Uninterrupted CAG repeat drives striatum-selective transcriptionopathy and nuclear pathogenesis in human Huntingtin BAC mice

Xiaofeng Gu,Jeffrey Richman,Gillian P Bates,Laura P.W Ranum,Lalini Ramanathan,Linna Deng,Fuying Gao,Chang Sin Park,Jeffrey P Cantle,Huei-Bin Wang,Steve Horvath,Michelle Gray,Shasha Zhang,Giovanni Coppola,Nan Wang,Peter Langfelder,Lucia Yang,Christopher S Colwell,Mónica Báñez-Coronel ,Christopher R Choi ,Xiangdong Yang

doi:10.1016/j.neuron.2022.01.006

Abstract

In Huntington's disease (HD), the uninterrupted CAG repeat length, but not the polyglutamine length, predicts disease onset. However, the underlying pathobiology remains unclear. Here, we developed bacterial artificial chromosome (BAC) transgenic mice expressing human mutant huntingtin (mHTT) with uninterrupted, and somatically unstable, CAG repeats that exhibit progressive disease-related phenotypes. Unlike prior mHTT transgenic models with stable, CAA-interrupted, polyglutamine-encoding repeats, BAC-CAG mice show robust striatum-selective nuclear inclusions and transcriptional dysregulation resembling those in murine huntingtin knockin models and HD patients. Importantly, the striatal transcriptionopathy in HD models is significantly correlated with their uninterrupted CAG repeat length but not polyglutamine length. Finally, among the pathogenic entities originating from mHTT genomic transgenes and only present or enriched in the uninterrupted CAG repeat model, somatic CAG repeat instability and nuclear mHTT aggregation are best correlated with early-onset striatum-selective molecular pathogenesis and locomotor and sleep deficits, while repeat RNA-associated pathologies and repeat-associated non-AUG (RAN) translation may play less selective or late pathogenic roles, respectively.

Highlights

Huntington’s disease (HD) is one of the most common autosomal dominant neurodegenerative disorders characterized by progressive movement disorder, cognitive impairment, and psychiatric symptoms (Ross et al, 2014)
A bacterial artificial chromosome (BAC) transgenic mouse model expressing human mutant huntingtin (mHTT) with a long uninterrupted CAG repeat To create a human genomic transgenic mouse model of HD with a long uninterrupted CAG repeat in HTT, we engineered a human mHTT-exon 1 construct with about 120 pure CAG repeats followed by one CAA, 8 CAGs, and another CAA-CAG, encoding about 131 glutamine repeats (Figure 1A; Gray et al, 2008; Gu et al, 2009, Gu et al, 2015; Yang et al, 1997)
Besides the long uninterrupted CAG repeat, another key difference in the new mHTT BAC construct from the BACHD is the absence of LoxP sites and some silent coding variants for the N17 domain of mHTT

Summary

Introduction

Huntington’s disease (HD) is one of the most common autosomal dominant neurodegenerative disorders characterized by progressive movement disorder (i.e., chorea and dystonia), cognitive impairment, and psychiatric symptoms (Ross et al, 2014). HD is caused by a CAG trinucleotide repeat expansion encoding a polyglutamine (polyQ) stretch near the N terminus of huntingtin (HTT) (The Huntington’s Disease Collaborative Research Group, 1993). Emerging evidence suggests the potential for mutant CAG repeats to contribute to disease pathogenesis beyond encoding polyQ peptides (Lieberman et al, 2019; Malik et al, 2021). This may include distinct toxic molecular entities originating from the Neuron 110, 1–20, April 6, 2022 a 2022 The Authors.

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