Genetic background modulates behavioral impairments in R6/2 mice and suggests a role for dominant genetic modifiers in Huntington’s disease pathogenesis

Variability and modification of the symptoms of Huntington’s disease (HD) are commonly observed in both patient populations and animal models of the disease. Utilizing a stable line of the R6/2 HD mouse model, the present study investigated the role of genetic background in the onset and severity of HD symptoms in a transgenic mouse. R6/2 congenic C57BL/6J and C57BL/6J × DBA/2J F1 (B6D2F1) mice were evaluated for survival and a number of behavioral phenotypes. This study reports that the presence of the DBA/2J allele results in amelioration or exacerbation of several HD-like phenotypes characteristic of the R6/2 mouse model and indicates the presence of dominant genetic modifiers of HD symptoms. This study is the first step in identifying genes that confer natural genetic variation and modify the HD symptoms. This identification may lead to novel targets for treatment and help elucidate the molecular mechanisms of HD pathogenesis

Onset and Progression of Behavioral and Molecular Phenotypes in a Novel Congenic R6/2 Line Exhibiting Intergenerational CAG Repeat Stability

In the present study we report on the use of speed congenics to generate a C57BL/6J congenic line of HD-model R6/2 mice carrying 110 CAG repeats, which uniquely exhibits minimal intergenerational instability. We also report the first identification of the R6/2 transgene insertion site. The relatively stable line of 110 CAG R6/2 mice was characterized for the onset of behavioral impairments in motor, cognitive and psychiatric-related phenotypes as well as the progression of disease-related impairments from 4 to 10 weeks of age. 110Q mice exhibited many of the phenotypes commonly associated with the R6/2 model including reduced activity and impairments in rotarod performance. The onset of many of the phenotypes occurred around 6 weeks and was progressive across age. In addition, some phenotypes were observed in mice as early as 4 weeks of age. The present study also reports the onset and progression of changes in several molecular phenotypes in the novel R6/2 mice and the association of these changes with behavioral symptom onset and progression. Data from TR-FRET suggest an association of mutant protein state changes (soluble versus aggregated) in disease onset and progression

A Mouse Model of the Human Fragile X Syndrome I304N Mutation

The mental retardation, autistic features, and behavioral abnormalities characteristic of the Fragile X mental retardation syndrome result from the loss of function of the RNA–binding protein FMRP. The disease is usually caused by a triplet repeat expansion in the 5′UTR of the FMR1 gene. This leads to loss of function through transcriptional gene silencing, pointing to a key function for FMRP, but precluding genetic identification of critical activities within the protein. Moreover, antisense transcripts (FMR4, ASFMR1) in the same locus have been reported to be silenced by the repeat expansion. Missense mutations offer one means of confirming a central role for FMRP in the disease, but to date, only a single such patient has been described. This patient harbors an isoleucine to asparagine mutation (I304N) in the second FMRP KH-type RNA–binding domain, however, this single case report was complicated because the patient harbored a superimposed familial liver disease. To address these issues, we have generated a new Fragile X Syndrome mouse model in which the endogenous Fmr1 gene harbors the I304N mutation. These mice phenocopy the symptoms of Fragile X Syndrome in the existing Fmr1–null mouse, as assessed by testicular size, behavioral phenotyping, and electrophysiological assays of synaptic plasticity. I304N FMRP retains some functions, but has specifically lost RNA binding and polyribosome association; moreover, levels of the mutant protein are markedly reduced in the brain specifically at a time when synapses are forming postnatally. These data suggest that loss of FMRP function, particularly in KH2-mediated RNA binding and in synaptic plasticity, play critical roles in pathogenesis of the Fragile X Syndrome and establish a new model for studying the disorder

Mice with Truncated MeCP2 Recapitulate Many Rett Syndrome Features and Display Hyperacetylation of Histone H3

Mutations in the methyl-CpG binding protein 2 ( MECP2) gene cause Rett syndrome (RTT), a neurodevelopmental disorder characterized by the loss of language and motor skills during early childhood. We generated mice with a truncating mutation similar to those found in RTT patients. These mice appeared normal and exhibited normal motor function for about 6 weeks, but then developed a progressive neurological disease that includes many features of RTT: tremors, motor impairments, hypoactivity, increased anxiety-related behavior, seizures, kyphosis, and stereotypic forelimb motions. Additionally, we show that although the truncated MeCP2 protein in these mice localizes normally to heterochromatic domains in vivo, histone H3 is hyperacetylated, providing evidence that the chromatin architecture is abnormal and that gene expression may be misregulated in this model of Rett syndrome

EM48 staining for aggregated mutant HTT appears to increase in amount and intensity with increasing age and disease progression.

<p>In 4 week old B6 110Q R6/2 mice, very few small, circular intranuclear inclusions characteristic of 110Q R6/2 mice can be identified. However, by 10 weeks of age such inclusions are readily observed neuronal nuclei. Intensity of EM48 staining appears increase from 4 and 10 weeks of age, further suggesting an increase in aggregate load.</p

Behavioral responses of B6 R6/2 transgenic and wildtype mice across 4 ages in the open-field test.

<p>(<b>A</b>) Exploratory activity as measured by total distance traveled in an open-field. (<b>B</b>) Rearing behavior as measured by vertical activity (beam interruptions). In both <b>A</b> and <b>B</b>, ‘<i>a</i>’ represents that R6/2 mice are significantly less active than wildtype littermates at 6, 8 and 10 weeks of age. ‘<i>b</i>’ represents that 8 and 10 week old R6/2 mice are less active than 4 and 6 week old R6/2 mice. Wildtype mice increase in activity from 6 to 8 weeks of age. (<b>C</b>) Anxiety-like responses as measured by the center∶total distance ratio. ‘*’ represents that overall B6 R6/2 transgenic mice spend less of their exploration in the center of the open-field relative to wildtype mice. All p-values≤0.022 by three-way ANOVA and simple effects analysis.</p

Location of the insertion site of the R6/2 transgene.

<p>The insertion site localized to a position on mouse chromosome 4 in an intron of predicted gene GM12695. The junction of the 5′ insertion site was sequenced through approximately 1400 bp of the transgene. The represented structure of the R6/2 insertion (for areas not sequenced) is based on the report by Mangiarini et al. (1996). Underlined sequences represent the primers used to amplify across the junction. Sequence given in black represents chromosome 4. Sequence given in grey represents transgene.</p