Huntington’s disease (HD) is an autosomal dominant neurodegenerative disease with the age at which characteristic symptoms manifest strongly influenced by inherited HTT CAG length. Somatic CAG expansion occurs throughout life and understanding the impact of somatic expansion on neurodegeneration is key to developing therapeutic targets. In 57 HD gene expanded (HDGE) individuals, ~23 years before their predicted clinical motor diagnosis, no significant decline in clinical, cognitive or neuropsychiatric function was observed over 4.5 years compared with 46 controls (false discovery rate (FDR) > 0.3). However, cerebrospinal fluid (CSF) markers showed very early signs of neurodegeneration in HDGE with elevated neurofilament light (NfL) protein, an indicator of neuroaxonal damage (FDR = 3.2 × 10−12), and reduced proenkephalin (PENK), a surrogate marker for the state of striatal medium spiny neurons (FDR = 2.6 × 10−3), accompanied by brain atrophy, predominantly in the caudate (FDR = 5.5 × 10−10) and putamen (FDR = 1.2 × 10−9). Longitudinal increase in somatic CAG repeat expansion ratio (SER) in blood was a significant predictor of subsequent caudate (FDR = 0.072) and putamen (FDR = 0.148) atrophy. Atypical loss of interruption HTT repeat structures, known to predict earlier age at clinical motor diagnosis, was associated with substantially faster caudate and putamen atrophy. We provide evidence in living humans that the influence of CAG length on HD neuropathology is mediated by somatic CAG repeat expansion. These critical mechanistic insights into the earliest neurodegenerative changes will inform the design of preventative clinical trials aimed at modulating somatic expansion. ClinicalTrials.gov registration: NCT06391619 . A comprehensive longitudinal analysis of individuals with preclinical Huntington’s disease identifies biomarkers of neurodegeneration and somatic expansion in blood DNA, detectable years before symptom onset.

Huntington's Disease Integrated Staging System (HD-ISS) stages are likely inclusion criteria in future clinical trials. Stage 1 volumetric cut-offs were derived using the FreeSurfer longitudinal stream (LG). However, trials will require cross-sectional stream (CS) application with one MRI. Volumetric outputs are not robust to software type or version. T1-weighted images from 88 participants with MRIs from baseline and follow-up HD-YAS visits were segmented using both streams. CS calculated smaller caudate and putamen volumes adjusted for total intracranial volume, with greater reduction for larger volumes, shifting towards HD-ISS stage 1. CS-specific cut-offs need to be established before application to clinical trials.

Promising blood-based biomarkers of neuropathology have emerged with potential for therapeutic development and disease monitoring. However, these tools will require specialist tertiary services for integration into clinical management. Remote sampling for biomarker assessment could ease the burden of in-person clinical visits for such tests and allow for frequent sampling. Here we evaluated a capillary finger-prick collection for remote quantification of blood neurofilament light (NfL), a common blood-based biomarker evident in various neurological disorders, and other exploratory markers of neuronal injury and neuroinflammation (GFAP, tau). Matched samples from venepuncture and finger-prick were collected and processed into plasma and/or serum to directly compare NfL levels across four different neurological conditions (HD, MS, ALS, PD). Two delayed processing conditions were compared, three- and seven-day delay, simulating ambient shipment. Capillary NfL and GFAP concentrations were equivalent to those in venous blood serum and plasma. Only NfL remained stable after seven-day processing delay. Capillary NfL replicated disease group differences displayed in venous blood. This data supports our finger-prick method for remote collection and quantification of NfL. With the widespread applications for NfL across the spectrum of neurological disorders, this has the potential to transform disease monitoring, prognosis, and therapeutic development within clinical practice and research. Graphical abstract: Figure 1 ### Competing Interest Statement LMB holds consultancy contracts with Annexon Biosciences, Remix Therapeutics, PTC Therapeutics, Alchemab Therapeutics, and