Experiment
Mitochondrial respiratory chain complex II dysfunction is associated with neuronal degeneration in several diseases, including Huntington's disease (HD) and multiple system atrophy (MSA), and is likely to play a pivotal role in the progression of these disorders. Livestock poisoning by either plants or fungi containing 3-nitropropionic acid (3-NP), a naturally occurring neurotoxin that irreversibly inhibits succinate dehydrogenase (SDH), the main constituent of the mitochondrial respiratory chain complex II has been extensively documented.
In rodents, 3-NP-induced striatal neurodegeneration depends on the strain background suggesting that genetic differences among genotypes modulate toxicant variability and mechanisms that underlie 3-NP-induced neuronal cell death. Using the large BXD family of recombinant inbred (RI) strains the authors demonstrate that variants in Ccnd1 - the gene encoding cyclin D1 - of the DBA/2J parent underlie the resistance to 3-NP-induced striatal neurodegeneration. In contrast, the Ccnd1 variant inherited from the widely used C57BL/6J parental strain confers sensitivity.
Quantitative trait locus (QTL) mapping was performed using the GeneNetwork web resource (www.genenetwork.org). Interval mapping was performed by genotype regression against trait values in combination with whole genome permutation to determine the significance threshold for the association between genotype and trait expression. A permutation test establishes genome-wide significance criteria of p < 0.05. Three-month old females (n = 2 or 3 per strain) were treated with 3-NP as described above, always paired with at least one sensitive agematched C57BL/6J female as a positive control. Mice were weighed daily during the course of the 3-NP treatment and assessed daily for behavioral changes. Sensitivity or resistance to 3-NP was scored by histological analyses: presence of lesion with overt neuronal cell loss was scored as sensitive (-1) while lack of lesion with no overt signs of neuronal cell loss was scored as resistant (+1).
The heritability of the resistance trait allowed the authors to identify one strong QTL linked to 3-NP resistance (genome coordinates relative to GRCm38/mm10):
R3np1 (resistance to 3-nitropropionic acid neurotoxicity) maps to Chr 7: 144.1 - 145.3 Mb. BXD strains with the DBA/2J Ccnd1 allele were resistant to 3-NP-induced striatal neurodegeneration, while the BXD strains sensitive to 3-NP all carried the C57BL/6J Ccnd1 allele.
The authors confirm that 3-NP induces cyclin D1 expression in striatal neuronal cells of C57BL/6J, but this response is blunted in the DBA/2J. They further show that striatal-specific alternative processing of a highly conserved 3' UTR negative regulatory region of Ccnd1 co-segregates with the C57BL/6J parental Ccnd1 allele in BXD strains and that its differential processing accounts for sensitivity or resistance to 3-NP.
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