Characterising the biology of neurofilament light protein as a translational biomarker for Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by a multitude of severe motor, cognitive, and psychiatric symptoms. It is caused by the inheritance of an expanded CAG repeat in the huntingtin gene1. Symptoms typically manifest between the ages of 30 and 50 years old, with underlying pathological changes occurring decades prior to onset2. These landmarks have been utilized alongside later cognitive, motor and functional markers to develop the HD Integrated Staging System (HD-ISS), a framework for characterising mutant huntingtin (mHTT) gene carriers for clinical trials and research purposes3.

Whilst there are currently no disease-modifying therapies available, numerous potential drug candidates exist within the developmental pipeline. However, the success of these potential treatments may be limited by the absence of biological tools to sensitively measure clinical benefit across short time periods. Prior to clinical manifestation, disease progression in HD is marked solely by MRI measurements of brain atrophy within the caudate and putamen of the basal ganglia3. Accurate measurement of potential therapeutic efficacy is challenging due to the slow rate of brain atrophy, the posited irreversible nature of brain loss in the earliest phases of the disease, and the small trial sample size in rare diseases such as HD. Moreover, atrophy is thought to be preceded by pathological changes at the molecular level4. The validation of an accessible, sensitive molecular biomarker would prove extremely useful both as a means of enriching the HD-ISS to add granularity to the early premanifest stages, and as a potential surrogate endpoint for future disease modifying clinical trials. Neurofilament light (NfL) has emerged as a promising candidate for this; it serves as an indicator of neuronal damage, measurable in both CSF and blood, and is significantly correlated with multiple clinical severity scores in HD5. Moreover, NfL levels in CSF have been shown to decrease in response to neuroprotective treatment in children with spinal muscular atrophy, thus providing a measure of the ongoing rate of neurodegeneration6.

My PhD project seeks to explore NfL as a translational biomarker for Huntington's disease. This aim will be pursued through two concurrent approaches: utilizing observational study data and computational methods to investigate NfL as a prognostic biomarker in mHTT carriers, whilst simultaneously investigating NfL's viability as both an efficacy and safety biomarker in induced pluripotent stem cell (iPSC) derived cell models of HD.

At the computational level, existing data from multiple observational studies will be combined and harmonised, before being categorised according to the HD-ISS framework3. This will generate the largest NfL HD dataset to date, allowing NfL variations to be mapped throughout the progression of the disease, representative of diverse ages and CAG repeat lengths. Retrospective analyses will then be performed to investigate NfL's potential as a prognostic biomarker in HD, with significant implications for its potential future use as a surrogate endpoint in clinical trials. The project will build upon the skills I acquired during my third rotation with Dr Lauren Byrne, in which I learnt to code in Stata to retrospectively analyse plasma NfL levels within a cohort of mHTT carriers. I will develop the code I wrote during this rotation whilst also learning MATLAB to develop a novel harmonization protocol for NfL measurements across studies.
In parallel, NfL dynamics will be investigated in vitro to assess its potential as both a safety biomarker and measure of drug efficacy in cell models of HD. Both 2D iPSC derived striatal cell cultures and 3D cortico-striatal organoids will be utilized, where cells and media will be collected, and extracellular vesicles (EVs) will be subsequently isolated from the media. The 2D models will utilize medium spiny neurons (MSNs) and cortical