LoQus23 Therapeutics Ltd via UCL Consultants Ltd. HZ has served at scientific advisory boards and/or as a consultant for Abbvie, Acumen, Alector, Alzinova, ALZPath, Annexon, Apellis, Artery Therapeutics, AZTherapies, Cognito Therapeutics, CogRx, Denali, Eisai, Merry Life, Nervgen, Novo Nordisk, Optoceutics, Passage Bio, Pinteon Therapeutics, Prothena, Red Abbey Labs, reMYND, Roche, Samumed, Siemens Healthineers, Triplet Therapeutics, and Wave, has given lectures in symposia sponsored by Alzecure, Biogen, Cellectricon, Fujirebio, Lilly, and Roche, and is a co-founder of Brain Biomarker Solutions in Gothenburg AB (BBS), which is a part of the GU Ventures Incubator Program (outside submitted work). AN received consultancy fees during the design phase of AccessPD and reports consultancy and personal fees from AstraZeneca, AbbVie, Profile, Roche, Biogen, UCB, Bial, Charco Neurotech, Alchemab, Sosei Heptares and Britannia, outside the submitted work. AN is an Associate Editor for the Journal of Parkinsons Disease. In the previous 12 months SJT has received research grant funding from the CHDI Foundation, the National Institute for Health Research, and the UK Medical Research Council. Through the offices of UCL Consultants Ltd, a wholly owned subsidiary of University College London, SJT has undertaken consultancy services in the past 12 months for Alnylam Pharmaceuticals, Annexon, Ascidian Therapeutics, Arrowhead Pharmaceuticals, Atalanta Therapeutics, Design Therapeutics, F. Hoffman-La Roche, Iris Medicine, Latus Bio, LifeEdit, Novartis Pharma, Pfizer, Prilenia Neurotherapeutics, PTC Therapeutics, Rgenta Therapeutics, Takeda Pharmaceuticals, UniQureBiopharma, Vertex Pharmaceuticals. In the past 12 months, University College London Hospitals NHS Foundation Trust, Professor Tabrizis host clinical institution, received funding to run clinical trials for F. Hoffman-La Roche, Novartis Pharma, PTC Therapeutics, and UniQure Biopharma. ### Funding Statement This work and the salaries of LMB, AC, and AT were supported by a Medical Research Council Career Development Award (MR/W026686/1). HZ is a Wallenberg Scholar supported by grants from the Swedish Research Council (#202 01018 and #2019 02397), the European Unions Horizon Europe research and innovation programme under grant agreement No 101053962, Swedish State Support for Clinical Research (#ALFGBG 71320), the Alzheimer Drug Discovery Foundation (ADDF), USA (#201809 2016862), the AD Strategic Fund and the Alzheimers Association (#ADSF 21 831376 C, #ADSF 21 831381 C, and #ADSF 21 831377 C), the Bluefield Project, the Olav Thon Foundation, the Erling Persson Family Foundation, Stiftelsen for Gamla Tjanarinnor, Hjarnfonden, Sweden (#FO2022 0270), the European Unions Horizon 2020 research and innovation programme under the Marie Sklodowska Curie grant agreement No 860197 (MIRIADE), the European Union Joint Programme Neurodegenerative Disease Research (JPND2021 00694), the National Institute for Health and Care Research University College London Hospitals Biomedical Research Centre, and the UK Dementia Research Institute at UCL (UKDRI 1003). AN reports grants from Parkinsons UK, Barts Charity, Cure Parkinsons, National Institute for Health and Care Research, Innovate UK, Virginia Keiley benefaction, Solvemed, the Medical College of Saint Bartholomews Hospital Trust, Alchemab, Aligning Science Across Parkinsons Global Parkinsons Genetics Program (ASAP GP2) and the Michael J Fox Foundation. SJT received research grant funding from the Wellcome Trust (223082/Z/21/Z), and the UK Dementia Research Institute that receives its funding from DRI Ltd., funded by the UK MRC, Alzheimers Society, and Alzheimers Research UK. We are grateful to the United to End MND (U2EM) and UK MND Research Institute consortium for their contributions for the recruitment of the ALS patients in this study. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The National Hospital for Neurology and Neurosurgery and the Institute of Neurology Joint Research Ethics Committee of University College London gave ethical approval for this work. London, City & East Research Ethics committee of Queen Mary University of London gave ethical approval for this work. South West, Central Bristol Research Ethics Committee of Queen Mary University of London gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors