An enhanced Q175 knock-in mouse model of Huntington disease with higher mutant huntingtin levels and accelerated disease phenotypes

Abstract

Huntington disease (HD) model mice with heterozygous knock-in (KI) of an expanded CAG tract in exon 1 of the mouse huntingtin (Htt) gene homolog genetically recapitulate the mutation that causes HD, and might be favoured for preclinical studies. However, historically these mice have failed to phenotypically recapitulate the human disease. Thus, homozygous KI mice, which lack wildtype Htt, and are much less relevant to human HD, have been used. The zQ175 model was the first KI mouse to exhibit significant HD-like phenotypes when heterozygous. In an effort to exacerbate HD-like phenotypes and enhance preclinical utility, we have backcrossed zQ175 mice to FVB/N, a strain highly susceptible to neurodegeneration. These Q175F mice display significant HD-like phenotypes along with sudden early death from fatal seizures. The zQ175 KI allele retains a floxed neomycin resistance cassette upstream of the Htt gene locus and produces dramatically reduced mutant Htt as compared to the endogenous wildtype Htt allele. By intercrossing with mice expressing cre in germ line cells, we have excised the neo cassette from Q175F mice generating a new line, Q175FΔneo (Q175FDN). Removal of the neo cassette resulted in a ∼2 fold increase in mutant Htt and rescue of fatal seizures, indicating that the early death phenotype of Q175F mice is caused by Htt deficiency rather than by mutant Htt. Additionally, Q175FDN mice exhibit earlier onset and a greater variety and severity of HD-like phenotypes than Q175F mice or any previously reported KI HD mouse model, making them valuable for preclinical studies.

Global ribosome profiling reveals that mutant huntingtin stalls ribosomes and represses protein synthesis independent of fragile X mental retardation protein

“…4P) in these mice compared with control groups (n = 6-16/equal sex ratio). But, while, the HD mice had no observable phenotype, consistent with a previous report (45), and acquired motor learning skills (rotarod) comparable to WT or Fmr1 +/mice, the Q175FDN-Het; Fmr1 +/mice showed a subtle defects to learn motor skills, compared to WT mice ( Fig. 4Q).…”

“…Next, we investigated how wtHtt and mHtt interacted with ribosomes in the HD mouse brain (zQ175DN) (45). As expected, the treatment of HD cortical brain lysate with EDTA, which disassembled the ribosomes (Fig.…”

A Patient-Derived Cellular Model for Huntington’s Disease Reveals Phenotypes at Clinically Relevant CAG Lengths

“…The functional implications of this expanded, mutant huntingtin are not fully understood. Much of the existing research on HD cell biology in relevant neuronal cell types has been limited to primary post-mitotic neurons from murine brain tissue or transformed cell lines, which have several limitations, including the use of synthetically long CAG lengths in order to mimic human disease in mice [2][3][4][5][6][7][8][9][10][11][12][13][14][15] . These alleles actually genetically model juvenile or Westphal variant HD, which are not age-onset diseases.…”

Clinical manifestations of homozygote allele carriers in Huntington disease

“…1,2 In this regard, some publications in animal models and small HD human retrospective studies have reported a more aggressive HD progression and brain atrophy in homozygotes. 2,3 In contrast, other authors have reported an indistinguishable phenotype in HD homozygotes and heterozygotes. [4][5][6][7][8] Given these contradictory data and the sparse longitudinal information, we aimed to further investigate the clinical differences between HD homozygotes (with both alleles ≥36 CAG repeats) and heterozygotes (with one allele ≥36 CAG repeats) in terms of age at onset, phenotypic presentation, and disease progression.…”