Phenotype onset in Huntington's disease knock-in mice is correlated with the incomplete splicing of the mutant huntingtin gene.

Nicholas R Franich,Vincent Lemesre,Nicholas H Bove,Chunni Zhu,Scott O Zeitlin,Miriam A Hickey,Gillian P Bates,Tiffany Chu,Marie‐Francoise Chesselet,David Howland,Nadira Ali,Andreas Neueder,Georgina F Osborne,Christian Landles,Renata P Lerner

doi:10.1002/jnr.24493

Abstract

Huntington’s disease (HD) is a progressive neurodegenerative disorder caused by an expanded CAG repeat within the huntingtin (HTT) gene. The Q140 and HdhQ150 knock‐in HD mouse models were generated such that HdhQ150 mice have an expanded CAG repeat inserted into the mouse Htt gene, whereas in the Q140s, mouse exon 1 Htt was replaced with a mutated version of human exon 1. By standardizing mouse strain background, breeding to homozygosity and employing sensitive behavioral tests, we demonstrate that the onset of behavioral phenotypes occurs earlier in the Q140 than the HdhQ150 knock‐in mouse models and that huntingtin (HTT) aggregation appears earlier in the striata of Q140 mice. We have previously found that the incomplete splicing of mutant HTT from exon 1 to exon 2 results in the production of a small polyadenylated transcript that encodes the highly pathogenic mutant HTT exon 1 protein. In this report, we have identified a functional consequence of the sequence differences between these two models at the RNA level, in that the level of incomplete splicing, and of the mutant exon 1 HTT protein, are greater in the brains of Q140 mice. While differences in the human and mouse exon 1 HTT proteins (e.g., proline rich sequences) could also contribute to the phenotypic differences, our data indicate that the incomplete splicing of HTT and approaches to lower the levels of the exon 1 HTT transcript should be pursued as therapeutic targets.

Highlights

Huntington’s disease (HD) is an autosomal dominant, progressive, neurodegenerative disorder caused by a created by replacing the mouse (CAG) repeat expansion within exon 1 of the huntingtin gene (HTT), that encodes a polyglutamine tract in the HTT protein (Bates et al, 2015)
Previous studies had suggested that the onset of behavioral deficits in Q140 mice might occur much earlier than in HdhQ150 mice (Brooks et al, 2012; Dorner et al, 2007; Heng et al, 2007; Hickey et al, 2008; Lerner, Trejo Martinez Ldel, Zhu, Chesselet, & Hickey, 2012; Lin et al, 2001; Menalled et al, 2003; Rising et al, 2011; Simmons et al, 2009; Woodman et al, 2007)
To systematically evaluate these potential differences, we standardized the genetic background by generating Q140 and HdhQ150 C57BL/6J congenic lines, bred them to homozygosity and maintained them under identical husbandry conditions

Summary

| INTRODUCTION

Huntington’s disease (HD) is an autosomal dominant, progressive, neurodegenerative disorder caused by a CAG repeat expansion within exon 1 of the huntingtin gene (HTT), that encodes a polyglutamine (polyQ) tract in the HTT protein (Bates et al, 2015). A comprehensive analysis of the HdhQ150 mice using the same tests has not been performed, the available data indicate that behavioral phenotypes present much later We show that the onset of behavioral phenotypes and the striatal deposition of HTT aggregates occurs earlier in the Q140 than in the HdhQ150 model This correlated to increased levels of the incompletely spliced Httexon transcript and the highly aggregation prone and pathogenic exon 1 HTT protein that it encodes. Heng, Tallaksen‐Greene, Detloff, & Albin, 2007; Lin et al, 2001; Woodman et al, 2007) These data suggest that sequence differences in the Q140 and HdhQ150 mutant Htt genes result in considerable differences in the onset of behavioral deficits. Our work suggests that increased production of the mutant exon 1 HTT protein in Q140 mice may contribute to an earlier phenotype onset and supports the design of therapeutics that decrease the levels of the small Httexon transcript

| MATERIALS AND METHODS

| DISCUSSION

Findings

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