VRK1-Related Motor Neuropathy With Upper Motor Neuron Signs and Selective Muscle Involvement

Hereditary motor neuropathies (HMN) represent a heterogeneous group of disorders with wide clinical and genetic variability. Despite advances in molecular diagnostics, approximately 50% of cases remain genetically unresolved, particularly those where distinguishing length-dependent motor neuropathy from motor neuron disorder with disproportionate segmental involvement is a challenge. Variants in the VRK1 gene, originally described in association with pontocerebellar hypoplasia, are now known to produce a broad clinical spectrum, including amyotrophic lateral sclerosis, dHMN, and less frequently, spastic paraplegia.

This study's aim was to characterize the clinical presentation, electrophysiological findings, and muscle MRI patterns associated with VRK1-related motor neuron disease in a cohort of nine patients from five unrelated families.

Five unrelated families with inherited motor neuropathy were investigated using next-generation sequencing techniques, including targeted gene panels or whole-exome sequencing, with subsequent confirmation by Sanger sequencing. A total of nine affected individuals underwent detailed clinical evaluation, nerve conduction studies (NCS), electromyography (EMG), and whole-body muscle MRI (wbMRI).

Nine affected individuals carrying biallelic VRK1 variants were evaluated. In most cases (66%), symptom onset occurred during the first decade of life. All patients presented with gradually progressive distal muscle weakness. Mean Medical Research Council (MRC) scores were 2.8 for ankle dorsiflexion and 1.9 for plantar flexion. Sensory nerve conduction studies were normal in all individuals evaluated (8/9), although mild sensory complaints were reported in four patients. Muscle cramps were observed in two-thirds of the cohort, while fasciculations were uncommon (11%). EMG findings consistently demonstrated a neurogenic pattern with predominant distal involvement, and evidence of both acute and chronic denervation was present in four patients. Whole-body muscle MRI, available for all patients, revealed a consistent pattern of fatty infiltration predominantly affecting posterior muscle compartments, with minimal STIR signal changes. Brain and spinal imaging, performed in all individuals, showed no abnormalities.

Biallelic mutations in VRK1 are associated with a recognizable form of motor neuron disease characterized by features of dHMN combined with upper motor neuron involvement, along with a distinctive posterior-predominant pattern on muscle MRI. Identifying this phenotype, a known presentation of VRK1-related disorders, highlights the importance of targeted genetic testing in unresolved cases of hereditary motor neuropathy.