Beyond Monogenic Inheritance: Emerging Genetic Architecture of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by marked genetic and phenotypic heterogeneity. Although pathogenic variants in major ALS genes, including C9ORF72, SOD1, TARDBP, and FUS, have provided fundamental insights into disease pathogenesis, many familial and most sporadic cases remain without a molecular diagnosis, suggesting a genetic architecture extending beyond coding variants and monogenic inheritance. Advances in genomic technologies have revealed contributions from deep-intronic and regulatory variants, structural rearrangements, repeat expansions, and other forms of genomic variation incompletely captured by standard diagnostic approaches. Evidence also indicates that combinations of rare variants with moderate or incomplete penetrance may contribute to disease susceptibility and clinical variability, supporting a multilayer model of ALS genetics. Population-specific variation and convergence of genetically diverse defects on common molecular pathways further complicate variant interpretation and disease classification. This review examines emerging components of ALS genetic architecture, including non-coding variation, structural variants, repeat expansions, oligogenic burden, ancestry-related gaps, and convergent neurodegenerative pathways. Integrating these genomic layers with functional and transcriptomic evidence may improve variant interpretation, diagnostic yield, patient stratification, and the development of mechanism-based precision therapies.