Pfeiffer, Ronald F.

Abstract Background Spinocerebellar ataxia type 3 (SCA3), the most common autosomal dominant ataxia, is driven by the accumulation of polyglutamine‐expanded (polyQ) ATXN3 proteins. While promising as biomarkers, their longitudinal trajectories across multiple biofluids remain poorly defined. Objectives To quantify polyQ ATXN3 levels in cerebrospinal fluid (CSF), plasma, and urine within a comprehensive cohort, utilizing serial measurements to map protein dynamics. Methods We employed a validated immunoassay to quantify polyQ ATXN3 in 97 symptomatic and 13 presymptomatic SCA3 patients, correlating levels with clinical features, ancestry, disease status, and longitudinal progression. Results Asian participants exhibited lower plasma but elevated urinary polyQ ATXN3 levels relative to other ancestries. While CSF levels were higher in symptomatic patients at baseline, they showed a significant longitudinal decline. Conclusions PolyQ ATXN3 is a viable multi‐biofluid biomarker. Declining CSF levels likely reflect neurodegeneration, supporting its role in tracking progression and emphasizing the need for ancestry‐based adjustment in trials. © 2026 International Parkinson and Movement Disorder Society.

PolyQ ATXN3 is a pharmacodynamic marker for SCA3, and a SNP associated with the expanded allele may serve to specifically target mutant ATXN3 .

AbstractAlthough coding variants in THAP1 have been causally associated with primary dystonia, the contribution of noncoding variants remains uncertain. Herein, we examine a previously identified Intron 1 variant (c.71+9C>A, rs200209986). Among 1672 subjects with mainly adult‐onset primary dystonia, 12 harbored the variant in contrast to 1/1574 controls (P < 0.01). Dystonia classification included cervical dystonia (N = 3), laryngeal dystonia (adductor subtype, N = 3), jaw‐opening oromandibular dystonia (N = 1), blepharospasm (N = 2), and unclassified (N = 3). Age of dystonia onset ranged from 25 to 69 years (mean = 54 years). In comparison to controls with no identified THAP1 sequence variants, the c.71+9C>A variant was associated with an elevated ratio of Isoform 1 (NM_018105) to Isoform 2 (NM_199003) in leukocytes. In silico and minigene analyses indicated that c.71+9C>A alters THAP1 splicing. Lymphoblastoid cells harboring the c.71+9C>A variant showed extensive apoptosis with relatively fewer cells in the G2 phase of the cell cycle. Differentially expressed genes from lymphoblastoid cells revealed that the c.71+9C>A variant exerts effects on DNA synthesis, cell growth and proliferation, cell survival, and cytotoxicity. In aggregate, these data indicate that THAP1 c.71+9C>A is a risk factor for adult‐onset primary dystonia.