From molecular convergence to clinical divergence: Comparative pathogenic mechanisms and therapeutic trajectories in C9orf72-ALS/FTD and myotonic dystrophy

Short tandem repeat expansions in C9orf72, DMPK, and CNBP genes cause amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD) and myotonic dystrophy types 1 and 2 (DM1/DM2), respectively. Despite distinct clinical phenotypes, these disorders share convergent molecular mechanisms with tissue-specific vulnerability, offering a framework to inform precision therapeutic strategies. Shared pathogenic features include nuclear RNA foci sequestering RNA-binding proteins that disrupt splicing, and repeat-associated non-AUG translation generating toxic dipeptide repeat proteins. In C9orf72, GGGGCC repeats form RNA-driven condensates, including protein-free condensates, via G-quadruplex formation. Evidence also implicates autophagy-lysosome and mitochondrial dysfunction, suggesting a potential "two-hit" loss/gain-of-function model. Clinically, C9orf72 expansions primarily affect motor neurons and frontotemporal circuits, with ALS progression typically occurring over 2-5 years. Conversely, myotonic dystrophy manifests as a muscle-predominant multisystem disorder progressing over decades. Genomic instability contributes to disease variability, with anticipation and parent-of-origin effects strongest in DM1, not confirmed in DM2 and controversial in C9orf72. Sequence interruptions modulate repeat stability and phenotype, influencing diagnostic interpretation. Therapeutic development has yielded contrasting outcomes. Antisense oligonucleotides targeting C9orf72 achieved target engagement and reduced dipeptide repeat proteins but failed clinically, potentially due to sense-strand selectivity and persistence of TDP-43 pathology. In contrast, RNA-targeting conjugates for DM1 (delpacibart etedesiran and DYNE-101) received FDA Breakthrough Therapy designation. Therapeutic success depends on tissue accessibility and addressing both shared and circuit-specific pathogenic cascades. While nuclear RNA targets appear druggable in myotonic dystrophy, the bidirectional transcription and compartmentalized pathology of C9orf72 ALS/FTD may require multi-targeted approaches for precision medicine.