non-coding sequences (those that don’t encode protein). Far from being the “junk” sequences in the human genome, these non-coding sequences are increasingly being recognized as playing important roles such as regulating gene expression and contributing to human diseases. Alu elements, repeated sequences unique to primates and humans, are a family of non-coding sequences, comprising about 12% of the human genome.¹ Alu elements promote circular RNA (circRNAs) formation. With support from an FY18 Peer Reviewed Alzheimer’s Research Program award, Dr. Stamm and his team found that the human tau gene MAPT generates two major tau circRNAs, known as tau 12→7 and tau 12→10², which is helped by Alu-elements in the pre-mRNA.

Dr. Stamm’s group found the tau 12→7 circRNAs contained a “go” signal to start making proteins. Conversely, the tau 12→10 circRNA did not contain any signals to start protein productions. However, two of the FTLD/FTD mutations, K317M and V337M, introduced go signals into the tau 12→10 circRNAs which then enabled protein production in that normally “silent” tau circRNA.

In the human brain, the tau 12→10 circRNAs are 10 times more abundant than the tau 12→7 circRNAs; both are stable and form rod-like secondary structures, making them frequent targets of the adenosine deaminase RNA-specific (ADAR) enzymes. Through deamination, the ADAR enzymes convert adenosines (A) to inosines (I) (a process termed ‘A-to-I editing’). Inosines are usually interpretated as guanosine (G) by the protein translational machinery, the ribosome. Thus A-to-I editing can create a start codon (AUI) that resembles the canonical AUG start codon which enables translational initiation. Dr. Stamm’s group further demonstrated that the activity of the ADAR enzymes did introduce the AUI into the tau 12→10 circRNAs to work as a start codon to promote its translation. The presence of the ADAR enzyme also strongly increased the generation of both the tau 12→7 and 12→10 circRNAs.

Since neither tau circRNA contains a stop codon (stop signaling for translation), translation from the tau circRNAs follows a rolling circle pattern with the protein products showing up as “smear”-like products in lab analysis, likely indicating numerous stop signals halting production of proteins that could be caused by circRNA structures. Tau proteins made from the circRNAs tended to clump, or aggregate and evidence indicated they were capable of promoting the formation of pathological tau known as neurofibrillary tangles. These data suggested a potential contribution of the tau circRNAs to the tauopathy³ (Figure 1).

Figure 1: Working Model of Tau Circular RNA translation in Tauopathy.
Intron sequences flanking exon 7 and exon 10/12 predetermine generation of tau circular RNA 12→7 or 12?10. Harboring a start codon (AUG) in its sequence, tau circular RNA 12?7 is capable of being translated physiologically at a low rate. Tau circular RNA 12?10 is normally not translated due to lack of a start codon. FTD/FTLD-Tau mutations (K317M and V337M) or RNA editing by ADAR enzymes introduce start codons into Tau circular RNA 12?10 to facilitate its translation. Protein products from the rolling circle translation of tau circular RNAs 12?7 or 12?10 tend to self-aggregate and promote neurofibrillary tangles from linear tau proteins, potentially contribute to AD/FTD disease progression. (Figure Provided - Adapted from Welden JR et al., 2022)

In addition to the evidence accumulated for the expression and translation from the Tau circRNAs in cell models, the group investigated circRNAs in the brains of AD patients at different clinical stages. More than 200 circRNAs, but not their linear RNA counterparts, were found to correlate with AD progression in an area of the brain that is affected early in AD. The A-to-I RNA editing that was shown to facilitate translation was increased in all circRNAs. These human autopsy data suggest circRNAs may play a role in processes underlying AD severity by forming proteins.

Taken together, Dr. Stamm’s group unveiled a novel mechanism where circRNAs may play a pivotal role in AD/FTD disease progression. Are these circular RNAs the hidden key to the mystery of the tauopathies that everyone is looking for? Further investigation is warranted; research in the circRNAs may mark a new frontier in our understanding toward the AD/FTD disease mechanisms. Circular RNAs are not only novel biomarkers (measurements of disease severity) but may provide new avenues for treatment for these debilitating diseases. There is hope for our beloved actor and those affected similarly after all!

References:

¹Gussakovsky D, McKenna SA. 2021. Alu RNA and their roles in human disease states. RNA Biology 18(sup2):574-585. doi: 10.1080/15476286.2021.1989201. Epub 2021 Oct 21. PMID: 34672903; PMCID: PMC8782168.

²Welden JR, van Doorn J, Nelson PT, Stamm S. 2018. The human MAPT locus generates circular RNAs. Biochimica et Biophysica Acta – Molecular Basis of Disease 1864(9 Pt B):2753-2760. doi:10.1016/j.bbadis.2018.04.023. Epub 2018 May 3. PMID: 29729314; PMCID: PMC6434521.

³Welden JR, Margvelani G, Arizaca Maquera KA, et al. 2022. RNA editing of microtubule-associated protein tau circular RNAs promotes their translation and tau tangle formation. Nucleic Acids Research 50(22):12979-12996. doi: 10.1093/nar/gkac1129. PMID: 36533443; PMCID: PMC9825173.

⁴Arizaca Maquera KA, Welden JR, Margvelani G, et al. Alzheimer progression is associated with changes in regulated retained introns and editing of circular RNAs. In press, Front. Mol. Neurosci., 2023.

Links:

Public and Technical Abstracts: Determine the Role of Human-Specific Tau Circular RNAs in Tauopathies

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