Functional Activity of TDP-43: A Direct Biomarker for ALS.

TDP-43 dysfunction is a defining feature of amyotrophic lateral sclerosis (ALS), yet no biofluid biomarker directly measures its functional activity. We developed a serum-based homogeneous time-resolved FRET (hTR-FRET) assay that quantifies TDP-43 RNA binding activity using synthetic UU-rich RNA probes. We analyzed 1080 serum samples from controls, sporadic ALS, and genetic subgroups (C9orf72, SOD1) across multiple biorepositories. Cross-sectionally, TDP-43 functional activity was elevated in ALS (mean 390 a.u.) versus controls (302 a.u.), yielding AUC = 0.79. Genotype means were 392 a.u. (sporadic), 382 a.u. (C9orf72), and 323 a.u. (SOD1); a 366 a.u. threshold achieved 95% specificity against controls. Longitudinally, Target ALS showed a modest but significant inverse correlation between TDP-43 activity and ALSFRS-R, while other cohorts exhibited similar non-significant trends. Elevated signal in serum likely reflects increased extracellular release of probe-competent TDP-43 species during cell death and exosomal shedding, rather than restored intracellular nuclear splicing function. This assay provides a proof-of-concept platform for the direct functional measurement of probe-competent TDP-43 species in serum. While it demonstrates moderate group-level discrimination, individual diagnostic performance requires prospective validation. The assay may support exploratory applications in genotype stratification and progression monitoring in future clinical studies.