Behavioral and neurological effects of Vrk1 deficiency in zebrafish

CMT2 and distal hereditary motor neuropathy associated with VRK1 variants: Case series

Axonal Charcot-Marie-Tooth disease (CMT2) and distal hereditary motor neuropathy (dHMN) are associated with a heterogeneous group of genes encoding proteins that are involved in axonal transport, control of RNA metabolism, mitochondrial dynamics and DNA repair. VRK1 (vaccinia-related kinase 1) is a serine/threonine kinase which is widely expressed in human tissue and plays a role in RNA maturation and processing and in DNA damage response. Variants of VRK1 have been associated with neurodevelopmental and neuromuscular disorders including pontocerebellar hypoplasia, motor neuron disorders and distal hereditary motor neuropathy. We present 3 cases of VRK1-associated neuromuscular disorders without neurodevelopmental abnormalities including CMT2 associated with homozygous variant of VRK1 at Arg387His and dHMN with combination of heterozygous variants at Arg133His and Asp243Asn. While our case series expands the clinical spectrum of VRK1-associated neuromuscular disorders, additional studies are needed to elucidate pathophysiology of neuromuscular disorders associated with VRK1 variants.

Nuclear functions regulated by the VRK1 kinase

In the nucleus, the VRK1 Ser-Thr kinase is distributed in nucleoplasm and chromatin, where it has different roles. VRK1 expression increases in response to mitogenic signals. VRK1 regulates cyclin D1 expression at G0 exit and facilitates chromosome condensation at the end of G2 and G2/M progression to mitosis. These effects are mediated by the phosphorylation of histone H3 at Thr3 by VRK1, and later in mitosis by haspin. VRK1 regulates the apigenetic patterns of histones in processes requiring chromating remodeling, such as transcription, replication and DNA repair. VRK1 is overexpressed in tumors, facilitating tumor progression and resistance to genotoxic treatments. VRK1 also regulates the organization of Cajal bodies assembled on coilin, which are necessary for the assembly of different types of RNP complexes. VRK1 pathogenic variants cuase defects in Cajal bodies, functionally altering neurons with long axons and leading to neurological diseases, such as amyotrophic laterla sclerosis, spinal muscular atrophy, distal hereditay motor neuropathies and Charcot-Marie-Tooth.

DHCR7 links cholesterol synthesis with neuronal development and axonal integrity

The DHCR7 enzyme converts 7-DHC into cholesterol. Mutations in DHCR7 can block cholesterol production, leading to abnormal accumulation of 7-DHC and causing Smith-Lemli-Opitz syndrome (SLOS). SLOS is an autosomal recessive disorder characterized by multiple malformations, including microcephaly, intellectual disability, behavior reminiscent of autism, sleep disturbances, and attention-deficit/hyperactivity disorder (ADHD)-like hyperactivity. Although 7-DHC affects neuronal differentiation in ex vivo experiments, the precise mechanism of SLOS remains unclear. We generated Dhcr7 deficient (dhcr7-/-) zebrafish that exhibited key features of SLOS, including microcephaly, decreased neural stem cell pools, and behavioral phenotypes similar to those of ADHD-like hyperactivity. These zebrafish demonstrated compromised myelination, synaptic anomalies, and neurotransmitter imbalances. The axons of the dhcr7-/- zebrafish showed increased lysosomes and attenuated autophagy, suggesting that autophagy-related neuronal homeostasis is disrupted.