Peripheral motor neuropathy is associated with defective kinase regulation of the KCC3 cotransporter

Abstract

Using exome sequencing, we identified a de novo mutation (c.2971 A>G; T991 A) in SLC12A6, the gene encoding the K+-Cl- cotransporter KCC3, in a patient with an early-onset, progressive, and severe peripheral neuropathy primarily affecting motor neurons. Normally, the WNK kinase—dependent phosphorylation of T991 tonically inhibits KCC3; however, cell swelling triggers Thr991 dephosphorylation to activate the transporter and restore cell volume. KCC3 T991A mutation in patient cells abolished Thr991 phosphorylation, resulted in constitutive KCC3 activity, and compromised cell volume homeostasis. KCC3T991A/T991A mutant mice exhibited constitutive KCC3 activity and recapitulated aspects of the clinical, electrophysiological, and histopathological findings of the patient. These results suggest that the function of the peripheral nervous system depends on finely tuned, kinase-regulated KCC3 activity and implicate abnormal cell volume homeostasis as a previously unreported mechanism of axonal degeneration.

A new splice-site mutation in SLC12A6 causing Andermann syndrome with motor neuronopathy

“…Main symptoms of AS are as follows: mental retardation along with such structural problems as total or partial agenesis of the CC, progressive sensorimotor polyneuropathy and amyotrophy. In the neurodegenerative process, sensorimotor polyneuropathy is always encountered, and involvement of the motor fibres can be predominant 3. This present report may be noteworthy in that the family has AS due to a novel c.1073+1G>A mutation without sensory involvement but with severe motor neuronopathy during the course of the disease.…”

The KCC3 cotransporter as a therapeutic target for peripheral neuropathy

“…Using CRISPR/cas9, we were able to reproduce the mutation in mice. Remarkably, it was only when the mice were homozygotes that significant deficits in locomotion (accelerated rotarod), fine motor movements (beam walk task), and nerve conduction were observed [7]. These observations established causality between the KCC3-T991A mutation and the neuropathy phenotype of the unique human patient, but also underscored species differences, as one copy of mutant was enough to produce the phenotype in a human patient, whereas two copies were necessary in mice.…”

“…Interestingly, the KCC3-T991A patient did not exhibit the additional CNS deficits and bone deformities observed in ACCPN patients. Also, in contrast to HSMN/ACC patients, the team of neurologists taking care of this unique patient characterized the neuropathy as predominantly motor, a discrepancy that will need to be addressed in future work [7].…”

“…The fact that phosphorylation of K-Cl cotransporter leads to inhibition while dephosphorylation leads to its activation is well appreciated in the field since the early 1990s [12]. In 2016, we published the case of a 10-year-old boy suffering from severe motor neuropathy found to harbor a novel de novo gain-of-function (GoF) mutation (Thr991Ala) in KCC3 [7]. The mutation involves one of the two key phospho-threonine residues (Thr991 and Thr1048) that are critical for regulating the activity of the transporter by the WNK-SPAK kinase complex [13].…”

“…To date, no studies have targeted KCC3 exclusively in motor neurons. It is notable that the strong locomotor phenotype observed in the KCC3-T991A knock-in mouse did not reproduce the hind limb clasping phenotype that points to deficits in proprioception [7]. Could it be that both sensory and motor neurons are implicated?…”