Development of a sensitive trial-ready poly(GP) CSF biomarker assay for <i>C9orf72</i>-associated frontotemporal dementia and amyotrophic lateral sclerosis.

Abstract

A GGGGCC repeat expansion in the <i>C9orf72</i> gene is the most common cause of genetic frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). As potential therapies targeting the repeat expansion are now entering clinical trials, sensitive biomarker assays of target engagement are urgently required. Our objective was to develop such an assay. We used the single molecule array (Simoa) platform to develop an immunoassay for measuring poly(GP) dipeptide repeat proteins (DPRs) generated by the <i>C9orf72</i> repeat expansion in cerebrospinal fluid (CSF) of people with <i>C9orf72</i>-associated FTD/ALS. We show the assay to be highly sensitive and robust, passing extensive qualification criteria including low intraplate and interplate variability, a high precision and accuracy in measuring both calibrators and samples, dilutional parallelism, tolerance to sample and standard freeze-thaw and no haemoglobin interference. We used this assay to measure poly(GP) in CSF samples collected through the Genetic FTD Initiative (N=40 <i>C9orf72</i> and 15 controls). We found it had 100% specificity and 100% sensitivity and a large window for detecting target engagement, as the <i>C9orf72</i> CSF sample with the lowest poly(GP) signal had eightfold higher signal than controls and on average values from <i>C9orf72</i> samples were 38-fold higher than controls, which all fell below the lower limit of quantification of the assay. These data indicate that a Simoa-based poly(GP) DPR assay is suitable for use in clinical trials to determine target engagement of therapeutics aimed at reducing <i>C9orf72</i> repeat-containing transcripts.