A de novo variant of POLR3B causes demyelinating Charcot-Marie-Tooth disease in a Chinese patient: a case report

Background Charcot-Marie-Tooth (CMT) disease is a group of inherited peripheral neuropathies, which are subdivided into demyelinating and axonal forms. Biallelic mutations in POLR3B are the well-established cause of hypomyelinating leukodystrophy, which is characterized by hypomyelination, hypodontia, and hypogonadotropic hypogonadism. To date, only one study has reported the demyelinating peripheral neuropathy phenotype caused by heterozygous POLR3B variants. Case presentation A 19-year-old male patient was referred to our hospital for progressive muscle weakness of the lower extremities. Physical examination showed muscle atrophy, sensory loss and deformities of the extremities. Nerve conduction studies and electromyography tests revealed sensorimotor demyelinating polyneuropathy with secondary axonal loss. Trio whole-exome sequencing revealed a de novo variant in POLR3B (c.3137G > A). Conclusions In this study, we report the case of a Chinese patient with a de novo variant in POLR3B (c.3137G > A), who manifested demyelinating CMT phenotype without additional neurological or extra-neurological involvement. This work is the second report on POLR3B-related CMT.

Activation of mTORC1 and c-Jun by Prohibitin1 loss in Schwann cells may link mitochondrial dysfunction to demyelination

Schwann cell (SC) mitochondria are quickly emerging as an important regulator of myelin maintenance in the peripheral nervous system (PNS). However, the mechanisms underlying demyelination in the context of mitochondrial dysfunction in the PNS are incompletely understood. We recently showed that conditional ablation of the mitochondrial protein Prohibitin 1 (PHB1) in SCs causes a severe and fast progressing demyelinating peripheral neuropathy in mice, but the mechanism that causes failure of myelin maintenance remained unknown. Here, we report that mTORC1 and c-Jun are continuously activated in the absence of Phb1, likely as part of the SC response to mitochondrial damage. Moreover, we demonstrate that these pathways are involved in the demyelination process, and that inhibition of mTORC1 using rapamycin partially rescues the demyelinating pathology. Therefore, we propose that mTORC1 and c-Jun may play a critical role as executioners of demyelination in the context of perturbations to SC mitochondria.

Prohibitin 1 is essential to preserve mitochondria and myelin integrity in Schwann cells

In peripheral nerves, Schwann cells form myelin and provide trophic support to axons. We previously showed that the mitochondrial protein prohibitin 2 can localize to the axon-Schwann-cell interface and is required for developmental myelination. Whether the homologous protein prohibitin 1 has a similar role, and whether prohibitins also play important roles in Schwann cell mitochondria is unknown. Here, we show that deletion of prohibitin 1 in Schwann cells minimally perturbs development, but later triggers a severe demyelinating peripheral neuropathy. Moreover, mitochondria are heavily affected by ablation of prohibitin 1 and demyelination occurs preferentially in cells with apparent mitochondrial loss. Furthermore, in response to mitochondrial damage, Schwann cells trigger the integrated stress response, but, contrary to what was previously suggested, this response is not detrimental in this context. These results identify a role for prohibitin 1 in myelin integrity and advance our understanding about the Schwann cell response to mitochondrial damage.

Early onset familial relapsing polyneuropathy, mimicking CIDP; A lesson from clinical genetics

Background: In children, chronic immune-mediated neuropathies present with slowly progressive or relapsing episodes of gait difficulty, symmetric weakness and sometimes paraesthesia. Infancy and early childhood age of presentation and familial recurrence are believed to be atypical features. Case presentation: Herein, we describe two brothers from a non- consanguineous Iraqi family, who presented with episodes of acute immune-mediated demyelinating peripheral neuropathy in early infancy that relapsed recurrently. Mild haemolytic anaemia was also reported. Inherited metabolic disorders were suspected and Whole Exome Sequencing of the youngest brother revealed homozygous frame shift mutation in CD59 gene, confirming the diagnosis of autosomal recessive hemolytic anemia, CD59-mediated, with or without immune-mediated polyneuropathy (HACD59). Conclusion: The report highlights the advantage of genetic testing in such rare and inherited conditions. In the lack of necessary non-traditional diagnostic methods, it is substantial to maintain the accustomed medical practice and strategies, based on available clinical data.

mTORC1 and JUN are activated after deletion of Prohibitin 1 in Schwann cells and may link mitochondrial dysfunction to demyelination

Schwann cell (SC) mitochondria are quickly emerging as an important regulator of myelin maintenance in the peripheral nervous system (PNS). However, the mechanisms underlying demyelination in the context of mitochondrial dysfunction in the PNS are incompletely understood. We recently showed that conditional ablation of the mitochondrial protein Prohibitin 1 (Phb1) in SCs causes a severe and fast progressing demyelinating peripheral neuropathy, but the mechanism that causes failure of myelin maintenance remained unknown. Here, we report that mTORC1 and JUN are continuously activated in the absence of Phb1, likely due to mitochondrial damage. Moreover, we demonstrate that these pathways are involved in the demyelination process, and that inhibition of mTORC1 using rapamycin partially rescues the demyelinating pathology. Therefore, we propose that mTORC1 and JUN may play a critical role as executioners of demyelination in the context of perturbations to SC mitochondria.

