Nathalie Bonello-Palot

Charcot–Marie–Tooth disease type 2A (CMT2A) is the most common axonal CMT and is associated with an early onset and severe motor neuropathy. CMT2A is mainly caused by dominant mutations in the MFN2 gene, encoding mitofusin-2, a GTPase located in the outer membrane of the mitochondria and endoplasmic reticulum (ER). Mutations in MFN2 affect mitochondrial dynamics. We previously demonstrated that mutated MFN2 further disrupts contacts between the ER and the mitochondria, leading to axonal degeneration. There are no treatments for CMT2A, and those currently under development primarily focus on restoring mitochondrial function. Here, we provide proof of concept that neuronal overexpression of wild-type MFN2 (MFN2 ^WT ) provides therapeutic benefit in transgenic CMT2A mice as well as in CMT2A-motor neurons derived from induced pluripotent stem cells. Intrathecal delivery of an AAV9 vector expressing MFN2 ^WT effectively targets motor and sensory neurons, restoring ER–mitochondria contacts and mitochondrial morphology, thereby preserving both neuromuscular junction integrity and motor function. Strikingly, therapeutic efficacy is also achieved by administering the vector after the onset of symptoms. Importantly, AAV administration was well tolerated, with no evidence of hepatotoxicity or dorsal root ganglion inflammation. We further show that CMT2A pathology can be corrected in vitro and in vivo using an ER-targeting MFN1 isoform that selectively enhances ER–mitochondria contacts. These results establish that restoring contacts between the ER and mitochondria using gene therapy is a promising therapeutic avenue for CMT2A.

Charcot-Marie-Tooth disease type 2A (CMT2A) is the most common axonal CMT and is associated with an early onset and severe motor-dominant phenotype. CMT2A is mainly caused by dominant mutations in the MFN2 gene, encoding Mitofusin-2, a GTPase located in the outer membrane of the mitochondria and endoplasmic reticulum (ER). Mutations in MFN2 are known to affect mitochondrial dynamics. We previously demonstrated that the mutated MFN2 ^Arg94Gln further disrupts contacts between the ER and the mitochondria, leading to progressive axonal degeneration. There is no effective therapeutic approach to slow or reverse the progression of CMT2A, and treatments currently under development primarily focus on restoring mitochondrial function. Here, we provide proof-of-concept that neuronal overexpression of wild-type MFN2 (MFN2 ^WT ) provides therapeutic benefit in transgenic CMT2A mice carrying the mutated MFN2 ^Arg94Gln . Intrathecal delivery of an AAV9 vector expressing MFN2 ^WT effectively targets motor and sensory neurons, restoring ER-mitochondria contacts and mitochondrial morphology, thereby preserving both neuromuscular junction integrity and motor function. Strikingly, therapeutic efficacy is also achieved following vector injection after the onset of symptoms, rescuing the molecular hallmarks of CMT2A pathology and reversing locomotor. Notably, AAV administration was well tolerated, with no evidence neither of hepatotoxicity nor dorsal root ganglion inflammation. These results establish that boosting MFN2’s levels using gene therapy is a promising therapeutic avenue for CMT2A.

Objective: Charcot–Marie–Tooth (CMT) disease is a heterogeneous group of genetic neuropathies, with >90 genes identified. Several aminoacyl‐tRNA synthetases have been linked to CMT. DARS2 , encoding the mitochondrial aspartyl‐tRNA synthetase, has been typically associated with leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation. This study aimed to investigate the association between biallelic DARS2 variants and axonal CMT. Methods: We investigated 5 individuals from 3 unrelated families with axonal CMT and biallelic DARS2 variants. Functional studies in fibroblasts assessed their effects on DARS2 expression, localization, and mitochondrial function. Enzymatic activity was evaluated in HEK293 cells. Results: The 5 individuals, including 4 adults, presented with childhood‐onset progressive axonal CMT. None had leukoencephalopathy, but one showed central nervous system involvement, with intellectual disability and epilepsy. Genetic analysis identified compound heterozygous DARS2 variants: family A, p.Ser238Phe and p.Arg336Cys; family B, p.Ser238Phe and p.Ile25Thrfs*38; family C, c.492+2T>C and p.Pro503Leu. Functional studies revealed reduced DARS2 protein levels, mitochondrial network abnormalities, and impaired mitochondrial function. p.Ser238Phe behaves as a hypomorphic allele, whereas p.Pro503Leu reduced DARS2 enzymatic activity by 75%. Interpretation: Our findings expand the DARS2 ‐related disease spectrum, establishing a novel association with axonal CMT. Hypomorphic variants, such as p.Ser238Phe, when paired with more deleterious variants, result in isolated axonal CMT, whereas more severe combinations—although not as deleterious as those seen in leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation—result in axonal CMT with central nervous system involvement, albeit without leukoencephalopathy. These observations raise the possibility that DARS2 ‐associated diseases form a continuum rather than representing strictly distinct central or peripheral nervous system disorders. ANN NEUROL 2025;98:1335–1351

