Phenotype onset in Huntington’s disease knock‐in mice is correlated with the incomplete splicing of the mutant huntingtin gene

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. Whilst 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.

Truncation of mutant huntingtin in knock-in mice demonstrates exon1 huntingtin is a key pathogenic form

“…Exon 1 HTT fragment was found to be generated by aberrant exon 1-intron 1 RNA, which was observed only in the HD brains in which a large CAG repeat is present 20,24 . Using RT-PCR of WT and KI mouse, we confirmed the selective expression of aberrant exon 1-intron 1 RNA in the KI mouse striatum when compared with WT mouse striatum ( Supplementary Fig.…”

D1R- and D2R-Medium-Sized Spiny Neurons Diversity: Insights Into Striatal Vulnerability to Huntington’s Disease Mutation

“…To dissect why D1R- and D2R-MSNs are differentially affected by HD, Lee et al (2020) highlighted thousands dysregulated protein-coding genes implicated in OXPHOS, synaptic functioning and circadian entrainment by using translating ribosome affinity purification and snRNA-seq of D1R and D2R neurons of HD patients and mouse models (R6/2 and zQ175DN, a knock-in zQ175 line without neomycin cassette) ( Franich et al, 2019 ). Strikingly, downregulation of OXPHOS and upregulation of neurotrophin pathway genes in D2R neurons indicated a cell-type specific response to the disease ( Figure 1D ).…”

Subcellular Localization And Formation Of Huntingtin Aggregates Correlates With Symptom Onset And Progression In A Huntington’S Disease Model

“…The HTT gene contains 67 exons, and we have previously shown that exon 1 does not always splice to exon 2 resulting in the production of a small transcript ( Httexon1 ) that terminates at cryptic polyadenylation sites within intron 1. This occurs in knock-in mouse models ( Sathasivam et al , 2013 ; Franich et al , 2019 ), where the level of incomplete splicing correlates with the appearance of disease phenotypes, and in Huntington’s disease post-mortem brains and cell lines ( Neueder et al , 2017 ). The Httexon1 transcript contains a stop codon at the beginning of intron 1 and is translated to produce the highly pathogenic exon 1 HTT protein ( Sathasivam et al , 2013 ; Neueder et al , 2018 ), by the same mechanism as occurs in R6/2 mice ( Mangiarini et al , 1996 ).…”