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Absence of neurological abnormalities in mice homozygous for the Polr3a G672E hypomyelinating leukodystrophy mutation
Karine Choquet1,2,3, Sharon Yang1, Robyn D Moir4
1Montreal Neurological Institute, McGill University, 3801 University Street, room 622, Montréal, Québec, H3A 2B4, Canada.
Abstract:
Recessive mutations in the ubiquitously expressed POLR3A gene cause one of the most frequent forms of childhood-onset hypomyelinating leukodystrophy (HLD): POLR3-HLD. POLR3A encodes the largest subunit of RNA Polymerase III (Pol III), which is responsible for the transcription of transfer RNAs (tRNAs) and a large array of other small non-coding RNAs. In order to study the central nervous system pathophysiology of the disease, we introduced the French Canadian founder Polr3a mutation c.2015G > A (p.G672E) in mice, generating homozygous knock-in (KI/KI) as well as compound heterozygous mice for one Polr3a KI and one null allele (KI/KO). Both KI/KI and KI/KO mice are viable and are able to reproduce. To establish if they manifest a motor phenotype, WT, KI/KI and KI/KO mice were submitted to a battery of behavioral tests over one year. The KI/KI and KI/KO mice have overall normal balance, muscle strength and general locomotion. Cerebral and cerebellar Luxol Fast Blue staining and measurement of levels of myelin proteins showed no significant differences between the three groups, suggesting that myelination is not overtly impaired in Polr3a KI/KI and KI/KO mice. Finally, expression levels of several Pol III transcripts in the brain showed no statistically significant differences. We conclude that the first transgenic mice with a leukodystrophy-causing Polr3a mutation do not recapitulate the childhood-onset HLD observed in the majority of human patients with POLR3A mutations, and provide essential information to guide selection of Polr3a mutations for developing future mouse models of the disease.
Insights
Mice with a POLR3A mutation, a cause of childhood hypomyelinating leukodystrophy (HLD), did not develop motor or myelination deficits. These models do not fully replicate the human disease, guiding future research for better POLR3A-HLD models.
Area of Science:
- Genetics
- Neuroscience
- Molecular Biology
Background:
- Recessive mutations in POLR3A cause POLR3-hypomyelinating leukodystrophy (HLD), a frequent childhood neurological disorder.
- POLR3A is essential for RNA Polymerase III (Pol III) function, transcribing tRNAs and small non-coding RNAs.
Purpose of the Study:
- To create and characterize mouse models expressing a common POLR3A mutation (c.2015G>A, p.G672E) to study central nervous system pathophysiology.
- To determine if these Polr3a knock-in mice exhibit phenotypes relevant to POLR3-HLD.
Main Methods:
- Generated homozygous knock-in (KI/KI) and compound heterozygous (KI/KO) mice with the Polr3a mutation.
- Conducted a one-year battery of behavioral tests assessing motor function, balance, and locomotion.
- Performed Luxol Fast Blue staining and myelin protein level measurements; analyzed Pol III transcript levels in the brain.
Main Results:
- KI/KI and KI/KO mice were viable and fertile, showing normal balance, muscle strength, and locomotion.
- Cerebral and cerebellar myelination, assessed by staining and protein levels, was not significantly impaired.
- Brain expression levels of key Pol III transcripts remained unchanged in the mutant mice.
Conclusions:
- The first mouse models with a leukodystrophy-associated POLR3A mutation do not fully recapitulate childhood-onset POLR3-HLD.
- These findings are crucial for selecting appropriate Polr3a mutations for developing more accurate future mouse models of the disease.