Background: Autosomal recessive mutations in the SH3TC2 gene cause Charcot–Marie‐Tooth type 4C (CMT4C) demyelinating peripheral neuropathy.Methods: In this nationwide observational retrospective study involving 27 French University Hospitals, we analyzed the clinical, electrophysiological, and genetic features of 103 patients from 89 families with homozygous and compound heterozygous SH3TC2 gene mutations identified between 2003 and 2023.Results: Mean age was 42 years (2–80), and 49% of patients were female. Mean age at disease onset was 14 years (0–52), 60% of patients started the disease before age 10 years, and 24% after age 20 years. Patients presented with distal motor weakness (93% of cases), sensory loss (86%), foot deformities (83%), scoliosis (73%), proximal limb weakness (40%), cranial nerve involvement (48%), hearing loss (37%), scoliosis‐related respiratory insufficiency (14%), and genitourinary disorders (6%). Half the patients (48%) walked independently before age 50 years, in contrast with only 13% after age 50 years. After age 50 years, 23% of patients were wheelchair‐bound. Nerve conduction studies showed sensorimotor abnormalities within the demyelinating range in all cases. We identified 56 different pathogenic variants in the SH3TC2 gene, including 22 previously undescribed. Patients with two SH3TC2 gene truncating variants had more severe symptoms than patients with one or zero truncating variants.Interpretation: This study shows CMT4C is a severe childhood‐ and adult‐onset demyelinating peripheral neuropathy often associated with scoliosis, hearing loss, and ambulation loss in a significant proportion of patients after age 50 years. Genotype–phenotype correlations suggest two truncating SH3TC2 gene variants cause a more severe phenotype.

Charcot-Marie-Tooth disease (CMT) is the most common inherited neuromuscular disorder, characterized by progressive, length-dependent degeneration of peripheral nerves, resulting in distal muscle atrophy and weakness, foot and hand deformities, and sensory deficits. The disease is clinically and genetically heterogeneous, with over 125 disease-causing genes identified to date. Here, genetic studies in ten patients from 5 unrelated families of diverse ethnic background, led to the identification of KCTD11 as a novel CMT gene, responsible for a new autosomal recessive intermediate CMT subtype, RI-CMTE. The variants identified are loss of function. KCTD11 encodes KCTD11/REN, a protein of yet unknown function in the Peripheral Nervous System, known to regulate HDAC1, β-catenin, and mTORC1, key regulators of myelination and neuronal differentiation in the PNS. To explore KCTD11’s role in the PNS, we used a constitutive Kctd11-/- mouse model and the derived in vitro myelin model of sensory neuron and Schwann cell co-culture (DRGN/SC), to mimic the loss-of-function induced by patient mutations. We first demonstrate that the loss of KCTD11 is due to enhanced degradation of the mutated protein via autophagy. Both in vitro and in vivo, we demonstrate abnormal myelination in vivo and altered myelination dynamics in vitro. These defects were associated with dysregulation of the expression of key transcription factors in Schwann cells, such as Egr2 and Sox10, along with other myelin-related genes, as revealed by mRNA-sequencing data. Regarding pathophysiological mechanisms, we identified dysregulation of HDAC1 expression, as well as alterations in the Wnt/β-catenin, Sonic Hedgehog and Hippo/YAP signaling pathways. The deregulation of these pathways seem to converge to altered autophagy and altered balance between proliferation, differentiation and apoptosis, at least in Schwann cells. These mechanisms remain to be explored in axons from PNS neurons. Altogether, our results identify KCTD11 as a novel gene defective in autosomal recessive intermediate RI-CMTE and highlight the key role of KCTD11 in maintaining myelin homeostasis through regulation of HDAC1 and phosphorylated β-catenin levels, thereby preventing late-onset myelin abnormalities and degradation.

Background and Aims: Charcot–Marie–Tooth disease (CMT) is a rare hereditary neuropathy that affects peripheral nerves in the upper and lower limbs. To distinguish between the different forms of the disease, electrophysiological criteria are essential. Furthermore, identifying the genetic cause is crucial for providing accurate genetic counseling. The genetic complexity of CMT is partly explained by digenism, where mutations in two distinct genes might contribute to the disease. Two genes involved in mitochondrial dynamics, MFN2 and GDAP1, have been identified in digenic cases of CMT. This retrospective study reports MFN2/GDAP1 digenism cases identified in patients affected by CMT in our laboratory.Methods: We conducted a retrospective analysis of 1665 patients who underwent NGS using the CMT gene panel between 2016 and 2024. These patients affected by CMT were addressed from neurology reference centers in France. The results were analyzed with bioinformatics tools, initially using the hg19 reference genome and then the hg38 version.Results: Out of 1665 patients, 367 positive cases were identified, corresponding to a 22% molecular diagnostic rate, excluding PMP22 duplications. Among these, 15 cases involved variants in two distinct genes, resulting in a 4% digenism rate. Five cases involved MFN2/GDAP1 variants, accounting for 1.4% of the total positive results and 33% of all digenic cases.Interpretation: The cases of digenism have a significant prevalence in CMT disease and may explain the severity of the phenotype in our patients. Multilocus variants complicate genetic counseling due to non‐Mendelian inheritance. In addition, it is important to distinguish between digenism and modifier genes.

