HD CAG-correlated gene expression changes support a simple dominant gain of function

Abstract

Huntington's disease is initiated by the expression of a CAG repeat-encoded polyglutamine region in full-length huntingtin, with dominant effects that vary continuously with CAG size. The mechanism could involve a simple gain of function or a more complex gain of function coupled to a loss of function (e.g. dominant negative-graded loss of function). To distinguish these alternatives, we compared genome-wide gene expression changes correlated with CAG size across an allelic series of heterozygous CAG knock-in mouse embryonic stem (ES) cell lines (Hdh(Q20/7), Hdh(Q50/7), Hdh(Q91/7), Hdh(Q111/7)), to genes differentially expressed between Hdh(ex4/5/ex4/5) huntingtin null and wild-type (Hdh(Q7/7)) parental ES cells. The set of 73 genes whose expression varied continuously with CAG length had minimal overlap with the 754-member huntingtin-null gene set but the two were not completely unconnected. Rather, the 172 CAG length-correlated pathways and 238 huntingtin-null significant pathways clustered into 13 shared categories at the network level. A closer examination of the energy metabolism and the lipid/sterol/lipoprotein metabolism categories revealed that CAG length-correlated genes and huntingtin-null-altered genes either were different members of the same pathways or were in unique, but interconnected pathways. Thus, varying the polyglutamine size in full-length huntingtin produced gene expression changes that were distinct from, but related to, the effects of lack of huntingtin. These findings support a simple gain-of-function mechanism acting through a property of the full-length huntingtin protein and point to CAG-correlative approaches to discover its effects. Moreover, for therapeutic strategies based on huntingtin suppression, our data highlight processes that may be more sensitive to the disease trigger than to decreased huntingtin levels.

Large-scale phenome analysis defines a behavioral signature for Huntington's disease genotype in mice

“…This line did not exhibit a behavioral signature consistent with the CAG-repeat-dependence signal captured by our CAG model, supporting the view that loss of function due to reduced HTT levels is not in itself sufficient to explain the behavioral signatures observed in this study. Previous work with a cellular allelic KI series revealed a CAG signature consisting of 73 CAG-length-dependent gene expression profiles involved in 172 CAG-length-correlated pathways 15 . Here we have extended that work to a CAG-correlative approach inclusive of downstream functional effects using a comprehensive behavioral battery and careful experimental design.…”

“…Longer CAG repeats are associated with onset during adolescence, and >110 CAG repeats result in early (juvenile) onset and severe signs of disease [12][13][14] . We used an allelic series consisting of three HET Htt CAG-KI lines expressing CAG-repeat lengths in the higher range (HdhQ 80 , HdhQ 92 and HdhQ 111 , where Q indicates an expected average glutamine tract length expressed from CAG codons; Online Methods) 15 and extended the upper range with the HET CAG140 (ref. 16) and zQ175 KI 17 lines.…”

Huntingtin protein is essential for mitochondrial metabolism, bioenergetics and structure in murine embryonic stem cells

“…In contrast to the lethality of htt -/- embryos, mutation of a single htt allele that expands the poly-Q (CAG) repeat at the N-terminal domain of Htt protein results in the dominantly inherited Huntington's Disease (HD) [1], [3]. HD is a devastating neurodegenerative disease typified by a progressive movement disorder, cognitive decline, and psychological impairment due to the death of medium spiny neurons in the striatum [4], and other areas of the brain [5].…”

Experimental Models for Identifying Modifiers of Polyglutamine-Induced Aggregation and Neurodegeneration

“…The simplest models have high throughput, but limited biological complexity, whereas tissue and organismal models retain high biological complexity, but have more modest throughput capacities. Note that the categories shown are not intended to represent hard distinctions, but rather a continuum of screening solutions balancing throughput against biological complexity gain-of-function mechanisms [15,16], suggest that molecular screening assays based on mutant HTT (mHTT) proteins could be used to identify proximal events in disease initiation. In fact, one of the first observable molecular phenotypes of polyQ diseases was the presence of large visible protein inclusions, comprised of the mutant misfolded protein and associated cellular proteins [17][18][19].…”