Two new mouse models of Gjb1-associated Charcot-Marie-Tooth disease type 1X

BACKGROUND

Charcot-Marie-Tooth disease type 1X is caused by mutations in GJB1, which is the second most common gene associated with inherited peripheral neuropathy. The GJB1 gene encodes connexin 32 (CX32), a gap junction protein expressed in myelinating glial cells. The gene is X-linked, and the mutations cause a loss of function.

AIMS

A large number of disease-associated variants have been identified, and many result in mistrafficking and mislocalization of the protein. An existing knockout mouse lacking Gjb1 expression provides a valid animal model of CMT1X, but the complete lack of protein may not fully recapitulate the disease mechanisms caused by aberrant CX32 proteins. To better represent the spectrum of human CMT1X-associated mutations, we have generated a new Gjb1 knockin mouse model.

METHODS

CRISPR/Cas9 genome editing was used to produce mice carrying the R15Q mutation in Gjb1. In addition, we identified a second allele with an early frame shift mutation in codon 7 (del2). Mice were analyzed using clinically relevant molecular, histological, neurophysiological, and behavioral assays.

RESULTS

Both alleles produce protein detectable by immunofluorescence in Schwann cells, with some protein properly localizing to nodes of Ranvier. However, both alleles also result in peripheral neuropathy with thinly myelinated and demyelinated axons, as well as degenerating and regenerating axons, predominantly in distal motor nerves. Nerve conduction velocities were only mildly reduced at later ages and compound muscle action potential amplitudes were not reduced. Levels of neurofilament light chain in plasma were elevated in both alleles. The del2 mice have an onset at ~3 months of age, whereas the R15Q mice had a later onset at 5-6 months of age, suggesting a milder loss of function. Both alleles performed comparably to wild type littermates in accelerating rotarod and grip strength tests of neuromuscular performance.

INTERPRETATION

We have generated and characterized two new mouse models of CMT1X that will be useful for future mechanistic and preclinical studies.

Mouse models of NADK2 deficiency analyzed for metabolic and gene expression changes to elucidate pathophysiology

NADK2 encodes the mitochondrial form of nicotinamide adenine dinucleotide (NAD) kinase, which phosphorylates NAD. Rare recessive mutations in human NADK2 are associated with a syndromic neurological mitochondrial disease that includes metabolic changes, such as hyperlysinemia and 2,4 dienoyl CoA reductase (DECR) deficiency. However, the full pathophysiology resulting from NADK2 deficiency is not known. Here, we describe two chemically induced mouse mutations in Nadk2-S326L and S330P-which cause severe neuromuscular disease and shorten lifespan. The S330P allele was characterized in detail and shown to have marked denervation of neuromuscular junctions by 5 weeks of age and muscle atrophy by 11 weeks of age. Cerebellar Purkinje cells also showed progressive degeneration in this model. Transcriptome profiling on brain and muscle was performed at early and late disease stages. In addition, metabolomic profiling was performed on the brain, muscle, liver and spinal cord at the same ages and on plasma at 5 weeks. Combined transcriptomic and metabolomic analyses identified hyperlysinemia, DECR deficiency and generalized metabolic dysfunction in Nadk2 mutant mice, indicating relevance to the human disease. We compared findings from the Nadk model to equivalent RNA sequencing and metabolomic datasets from a mouse model of infantile neuroaxonal dystrophy, caused by recessive mutations in Pla2g6. This enabled us to identify disrupted biological processes that are common between these mouse models of neurological disease, as well as those processes that are gene-specific. These findings improve our understanding of the pathophysiology of neuromuscular diseases and describe mouse models that will be useful for future preclinical studies.

The integrated stress response contributes to tRNA synthetase-associated peripheral neuropathy

Dominant mutations in ubiquitously expressed transfer RNA (tRNA) synthetase genes cause axonal peripheral neuropathy, accounting for at least six forms of Charcot-Marie-Tooth (CMT) disease. Genetic evidence in mouse and Drosophila models suggests a gain-of-function mechanism. In this study, we used in vivo, cell type–specific transcriptional and translational profiling to show that mutant tRNA synthetases activate the integrated stress response (ISR) through the sensor kinase GCN2 (general control nonderepressible 2). The chronic activation of the ISR contributed to the pathophysiology, and genetic deletion or pharmacological inhibition of Gcn2 alleviated the peripheral neuropathy. The activation of GCN2 suggests that the aberrant activity of the mutant tRNA synthetases is still related to translation and that inhibiting GCN2 or the ISR may represent a therapeutic strategy in CMT.