Objectives
Define structural changes in cutaneous peripheral nerve innervation and identify neurodegenerative protein deposition in patients with amyotrophic lateral sclerosis (ALS).
Background
ALS is a progressive neurodegenerative disorder with no known cure. One limitation in the development of disease modifying therapies is the absence of a reliable diagnostic and therapeutic biomarker. Over the past decade, TAR DNA-binding protein of 43 kDa (TDP-43) has been identified as a misfolded protein in ALS. Hyperphosphorylated, fragmented, and ubiquitinated forms of TDP-43 have been identified as core components of cytosolic inclusions in cortex and motor neurons in sporadic ALS patients. We hypothesized that TDP-43 could be detected in cutaneous peripheral nerves and could result in axonal structural changes.
Methods
21 patients with ALS (N=11) and healthy controls (N=10) underwent ALS-FRS, ALS-CBS and three skin biopsies were performed at the distal leg, distal thigh, and posterior cervical sites. All skin biopsies were evaluated for cutaneous TDP-43 and phosphorylated TDP-43 deposition using immunofluorescence techniques at CND Life Sciences. Peripheral sensory and autonomic nerves were immunostained with protein gene product 9.5 and nerve fiber densities quantified.
Results
Of the sporadic ALS subjects (N=11), we detected phosphorylated TDP-43 in cutaneous nerves of two subjects. There was a length-dependent reduction in intra-epidermal nerve fiber density (P<0.001 vs controls), a severe reduction in sympathetic adrenergic innervation within affected limbs (P<0.001 vs controls) that correlated with disease severity. There was no impact on sympathetic cholinergic innervation.
Conclusions
We report the detection of cutaneous intra-axonal phosphorylated TDP-43 in a small subset of patients with ALS. Although preliminary, further investigation with technical modifications could increase rates of protein detection. We also identified a significant reduction in sympathetic adrenergic innervation within the affected limbs of individuals with ALS, a potential novel marker of disease severity. Further studies are needed to expand upon these findings and determine if skin biopsy has a potential role in the investigation of ALS.