Research Models

C9-BAC500 (Brown)

Synonyms: C9-BAC[GGGGCC]500

Species: Mouse
Genes: C9orf72
Mutations: Hexanucleotide repeat in C9ORF72
Modification: C9orf72: Transgenic
Disease Relevance: Amyotrophic Lateral Sclerosis, Frontotemporal Dementia
Strain Name: N/A
Genetic Background: SJL/B6
Availability: Available through Robert Brown

Summary

This transgenic mouse expresses the C9orf72 repeat expansion (GGGGCC), which is associated with ALS and FTD. This model, generated by Robert Brown and colleagues, uses a bacterial artificial chromosome (BAC) to deliver a sizeable chunk of the human C9orf72 sequence, including about 500 copies of the repeat expansion (Peters et al., 2015). Hemizygous C9BAC mice are viable, fertile, and born at expected Mendelian frequencies. They live a normal lifespan and do not develop overt behavioral abnormalities. However, they do recapitulate distinctive histopathological features of C9orf72 ALS/FTD, including intranuclear RNA foci and dipeptide repeats within the nervous system.

Male C9BAC mice live a normal lifespan with no overt behavioral abnormalities. Assessment of Rotarod performance and grip strength revealed no motor deficits over a wide range of ages (three to 24 months). Social behavior also appeared normal as assessed by the intruder assay. Survival analysis and behavioral data for females have not yet been reported.

Consistent with the absence of motor impairment, spinal motor axons were normal with respect to overall number and morphology. Furthermore, there was no evidence of increased denervation of neuromuscular junctions in 24-month-old male mice.

In the brain, there was no evidence of increased microgliosis, astrogliosis, or neuronal loss. Likewise, dendritic spine density in layer 2/3 of the prefrontal cortex was unaffected and TDP-43 was not mislocalized to the cytoplasm. Notably, C9BAC mice develop prominent RNA foci and dipeptide repeats. Intranuclear RNA foci were detectable as early as three months of age, and were abundant throughout the CNS, including the spinal cord and motor cortex, by 10 to 24 months of age. RNA foci were observed in both neurons and glia. Foci containing sense strand RNA were more abundant than foci containing antisense RNA.

The mice also express dipeptide repeat proteins produced by repeat-associated non-ATG (RAN) translation. Specifically, poly-glycine-proline peptides (poly-GP) were observed in the brain and spinal cord by four months of age. The poly-GP peptides, which are synthesized from both sense and antisense transcripts, were primarily soluble in young mice, but aggregated into small perinuclear inclusions in older mice, leading to a decrease in levels of soluble protein. At four months of age, poly-GP peptides were most abundant in the cerebellum. It is not yet clear whether, and to what extent, other RAN translation products (e.g., GA, GP, GR, PR, and PA) may be present.

In summary, these mice model certain pathological aspects of disease (e.g., RNA foci and dipeptide repeats). The fact that they do not develop neurodegeneration or behavioral impairment suggests that RNA foci and poly-dipeptides alone are not sufficient to drive these phenotypes.

Modification Details

The BAC construct was constructed from DNA isolated from a familial FTD/ALS patient carrying the C9 expansion. The construct included about 140 kb upstream sequence, exons 1 to 6 of the C9orf72 gene, including part of the 3┬┤UTR of the short isoform V1, and 20 kb downstream of the locus. The construct contained about 500 GGGGCC motifs between exons 1 and 2. The repeat sizes were relatively stable across tissues (i.e., minimal somatic instability), and appeared to be stable across generations as well, although this should be monitored.

Phenotype Characterization

When visualized, these models will distributed over a 18 month timeline demarcated at the following intervals: 1mo, 3mo, 6mo, 9mo, 12mo, 15mo, 18mo+.

Absent

  • Motor Impairment
  • Cortical Neuron Loss
  • Lower Motor Neuron Loss
  • NMJ Abnormalities
  • Body Weight
  • Premature Death
  • Gliosis

No Data

  • Muscle Atrophy

Cortical Neuron Loss

Not observed.

Lower Motor Neuron Loss

Not observed.

Cytoplasmic Inclusions

No cytoplasmic mislocalization, or aggregation of TDP-43 in the motor cortex. However, dipeptide repeats accumulated at advanced age and formed small perinuclear inclusion bodies positive for poly-GP.

Gliosis

No signs of increased activation of microglia or astrocytes in the brain or spinal cord.

NMJ Abnormalities

No difference in denervation of neuromuscular junctions at 24 months of age. No difference in motor or sensory spinal nerve root axon number or morphology.

Muscle Atrophy

Muscle histology has not been reported, but no overt muscle atrophy was observed.

Motor Impairment

No overt motor deficit as measured by the Rotarod and grip strength.

Body Weight

Non-significant trend for male C9BAC mice to be heavier than non-Tg controls. Female data have not yet been reported.

Premature Death

Normal lifespan beyond 2 years in male mice. Female data have not yet been reported.

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References

Paper Citations

  1. . Human C9ORF72 Hexanucleotide Expansion Reproduces RNA Foci and Dipeptide Repeat Proteins but Not Neurodegeneration in BAC Transgenic Mice. Neuron. 2015 Dec 2;88(5):902-9. PubMed.

Further Reading