Research Models

Endogenous Sod1 D83G

Species: Mouse
Genes: SOD1
Mutations: D83G in SOD1
Modification: SOD1: Other
Disease Relevance: Amyotrophic Lateral Sclerosis
Strain Name: B6(C3H)-Sod1m1H/J
Genetic Background: C57BL/6J
Availability: The Jackson Lab: Stock# 020440; Cryopreserved

Summary

In contrast to most SOD1 mouse models, which overexpress either mutant or wild-type human SOD1, this model has a mutation in the endogenous murine Sod1 gene (Joyce et al., 2015). Although human SOD1 is not present, the A>G point mutation carried by these mice corresponds to the D83G mutation in human SOD1, which causes familial ALS.

The mutation in these mice did not arise spontaneously, but was the result of treatment with the chemical N-ethyl-N-Nitrosourea (ENU), which causes random mutagenesis. Both homozygous and heterozygous mice are viable. ALS-related phenotypes are more pronounced in homozygotes, which are the focus of this entry. Mice carrying the mutation exhibited reduced levels of Sod1 protein in the spinal cord. Homozygous D83G mice had protein levels only about 12 percent of levels in wild-type mice, whereas heterozygotes had intermediate protein levels (about 70 percent of wild-type). Augmenting this loss of function, is the fact that the D83G mutation alters SOD1’s zinc-binding site, destabilizing the protein and inactivating the enzyme.

Homozygous D83G mice exhibit ALS-related neuropathology (e.g., upper and motor neuron loss, degenerating neuromuscular junctions, gliosis) and progressive motor impairment (e.g., tremor, muscle weakness, gait abnormalities). They do not develop paralysis, although humane euthanasia around 16 months of age may preclude end-stage motor impairment. Euthanasia is necessitated primarily by excessive weight loss, probably due to the corresponding development of hepatocellular carcinomas. In homozygotes, body weight started to diverge from wild-type mice as early as four weeks of age and worsened with age. Males reached end-stage sooner than females (495 ± 22 versus 588 ± 24 days).

In terms of neuropathology, lower motor neuron loss appears first, between 6 and 15 weeks of age. It then appears to stabilize and is no more severe at 52 weeks. In the cerebral cortex, neuronal numbers were comparable to wild-type at 15 weeks of age, but cellular degeneration occurred by 29 weeks in layer V of the motor cortex, which showed selective vulnerability. Astrocytosis and microgliosis were evident by 15 weeks, and became more severe with age.

Denervation of a hindlimb muscle, the extensor digitorum longus (EDL), was detected by 52 weeks of age. A decreased number of motor units were seen in the EDL muscle of homozygous D83G mice compared with wild-type littermates.

D83G homozygous mice also develop progressive motor symptoms, including gait abnormalities, tremor, and impaired performance on motor tasks. Grip strength was reduced in both male and female mice at 6 weeks of age. Tremors developed by about five months of age in both sexes, and became more severe over time. Deficits in Rotarod performance were also apparent, at 23 and 67 weeks in females and males, respectively. Select sensory deficits were also observed, such as to a noxious heat stimulus.

Due to the random-mutagenesis technique used to generate these mice, they also carry other mutations throughout the genome that theoretically could contribute to phenotypic abnormalities. However, random mutations in the initial ENU-treated gametes were diluted by at least 10 generations of backcrossing to wild-type C57BL/6J. In addition, the majority of the ENU-induced genomic changes are not linked to the SOD locus, and therefore do not co-segregate with SOD1, further diluting their potential effects.

Modification Details

This mouse has a point mutation (A>G) in the endogenous murine Sod1 gene, resulting from exposure to the chemical mutagen N-ethyl-N-nitrosourea (ENU). The endogenous murine Sod1 promoter drives expression of the mutant protein, which carries a D83G missense mutation.

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

  • Cytoplasmic Inclusions

No Data

  • Muscle Atrophy

Cortical Neuron Loss

Neuronal numbers comparable to wild-type at 15 weeks, but about 20 percent loss of upper motor neurons by 29 weeks. Neurons in layer V of the motor cortex appeared selectively vulnerable.

Lower Motor Neuron Loss

Neuronal numbers comparable to wild-type at 6 weeks, but loss occurred by 15 weeks. The neuronal loss then stabilized; it was not more severe at 52 weeks of age.

Cytoplasmic Inclusions

Not observed.

Gliosis

Gliosis, of both astrocytes and microglia, was evident in the spinal cord by 15 weeks. It was further elevated at 52 weeks.

NMJ Abnormalities

Denervation of a hindlimb muscle, the extensor digitorum longus, was detected by 52 weeks of age, and a decreased number of motor units in the EDL muscle.

Muscle Atrophy

Muscle atrophy was not reported, although changes to the muscle composition and histochemistry were observed.

Motor Impairment

Both male and female mice develop a variety of progressive motor symptoms. Grip strength was reduced at 6 weeks of age. Tremors developed by about 5 months of age. Rotarod performance was impaired, at 23 weeks in females and 67 weeks in males.

Body Weight

Homozygous mice, both male and female, showed reduced body weight by 4 weeks of age in contrast to wild-type littermates. Loss of excessive body weight was the primary factor leading to euthanasia.

Premature Death

Males reach end-stage sooner than females (495 ± 22 versus 588 ± 24 days). Animals were sacrificed when weight loss exceeded 20 percent of maximum weight, in accordance with animal-use guidelines. This is likely explained by the development of hepatocellular carcinomas due to SOD1 loss of function.

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References

Paper Citations

  1. . A novel SOD1-ALS mutation separates central and peripheral effects of mutant SOD1 toxicity. Hum Mol Genet. 2015 Apr 1;24(7):1883-97. Epub 2014 Dec 2 PubMed.

External Citations

  1. The Jackson Lab: Stock# 020440

Further Reading