Insights into the pathogenesis of ATP1A1 ‐related CMT disease using patient‐specific iPSCs

Current Treatment Methods for Charcot-Marie-Tooth Diseases

Charcot-Marie-Tooth (CMT) disease, the most common inherited neuromuscular disorder, exhibits a wide phenotypic range, genetic heterogeneity, and a variable disease course. The diverse molecular genetic mechanisms of CMT were discovered over the past three decades with the development of molecular biology and gene sequencing technologies. These methods have brought new options for CMT reclassification and led to an exciting era of treatment target discovery for this incurable disease. Currently, there are no approved disease management methods that can fully cure patients with CMT, and rehabilitation, orthotics, and surgery are the only available treatments to ameliorate symptoms. Considerable research attention has been given to disease-modifying therapies, including gene silencing, gene addition, and gene editing, but most treatments that reach clinical trials are drug treatments, while currently, only gene therapies for CMT2S have reached the clinical trial stage. In this review, we highlight the pathogenic mechanisms and therapeutic investigations of different subtypes of CMT, and promising therapeutic approaches are also discussed.

The phenotypic spectrum of pathogenic ATP1A1 variants expands: the novel p.P600R substitution causes demyelinating Charcot-Marie-Tooth disease

BACKGROUND

Charcot-Marie-Tooth disease (CMT) is a genetically and clinically heterogeneous group of inherited neuropathies. Monoallelic pathogenic variants in ATP1A1 were associated with axonal and intermediate CMT. ATP1A1 encodes for the catalytic α1 subunit of the Na⁺/ K⁺ ATPase. Besides neuropathy, other associated phenotypes are spastic paraplegia, intellectual disability, and renal hypomagnesemia. We hereby report the first demyelinating CMT case due to a novel ATP1A1 variant.

METHODS

Whole-exome sequencing on the patient's genomic DNA and Sanger sequencing to validate and confirm the segregation of the identified p.P600R ATP1A1 variation were performed. To evaluate functional effects, blood-derived mRNA and protein levels of ATP1A1 and the auxiliary β1 subunit encoded by ATP1B1 were investigated. The ouabain-survival assay was performed in transfected HEK cells to assess cell viability, and two-electrode voltage clamp studies were performed in Xenopus oocytes.

RESULTS

The variant was absent in the local and global control datasets, falls within a highly conserved protein position, and is in a missense-constrained region. The expression levels of ATP1A1 and ATP1B1 were significantly reduced in the patient compared to healthy controls. Electrophysiology indicated that ATP1A1^p.P600R injected Xenopus oocytes have reduced Na⁺/ K⁺ ATPase function. Moreover, HEK cells transfected with a construct encoding ATP1A1^p.P600R harbouring variants that confers ouabain insensitivity displayed a significant decrease in cell viability after ouabain treatment compared to the wild type, further supporting the pathogenicity of this variant.

CONCLUSION

Our results further confirm the causative role of ATP1A1 in peripheral neuropathy and broaden the mutational and phenotypic spectrum of ATP1A1-associated CMT.

Modelling Charcot-Marie-Tooth disease in a dish reveals common cell type-specific alterations

This scientific commentary refers to ‘Induced pluripotent stem cell-derived motor neurons of CMT type 2 patients reveal progressive mitochondrial dysfunction’, by Van Lent et al. (doi:10.1093/brain/awab226).

Diseases caused by mutations in the Na+/K+ pump α1 gene ATP1A1

Human cell survival requires function of the Na⁺/K⁺ pump; the heteromeric protein that hydrolyzes ATP to extrude Na⁺ and import K⁺ across the plasmalemma, thereby building and maintaining these ions' electrochemical gradients. Numerous dominant diseases caused by mutations in genes encoding for Na⁺/K⁺ pump catalytic (α) subunit isoforms highlight the importance of this protein. Here, we review literature describing disorders caused by missense mutations in ATP1A1, the gene encoding the ubiquitously expressed α1 isoform of the Na⁺/K⁺ pump. These various maladies include primary aldosteronism with secondary hypertension, an endocrine syndrome, Charcot-Marie-Tooth disease, a peripheral neuropathy, complex spastic paraplegia, another neuromuscular disorder, as well as hypomagnesemia accompanied by seizures and cognitive delay, a condition affecting the renal and central nervous systems. This article focuses on observed commonalities among these mutations' functional effects, as well as on the special characteristics that enable each particular mutation to exclusively affect a certain system, without affecting others. In this respect, it is clear how somatic mutations localized to adrenal adenomas increase aldosterone production without compromising other systems. However, it remains largely unknown how and why some but not all de novo germline or familial mutations (where the mutant must be expressed in numerous tissues) produce a specific disease and not the other diseases. We propose hypotheses to explain this observation and the approaches that we think will drive future research on these debilitating disorders to develop novel patient-specific treatments by combining the use of heterologous protein-expression systems, patient-derived pluripotent cells, and gene-edited cell and mouse models.

Hereditary motor neuropathies

PURPOSE OF REVIEW

Hereditary motor neuropathies (HMN) comprise a broad genotypic and phenotypic spectrum of rare, progressively disabling diseases manifesting with length-dependent muscle weakness and atrophy. To date, more than half of the cases cannot be genetically explained. To provide symptomatic and disease-modifying treatments in the future, a better understanding of disease mechanisms is required.

RECENT FINDINGS

By whole exome and genome sequencing, the discovery of several novel genes (SCO2, TDRKH, SPTAN1, CADM3, and SORD) involved in the pathogenesis of HMN has now relevantly changed the pathophysiological knowledge. This recent success in causative understanding has mainly been driven by the development of functional models including cell culture, animal, and patient-derived induced pluripotent stem cell platforms. These models have an important impact on therapeutic advances including broader approaches to prevent or reverse axonal degeneration and individualized gene silencing attempts using sequence-specific RNA degradation mechanisms.

SUMMARY

In rare diseases such as HMN, the recent development of genetic sequencing and data interpretation methods has enabled a broader diagnostic approach, whereas treatment strategies are becoming more individualized. Significant milestones have been reached in the discovery of new genes, the establishment of functional disease models, and the preclinical development of mechanistic-based therapies.