A novel, ataxic mouse model of ataxia telangiectasia caused by a clinically relevant nonsense mutation

Abstract

Ataxia Telangiectasia (A-T) and Ataxia with Ocular Apraxia Type 1 (AOA1) are devastating neurological disorders caused by null mutations in the genome stability genes, A-T mutated (ATM) and Aprataxin (APTX), respectively. Our mechanistic understanding and therapeutic repertoire for treating these disorders are severely lacking, in large part due to the failure of prior animal models with similar null mutations to recapitulate the characteristic loss of motor coordination (i.e., ataxia) and associated cerebellar defects. By increasing genotoxic stress through the insertion of null mutations in both the Atm (nonsense) and Aptx (knockout) genes in the same animal, we have generated a novel mouse model that for the first time develops a progressively severe ataxic phenotype associated with atrophy of the cerebellar molecular layer. We find biophysical properties of cerebellar Purkinje neurons (PNs) are significantly perturbed (e.g., reduced membrane capacitance, lower action potential [AP] thresholds, etc.), while properties of synaptic inputs remain largely unchanged. These perturbations significantly alter PN neural activity, including a progressive reduction in spontaneous AP firing frequency that correlates with both cerebellar atrophy and ataxia over the animal’s first year of life. Double mutant mice also exhibit a high predisposition to developing cancer (thymomas) and immune abnormalities (impaired early thymocyte development and T-cell maturation), symptoms characteristic of A-T. Finally, by inserting a clinically relevant nonsense-type null mutation in Atm, we demonstrate that Small Molecule Read-Through (SMRT) compounds can restore ATM production, indicating their potential as a future A-T therapeutic.

Vulnerability of Human Cerebellar Neurons to Degeneration in Ataxia-Causing Channelopathies

“…While there are instances of Purkinje degeneration that occurs in a similar fashion both mice and in humans as outlined above, there are other causes of ataxia, such as in disorders of DNA repair, which also appear to have a disconnect between degeneration that is profound in human disease yet is absent in mouse knockout models. In models of ataxia telangiectasia and ataxia with oculomotor apraxia type 1, complete knockout of the proteins ATM ( Lavin, 2013 ) and APTX ( Ahel et al, 2006 ), respectively, fails to produce motor impairment, cerebellar atrophy, or Purkinje neuron loss ( Lavin, 2013 ; Perez et al, 2021 ). In a recently developed mouse model, a combination of a biallelic human ATM mutation and knockout of APTX produces motor impairment and cerebellar volume loss without loss of Purkinje neurons by post-natal day 400.…”

Mechanisms of DNA damage‐mediated neurotoxicity in neurodegenerative disease

“…Notably, the development of a new mouse model of AT harboring both ATM nonsense mutation and APTX knockout was found to better recapitulate the neurologic deficits observed in AT (Perez et al , 2021 ). While progressive cerebellar atrophy and ataxia were not observed in mice with individual mutations in either ATM or APTX, the combination of both mutations lowered the threshold for neuronal genomic instability, resulting in neurological deficits (Perez et al , 2021 ). This indicates that at least for murine models of AT, the ATM knockout alone is fairly well tolerated, and additional genomic stress is needed to bring about toxicity in neurons (Tal et al , 2018 ).…”

The DNA damage response - from cell biology to human disease

“…(1) A-T mice faithfully recapitulate tissue radiosensitivity and immunodeficiency seen in patients with A-T; however, earlier mouse models lacked the typical neurological symptoms and cerebellar dysfunction seen in humans. A-T mice faithfully recapitulate tissue radiosensitivity...cerebellar dysfunction seen in humans [119][120][121][122][123][124] . A more recent report using the insertion recent report using the insertion of null mutations in both the ATM and APTX genes in a mouse model of A-T, successfully generated mice that developed severe ataxic symptoms and atrophy of the cerebellar molecular layer [125] .…”