Toward allele-specific targeting therapy and pharmacodynamic marker for spinocerebellar ataxia type 3

Abstract

Spinocerebellar ataxia type 3 (SCA3), caused by a CAG repeat expansion in the ataxin-3 gene (ATXN3), is characterized by neuronal polyglutamine (polyQ) ATXN3 protein aggregates. Although there is no cure for SCA3, gene-silencing approaches to reduce toxic polyQ ATXN3 showed promise in preclinical models. However, a major limitation in translating putative treatments for this rare disease to the clinic is the lack of pharmacodynamic markers for use in clinical trials. Here, we developed an immunoassay that readily detects polyQ ATXN3 proteins in human biological fluids and discriminates patients with SCA3 from healthy controls and individuals with other ataxias. We show that polyQ ATXN3 serves as a marker of target engagement in human fibroblasts, which may bode well for its use in clinical trials. Last, we identified a single-nucleotide polymorphism that strongly associates with the expanded allele, thus providing an exciting drug target to abrogate detrimental events initiated by mutant ATXN3. Gene-silencing strategies for several repeat diseases are well under way, and our results are expected to improve clinical trial preparedness for SCA3 therapies.

Polyglutamine-expanded ataxin-3: a target engagement marker for Spinocerebellar ataxia type 3 in peripheral blood

“…We did not nd an association of polyQ-expanded ataxin-3 levels in plasma and CSF. Therefore, the pool of polyQ-expanded ataxin-3 in CSF differs from that in peripheral blood and blood cells, as reported earlier [2]. This notion is further supported by the observation that levels of polyQ-expanded ataxin-3 were > 10 times higher in plasma as in CSF.…”

“…Here we report, on the successful generation and validation of a new ultrasensitive and quantitative immunoassay to speci cally measure low concentrations in pg/mL range of polyQ-expanded ataxin-3 in human bio uids like blood plasma and CSF. Our SMC™ immunoassay perfectly discriminated between healthy controls and SCA3 mutation carriers, yielding discrimination values similar to a recent published ataxin-3-speci c mesoscale assay [2]. In addition, our assay allowed for a discrimination between preataxic and ataxic mutation carriers in plasma.…”

“…To demonstrate target engagement in such trials, availability of an ultrasensitive quantitative immunoassay to measure concentration of polyQ-expanded ataxin-3 in body uids is mandatory. Recently, an immunoassay that detected polyQ-expanded ataxin-3 in body uids including cerebrospinal uid (CSF) and discriminated between SCA3 patients and healthy controls was developed [2]. In addition, an assay based on time-resolved uorescence energy transfer (TR-FRET) was reported that was capable of measuring ataxin-3 concentrations in peripheral blood mononuclear cells (PBMCs) but failed to detect ataxin-3 in body uids [3].…”

Spinocerebellar ataxia clinical trials: opportunities and challenges

“…While no fluid biomarkers have yet been validated for the SCAs, there are some promising candidates (Table 2 ). 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 The first candidate is the mutant protein itself in the case of the polyglutamine SCAs. In all these ataxias, the protein tends to accumulate and aggregate.…”

“… 125 For instance, a recent strategy for monitoring ataxin‐3 levels in blood has been identified, although it is still unclear the extent to which blood levels of ataxin‐3 reflect the levels in the central nervous system. 105 …”