High levels of mutant huntingtin protein in tear fluid from Huntington's Disease Gene Expansion Carriers

OBJECTIVE: Huntington's disease (HD) is an autosomal dominant, fully penetrant, neurodegenerative disease that most commonly affects adults in mid-life. HD is caused by a CAG repeat expansion in the HTT gene, resulting in the expression of mutant huntingtin (mHTT). Our aim was to detect and quantify mHTT in tear fluid, which to our knowledge has never been measured before. METHODS: We recruited 20 manifest, 13 premanifest HD gene expansion carriers (HDGECs) and 20 age-matched controls. All patients underwent detailed assessments, including Unified Huntington's Disease Rating Scale (UHDRS) total motor score (TMS) and total function capacity score. Tear fluid was collected using paper Schirmer's strips. The level of tear mHTT was determined using the Single Molecule Counting SMCxPRO technology. RESULTS: Average tear mHTT levels in manifest (67,223 ± 80,360 fM) and premanifest patients (55,561 ± 45,931 fM) were significantly higher than in controls (1622 ± 2179 fM). We noted significant correlations between tear mHTT levels and CAG repeat length, 'estimated years to diagnosis', disease burden score and UHDRS TMS and TFC. The ROC curve demonstrated an almost perfect score (AUC = 0.9975) when comparing controls to manifest patients. Similarly, the AUC between controls and premanifest patients was 0.9846. The optimal cut-off value to distinguish between controls and manifest patients was 4544 fM, whereas it was 6596 fM for the distinction between controls and premanifest patients. CONCLUSIONS: Tear mHTT levels have the potential for early and non-invasive detection of alterations in HD and could be integrated into both clinical trials and clinical diagnostics.