A critical tool in the quest for treatments
By: Mahmoud A. Pouladi, PhD
Our current understanding of the pathological processes underlying HD has benefited immensely from a large number of animal models developed throughout the past two decades. The establishment of such models was facilitated by refinements in transgenic technologies such as yeast and bacterial artificial chromosome (YAC and BAC) transgenesis, which proved essential for manipulation of large genes such as HTT. Coupled with the monogenic, dominant, and highly-penetrant nature of HD, these approaches to transgenesis yielded animal models that have relatively high construct (genetic) and face validities that have been applied to derive important insights into HD pathophysiology.1
An early focus of studies that used animal models of HD was the characterization of disease-related phenotypes. These phenotypes included measures of motor, cognitive, and affective function, as well as biochemical and neuropathological changes such as dysregulated mRNA transcripts; loss of neurons and neurochemical markers; and atrophy of striatal and cortical structures. Many of the phenotypes established as a result of these studies have subsequently been used as endpoints in preclinical trials of pharmaceutical and genetic interventions, and continue to be useful in ongoing efforts to develop therapies for HD.
Genetic manipulation of animal models of HD has also helped provide holistic answers to many important mechanistic questions. For example, transgenic animals that express mHTT variants in which specific sites of post-translational modifications (PTMs), such as phosphorylation or cleavage, are mutated have allowed examination of the role of these PTMs of mHTT on behavioral, neuropathological, and biochemical aspects of disease phenotypes in vivo.2 Similarly, the use of Cre/loxP recombination and tetracycline-inducible technologies have provided important insight into specific effects of mHTT with respect to age, tissue, and cell type. More recently, the creation of a fully humanized mouse model of HD, carrying two human HTT alleles (one wild-type, one mutant) and no mouse Hdh genes, has been of immediate value in testing allele-specific HTT silencing approaches, among other uses.3
Studies of animal models of disease have benefited from technological developments that provide new tools with which to tackle biological problems. Optogenetic and genomic engineering technologies are two advances that have gained popularity in animal studies in recent years. Optogenetics provides genetic-based tools that allow precise control over the firing and function of defined neuronal populations.4 Coupled with electrophysiological techniques, optogenetic studies are providing novel insights into the role of specific neuronal populations and neural circuits in HD.5,6 Genomic engineering technologies such as zinc finger nucleases, transcription activator – like effector nucleases, and more recently CRISPR-Cas9 nucleases, which permit precise editing of the genome in an efficient manner, are revolutionizing disease modeling by allowing the rapid creation of genetically-modified animals, and in species that have traditionally been challenging to genetically modify.7 Genome engineering technologies also offer the tantalizing possibility of modeling genetic disorders in large animals, allowing greater approximation of the human condition. These tools are bound to benefit ongoing efforts to establish refined animal models of increasing sophistication and relevance to HD.
Much has undoubtedly been learned from animal models of HD. While their true predictive value can only be assessed once the ultimate goal of developing an effective treatment for HD is achieved, there is great hope that the experience gained over the past two decades in establishing relevant models of disease, coupled with recent technological advancements, will help accelerate the realization of this objective in the very near future.
1 Pouladi MA, Morton AJ, Hayden MR. Choosing an animal model for the study of Huntington’s disease. Nat Rev Neurosci. 2013 Oct;14(10):708-21. doi: 10.1038/nrn3570.
2 Ehrnhoefer DE, Sutton L, Hayden MR. Small changes, big impact: posttranslational modifications and function of huntingtin in Huntington disease. Neuroscientist. 2011 Oct; 17(5): 475-92. doi: 10.1177/1073858410390378. Epub 2011 Feb 10.
3 Southwell AL, Warby SC, Carroll JB, Doty CN, et al. A fully humanized transgenic mouse model of Huntington disease. Hum Mol Genet. 2013 Jan 1; 22(1):18-34. doi: 10.1093/hmg/dds397. Epub 2012 Sep 21.
4 Fenno, L, Yizhar O, Deisseroth K. The development and application of optogenetics. Annu Rev Neurosci. 2011;34:389-412.doi: 10.1146/annurev-neuro-061010-113817.
5 Plotkin JL, Day M, Peterson JD, Xie Z, et al. Impaired TrkB receptor signaling underlies corticostriatal dysfunction in Huntington’s disease. Neuron. 2014 Jul 2;83(1):178-88. doi: 10.1016/j.neuron.2014.05.032.
6 Cepeda C, Galvan L, Holley SM, Rao SP, et al. Multiple sources of striatal inhibition are differentially affected in Huntington’s disease mouse models. J Neurosci. 2013 Apr 24;33(17):7393 406. doi: 10.1523/JNeurosci.2137-12.2013.
7 Gaj T, Gersbach CA, Barbas CF. ZFN, TALEN, and CRISPR/Cas-based methods for genome engineering. Trends Biotechnol. 2013 Jul;31(7):397-405. doi: 10.1016/j.tibtech.2013.04.004. Epub 2013 May 9.