Time course of early motor and neuropathological anomalies in a knock‐in mouse model of Huntington's disease with 140 CAG repeats

Abstract

Huntington's disease (HD) is caused by an abnormal expansion of CAG repeats in the gene encoding huntingtin. The development of therapies for HD requires preclinical testing of drugs in animal models that reproduce the dysfunction and regionally specific pathology observed in HD. We have developed a new knock-in mouse model of HD with a chimeric mouse/human exon 1 containing 140 CAG repeats inserted in the murine huntingtin gene. These mice displayed an increased locomotor activity and rearing at 1 month of age, followed by hypoactivity at 4 months and gait anomalies at 1 year. Behavioral symptoms preceded neuropathological anomalies, which became intense and widespread only at 4 months of age. These consisted of nuclear staining for huntingtin and huntingtin-containing nuclear and neuropil aggregates that first appeared in the striatum, nucleus accumbens, and olfactory tubercle. Interestingly, regions with early pathology all receive dense dopaminergic inputs, supporting accumulating evidence for a role of dopamine in HD pathology. Nuclear staining and aggregates predominated in striatum and layer II/III and deep layer V of the cerebral cortex, whereas neuropil aggregates were found in the globus pallidus and layer IV/superficial layer V of the cerebral cortex. The olfactory system displayed early and marked aggregate accumulation, which may be relevant to the early deficit in odor discrimination observed in patients with HD. Because of their early behavioral anomalies and regionally specific pathology, these mice provide a powerful tool with which to evaluate the effectiveness of new therapies and to study the mechanisms involved in the neuropathology of HD.

Mouse models of Huntington's disease

“…Therefore, knowing how many CAG repeats HD transgenic mice carry, and monitoring the CAG repeat lengths in breeding and experimental animals, is absolutely critical to reduce experimental variation. (Carter et al, 1999;Cummings et al, 2012;Mangiarini et al, 1996) R6/2 (Menalled et al, 2003) • Requests for mouse strains from CHDI Foundation should be directed to The Jackson Laboratory at 1-207-288-5845 or orderquest@jax.org.…”

“…A key differentiation among the HD knock-in mouse models reported to date, however, is whether the expanded CAG tract is inserted into an otherwise unaltered mouse Htt exon 1 or into a humanized exon 1 sequence. In the former case, the knock-in lines express a mutant form of mouse Htt (Lin et al, 2001;Sathasivam et al, 2013), but in the latter case the knock-in lines express a mutant Htt with a chimeric mouse-human HTT (Levine et al, 1999;Menalled et al, 2003;Shelbourne et al, 1999;Wheeler et al, 1999). Therefore, if preclinical strategies require the presence of a human HTT gene and protein sequence then the knock-in series of mice should not be used.…”

“…These mice, developed by Zeitlin, are heterozygous for a Htt allele with an expanded CAG repeat size of 71, 94 and 140 (Levine et al, 1999;Menalled et al, 2003). The 71 and 94 repeat size lines carry an arginine codon in position 42 of the polyglutamine tract.…”

“…However, because of this mutation, these mice have generally not been well studied. The 140 repeat size knock-in mouse (CAG140) has been well characterized and retains a pure CAG tract, (CAG)nCAACAG, encoding polyglutamine (no arginine codon) (Hickey et al, 2008;Menalled et al, 2003). Additionally, the targeting strategy taken by Zeitlin was to humanize the exon 1 amino acid sequence including the polyproline stretch.…”

“…Rodents are more active during the dark phase of the diurnal cycle; therefore, it is not surprising that discrimination between genotypes improves during testing in the dark phase, under dim red lights (Hossain et al, 2004;Menalled et al, 2012b). This is of particular importance when working with knock-in HD models, in which testing in the dark phase helps unveil more robust motor deficits (Hickey et al, 2008;Menalled et al, 2012b;Menalled et al, 2003). To avoid the inconvenience of testing at night, colonies can be maintained under reverse light cycles.…”