human amyotrophic lateral sclerosis (ALS)

Irreversible motor neuron loss results in widespread denervation of neuromuscular junctions (NMJs), leading to progressive neurogenic skeletal muscle atrophy, fasciculations, and spastic/flaccid paralysis. As degeneration spreads, loss of bulbar and respiratory musculature (notably the diaphragm and intercostal muscles) causes dysarthria, dysphagia, and progressive ventilatory failure. Without mechanical respiratory intervention, this condition typically leads to fatal hypoventilation within 2 to 5 years of clinical onset.

• Clinical Rating Scale (ALSFRS-R):
• • Serial assessment using the Revised ALS Functional Rating Scale (0–48 score), evaluating bulbar, fine motor, gross motor, and respiratory domains.
• Pulmonary Function Testing:
• • Longitudinal monitoring of forced vital capacity (%FVC in seated and supine positions), slow vital capacity (SVC), and sniff nasal inspiratory pressure (SNIP) to quantify diaphragmatic weakness.
• Electromyography (Needle EMG):
• • Demonstration of acute active denervation (fibrillation potentials, positive sharp waves) and chronic reinnervation changes (neurogenic motor unit action potentials) according to the Gold Coast and Awaji diagnostic criteria.
• Skeletal Muscle Biopsy Histology:
• • Verification of neurogenic atrophy characterized by grouped atrophy, small angulated esterase-positive fibers, and target fibers.

Taxonomic Applicability

• Primary Taxon: Homo sapiens (NCBI Taxon ID: 9606)
• Secondary / Model Taxa: Mus musculus (House mouse, NCBI Taxon ID: 10090), Rattus norvegicus (Norway rat, NCBI Taxon ID: 10116)
• Scientific Rationale: While human motor neuron physiology, neurofilament composition, and lifespan represent the primary domain, transgenic and knock-in rodent models harboring human ALS-associated mutations (e.g., SOD1 G93A, mutant TARDBP, FUS, C9orf72 repeat expansions) exhibit overlapping phenotypic features of motor neuron degeneration, axonal pathology, and muscle denervation.

Life Stage Applicability

• Domain: Adult, Aging / Senescent (Late adulthood)
• Scientific Rationale: ALS is primarily a late-onset neurodegenerative disorder. The median age of symptom onset in sporadic ALS is typically between 55 and 75 years, although familial forms with high-penetrance genetic mutations can present earlier in young to mid-adulthood (30s to 50s).

Sex Applicability

• Domain: Unspecific (Applies to both sexes)
• Scientific Rationale: ALS affects both males and females. A slight male predominance is clinically observed in sporadic ALS (male-to-female ratio of approximately 1.2:1 to 1.5:1), but the underlying cellular and molecular mechanisms of neurodegeneration operate across both sexes.

The development of ALS represents an apical neurotoxicological endpoint of paramount regulatory significance, falling directly under international safety assessment and human health protection frameworks:

1. Neurotoxicity Hazard Identification and Characterization:

• • Severe, irreversible neurodegeneration and motor system failure correspond directly to regulatory definitions of cumulative neurotoxicity and delayed neurotoxicity under chemical and pharmaceutical testing frameworks (e.g., OECD Test Guidelines for Neurotoxicity, US EPA Health Effects Test Guidelines, and ICH S7A/S7B safety pharmacology guidelines).

2. Support for Integrated Approaches to Testing and Assessment (IATA):

• • Defining the AO and upstream Key Events facilitates the transition toward New Approach Methodologies (NAMs)—such as human iPSC-derived motor neuron-glial coculture assays, microfluidic neuromuscular junction models, and microelectrode array (MEA) electrophysiology. This enables mechanistic chemical screening and hazard ranking without sole reliance on long-term animal studies.

3. Identification of Chemical and Environmental Risk Factors:

• • Regulatory agencies evaluate environmental contaminants, occupational neurotoxicants (e.g., heavy metals, organophosphates, solvents), and xenobiotics for their potential to trigger or accelerate motor neuron death via proteostasis failure, oxidative stress, and mitochondrial impairment, informing occupational exposure limits and environmental safety thresholds.

4. Pharmaceutical and Biotherapeutic Safety:

• • In drug discovery and preclinical safety evaluations, monitoring intermediate biomarkers aligned with this AO (e.g., elevated biofluid neurofilament light chain [NfL], cryptic exon inclusion) enables early de-risking of therapeutics that could inadvertently induce off-target axonal degeneration or disrupt RNA-binding protein homeostasis.

• Cedarbaum, J. M., et al. (1999). The ALSFRS-R: a revised ALS functional rating scale that incorporates assessments of respiratory function. Journal of the Neurological Sciences, 169(1-2), 13–21.
• Shefner, J. M., et al. (2020). A proposal for new diagnostic criteria for ALS: The Gold Coast Criteria. Clinical Neurophysiology, 131(8), 1975–1978.
• de Carvalho, M., et al. (2008). Electrodiagnostic criteria for diagnosis of ALS. Clinical Neurophysiology, 119(3), 497–503.