Accurate Quantification of Mutant and Wild-Type polyQ Proteins Using Simple Western Capillary Immunoassays

Polyglutamine (polyQ) diseases are monogenic fatal neurodegenerative disorders caused by a CAG repeat expansion that is translated into a toxic polyQ tract. There are nine polyQ diseases: Huntington's disease (HD), spinocerebellar ataxias 1, 2, 3, 6, 7 and 17 (SCA1, 2, 3, 6, 7, 17), dentatorubral-pallidoluysian atrophy (DRPLA) and spinal and bulbar muscular atrophy (SBMA). Although no disease-modifying therapies are available, lowering levels of the causative mutant polyQ protein is a promising potential treatment. Preclinically, the efficacy of polyQ protein-lowering compounds is often assessed using time-consuming Western blots (WB), which can produce variable results. Therefore, to improve throughput and accuracy of polyQ protein level quantification, Simple Western (SW) capillary immunoassays were developed. A panel of antibodies was screened for reactivity to the polyQ proteins on SW. The most promising antibodies were selected for further assay development. This resulted in optimised SW immunoassays for huntingtin (HTT), ataxin 1, 2 and 3 (ATXN1, 2, 3), atrophin 1 (ATN1) and androgen receptor (AR). Additionally, size-separation of the wild-type and polyQ-expanded mutant protein isoforms on SW was shown for ATXN1, ATXN3 and ATN1, allowing for their separate quantification. To facilitate size-separation of the larger HTT protein (≥ 348 kDa), a novel caspase 3-based assay was developed to generate N-terminal wild-type and mutant HTT fragments that could be separately quantified on SW in contrast to full-length HTT. In conclusion, SW capillary immunoassays were developed for polyQ proteins to improve preclinical research and aid the development of polyQ-lowering therapies for polyQ diseases.