The GAA triplet-repeat is unstable in the context of the human FXN locus and displays age-dependent expansions in cerebellum and DRG in a transgenic mouse model

Abstract

Friedreich ataxia (FRDA) is caused by homozygosity for FXN alleles containing an expanded GAA triplet-repeat (GAA-TR) sequence. This expanded GAA-TR sequence is unstable in somatic cells of FRDA patients, showing age-dependent expansions in dorsal root ganglia (DRG), the tissue where pathology occurs earliest and is most significant. This is thought to be the basis for the progressive, tissue-specific pathology seen in FRDA, but the mechanism(s) for this somatic instability is unknown. We show that transgenic mice containing the expanded GAA-TR sequence (190 or 82 triplets) in the context of the human FXN locus show tissue-specific and age-dependent somatic instability that mimics the human condition. Small pool PCR analysis, which allows quantitative analysis of instability by assaying individual transgenes in vivo, showed age-dependent expansions specifically in the cerebellum and DRG. The (GAA) 190 allele showed some instability by 2 months, progressed at about 0.3 -0.4 triplets/week, resulting in a significant number of expansions by 12 months. Repeat length determined the age of onset of somatic instability, and the rate and magnitude of expansion.Whereas the GAA-TR was unstable in the context of the human FXN locus, pure GAA-TR sequences at other genetic loci in the human and murine genomes showed no instability. These data indicate that somatic instability of the GAA-TR sequence in the human FXN gene is determined by a combination of unique cis and trans-acting factors.This mouse model will serve as a useful tool to delineate the mechanism(s) of diseasespecific somatic instability in FRDA.3

Motor uncoordination and neuropathology in a transgenic mouse model of Machado–Joseph disease lacking intranuclear inclusions and ataxin-3 cleavage products

“…8A). Our results are in accordance with those obtained in animal models of different polyQ diseases, in which age has been described as a CAG repeat instability modifier (Clark et al, 2007;Ishiguro et al, 2001;Sato et al, 1999). Interestingly, we also observed that different tissues exhibited different patterns of MI increase with age.…”

DNA Mismatch Repair Complex MutSβ Promotes GAA·TTC Repeat Expansion in Human Cells

“…However, a somatic expansion bias exists within post-mitotic neurons of the spinal cord in FRDA patients (29) and mouse models (44,45). To investigate whether a lack of replication contributed to this bias, we performed parallel experiments with both dividing and non-dividing primary FRDA patient fibroblasts.…”

Small molecules affecting transcription in Friedreich ataxia

“…Three animal models are currently available for testing the in vivo efficacy of potential FXN activator molecules: these are the heterozygous [GAA·TTC] 230 KIKO (knock-in/knockout) and homozygous KIKI (knock-in) mice developed by Pandolfo and colleagues (Miranda et al, 2002), the human YAC FXN mouse developed by Pook and colleagues Clark et al, 2006), and the FXN-EGFP reporter mouse developed by Sarsero and colleagues (Sarsero et al, 2005;Sarsero et al, 2003). In the Pandolfo knock-in mice, a [GAA·TTC] 230 repeat was inserted into the mouse FXN locus at the position of the expansion in human FRDA alleles (FXN intron 1).…”

“…Both transgenic lines exhibit intergenerational and age-related somatic instability of the repeat, with the most prominent expansions occurring specifically in the cerebellum (as seen in FRDA autopsy tissues) and in the dorsal root ganglia (DRG) Clark et al, 2006). By cross breeding the FXN YAC transgenic mice with heterozygous FXN knockout mice from Puccio and colleagues, the Pook laboratory has further shown that both transgenes are able to successfully rescue homozygous FXN knockout embryonic lethality and that the rescue mice exhibit an FRDA-like phenotype .…”