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 themouse huntingtin (Htt) gene homolog genetically recapitulate themutation that causes HD, andmight be favoured for preclinical studies. However, historically thesemice have failed to phenotypically recapitulate the human disease. Thus, homozygous KImice, which lack wildtype Htt, and aremuch less relevant to human HD, have been used. The zQ175model was the first KImouse to exhibit significant HD-like phenotypes when heterozygous. In an effort to exacerbate HD-like phenotypes and enhance preclinical utility, we have backcrossed zQ175mice to FVB/N, a strain highly susceptible to neurodegeneration. These Q175Fmice 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 reducedmutant Htt as compared to the endogenous wildtype Htt allele. By intercrossing withmice expressing cre in germline cells, we have excised the neo cassette fromQ175Fmice generating a new line, Q175FDneo (Q175FDN). Removal of the neo cassette resulted in a ~2 fold increase inmutant Htt and rescue of fatal seizures, indicating that the early death phenotype of Q175Fmice is caused by Htt deficiency rather than bymutant Htt. Additionally, Q175FDNmice exhibit earlier onset and a greater variety and severity of HD-like phenotypes than Q175Fmice or any previously reported KI HDmousemodel,making themvaluable for preclinical studies.