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

Background and objectives: 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. Methods: 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. Results: 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. Discussion: 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.