Research Article: The expanded CAG repeat in the huntingtin gene as target for therapeutic RNA modulation throughout the HD mouse brain

Date Published: February 9, 2017

Publisher: Public Library of Science

Author(s): Nicole A. Datson, Anchel González-Barriga, Eleni Kourkouta, Rudie Weij, Jeroen van de Giessen, Susan Mulders, Outi Kontkanen, Taneli Heikkinen, Kimmo Lehtimäki, Judith C. T. van Deutekom, Yuqing Li.


The aim of these studies was to demonstrate the therapeutic capacity of an antisense oligonucleotide with the sequence (CUG)7 targeting the expanded CAG repeat in huntingtin (HTT) mRNA in vivo in the R6/2 N-terminal fragment and Q175 knock-in Huntington’s disease (HD) mouse models. In a first study, R6/2 mice received six weekly intracerebroventricular infusions with a low and high dose of (CUG)7 and were sacrificed 2 weeks later. A 15–60% reduction of both soluble and aggregated mutant HTT protein was observed in striatum, hippocampus and cortex of (CUG)7-treated mice. This correction at the molecular level resulted in an improvement of performance in multiple motor tasks, increased whole brain and cortical volume, reduced levels of the gliosis marker myo-inositol, increased levels of the neuronal integrity marker N-aceyl aspartate and increased mRNA levels of the striatal marker Darpp-32. These neuroanatomical and neurochemical changes, together with the improved motor performance, suggest that treatment with (CUG)7 ameliorates basal ganglia dysfunction. The HTT-lowering was confirmed by an independent study in Q175 mice using a similar (CUG)7 AON dosing regimen, further demonstrating a lasting reduction of mutant HTT protein in striatum, hippocampus and cortex for up to 18 weeks post last infusion along with an increase in motor activity. Based on these encouraging results, (CUG)7 may thus offer an interesting alternative HTT-lowering strategy for HD.

Partial Text

Huntington’s disease (HD) is a rare inherited neurodegenerative disorder with a progressive and fatal course characterized by movement disorders, cognitive impairment, dementia and psychiatric manifestations including depression and psychosis. These symptoms result from the selective death and dysfunction of specific neuronal subpopulations within the central nervous system, in particular of medium spiny GABAergic projection neurons in the striatum, although marked alterations have also been observed in other areas of the brain, including the cerebellar cortex, thalamus and cerebellum [1, 2]. Generally, first symptoms occur in midlife, eventually leading to premature death within 15–20 years after disease onset. The disease causing mutation is the expansion of a CAG-trinucleotide repeat in the coding region of exon 1 of the huntingtin (HTT) gene, which results in a mutant protein with an elongated polyglutamine (also known as polyQ) stretch at its N-terminus [3]. This expanded polyglutamine stretch confers a toxic gain-of-function to mutant protein forms, ultimately resulting in widespread neuronal death. Besides a toxic gain-of-function of mutant HTT (mHTT), loss of the normal functions of wildtype HTT (wtHTT) likely also plays a role in pathological mechanisms of HD [4]. In addition, there is emerging evidence that mRNA species with extended CAG repeats may themselves be directly involved in toxicity, possibly by sequestration of diverse proteins [5]. Today there is still a significant unmet need in disease-modifying therapies for the treatment and management of HD, with drugs that can alleviate some of the movement and psychiatric symptoms, but no curative treatments available [6].

The aim of these studies was to demonstrate the therapeutic capacity of a 2OMePS RNA (CUG)7 AON that does not activate RNase H and targets the expanded CAG repeat sequence in exon 1 of mHTT. In 2 different HD mouse models, the R6/2 N-terminal fragment model and the Q175 knock-in model, only 6 repeated weekly ICV administrations of (CUG)7 already reduced levels of the mHTT protein up by approximately 15–60% in 3 key brain regions affected in HD, i.e. striatum, cortex and hippocampus. This level of mHTT-lowering resulted in correction of the motor phenotype in multiple motor tests, a brain volume increase, positive changes in striatal metabolite profile and an increase of the striatal marker Darpp-32. The Q175 study further demonstrated that the HTT-lowering was persistent and lasted for up to 18 weeks post infusion, which suggests that a less frequent dose regimen is feasible.