Live Blogger: Ryan Schildcrout
Editor: Madison Fitzgerald
This piece was written live during the 8th annual RNA Symposium, “Unmasking the Power of RNA: From Structure to Medicine” hosted by the University of Michigan’s Center for RNA Biomedicine. Follow MiSciWriters’ coverage of this event on Twitter with the hashtag #umichrna.
When we think of “DNA,” we normally think about genes that encode proteins. However, the vast majority of the human genome is thought to be “non-coding,” in that the DNA does not encode proteins. Non-coding DNA has been long thought of as biologically inert, but in the last few decades, scientists have started exploring its purpose. Since then, it has been recognized to be a key element in regulating gene expression. Within those non-coding regions exist millions of Short Tandem Repeats, or microsatellites, which are repetitive DNA sequences of up to 6 base pairs. The number of repeats within a microsatellite can vary drastically across the genome. We know very little about the function of these microsatellites because next-generation sequencing (NGS) technologies are unable to sequence highly repetitive regions of DNA. Some of these repeats have been linked to neurological disorders such as Autism, ALS, and dementia, thus highlighting the need to study the mechanisms by which they cause disease. Dr. Peter Todd from the University of Michigan has recently uncovered some of these mechanisms in context with neurological diseases.
Dr. Peter Todd starts his talk by providing a relatable example of Short Tandem Repeats in a biological context – the many repeat sequences in the Flocculence 1 (FLO1) gene in beer-producing yeast. Flocculation is the process by which biomass is separated at the end of brewing, and increased repeats in the FLO1 gene correlates with increased flocculation. He then explains that repeats allow for rapid evolution (much faster than point mutations) but can lead to diseases such as Huntington’s disease when expanded erroneously. Although most microsatellites exist outside protein coding regions, there are cases where they reside within them, such as the fragile X messenger ribonucleoprotein 1 (FMR1) gene which is implicated in neuronal disorders. FMR1 encodes the FMRP protein and typically contains ~30 repeats of Cysteine-Cysteine-Guanine (CCG) at the start of the gene. When these CGG repeats are expanded beyond the typical ~30 at the start of the sequence, it leads to Fragile X-associated disorders. These disorders include Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) which has between 55-200 expanded repeats, resulting in overtranscription, and Fragile X Syndrome, which has >200 repeats, resulting in loss of transcription. Fragile X Syndrome can cause autism and intellectual disability while FXTAS causes symptoms similar to Parkinson’s or Alzheimer’s disease, including tremors, memory loss, and slowed speech.
Why does the same genetic mechanism that causes loss of transcription in Fragile X Syndrome lead to a completely different disease when a different number of repeats are present? Dr. Todd proposes that in FXTAS, the extra repeats are included in the translation of FMR1 RNA transcripts, resulting in a toxic form of FRMP. Additionally, he explains that FXTAS causes cells to make too much FRMP through repeat-associated non-AUG-initiated (RAN) translation driven by the expanded CGG repeats. Normally, an AUG start codon signals where to begin translating the messenger RNA (mRNA) into protein. In the case of FXTAS, CGG repeats initiate RAN translation without a canonical start codon, leading to an accumulation of toxic FMRP in neurons. However, these phenomena are not sufficient to explain the entire disease state.
The impact of microsatellites, both in the transcribed RNA and RAN translated proteins, has broader implications. For example, microsatellites can force RNAs to exit the nucleus by inducing stress granules or inhibit translational function more broadly. Some kinases are even activated by repeat elements – repeats in FMR1 cause dsRNA-dependent protein kinase R (PKR) to suppress canonical translation, but allows for RAN translation. Alternatively, when PKR is inhibited, RAN translation is less efficient. These data indicate that perhaps RAN is contributing to genetic regulation in a more complex way than previously thought.
Once Dr. Todd and his team determined the physiological role of these short repeats in disease, they sought to identify therapeutic targets. One viable option for inhibiting RAN translation is using antisense oligonucleotides (ASOs) to degrade specific mRNAs. Unfortunately, this isn’t feasible for the FMRP protein because it is necessary for normal physiological function. To overcome this, Dr. Todd’s group produces ASOs that are chemically modified and specific to microsatellites to prevent RAN translation, thus allowing non-pathogenic FMRP to be translated without producing toxic FMRP. This approach corrected FMRP expression to normal levels while limiting toxicity in HEK239 cells.
Dr. Todd’s work successfully determined disease mechanisms driven by microsatellites, informing novel therapeutic avenues for previously untreatable diseases. Short Tandem Repeats are an understudied phenomenon, and Dr. Todd’s research has the ability to uncover their purpose, ultimately leading to a better understanding of neurobiology and its key regulators.
Dr. Peter Todd is the Chester and Anne Sackett Endowed Professor of Neurology and Human Genetics at the University of Michigan, Ann Arbor, where he serves as the associate chair for research in the department of Neurology. He earned his BS in Biology from the University of California, San Diego, and his MD and PhD in Neuroscience from the University of Wisconsin, Madison. His research investigates the molecular mechanisms of neurological disorders driven by repeat expansions in Fragile X-associated disorders. More information about Dr. Todd and his work can be found here.
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