Raghu R. Chivukula, MD, Ph.D., a physician-investigator at Massachusetts General Hospital and Harvard Medical School, has led a study recently published in Science. The paper is titled "Polyglycine-mediated aggregation of FAM98B disrupts tRNA processing in GGC repeat disorders."
ScienceThe following interview explores his work.
Neurodegenerative diseases like Alzheimer's and Parkinson's are devastating conditions without a cure. While many such diseases involve abnormal protein aggregation in the brain, there's limited understanding of how these aggregates cause brain cell dysfunction and death—a hurdle to developing effective treatments.
Following approaches from cardiovascular and cancer research, we focused on rare genetic forms of neurodegeneration as a strategy to reveal fundamental mechanisms linking protein aggregation to brain disorders. Unexpectedly, our work connected protein aggregation in these genetic conditions to disrupted processing of transfer RNAs (tRNAs), identifying a potential therapeutic target for such diseases.
Our interest was piqued by neurodegenerative disorders caused by GGC trinucleotide repeat expansions—mutations resulting from repeated sequences of three DNA letters. These mutations produce aggregation-prone proteins with extended stretches of glycine, forming "polyglycine" aggregates detected in various tissues and cell types of affected patients. However, GGC repeat expansion disorders primarily impact only the central nervous system.
To understand what exactly polyglycine aggregates do to cells and why they selectively harm brain cells, we employed a biochemical approach. We produced polyglycine proteins in cultured cells and purified the resulting protein aggregates. Using mass spectrometry, which analyzes molecule quantities in samples, we comprehensively identified host cell proteins recruited to these aggregates, effectively depleting them from the cells.
Next, we examined how polyglycine aggregation affects RNA processing in cultured cells, verified our results with human disease tissue samples, and developed mouse models to assess functional consequences of tRNA processing defects in the brain.
We discovered that polyglycine aggregates, both in vitro and in patients, recruit the tRNA ligase complex (tRNA-LC), which is essential for processing spliced tRNAs. Mutations in other tRNA splicing genes also cause early-onset neurodegenerative disorders similar to GGC repeat expansion conditions. We found that tRNA-LC aggregation results in misprocessed tRNAs in cultured cells and brain samples from patients.
Moreover, mice with depleted tRNA-LC in their brains exhibited neurodegeneration and motor coordination impairments akin to those seen in GGC repeat expansion diseases.
Our research reveals an unexpected link between protein aggregation and RNA processing disorders in GGC repeat diseases.
The striking parallels between GGC repeat disorders and previously documented tRNA splicing disorders suggest that polyglycine-dependent disruption of tRNA splicing may be a key mechanism behind selective neuronal death. Importantly, our discoveries offer proof-of-concept evidence that disrupting tRNA-LC aggregation may shield cells from the harmful effects of GGC repeat expansions.
Our laboratory is now actively exploring the cellular and molecular consequences in vivo of altered tRNA splicing within the brain. We are keenly interested in developing therapeutic strategies to block this pathogenic mechanism in neurodegenerative GGC repeat disorders.