Phosphorylated TDP-43 Pathology in Skin and Muscle Tissue of Patients With Amyotrophic Lateral Sclerosis

Phosphorylated TAR DNA-binding protein 43 (pTDP-43) is the pathologic hallmark of amyotrophic lateral sclerosis (ALS), yet no peripheral premortem biomarker is available. We evaluated pTDP-43 distribution in skin and tongue tissues and its association with ALS and clinical stage.

This cross-sectional case-control study included patients with ALS meeting revised El Escorial criteria who underwent skin and tongue biopsies. Control groups included healthy controls (HC), patients with non-ALS neuropathy or neuronopathy (NANN), and patients with burning mouth syndrome (BMS). pTDP-43 was quantified in Meissner corpuscles (MC) and keratinocytes using standardized immunofluorescence. In MC, PGP (%), pTDP-43 (%), and the pTDP-43/PGP ratio were assessed. A subset of skin samples underwent western blot analysis. ALS severity was classified using King's staging system. Diagnostic performance was evaluated using receiver operating characteristic analysis.

Fifty patients with ALS were included, median age 66.5 years, 36% female. Control groups included 20 HC, median age 60 years, 20% female, and 20 patients with NANN, median age 60 years, 50% female. Tongue biopsy was performed in 10 patients with ALS and 10 patients with BMS. pTDP-43 deposits were detected in ALS across epidermis and dermis structures, whereas they were almost absent in HC and low in NANN. Accordingly, MC pTDP-43% differed across groups (H = 53.30; p < 0.001), with median values of 0.35 (interquartile range [IQR] 0.39) in ALS, 0.04 (IQR 0.03) in NANN, and 0.00 in HC. The pTDP-43/PGP ratio increased with clinical stage at subject level (Z = 2.20, p = 0.028) and single-corpuscle level (H = 21.72, p < 0.001). Western blot showed higher pTDP-43/GAPDH ratio in ALS skin than in HC (median 3.45, IQR 7.23 vs 1.30, IQR 0.80; p = 0.007). Nonphosphorylated TDP-43 did not differ across groups. In keratinocytes, pTDP-43 was higher in ALS than in NANN and HC (median 0.047, IQR 0.023; 0.019, IQR 0.049; and 0.005, IQR 0.047; p < 0.001). Combined cutaneous pTDP-43 measures discriminated ALS from HC (area under the curve [AUC] 0.94, p < 0.001) and from NANN (AUC 0.90, p < 0.001). Tongue biopsies showed pTDP-43 aggregates in intramuscular nerves, denervated endplates, and muscle fibers.

Peripheral pTDP-43 deposition distinguishes ALS from controls and reflects disease stage, supporting its potential role as a biomarker of ALS and disease severity. Larger and longitudinal studies are required for validation.