Systematic proteomics reveals plasma NEFL as a robust predictor and pathological associate in <i>C9ORF72</i>-related neurodegeneration

<h4>Background</h4>The <i>C9ORF72</i> repeat expansion is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). While neurofilament light chain (NEFL) is an established biomarker of neuroaxonal damage, its specific dose-response relationship with the <i>C9ORF72</i> expansion and its potential role beyond a passive bystander require systematic investigation. We performed a proteome-wide screen to identify plasma proteins linked to the <i>C9ORF72</i> expansion and evaluated their predictive value for motor neuron disease (MND).<h4>Methods</h4>We utilized whole-genome sequencing and plasma proteomics from the UK Biobank, analyzing 106 individuals with <i>C9ORF72</i> expansions (defined as >30 repeats) and 212 age- and sex-matched controls. We screened ~3,000 proteins for associations with the continuous repeat count. The top candidate was evaluated using restricted cubic splines (RCS) to assess non-linearity and threshold effects. Its ability to independently predict MND risk was tested using regression models and a machine learning approach.<h4>Results</h4>Our unbiased screen identified NEFL as the sole protein significantly associated with the <i>C9ORF72</i> repeat count (FDR-adjusted <i>P</i> = 8.39 × 10^-4). NEFL levels demonstrated a step-wise increase with expansion size, which followed a stable linear trajectory across the repeat spectrum (<i>P</i> _{non - linear} = 0.4435). Elevated NEFL independently predicted MND risk (OR = 2.42; HR = 2.90), even after adjusting for the <i>C9ORF72</i> repeat count. Our predictive model, combining NEFL and repeat count, achieved an AUC of 0.941 with 100% sensitivity. These findings align with emerging evidence that secreted NEFL may actively modulate neuroinflammation.<h4>Conclusions</h4>NEFL emerges as a robust and specific plasma biomarker for <i>C9ORF72</i>-related neurodegeneration. Its strong linear association with repeat burden and independent predictive power, contextualized within its potential role in immune activation, suggest that NEFL is deeply integrated into the <i>C9ORF72</i> pathological landscape. These findings support NEFL-based screening and monitoring strategies for early intervention in <i>C9ORF72</i> carriers.