Neurofilament Light Chain in ALS: Roles, Subgroups, and More

Neurofilament light chain (NfL) has emerged as a key biomarker in amyotrophic lateral sclerosis (ALS), offering insights into disease progression and neuronal damage. Its presence in blood and cerebrospinal fluid reflects motor neuron breakdown, making it valuable for diagnosis and monitoring.

Understanding how NfL varies among ALS subgroups and what influences its levels can provide critical information for patient management. Additionally, exploring its relationship with disease severity may help refine prognosis and treatment strategies.

Role in Motor Neuron Disease Biology

Neurofilament light chain (NfL) is a structural component of neuronal cytoskeletons, particularly within axons, where it contributes to stability and intracellular transport. In ALS, motor neuron degeneration disrupts these structures, releasing NfL into the extracellular space and, subsequently, cerebrospinal fluid (CSF) and blood. Its concentration in these fluids serves as a measurable indicator of neuronal damage.

The elevation of NfL in ALS is directly linked to the rapid breakdown of motor neurons, driven by oxidative stress, mitochondrial dysfunction, and impaired axonal transport. Studies have shown that axonal swelling and fragmentation precede overt neuronal loss, suggesting NfL release occurs early in disease progression. Longitudinal research has detected increased NfL levels in pre-symptomatic individuals carrying ALS-associated genetic mutations, reinforcing its role as an early biomarker of neurodegeneration.

Beyond its structural function, disruptions in neurofilament homeostasis can exacerbate disease pathology by impairing axonal transport, leading to the accumulation of misfolded proteins and organelle dysfunction. Experimental models of ALS, including transgenic mice with mutant superoxide dismutase 1 (SOD1), show that altered neurofilament dynamics contribute to axonal dieback, a precursor to motor neuron death. These findings suggest NfL is not just a passive marker of injury but may actively influence disease progression.

Differences in Neurofilament Light Chain Among ALS Subgroups

NfL levels vary among ALS subgroups, reflecting differences in disease pathology, progression rates, and genetic factors. Patients with bulbar-onset ALS, where symptoms affect speech and swallowing first, consistently exhibit higher NfL concentrations in CSF and blood compared to those with limb-onset ALS. This aligns with the more aggressive disease course of bulbar-onset cases, where neuronal degeneration advances rapidly, leading to earlier respiratory compromise and shorter survival. Longitudinal studies confirm that NfL increases more steeply in bulbar-onset ALS, reinforcing its prognostic value.

Genetic forms of ALS also show variability in NfL levels. Individuals with C9orf72 mutations, the most common genetic cause of ALS, often present with elevated NfL even before symptoms appear, suggesting early axonal degeneration. Conversely, patients with certain SOD1 mutations, such as p.Ala4Val, tend to have lower NfL concentrations, reflecting slower disease progression. This highlights the molecular heterogeneity of ALS, where different mechanisms influence the extent and pace of neurodegeneration.

Sporadic ALS, the most common form, exhibits a broad range of NfL levels influenced by age at onset and functional decline rate. Rapidly progressing cases show NfL concentrations comparable to aggressive genetic variants, while slower-progressing individuals display lower values. Regional differences in motor neuron vulnerability, particularly corticospinal tract involvement, also contribute to NfL variability.

Factors Influencing Serum Levels

Serum NfL levels are shaped by multiple physiological and pathological factors. One major determinant is the efficiency of NfL clearance from the central nervous system (CNS) into the bloodstream. Since NfL is initially released into CSF, its serum levels depend on blood-brain barrier (BBB) permeability. Conditions that weaken the BBB, such as chronic inflammation or vascular dysfunction, can increase NfL diffusion into circulation, potentially amplifying serum concentrations independent of neuronal loss.

Age also affects baseline NfL levels, as neurofilament turnover increases with normal aging due to gradual axonal attrition. Even in healthy individuals, serum NfL concentrations rise over time, reflecting cumulative neuronal wear and tear. In ALS, distinguishing pathological neurodegeneration from age-related changes is critical for accurate biomarker interpretation. Some research efforts focus on establishing age-adjusted reference ranges to improve diagnostic accuracy.

Technical factors related to sample handling and assay variability also impact serum NfL readings. Pre-analytical conditions, such as blood collection timing, storage temperature, and freeze-thaw cycles, influence NfL stability. Standardization efforts, including ultrasensitive immunoassays like single-molecule array (Simoa) technology, have improved measurement precision, but inter-laboratory discrepancies remain. Harmonizing protocols across research centers and clinical trials is essential to ensure reliable NfL data.

Relationship to Disease Severity

NfL levels in serum and CSF correlate with ALS severity, with higher concentrations linked to faster functional decline and reduced survival. Patients with elevated NfL at diagnosis typically experience rapid deterioration in motor function, as measured by the ALS Functional Rating Scale-Revised (ALSFRS-R). Unlike subjective clinical assessments, NfL provides an objective, quantifiable measure of disease burden.

Longitudinal studies show that a steeper rise in serum NfL is associated with faster declines in muscle strength and respiratory function, hallmarks of worsening ALS. This trend is particularly evident in aggressive disease phenotypes, where NfL elevations precede noticeable symptom progression. In contrast, individuals with slower-progressing ALS tend to maintain relatively stable NfL levels. These findings suggest that tracking NfL over time could help guide treatment decisions based on predicted disease trajectories.