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Harding RJ, Xie Y, Caron NS, Findlay-Black H, Lyu C, Potluri N, Chandrasekaran R, Hayden MR, Leavitt BR, Langbehn DR, Southwell AL. Challenges and advances for huntingtin detection in cerebrospinal fluid: in support of relative quantification. Biomark Res 2025; 13:63. [PMID: 40259428 PMCID: PMC12012996 DOI: 10.1186/s40364-025-00772-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2024] [Accepted: 03/27/2025] [Indexed: 04/23/2025] Open
Abstract
Huntington disease (HD) is a progressive and devastating neurodegenerative disease caused by expansion of a glutamine-coding CAG tract in the huntingtin (HTT) gene above a critical threshold of ~ 35 repeats resulting in expression of mutant HTT (mHTT). A promising treatment approach being tested in clinical trials is HTT lowering, which aims to reduce levels of the mHTT protein. Target engagement of these therapies in the brain are inferred using antibody-based assays that measure mHTT levels in the cerebrospinal fluid (CSF). These levels are typically reported as the absolute concentration of mHTT concentration, derived from a standard curve generated using a single protein standard. However, patient biofluids are a complex milieu containing different mHTT protein species, suggesting that absolute quantitation is challenging. As a result, a single recombinant protein standard may not be sufficient to interpret assay signal as molar mHTT concentration. In this study, we used immunoprecipitation and flow cytometry (IP-FCM) to investigate different factors that influence mHTT detection assay signal. Our results show that HTT protein fragmentation, protein-protein interactions, affinity tag positioning, oligomerization and polyglutamine tract length affect assay signal intensity. These findings indicate that absolute HTT quantitation in heterogeneous biological samples is not possible with current technologies using a single standard protein. We also explore the binding specificity of the MW1 anti-polyglutamine antibody, commonly used in these assays as a mHTT-selective reagent and demonstrate that mHTT binding is preferred but not specific. Furthermore, we find that MW1 depletion of mHTT for quantitation of wildtype HTT is not only incomplete, leaving residual mHTT, but also non-specific, resulting in pull down of some wildtype HTT protein. Based on these observations, we recommend that mHTT detection assays report only relative mHTT quantitation using normalized arbitrary units of assay signal intensity, rather than molar concentrations, in the assessment of central nervous system HTT lowering in ongoing clinical and preclinical studies. Further, we recommend that MW1-depletion not be used as a method for quantifying wildtype HTT protein and that detergent be consistently added to samples during testing.
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Affiliation(s)
- Rachel J Harding
- Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, ON, M5S 3M2, Canada
- Structural Genomics Consortium, University of Toronto, Toronto, ON, M5G 1L7, Canada
- Department of Pharmacology and Toxicology, University of Toronto, Toronto, ON, M5S 1A8, Canada
| | - Yuanyun Xie
- Burnett School of Biomedical Sciences, University of Central Florida, Orlando, FL, 32827, USA
| | - Nicholas S Caron
- Centre for Molecular Medicine and Therapeutics, The University of British Columbia, Vancouver, BC, V5Z 4H4, Canada
| | - Hailey Findlay-Black
- Centre for Molecular Medicine and Therapeutics, The University of British Columbia, Vancouver, BC, V5Z 4H4, Canada
| | - Caroline Lyu
- Structural Genomics Consortium, University of Toronto, Toronto, ON, M5G 1L7, Canada
| | - Nalini Potluri
- Burnett School of Biomedical Sciences, University of Central Florida, Orlando, FL, 32827, USA
| | - Renu Chandrasekaran
- Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, ON, M5S 3M2, Canada
| | - Michael R Hayden
- Centre for Molecular Medicine and Therapeutics, The University of British Columbia, Vancouver, BC, V5Z 4H4, Canada
| | - Blair R Leavitt
- Centre for Molecular Medicine and Therapeutics, The University of British Columbia, Vancouver, BC, V5Z 4H4, Canada
| | - Douglas R Langbehn
- Department of Psychiatry, Carver College of Medicine, University of Iowa, Iowa City, IA, 52242, USA
| | - Amber L Southwell
- Burnett School of Biomedical Sciences, University of Central Florida, Orlando, FL, 32827, USA.
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Harvey IB, Chilewski SD, Bhosale D, Tobia SE, Gray C, Gleason C, Haulenbeek J. Overcoming Lot-to-Lot Variability in Protein Activity Using Epitope-Specific Calibration-Free Concentration Analysis. Anal Chem 2024; 96:6275-6281. [PMID: 38600735 PMCID: PMC11044105 DOI: 10.1021/acs.analchem.3c05607] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2023] [Revised: 04/01/2024] [Accepted: 04/02/2024] [Indexed: 04/12/2024]
Abstract
Concentration determination is a fundamental hallmark of protein reagent characterization, providing a means to ensure reproducibility and unify measurements from various assays. However, lot-to-lot differences in protein activity often still occur, leading to uncertainty in the accuracy of downstream measurements. Here, we postulate that those differences are caused by a misrepresentation of the protein concentration as measured by traditional total protein techniques, which can include multiple types of inactive protein species. To overcome this, we developed a standardized method to quantify a protein's active concentration via calibration-free concentration analysis (CFCA). As a pilot study, we compare the biophysical and immunoassay responses from three batches of recombinant soluble lymphocyte-activation gene 3 (sLAG3), as defined by either their total or active concentrations. Defining the sLAG3 reagents by their assay-specific concentration improved consistency in reported kinetic binding parameters and decreased immunoassay lot-to-lot coefficients of variation (CVs) by over 600% compared to the total protein concentration. These findings suggest that the total concentration of a protein reagent may not be the ideal metric to correlate in-assay signals between lots, and by instead quantifying the concentrations of a reagent's assay-specific epitopes, CFCA may prove a useful tool in overcoming lot-to-lot variability.
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Affiliation(s)
- Ian B. Harvey
- Translational
Sciences and Diagnostics, Bristol-Myers
Squibb, Princeton, New Jersey 08540, United States
| | - Shannon D. Chilewski
- Translational
Sciences and Diagnostics, Bristol-Myers
Squibb, Princeton, New Jersey 08540, United States
| | - Devyani Bhosale
- Translational
Sciences and Diagnostics, Bristol-Myers
Squibb, Princeton, New Jersey 08540, United States
| | - Sarah E. Tobia
- Translational
Sciences and Diagnostics, Bristol-Myers
Squibb, Princeton, New Jersey 08540, United States
| | - Christopher Gray
- Translational
Sciences and Diagnostics, Bristol-Myers
Squibb, Princeton, New Jersey 08540, United States
| | - Carol Gleason
- Global
Biometrics and Data Sciences, Bristol-Myers
Squibb, Princeton, New Jersey 08540, United States
| | - Jonathan Haulenbeek
- Translational
Sciences and Diagnostics, Bristol-Myers
Squibb, Princeton, New Jersey 08540, United States
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