From Mutation to Manifestation: Penetrance in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease characterized by progressive loss of motor neurons in the brain and spinal cord. While most cases are sporadic, around 10% are familial. Recent genetic studies show that many apparently isolated cases carry pathogenic mutations, highlighting the importance of penetrance, the probability that a causal mutation manifests clinically. This review focuses on mutation penetrance in ALS (C9orf72, SOD1, TARDBP, FUS genes), its variability across genes, age, and environmental or genetic modifiers, and its implications for genetic counseling. Identification of pathogenic mutations informs the monitoring of relatives and, in some cases, gives access to targeted therapies or clinical trials. Counseling of asymptomatic relatives must consider incomplete penetrance, which can lead to delayed or absent disease manifestation. ALS exists on a clinical and genetic continuum including related disorders, such as frontotemporal dementia, further influencing risk interpretation. Advances in panel, whole-exome and whole-genome sequencing refine our understanding of penetrance and enable precise diagnostics, and potential tailored therapies. Understanding penetrance is therefore essential to translate mutation discovery into informed clinical decisions and genetic counseling in ALS.