The motor neuron disease gene Vrk1: a conditional VRK1 knockout mouse as a novel model for neuromuscular disease

Mutations in Vaccinia-related kinase 1 (VRK1) gene cause a rare hereditary motor neuron disease (MND) which can begin in childhood, resulting in a spinal muscular atrophy (SMA)-like phenotype with brain anomalies (such as Pontocerebellar Hypoplasia), or as adult onset amyotrophic lateral sclerosis (ALS). VRK1 is a serine-threonine kinase originally identified and studied for its roles in cell proliferation and tumorigenesis. It was not known to have specific neuronal functions before the human phenotypes caused by VRK1 deficiency clearly demonstrated that it is critical for motor neuron function and maintenance. In previously works we demonstrated that VRK1 has important functions in brain development and neuronal migration.

To study VRK1’s role in MND, we generate a Vrk1fl/fl mouse using CRISPR/Cas9 technology. This Vrk1fl/fl mouse was crossed with PGK-cre mouse and the Vrk1+/- offspring were crossed with each other in order to create a Vrk1-/- homozygous mouse. With no offspring found to be Vrk1-/- homozygous, we have demonstrated that complete Vrk1 knockout is embryonically lethal. Next, we successfully created a neuronal tissue-specific Vrk1 knockout mouse by crossing the Vrk1fl/fl mice with ChAT-cre mice (cholinergic neurons). These homozygous Vrk1 cKO mice display a severe progressive motor disease phenotype followed by death at 25-32 days (median lifespan of 27 days). They were significantly smaller than their healthy littermates, and their spontaneous motor activity and grip strength were significantly impaired. Their body weight and locomotor activities gradually decrease and at the age of 20 days they develop progressive kyphosis and claw joints of their forelimb.