Phosphorylated Neurofilament Heavy Chain (pNfH) as a Biomarker in Motor Neuron Disease
Phosphorylated neurofilament heavy chain (pNfH) is the larger sibling to NfL in the neurofilament biomarker family. While both proteins are released when axons degenerate, they reflect different aspects of neuronal injury and may complement each other in a biomarker panel. pNfH's most important validation came from the MIROCALS trial — a landmark ALS study where pNfH stratification revealed a treatment effect that the overall trial population had obscured. For PLS specifically, direct evidence is limited, but the MIROCALS result has clear implications for how future PLS trials should be designed.
What pNfH is
Neurofilaments are the structural scaffolding of neuronal axons. The neurofilament complex has three components — light (NfL), medium (NfM), and heavy (NfH) chains — that interact to maintain axon caliber and integrity. The heavy chain is distinctive for its long phosphorylated tail, which is heavily modified by phosphorylation. This phosphorylated form (pNfH) is found at particularly high concentrations in large-diameter myelinated axons — the type of fiber that makes up the corticospinal tract, which is the pathway selectively affected in PLS.
When corticospinal axons degenerate — as they do in PLS — pNfH is released into the CSF and, to a lesser extent, into the blood. Because pNfH is concentrated in the large-diameter myelinated fibers of the motor system, some researchers have proposed that it may be more specific to motor neuron degeneration than NfL, which is released from axons of all sizes and in many neurological conditions.
Whether pNfH is truly more specific to motor neuron pathology than NfL remains debated. What is established is that it provides complementary information and that combining pNfH with NfL gives a more complete picture of disease severity than either alone.
How it is measured
pNfH is measurable from both cerebrospinal fluid (CSF, via lumbar puncture) and blood serum or plasma. CSF measurement is more sensitive — pNfH enters the blood at lower concentrations than NfL, making serum assays technically more challenging. The 2022 paper in Journal of Neurophysiology validated a serum pNfH assay that was complementary to NfL for characterizing disease severity in ALS patients — an important step toward blood-based monitoring.
Like NfL, pNfH is measured by immunoassay, with Simoa platforms offering the highest sensitivity for blood-based detection. CSF pNfH assays using ELISA are more established and were used in the MIROCALS trial.
MIROCALS trial: the landmark pNfH stratification finding
The MIROCALS trial tested low-dose interleukin-2 (IL-2) as a potential immunomodulatory treatment for ALS. Results were published in The Lancet and represented a significant advance in understanding how to use biomarkers in MND trial design.
The primary trial result was complex: the overall ALS population did not show a statistically significant treatment effect on survival. But when the investigators stratified participants by baseline CSF pNfH level, a striking difference emerged:
Patients with lower pNfH (approximately 80% of trial participants) showed a greater than 40% reduction in risk of death with low-dose IL-2 compared to placebo. The patients with higher pNfH (more advanced motor neuron degeneration at baseline) did not show the same benefit. This suggests that IL-2 may be more effective in patients with less severe biological disease activity — and that pNfH is a valid stratification tool for identifying those patients.
The MIROCALS finding established pNfH as more than a correlate — it showed that pNfH stratification could reveal clinically meaningful treatment effects that a non-stratified trial would miss. This has direct implications for ALS and, by extension, PLS trial design: any future treatment study that ignores baseline biological stratification risks diluting a real treatment effect by mixing patients who respond with patients who do not.
pNfH in ALS: broader evidence
Beyond MIROCALS, pNfH has been documented as a reliable indicator of motor neuron damage severity in multiple ALS cohorts. It correlates positively with disease progression rate and with clinical severity measures. The 2022 Journal of Neurophysiology study confirmed elevated serum pNfH in ALS patients compared to controls, showing that pNfH measured from blood (not just CSF) could complement NfL for characterizing disease severity and monitoring. pNfH has also been included in multi-biomarker panels alongside NfL and GFAP in systematic analyses of ALS biomarker performance (Poesen & Van Damme, Frontiers 2019).
pNfH in PLS: what is known and what is not
Direct PLS-specific pNfH data is limited. No large dedicated PLS cohort has yet characterized pNfH levels systematically. The available evidence consists of:
- PLS patients occasionally enrolled in ALS biomarker studies — typically as subgroup comparators — where pNfH tends to be lower in PLS than in ALS, consistent with the slower rate of motor neuron degeneration in PLS.
- The biological rationale that pNfH would be elevated in PLS — since the corticospinal tract, which is rich in heavy-chain neurofilaments, is the primary site of pathology — but at lower levels than in ALS.
- The MIROCALS finding implying that in any disease where pNfH stratification might predict treatment response, baseline pNfH measurement is clinically meaningful.
Whether pNfH performs as well in PLS as it does in ALS — given that PLS's slower degeneration rate may compress pNfH values toward the lower end of assay sensitivity — is not yet established from primary PLS data.
Why it matters for PLS
The MIROCALS result is the most compelling argument for including pNfH in future PLS trial design. If low-dose IL-2 — or any future treatment — has a differential effect based on biological disease activity at baseline, then a PLS trial that ignores pNfH stratification may miss a real treatment effect in the patients most likely to benefit. This is particularly important in PLS, where small sample sizes (due to rarity) and slow progression make detection of treatment effects difficult. Any tool that reduces variance or identifies a more responsive subgroup is valuable.
PLS-specific pNfH data is one of the gaps the PLS Natural History Study biorepository is positioned to fill. As PNHS biospecimens are analyzed, pNfH measurements will become available alongside the NfL data already published (p=0.001 with PLSFRS decline).
How this connects
pNfH complements the more extensively validated NfL biomarker in a panel approach described on the multi-biomarker panel page. The MIROCALS trial stratification strategy is the precedent for how pNfH could be used in future PLS trials. For the broader biomarker landscape in PLS, see the Biomarker Research hub. How these biomarkers might contribute to diagnosis is on the Diagnosis page.
Key studies referenced
MIROCALS trial investigators. Low-dose IL-2 in ALS: MIROCALS trial results.
The Lancet. [year of publication].
Phosphorylated neurofilament heavy chain as a potential diagnostic biomarker in ALS.
Journal of Neurophysiology. 2022.
Poesen K, Van Damme P. Diagnostic and prognostic performance of neurofilaments in ALS.
Frontiers in Neurology. 2019.