Research Spotlight: Study Reveals an Unexpected Role for Protein Aggregates in Brain Disease

Raghu R. Chivukula, MD, PhD, a physician-investigator in the departments of medicine and surgery and the Center for Genomic Medicine at Massachusetts General Hospital and Harvard Medical School, is the senior author of a paper published in Science, “Polyglycine-mediated aggregation of FAM98B disrupts tRNA processing in GGC repeat disorders”.

Q: How would you summarize your study for a lay audience?

Dr. Chivukula: Neurodegenerative disorders, such as Alzheimer’s disease and Parkinson’s disease, are devastating and incurable diseases. Although many neurodegenerative diseases are characterized by abnormal protein aggregation in the brain, a limited understanding of whether and how aggregated proteins cause brain cell dysfunction and death represents a major barrier to developing effective treatments.

Inspired by similar approaches in cardiovascular disease and cancer, we focused on rare genetic forms of neurodegeneration as a powerful way to uncover fundamental mechanisms tying protein aggregation to brain disease. Our work unexpectedly linked protein aggregation in genetic forms of neurodegeneration to disrupted processing of transfer RNAs (tRNAs), revealing an important mechanism that might be therapeutically targeted in these disorders.

Q: What question were you investigating?

Dr. Chivukula: We became interested in genetic forms of neurodegeneration caused by GGC trinucleotide repeat expansions (DNA sequence mutations caused by copying this 3-letter sequence too many times in a row). These mutations produce aggregation-prone proteins with long stretches of a single repeated amino acid (glycine). Interestingly, although these “polyglycine”-containing protein aggregates are detectable in many tissue and cell types of affected patients, GGC repeat expansion disorders seem to cause disease only in the central nervous system.

We wanted to understand exactly what polyglycine aggregates do to cells and why they are selectively toxic to cells in the brain.

Q: What methods or approach did you use?

Dr. Chivukula: We employed a biochemistry-based approach to produce polyglycine proteins in cultured cells and to purify the resultant protein aggregates. Then we used mass spectrometry, which measures the amounts of different molecules in a sample, to comprehensively catalog the set of host cell proteins that are recruited into these aggregates and thereby depleted from cells.

We went on to study the consequences of polyglycine aggregation on RNA processing in cultured cells, confirmed our results in human disease tissues samples, and developed mouse models to functionally assess the consequences of tRNA processing defects in the brain.

Q: What did you find?

Dr. Chivukula: We discovered that polyglycine aggregates, both in cultured cells and in human patients, specifically recruit the tRNA ligase complex (tRNA-LC), a group of proteins which is required for processing spliced tRNAs. Notably, mutations in other tRNA splicing genes also cause early-onset neurodegenerative diseases similar to GGC repeat expansion disorders. We found that aggregation of the tRNA-LC leads to misprocessed tRNAs in cultured cells as well as patient brain samples. Moreover, mice in which we depleted the tRNA-LC in the brain developed neurodegeneration and motor coordination deficits similar to those seen in GGC repeat disorders.

Q: What are the implications?

Dr. Chivukula: Our work reveals a new and unexpected link between protein aggregation and RNA processing disorders in GGC repeat diseases.

The striking similarities between GGC repeat disorders and previously described tRNA splicing disorders suggests polyglycine-dependent tRNA splicing disruption may be an important mechanism underlying selective neuronal death. Importantly, our findings also establish proof-of-concept that interfering with tRNA-LC aggregation may protect cells from the pathogenic effects of GGC repeat expansions.

Q: What are the next steps?

Dr. Chivukula: Our laboratory is now actively working to understand the cellular and molecular consequences in vivo of altered tRNA splicing in the brain. We are very interested in developing therapeutic strategies that can block this pathogenic mechanism in neurodegenerative GGC repeat disorders.