The genetics of autosomal recessive ALS: a review of the common forms and their phenotypes

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease marked by progressive degeneration of upper and lower motor neurons. Most forms of ALS associated with a suspected causal variant are inherited in an autosomal dominant manner. However, there is an important subset of autosomal recessive (AR) variants, often associated with early-onset or atypical clinical features. Advances in genetic sequencing have led to increased recognition of AR ALS. In this review, we focus on four key confirmed AR ALS-associated genes, which appear to be most common-ALS2, SPG11, OPTN, and the D90A variant of SOD1-reviewing their pathophysiology and unique clinical manifestations. We also highlight very rare AR mutations implicated in ALS, including SYNE1, ATP13A2, and FUS, and some associated with overlap syndromes or debated pathogenicity including SIGMAR1, ERLIN1, and ERLIN2. These genes are involved in an array of processes including axonal transport, endosomal trafficking, oxidative stress response, and autophagy, suggesting distinct mechanisms of motor neuron degeneration. Some forms of AR ALS more frequently present with juvenile onset and slower progression, but other genes are associated with broader phenotypic spectra. This includes overlap with hereditary spastic paraplegia (HSP) and hereditary ataxias. Understanding these AR forms of ALS may enhance diagnostic precision, improve prognostication, and may pave the way for targeted gene therapies. This review underscores the emerging significance of AR inheritance in ALS and calls for deeper investigation into its molecular and clinical dimensions.