| C9orf72 Dipeptide Repeat Proteins (DPRs) | |
|---|---|
| Gene | C9orf72 |
| UniProt | Q96LT7 |
| PDB | N/A |
| Mol. Weight | Variable (repeat-dependent) |
| Localization | Cytoplasm, nucleus (poly-GA, poly-GP, poly-GR, poly-PA, poly-PR) |
| Family | Repeat-associated non-AUG (RAN) translation products |
| Diseases | ALS, Frontotemporal Dementia |
C9orf72 Dipeptide Repeat Proteins (DPRs) are unconventional translation products generated by repeat-associated non-AUG (RAN) translation of the repeat in the C9orf expanded GGGGCC72 gene. The C9orf72 hexanucleotide repeat expansion is the most common genetic cause of both amyotrophic lateral sclerosis (ALS) andFTD), accounting for approximately 40 frontotemporal dementia (% of familial ALS and 25% of familial FTD cases. The expansion leads to three major pathogenic mechanisms: loss of C9orf72 function, toxic gain-of-function from DPRs, and RNA foci formation. Among these, DPRs are increasingly recognized as key drivers of neurodegeneration through their ability to disrupt multiple cellular processes including nucleocytoplasmic transport, stress granule dynamics, and autophagy.
[C9orf72[/entities/[c9orf72[/entities/[c9orf72[/entities/[c9orf72--TEMP--/entities)--FIX-- Dipeptide Repeat Proteins (DPRs) are proteins encoded by the [C9orf72[/genes/[c9orf72[/genes/[c9orf72[/genes/[c9orf72--TEMP--/genes)--FIX-- gene through RAN translation. The expanded GGGGCC repeat in the first intron of C9orf72 undergoes unconventional translation in all three reading frames, producing five different dipeptide repeat proteins: poly-GA, poly-GP, poly-GR, poly-PA, and poly-PR. These DPRs have distinct subcellular localizations and toxicities. Poly-GA forms the most abundant inclusions in patient brains, while poly-GR and poly-PR are highly toxic in cellular models. The repeat length correlates with age of onset and disease severity, with longer repeats causing earlier onset and more rapid progression.
Poly-GA (glycine-alanine) is the most abundant DPR found in patient tissue. It forms cytoplasmic inclusions that are ubiquitinated and contain p62. Poly-GA inclusions are thought to sequester proteins and disrupt cellular function. Studies show poly-GA can impair proteasome and autophagy function, and may act as a "sink" for important cellular proteins.
Poly-GP (glycine-proline) and poly-PA (proline-alanine) are relatively less toxic than other DPRs. They are produced in all three reading frames and can be detected in patient cerebrospinal fluid as potential biomarkers. Poly-GP may have some protective effects by sequestering more toxic DPRs.
Poly-GR (glycine-arginine) and poly-PR (proline-arginine) are the most toxic DPRs in cellular and animal models. These arginine-rich DPRs can enter the nucleus and disrupt nucleocytoplasmic transport by interacting with nuclear pore components. They also affect stress granule dynamics and RNA processing.
The arginine-rich DPRs (poly-GR and poly-PR) directly interact with nuclear pore complex (NPC) proteins, including NUP98 and NUP62. This disrupts nuclear envelope integrity and impairs the transport of proteins and RNA between the nucleus and cytoplasm. Nuclear pore dysfunction leads to transcriptional dysregulation and cellular stress.
DPRs localize to stress granules, membrane-less organelles formed under proteotoxic or oxidative stress. Poly-GR and poly-PR alter stress granule dynamics, causing aberrant granule formation and impaired disassembly. This leads to persistent stress granule accumulation and disrupted RNA metabolism.
DPRs, particularly poly-GA, impair both proteasome and autophagy function. This creates a feed-forward loop where impaired protein clearance leads to further DPR and other protein aggregate accumulation.
DPRs can localize to mitochondria and impair mitochondrial function. This includes reduced mitochondrial trafficking, impaired mitophagy, and increased oxidative stress—all contributing to neurodegeneration.
Arginine-rich DPRs can bind to RNA and DNA, potentially disrupting transcription and RNA processing. They may also sequester transcription factors and RNA-binding proteins.
Antisense oligonucleotides (ASOs) targeting C9orf72 can reduce DPR production by degrading the mutant RNA or blocking translation. Several ASO candidates have shown promise in preclinical studies and are advancing toward clinical trials.
Small molecules designed to inhibit DPR aggregation or enhance DPR clearance are under development. Compounds that enhance autophagy or proteasome function may help clear DPRs.
Compounds that bind to RNA G-quadruplex structures may reduce RNA foci formation and subsequent DPR production.
DPR levels in cerebrospinal fluid, particularly poly-GP, are being developed as biomarkers for disease progression and treatment response.
The study of C9Orf72 Dipeptide Repeat Proteins (Dprs) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.