A novelNDRG1mutation in a non-Romani patient with CMT4D/HMSN-Lom

Identification and Characterization of Novel Founder Mutations in NDRG1: Refining the Genetic Landscape of Charcot-Marie-Tooth Disease Type 4D in Bulgaria

Charcot-Marie-Tooth neuropathy type 4D (CMT4D) is a rare genetic disorder of the peripheral nervous system caused by biallelic mutations in the N-Myc Downstream Regulated 1 gene (NDRG1). Patients present with an early onset demyelinating peripheral neuropathy causing severe distal muscle weakness and sensory loss, leading to loss of ambulation and progressive sensorineural hearing loss. The disorder was initially described in the Roma community due to a common founder mutation, and only a handful of disease-causing variants have been described in this gene so far. Here, we present genetic and clinical findings from a large Bulgarian cohort of demyelinating CMT patients harboring recurrent and novel variants in the NDRG1 gene. Notably, two splice-site variants are exclusive to Bulgarian Muslims and reside in ancestral haplotypes, suggesting a founder effect. Functional characterization of these novel variants implicates a loss-of-function mechanism due to shorter gene products. Our findings contribute to a deeper understanding of the genetic and clinical heterogeneity of CMT4D and highlight novel founder mutations in the ethnic minority of Bulgarian Muslims.

A splice altering variant in NDRG1 gene causes Charcot-Marie-Tooth disease, type 4D

Charcot-Marie-Tooth disease, type 4D (CMT4D) is a progressive, autosomal recessive form of CMT, characterized by distal muscle weakness and atrophy, foot deformities, severe motor sensory neuropathy, and sensorineural hearing impairment. Mutations in NDRG1 gene cause neuropathy in humans, dogs, and rodents. Here, we describe clinical and genetic features of a 17-year-old male with wasting of hand muscle and foot and severe motor neuropathy. Whole exome sequencing was carried out on the patient and his unaffected parents. We identified a novel deletion of nine nucleotides (c.537 + 2_537 + 10del) on the splice donor site of intron 8 in NDRG1 gene. The Sanger sequencing confirmed the segregation of this mutation in autosomal recessive inheritance. Furthermore, transcript analysis confirmed a splice defect and reveals using of an alternate cryptic splice donor site on the downstream intronic region. It resulted in an insertion of 42 nucleotides to exon 8 of NDRG1. Translation of the resulting transcript sequence revealed an insertion of 14 amino acids in-frame to the existing NDRG1 protein. This insertion is predicted to disrupt an alpha helix which is involved in protein-protein interactions in homologous proteins. Our study expands the clinical and genetic spectrum of CMT4D. The splice defect we found in this patient reveals a novel splice isoform of NDRG1 as the potential cause for the neuropathy observed in this patient.

Aberrant Neuregulin 1/ErbB Signaling in Charcot-Marie-Tooth Type 4D Disease

Charcot-Marie-Tooth type 4D (CMT4D) is an autosomal recessive demyelinating form of CMT characterized by progressive motor and sensory neuropathy. N-myc downstream regulated gene 1 (NDRG1) is the causative gene for CMT4D. Although more CMT4D cases have been reported, the comprehensive molecular mechanism underlying CMT4D remains elusive. Here, we generated a novel knockout mouse model in which the fourth and fifth exons of the Ndrg1 gene were removed. Ndrg1-deficient mice develop early progressive demyelinating neuropathy and limb muscle weakness. The expression pattern of myelination-related transcriptional factors, including SOX10, OCT6, and EGR2, was abnormal in Ndrg1-deficient mice. We further investigated the activation of the ErbB2/3 receptor tyrosine kinases in Ndrg1-deficient sciatic nerves, as these proteins play essential roles in Schwann cell myelination. In the absence of NDRG1, although the total ErbB2/3 receptors expressed by Schwann cells were significantly increased, levels of the phosphorylated forms of ErbB2/3 and their downstream signaling cascades were decreased. This change was not associated with the level of the neuregulin 1 ligand, which was increased in Ndrg1-deficient mice. In addition, the integrin β4 receptor, which interacts with ErbB2/3 and positively regulates neuregulin 1/ErbB signaling, was significantly reduced in the Ndrg1-deficient nerve. In conclusion, our data suggest that the demyelinating phenotype of CMT4D disease is at least in part a consequence of molecular defects in neuregulin 1/ErbB signaling.

Crystal and solution structure of NDRG1, a membrane-binding protein linked to myelination and tumour suppression

N-myc downstream-regulated gene 1 (NDRG1) is a tumour suppressor involved in vesicular trafficking and stress response. NDRG1 participates in peripheral nerve myelination, and mutations in the NDRG1 gene lead to Charcot-Marie-Tooth neuropathy. The 43-kDa NDRG1 is considered as an inactive member of the α/β hydrolase superfamily. In addition to a central α/β hydrolase fold domain, NDRG1 consists of a short N terminus and a C-terminal region with three 10-residue repeats. We determined the crystal structure of the α/β hydrolase domain of human NDRG1 and characterised the structure and dynamics of full-length NDRG1. The structure of the α/β hydrolase domain resembles the canonical α/β hydrolase fold with a central β sheet surrounded by α helices. Small-angle X-ray scattering and CD spectroscopy indicated a variable conformation for the N- and C-terminal regions. NDRG1 binds to various types of lipid vesicles, and the conformation of the C-terminal region is modulated upon lipid interaction. Intriguingly, NDRG1 interacts with metal ions, such as nickel, but is prone to aggregation in their presence. Our results uncover the structural and dynamic features of NDRG1, as well as elucidate its interactions with metals and lipids, and encourage studies to identify a putative hydrolase activity of NDRG1. DATABASES: The coordinates and structure factors for the crystal structure of human NDRG1 were deposited to PDB (PDB ID: 6ZMM).

Identification and functional characterization of two missense mutations in NDRG1 associated with Charcot-Marie-Tooth disease type 4D

Charcot-Marie-Tooth disease type 4D (CMT4D) is an autosomal-recessive demyelinating form of CMT characterized by a severe distal motor and sensory neuropathy. NDRG1 is the causative gene for CMT4D. To date, only four mutations in NDRG1 -c.442C>T (p.Arg148*), c.739delC (p.His247Thrfs*74), c.538-1G>A, and duplication of exons 6-8-have been described in CMT4D patients. Here, using targeted next-generation sequencing examination, we identified for the first time two homozygous missense variants in NDRG1, c.437T>C (p.Leu146Pro) and c.701G>A (p.Arg234Gln), in two Chinese CMT families with consanguineous histories. Further functional studies were performed to characterize the biological effects of these variants. Cell culture transfection studies showed that mutant NDRG1 carrying p.Leu146Pro, p.Arg148*, or p.Arg234Gln variant degraded faster than wild-type NDRG1, resulting in lower protein levels. Live cell confocal microscopy and coimmunoprecipitation analysis indicated that these variants did not disrupt the interaction between NDRG1 and Rab4a protein. However, NDRG1-knockdown cells expressing mutant NDRG1 displayed enlarged Rab4a-positive compartments. Moreover, mutant NDRG1 could not enhance the uptake of DiI-LDL or increase the fraction of low-density lipoprotein receptor on the cell surface. Taken together, our study described two missense mutations in NDRG1 and emphasized the important role of NDRG1 in intracellular protein trafficking.