A dysfunctional endolysosomal pathway common to two sub-types of demyelinating Charcot–Marie–Tooth disease

Autosomal dominant mutations in LITAF are responsible for the rare demyelinating peripheral neuropathy, Charcot–Marie–Tooth disease type 1C (CMT1C). The LITAF protein is expressed in many human cell types and we have investigated the consequences of two different LITAF mutations in primary fibroblasts from CMT1C patients using confocal and electron microscopy. We observed the appearance of vacuolation/enlargement of late endocytic compartments (late endosomes and lysosomes). This vacuolation was also observed after knocking out LITAF from either control human fibroblasts or from the CMT1C patient-derived cells, consistent with it being the result of loss-of-function mutations in the CMT1C fibroblasts. The vacuolation was similar to that previously observed in fibroblasts from CMT4J patients, which have autosomal recessive mutations in FIG4. The FIG4 protein is a component of a phosphoinositide kinase complex that synthesises phosphatidylinositol 3,5-bisphosphate on the limiting membrane of late endosomes. Phosphatidylinositol 3,5-bisphosphate activates the release of lysosomal Ca2+ through the cation channel TRPML1, which is required to maintain the homeostasis of endosomes and lysosomes in mammalian cells. We observed that a small molecule activator of TRPML1, ML-SA1, was able to rescue the vacuolation phenotype of LITAF knockout, FIG4 knockout and CMT1C patient fibroblasts. Our data describe the first cellular phenotype common to two different subtypes of demyelinating CMT and are consistent with LITAF and FIG4 functioning on a common endolysosomal pathway that is required to maintain the homeostasis of late endosomes and lysosomes. Although our experiments were on human fibroblasts, they have implications for our understanding of the molecular pathogenesis and approaches to therapy in two subtypes of demyelinating Charcot–Marie–Tooth disease.

Prohibitin 1 is essential to preserve mitochondria and myelin integrity in Schwann cells.

Myelin is required for nervous system function. In the peripheral nervous system, Schwann cells (SCs) form myelin and trophically support the axons they ensheath. We previously showed that the mitochondrial protein prohibitin 2 (PHB2) can localize to the axon-SC interface and is required for developmental myelination. Whether the homologous protein PHB1 has a similar role, and whether prohibitins also play important roles in SC mitochondria is unknown. Here we show that deletion of Phb1 in SCs only minimally perturbs development, but later triggers a severe demyelinating peripheral neuropathy. Moreover, mitochondria are heavily affected by ablation of Phb1 and demyelination occurs preferentially in cells with apparent mitochondrial loss. Furthermore, in response to mitochondrial damage, SCs trigger the integrated stress response (ISR), but, contrary to what was previously suggested, the ISR is not detrimental and may be beneficial in this context. These results identify a new role for PHB1 in myelin integrity and advance our understanding of how SCs respond to mitochondrial damage.

Severe Demyelinating Neuropathy in an Advanced Melanoma Patient Treated with Nivolumab plus Ipilimumab Combined Therapy

Immune checkpoint inhibitors (ICIs) significantly prolong survival in patients with metastatic melanoma but can lead to serious immune-related adverse events. In this report, we described a case of atypical neuropathy caused by nivolumab plus ipilimumab combination therapy before primary tumor resection. In our case, not only demyelinating neuropathy, but also muscle weakness and unilateral facial nerve palsy developed and manifested as severe and diverse symptoms. Moreover, unlike spontaneously developing demyelinating peripheral neuropathy, the present case suggested the therapeutic effects of high-dose methylprednisolone monotherapy for the treatment of ICIs-induced immune-related demyelinating peripheral neuropathy.

A Novel Splice-Site Variant in SLC12A6 Causes Andermann Syndrome without Agenesis of the Corpus Callosum

Andermann syndrome, otherwise known as agenesis of the corpus callosum with peripheral neuropathy (ACCPN), is an autosomal recessive motor and sensory neuropathy known to be associated with ACC and mild-to-moderate intellectual disability. We present a 7-year-old girl with infantile-onset hypotonia, mild intellectual disability, and severe motor and sensory demyelinating peripheral neuropathy. Brain magnetic resonance imaging showed intact corpus callosum. Whole exome sequencing showed a novel splice-site pathogenic variant in the SLC12A6 gene. We confirm that ACC is not a mandatory feature and suggest that the term ACCPN may be misleading.