Nogo-A marks motor neuron disease

More than a century after Charcot's description of the disease, amyotrophic lateral sclerosis continues to be diagnosed, tracked, and treated based almost entirely on clinical findings. Electromyography has become an essential supplementary test for confirming lower motor neuron (LMN) degeneration. However, notwithstanding the evolving use of diffusion tensor MRI and transcranial magnetic stimulation, there are currently no reliable supplemental markers for upper motor neuron (UMN) pathology or for ALS in general.

In this issue of Annals, Pradat and colleagues add to their growing body of work suggesting that muscle expression of the Nogo-A protein serves as a biomarker for ALS (*ref). Previous findings by the same group had shown that Nogo-A expression increases in both humans and a widely-used mouse model of ALS¹; and that Nogo-A muscle levels correlate with severity on the ALS-functional rating scale². In the current study, Pradat et al. investigate the utility of Nogo as a biomarker in an important clinical scenario: Can we predict which patients presenting solely with LMN symptoms will progress toward full-blown ALS as opposed to a more benign pathology?

Pradat et al. examined muscle biopsies from 33 patients who presented with “lower motor neuron syndromes” (LMNS) that did not meet El Escorial criteria for diagnosis of probable or definite ALS **check**. The biopsies were probed by Western blotting for the presence of Nogo-A, a protein that inhibits nerve sprouting and regeneration, and that is normally found only in central nervous system tissue. Patients were then clinically followed **for up to 12 months** by investigators blinded to Nogo-A status. Remarkably, of the 17 patients in whom muscle Nogo-A was detected, 15 progressed to a diagnosis of ALS **chk prob/definite?**, whereas only one of 16 Nogo-A-negative patients progressed to ALS. These results demonstrate 94% sensitivity and 88% specificity for the prediction of progression from LMNS to ALS.

The potential benefits of a reliable biomarker for ALS are multiple:

1. Earlier diagnosis. Muscle-specific Nogo expression was detected as early as three months after LMN symptom onset. Earlier diagnosis allows earlier initiation of treatment (or treatment trials), potentially delaying or even preventing clinical onset of UMN symptoms.

2. More accurate diagnosis. Along with the findings presented in the current study, this group has previously shown that muscle-specific Nogo-A expression is not merely a consequence of denervation, as denervation under non-ALS conditions did not result in increased muscle Nogo-A expression¹.

3. Better objective tracking of disease progression. A previous paper by this group suggested that absolute levels of muscle-specific Nogo-A expression correlate with severity on the ALS-functional rating scale². However, this study by necessity only examined muscle biopsies at one time point per patient, precluding the ability to make any conclusions regarding Nogo levels and disease progression.

4. Insight into ALS pathogenesis. Nogo could play a role in motor neuron disease through one or more mechanisms. Its ability to inhibit axon sprouting and regeneration in the context of spinal cord injury and stroke has been well-documented³. Thus, if abnormally expressed in muscle, Nogo could conceivably limit surviving motor neurons' ability to form compensatory sprouts to replace dying neighboring motor units. Indeed, previous work by this group showed that Nogo over-expression in soleus muscle caused retraction of nerve terminals from the neuromuscular junction⁴. Alternatively, Nogo's membership in the Reticulon family of proteins suggests a different potential link to motor neuron disease. Though not well understood, Reticulons are thought to play a role in endoplasmic reticulum (ER) structure and trafficking^5-7. Mutations in the ER protein VAP-B, as well as the vesicle-trafficking factor Alsin, have been associated with familial forms of ALS^8-10. Perhaps Nogo plays an as-yet-unidentified role in ER metabolism that may even protect against motor neuron pathology. As none of the patients in the current study had a family history of ALS, Nogo may play more of a reactive rather than an initiating role in disease onset.

Though the study by Pradat et al. establishes Nogo-A as an interesting candidate biomarker for ALS, several issues need to be addressed before it could become a widely utilized supplemental test in neuromuscular clinics. Perhaps most importantly, an ideal disease marker should be easily detectable in serum or cerebrospinal fluid, rather than sacrificing precious muscle to a biopsy. Indeed, Nogo-A is detectable in CSF, as shown in the context of multiple sclerosis by two groups^{11, 12}. However, this raises the possibility that Nogo-A testing in ALS may not be as specific as desired. The small patient population studied by Pradat et al. all presented with a similar constellation of LMN symptoms without UMN symptoms. *There were no MS patients in the group*. Thus, this study needs to be repeated in a broader patient population to confirm the true specificity of Nogo-A testing in the general clinical setting.

Giants of neurology fear not – for the near term, we must still rely mostly on clinical acumen to diagnose and follow patients with ALS. But the day of a convenient and widely available ALS biomarker draws near. Until then, Pradat et al. have provided an intriguing clue into a potential pathway of ALS pathogenesis.