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

Abstract

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 ATP1A1p.P600R injected Xenopus oocytes have reduced Na+/ K+ ATPase function. Moreover, HEK cells transfected with a construct encoding ATP1A1p.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.

Auditory Neuropathy Spectrum Disorder Progressing with Motor and Sensory Neuropathy Caused by an <i>ATP1A1</i> Variant

“…A loss of the function of normal NKA activity has been reported to cause age-related hearing loss in a mouse model (13). A recent study reported a patient with the p.Pro600Arg variant in ATP1A1 who exhibited similar clinical manifestations to ours, including demyelinat-ing CMT and severe sensorineural deafness (8). Unfortunately, the diagnosis of ANSD in that case was not confirmed by further audiological evaluations, including ABR (8); however, ANSD may be a common cause of sensorineural deafness in this variant.…”

“…The c.1799C>G (p. Pro600Arg) variant was previously reported in a 50-year-old patient with CMT2DD and more severe sensorineural deafness (8). This variant has not been described in healthy individual databases, including the Exome Aggregation Consortium (https://gnomad.broadinstitu te.org).…”

ATP1A1-linked diseases require a malfunctioning protein product from one allele

“…NKAs formed by the α1 subunit are ubiquitous, while α2 pumps predominate in skeletal muscle and glia, and α3 pumps are expressed in neurons. Notably, sensory and motor neurons express both α1 and α3 isoforms, with α1 predominating in axons ( 4 ), which may explain why α1 disease-associated variants predominantly cause CMT type 2 in which axonal degeneration leads to progressive strength and sensory loss starting from the distal limbs, although an ATP1A1 variant causing demyelinating CMT was recently described ( 9 ).…”

“…Electrophysiological studies in Xenopus oocytes and adrenal NCI-H295R cells expressing disease-related NKA α1 variants reported depolarizing inward currents through most (but not all) variants linked to hyperaldosteronism ( 2, 8, 10, 11 ) or hypomagnesemia with refractory seizures ( 7, 12 ), providing a plausible gain-of-function mechanism for these disease-associated variants. However, variants causing neuropathies seem to lack such inward “leak” currents ( 4-7, 9-11 ). Moreover, functionally characterized CMT variants impair NKA function to distinct degrees.…”