The Genetic Architecture of ALS: From SOD1 to C9ORF72 and Beyond

Genetic discoveries over the past three decades have transformed our understanding of motor neuron degeneration. While familial ALS (fALS) accounts for only 5–10% of total cases, identifying its causative mutations provides invaluable insights into common pathogenic pathways shared by sporadic forms.

Major Familial ALS Genes

The discovery of mutations in the Superoxide Dismutase 1 (SOD1) gene on chromosome 21q22.1 in 1993 marked a major milestone in ALS research. Since then, numerous causative genes have been identified:

Gene	Locus	Inheritance Pattern	Key Clinical Features & Associated Phenotypes
C9ORF72	9p21	Autosomal Dominant	Accounts for >30% of fALS; strongly associated with Frontotemporal Dementia (FTD).
SOD1	21q22.1	Autosomal Dominant / Recessive	Accounts for ~20% of fALS; classic adult-onset motor neuron degeneration.
TARDBP	1p36.2	Autosomal Dominant	Encodes TDP-43 protein; forms characteristic neuronal protein aggregates.
FUS	16p11.2	Autosomal Dominant / Recessive	Associated with typical ALS as well as juvenile/early-onset forms.
FIG 4	6q21	Autosomal Dominant	Characterized by rapidly progressive disease with prominent corticospinal tract signs.
UBQLN2	Xp11	X-linked Dominant	Unique inheritance; upper motor neuron signs typically precede lower motor neuron signs.

Slowly Progressive and Variant Phenotypes

Not all genetic subtypes follow the typical rapid trajectory. Genes such as Alsin (ALS2), SETX (Senataxin/ALS4), Spatacsin (SPG11/ALS5), and Optineurin (OPTN/ALS12) are frequently linked to juvenile-onset or unusually slow-progressing forms of motor neuron disease.

Furthermore, genes like VCP, ANG, and C9ORF72 bridge neurodegenerative spectra, overlapping with frontotemporal dementia, parkinsonism, and primary open-angle glaucoma. In sporadic ALS (sALS), susceptibility risk variants have also been mapped to loci including ATXN2, NEFH, SMN1/SMN2, and VEGF.\