Nonsense mutations are involved in multiple peripheral neuropathies. These mutations induce the presence of a premature termination codon (PTC) at the mRNA level. As a result, a dysfunctional or truncated protein is synthesized, or even absent linked to nonsense-mediated mRNA degradation (NMD) system activation. Readthrough molecules or NMD inhibitors could be innovative therapies in these hereditary neuropathies, particularly molecules harboring the dual activity as amlexanox. Charcot–Marie–Tooth (CMT) is the most common inherited pathology of the peripheral nervous system, affecting 1 in 2500 people worldwide. Nonsense mutations in the GDAP1 gene have been associated with a severe form of CMT, prompting us to investigate the effect of readthrough and NMD inhibitor molecules. Although not clearly defined, GDAP1 could be involved in mitochondrial functions, such as mitophagy. We focused on the homozygous c.581C>G (p.Ser194*) mutation inducing CMT2H using patient human induced pluripotent stem cell (hiPSC)-derived neuronal cells. Treatment during 20 h with 100 µM of amlexanox on this cell model stabilized GDAP1 mRNAs carrying UGA-PTC and induced a restoration of the mitochondrial morphology. These results highlight the potential of readthrough molecules associated to NMD inhibitors for the treatment of genetic alterations in CMT, opening the way for future investigations and a potential therapy.

Next-generation sequencing (NGS) allows the detection of plentiful mutations increasing the rate of patients getting a positive diagnosis. However, while single-nucleotide variants (SNVs) or small indels can be easily detected, structural variations (SVs) such as copy number variants (CNVs) are often not researched. In Charcot–Marie–Tooth disease (CMT), the most common hereditary peripheral neuropathy, the PMP22-duplication was the first variation detected. Since then, more than 90 other genes have been associated with CMT, with point mutations or small indels mostly described. Herein, we present a personalized approach we performed to obtain a positive diagnosis of a patient suffering from demyelinating CMT. His NGS data were aligned to the human reference sequence but also studied using the CovCopCan software, designed to detect large CNVs. This approach allowed the detection of only one mutation in SH3TC2, the frequent p.Arg954*, while SH3TC2 is known to be responsible for autosomal recessive demyelinating CMT forms. Interestingly, by modifying the standard CovCopCan use, we detected the second mutation of this patient corresponding to a 922 bp deletion in SH3TC2 (Chr5:148,390,609-Chr5:148,389,687), including only one exon (exon 14). This highlights that SVs, different from PMP22 duplication, can be responsible for peripheral neuropathy and should be searched systematically. This approach could also be employed to improve the diagnosis of all inherited diseases.

Human induced pluripotent stem cells (hiPSCs) represent a powerful tool to investigate neuropathological disorders in which the cells of interest are inaccessible, such as in the Charcot–Marie–Tooth disease (CMT), the most common inherited peripheral neuropathy. Developing appropriate cellular models becomes crucial in order to both study the disease’s pathophysiology and test new therapeutic approaches. The generation of hiPS cellular models for disorders caused by a single nucleotide variation has been significantly improved following the development of CRISPR-based editing tools. In this study, we efficiently and quickly generated, by CRISPR editing, the two first hiPSCs cellular models carrying alterations involved in CMT4C, also called AR-CMTde-SH3TC2. This subtype of CMT is associated with alterations in the SH3TC2 gene and represents the most prevalent form of autosomal recessive demyelinating CMT. We aimed to develop models for two different SH3TC2 nonsense variants, c.211C>T, p.Gln71* and the most common AR-CMTde-SH3TC2 alteration, c.2860C>T, p.Arg954*. First, in order to determine the best CRISPR strategy to adopt on hiPSCs, we first tested a variety of sgRNAs combined with a selection of recent base editors using the conveniently cultivable and transfectable HEK-293T cell line. The chosen CRISPR base-editing strategy was then applied to hiPSCs derived from healthy individuals to generate isogenic CMT disease models with up to 93% editing efficiency. For point mutation generation, we first recommend to test your strategies on alternative cell line such as HEK-293T before hiPSCs to evaluate a variety of sgRNA-BE combinations, thus boosting the chance of achieving edited cellular clones with the hard-to-culture and to transfect hiPSCs.

Hereditary sensory neuropathies (HSN) are a heterogenous group of sensory neuropathies. Mutations in ATL3 have been described in patients presenting with hereditary sensory neuropathy IF (HSN1F), a subtype of HSN. Herein, by analyzing targeted-NGS data of a patient presenting with sensory neuropathy symptoms using the CovCopCan bioinformatic tool, we discovered the presence of a deletion of around 3kb in ATL3 from Chr11:63,401,422 to Chr11:63,398,182. This deletion affects ATL3 exons 11 and 12 and could lead to the mutation c.(1036-861_1539+329del), p.(Ala346_Gln513del). In addition, an analysis of the breakpoints’ sequences revealed the presence of Alu transposable elements at the position of the breakpoints, which pointed to a possible erroneous recombination event following a non-allelic-homologous-recombination mechanism in this area. Moreover, electronic microscopy analysis of the patient’s nerve biopsy revealed a severe rarefaction of the myelinated fibers, a demyelinating–remyelinating process, and an abnormal aspect of the endoplasmic reticulum. These findings suggest that this structural variation could potentially be responsible for the HSN symptoms of the patient. Research of structural variations in ATL3 in numerous other patients presenting similar symptoms should be broadly investigated in order to improve patients’ diagnoses.

Les neuropathies périphériques héréditaires (NPH) sont causées par des altérations génétiques entraînant des lésions des nerfs moteurs et/ou sensitifs. En 1991, la duplication de PMP22 a été décrite comme la première variation pathogène associée aux NPH. Aujourd'hui, grâce aux avancées des technologies de séquençage de nouvelle génération (NGS), plus de 100 gènes ont été identifiés chez les patients présentant des NPH. Cependant, la majorité des variants détectés dans ces gènes sont des mutations ponctuelles ou de petites insertions ou délétions, tandis que les variants du nombre de copies (CNVs) sont rarement décrits. Dans cette étude, nous avons d'abord réalisé une analyse bioinformatique en utilisant le logiciel CovCopCan, permettant de détecter des CNVs sur des données de NGS. 62 CNVs ont été détectés chez 12% de notre cohorte de 765 patients présentant de symptômes de NPH. Ces CNVs ont été repartis au sein de 32 gènes associés aux NPH et nous avons caractérisé certains de ces CNVs au sein des gènes KIF5A, ATL3, SACS et SH3TC2. En outre, nous nous sommes concentrés sur un CNV de type délétion détecté dans SH3TC2, et nous avons développé un modèle cellulaire pour caractériser cette variation. À partir d'une biopsie cutanée d'un patient hétérozygote composite atteint de CMT4C, portant à la fois un CNV et une altération non-sens, des cellules souches humaines pluripotentes induites (hiPSCs) ont été créées. Ces hiPSCs ont été ensuite différenciées en cellules de Schwann afin d’explorer les mécanismes pathologiques conduisant au phénotype du patient. Dans l'ensemble, notre étude souligne l'importance d'étudier les CNVs pour améliorer le diagnostic des NPH et a permis de développer pour la première fois un modèle cellulaire adapté à l’étude d'un CNV de type délétion dans un gène impliqué dans les NPH de transmission autosomique récessive.