Objective: The global increase in the prevalence of metabolic syndrome represents a significant public health concern. Rare biallelic pathogenic variants in ZMPSTE24 have been identified as the cause of mandibuloacral dysplasia type B, ie, a lipodystrophy syndrome associated with metabolic complications. The role of monoallelic pathogenic variants in ZMPSTE24 concerning metabolic syndrome remains uncertain. Design: Case report and systematic review of literature. Methods: We investigated a Wallisian family with FPLD and metabolic syndrome via whole-exome sequencing. We performed functional analyses of an identified rare ZMPSTE24 variant. To broadly assess the effect of heterozygous pathogenic ZMPSTE24 variants on FPLD-associated phenotypes, and metabolic syndrome, we used the Human Gene Mutation Database (HGMD) and 200 K exome data from UK Biobank. Results: We investigated a Wallisian family where a 40-year-old female with metabolic syndrome was found to carry a rare heterozygous missense variant in ZMPSTE24. Functional assays showed a decreased prelamin to lamin A maturation and accelerated senescence. In silico analysis demonstrated that this variant might disrupt the lamin A binding site. We then analyzed the impact of monoallelic pathogenic ZMPSTE24 variants on metabolic traits using data from the HGMD and the UK Biobank. In HGMD, ZMPSTE24 variants carriers presented with dyslipidemia and hepatic steatosis. In the UK Biobank, monoallelic pathogenic variants were associated with an increased risk of hypertriglyceridemia, with a trend toward metabolic syndrome. Conclusions: This study underscores the association of ZMPSTE24 rare variants with metabolic disorders and emphasizes the need for further research to clarify their clinical implications.

Background and purpose: X‐linked Charcot‐Marie‐Tooth disease type 1 (CMTX1) ranks as the second most prevalent hereditary neuropathy and, currently, has no definitive cure. Emerging preclinical trials offer hope for potential clinical studies in the near future. While it is widely accepted that experimental groups in these trials should be balanced for age and gender, there is a current shortfall in data regarding phenotype–genotype correlations. Our aim was to provide a more detailed understanding of these correlations to facilitate the formation of well‐matched patient groups in upcoming clinical trials.Methods: We conducted a retrospective evaluation of CMTX1 patients from 13 designated reference centers in France. Data on genetics, clinical features, and nerve conduction were systematically gathered.Results: We analyzed the genotype–phenotype correlations in 275 CMTX1 patients belonging to 162 families and carrying 87 distinct variants. Patients with variants affecting the transmembrane domains demonstrated significantly greater severity, as evidenced by a Charcot‐Marie‐Tooth Examination Score of 10.5, compared to 7.1 for those with intracellular domain variants and 8.7 for extracellular domain variants (p < 0.000). These patients also experienced an earlier age of onset, showed slower ulnar nerve conduction velocities and had more substantial loss of motor amplitude.Conclusions: This study confirms the presence of a correlation between the mutated protein domain and the clinical phenotype. Patients with a variant in the transmembrane domains demonstrated a more severe clinical and electrophysiological profile. Consequently, the genotype could play a prognostic role in addition to its diagnostic role, and it will be essential to consider this in future clinical trials.

Hereditary sensory and autonomic neuropathy type 1 is an autosomal dominant neuropathy caused by the SPTLC1 or SPTLC2 variants. These variants modify the preferred substrate of serine palmitoyl transferase, responsible for the first step of de novo sphingolipids synthesis, leading to accumulation of cytotoxic deoxysphingolipids. Diagnosis of HSAN1 is based on clinical symptoms, mainly progressive loss of distal sensory keep, and genetic analysis. Aim: Identifying new SPTLC1 or SPTLC2 “gain-of-function” variants raises the question as to their pathogenicity. This work focused on characterizing six new SPTLC1 variants using in silico prediction tools, new meta-scores, 3D modeling, and functional testing to establish their pathogenicity. Methods: Variants from six patients with HSAN1 were studied. In silico, CADD and REVEL scores and the 3D modeling software MITZLI were used to characterize the pathogenic effect of the variants. Functional tests based on plasma sphingolipids quantification (total deoxysphinganine, ceramides, and dihydroceramides) were performed by tandem mass spectrometry. Results: In silico predictors did not provide very contrasting results when functional tests discriminated the different variants according to their impact on deoxysphinganine level or canonical sphingolipids synthesis. Two SPTLC1 variants were newly described as pathogenic: SPTLC1 NM_006415.4:c.998A>G and NM_006415.4:c.1015G>A. Discussion: The combination of the different tools provides arguments to establish the pathogenicity of these new variants. When available, functional testing remains the best option to establish the in vivo impact of a variant. Moreover, the comprehension of metabolic dysregulation offers opportunities to develop new therapeutic strategies for these genetic disorders.