Model organisms reveal insight into human neurodegenerative disease: ataxin-2 intermediate-length polyglutamine expansions are a risk factor for ALS

Abstract

Model organisms include yeast Saccromyces cerevisae and fly Drosophila melanogaster. These systems have powerful genetic approaches, as well as highly conserved pathways, both for normal function and disease. Here, we review and highlight how we applied these systems to provide mechanistic insight into the toxicity of TDP-43. TDP-43 accumulates in pathological aggregates in ALS and about half of FTD. Yeast and fly studies revealed an interaction with the counterparts of human Ataxin-2, a gene whose polyglutamine repeat expansion is associated with spinocerebellar ataxia type 2. This finding raised the hypothesis that repeat expansions in ataxin-2 may associate with diseases characterized by TDP-43 pathology such as ALS. DNA analysis of patients revealed that intermediate-length polyglutamine expansions in ataxin-2 are a risk factor for ALS, such that repeat lengths are greater than normal, but lower than that associated with spinocerebellar ataxia type 2 (SCA2), and are more frequent in ALS patients than in matched controls. Moreover, repeat expansions associated with ALS are interrupted CAA-CAG sequences as opposed to the pure CAG repeat expansions typically associated with SCA2. These studies provide an example of how model systems, when extended to human cells and human patient tissue, can reveal new mechanistic insight into disease.

Fig. 1

TDP-43 is toxic to yeast and fly, and ataxin-2 enhances this toxicity. a Pbp1, the yeast homologue of ataxin-2, is a dose-sensitive modifier of TDP-43 toxicity in yeast. Fivefold serial dilutions of yeast cells spotted onto glucose (expression repressed) or galactose (expression induced). Spotting assays with yeast expressing TDP-43 showing toxicity. Upregulation of PBP1 enhances TDP-43 toxicity. b Ataxin-2 is a modifier of TDP-43 toxicity in the nervous system of Drosophila. Flies expressing TDP-43 or dAtx2 alone (dAtx2 ^EP) in the eye (with gmr-GAL4 driver) have a mild effect on retinal structure. TDP-43 toxicity is more severe upon upregulation of dAtx2 (dAtx2 ^EP). TDP-43 toxicity is markedly mitigated upon the reduction of dAtx2 (flies in trans to null allele dAtx2 ^X1). The figure and the legend are from Elden et al. (2010)

Fig. 2

Ataxin-2 shows altered localization in spinal cord motor neurons of ALS patients. Immunostaining for ataxin-2 in spinal cord. a, b In control spinal cord neurons, ataxin-2 is localized throughout the cytoplasm in a diffuse pattern. c, d In ALS spinal cord neurons, ataxin-2 was present in distinct cytoplasmic accumulations (arrows). In some cases, ataxin-2 positive accumulations were adjacent to clearings indicative of TDP-43 aggregates (asterisk in b). The figure and the legend are from Elden et al. (2010)

Fig. 3

Ataxin-2 intermediate-length polyglutamine repeat expansions are associated with increased risk for ALS. The ATXN2 gene contains a trinucleotide repeat encoding polyglutamine. The repeat length is normally 22–23Q. Expansions of >34 cause SCA2. We hypothesized that the intermediate-length polyglutamine expansions could be linked to ALS. The ataxin-2 polyglutamine length was defined by Genescan analysis of ALS cases and neurologically normal controls. a The distribution of ataxin-2 polyglutamine repeat lengths in ALS and control cases. Polyglutamine lengths ≥27 are significantly enriched in ALS. Further, polyQ lengths >31 were never observed in our controls, but we found ten ALS patients above this threshold. b Representative examples of Genescan analysis of polyglutamine lengths from control and ALS cases. The figure and the legend are from Elden et al. (2010)