Clinical features of inherited neuropathy with BSCL2 mutations in Japan

Rare Variants Cause Charcot-Marie-Tooth Disease in Malian Families

INTRODUCTION/AIMS

Charcot-Marie-Tooth disease (CMT), the most common inherited peripheral neuropathy, is clinically and genetically heterogeneous with over 100 genes identified to date. Recently, next-generation sequencing (NGS) has enabled molecular diagnosis in previously unidentified CMT cases. However, less progress has been achieved in sub-Saharan African (SSA) populations. We report rare CMT variants found in four unrelated Malian families.

METHODS

Patients went through a thorough neurological examination and Nerve Conduction Studies (NCS) were performed. DNA was extracted for genetic analysis (CMT gene panel testing and whole-exome/genome sequencing). Putative variants were confirmed with Sanger sequencing and segregation was checked in all available family members. Deleteriousness was checked using several in silico prediction tools and protein modeling.

RESULTS

Nine patients (three males and six females) from four families were enrolled. The mean age at onset and diagnosis were 15 and 22.7 years, respectively (ranges: 3 to 55 years, and 12 to 58 years). Walking difficulty was the first symptom commonly reported. Neurological examination found distal muscle weakness and wasting with sensory loss, reduced tendon reflexes, and skeletal deformities. In addition, some patients presented with ataxic gait associated with incoordination that are not in the forefront of CMT features. NCS was consistent with the axonal pattern in three families. Genetic analysis revealed rare pathogenic variants in BSCL2, SH3TC2, and PEX10, and of unknown significance in BAG3.

DISCUSSION

This study reports rare variants in these CMT genes for the first time in SSA populations, expanding the global epidemiological, clinical, and genetic spectrum of these diseases.

Lipid metabolism alterations in peripheral neuropathies

Alterations in lipid metabolism are increasingly recognized as central pathological hallmarks of inherited and acquired peripheral neuropathies. Correct lipid balance is critical for cellular homeostasis. However, the mechanisms linking lipid disturbances to cellular dysfunction and whether these changes are primary drivers or secondary effects of disease remain unresolved. This is particularly relevant in the peripheral nervous system, where the lipid-rich myelin integrity is critical for axonal function, and even subtle perturbations can cause widespread effects. This review explores the role of lipids as structural components as well as signaling molecules, emphasizing their metabolic role in peripheral neurons and Schwann cells. Additionally, we explore the genetic and environmental connections in both inherited and acquired peripheral neuropathies, respectively, which are known to affect lipid metabolism in peripheral neurons or Schwann cells. Overall, we highlight how understanding lipid-centric mechanisms could advance biomarker discovery and therapeutic interventions for peripheral nerve disorders.

Clinical Reasoning: A 19-Year-Old Woman With Progressive Weakness and Numbness in Her Arms and Legs

Acute worsening of chronic weakness and numbness can have diagnostic challenges when trying to distinguish between acquired and hereditary conditions. We present the case of a 19-year-old patient who had acute worsening of chronic symmetric sensory and motor neuropathy with no response to intravenous immunoglobulin. Her story highlights the diagnostic approach for patients with atypical features of acquired vs genetic neuropathies and the importance of interpreting a "pathogenic" gene variant in this clinical context. Readers will explore the diagnostic steps our group has considered to reach our final diagnosis and the management of patients with complex neuropathy.

Seipin Deficiency Impairs Motor Coordination in Mice by Compromising Spinal Cord Myelination

The integrity of the myelin sheath of the spinal cord (SC) is essential for motor coordination. Seipin is an endoplasmic reticulum transmembrane protein highly expressed in adipose tissue and motor neurons in the SC. It was reported Seipin deficiency induced lipid dysregulation and neurobehavioral deficits, but the underlying mechanism, especially in SC, remains to be elucidated. In present study, we found that Seipin and myelin basic protein (MBP) increased synchronously in SC of developmental stage of mice. Demyelination impaired motor coordination as well as MBP and Seipin expression, which were alleviated by remyelination. Moreover, Seipin deficiency impaired motor coordination of mice, accompanied by hypomyelination in spinal cord. Mechanistically, we further demonstrated that myelin content as labeled by Fluormyelin, myelin basic protein (MBP) was down-regulated by Seipin deficiency. Seipin deficiency led to reduction of myelin-forming oligodendrocytes (OLs) density in spinal cord. Notably, administration of rosiglitazone (RG), a classic PPARγ activator, successfully restored the phenotypes manifested by Seipin deficiency including reduced OLs density, hypomyelination, as well as motor dyscoordination. In summary, present study revealed that Seipin deficiency disrupted motor coordination by compromising myelination in SC, and RG treatment could rescue the phenotypes. This study throws light on the mechanism underlying Seipin deficiency associated disorders and paves ways for developing therapeutics toward those diseases.

Seipin deficiency-induced lipid dysregulation leads to hypomyelination-associated cognitive deficits via compromising oligodendrocyte precursor cell differentiation

Seipin is one key mediator of lipid metabolism that is highly expressed in adipose tissues as well as in the brain. Lack of Seipin gene, Bscl2, leads to not only severe lipid metabolic disorders but also cognitive impairments and motor disabilities. Myelin, composed mainly of lipids, facilitates nerve transmission and is important for motor coordination and learning. Whether Seipin deficiency-leaded defects in learning and motor coordination is underlined by lipid dysregulation and its consequent myelin abnormalities remains to be elucidated. In the present study, we verified the expression of Seipin in oligodendrocytes (OLs) and their precursors, oligodendrocyte precursor cells (OPCs), and demonstrated that Seipin deficiency compromised OPC differentiation, which led to decreased OL numbers, myelin protein, myelinated fiber proportion and thickness of myelin. Deficiency of Seipin resulted in impaired spatial cognition and motor coordination in mice. Mechanistically, Seipin deficiency suppressed sphingolipid metabolism-related genes in OPCs and caused morphological abnormalities in lipid droplets (LDs), which markedly impeded OPC differentiation. Importantly, rosiglitazone, one agonist of PPAR-gamma, substantially restored phenotypes resulting from Seipin deficiency, such as aberrant LDs, reduced sphingolipids, obstructed OPC differentiation, and neurobehavioral defects. Collectively, the present study elucidated how Seipin deficiency-induced lipid dysregulation leads to neurobehavioral deficits via impairing myelination, which may pave the way for developing novel intervention strategy for treating metabolism-involved neurological disorders.

Hereditary spastic paraplegia: Novel insights into the pathogenesis and management

Hereditary spastic paraplegia is a genetically heterogeneous neurodegenerative disorder characterised primarily by muscle stiffness in the lower limbs. Neurodegenerative disorders are conditions that result from cellular and metabolic abnormalities, many of which have strong genetic ties. While ageing is a known contributor to these changes, certain neurodegenerative disorders can manifest early in life, progressively affecting a person's quality of life. Hereditary spastic paraplegia is one such condition that can appear in individuals of any age. In hereditary spastic paraplegia, a distinctive feature is the degeneration of long nerve fibres in the corticospinal tract of the lower limbs. This degeneration is linked to various cellular and metabolic processes, including mitochondrial dysfunction, remodelling of the endoplasmic reticulum membrane, autophagy, abnormal myelination processes and alterations in lipid metabolism. Additionally, hereditary spastic paraplegia affects processes like endosome membrane trafficking, oxidative stress and mitochondrial DNA polymorphisms. Disease-causing genetic loci and associated genes influence the progression and severity of hereditary spastic paraplegia, potentially affecting various cellular and metabolic functions. Although hereditary spastic paraplegia does not reduce a person's lifespan, it significantly impairs their quality of life as they age, particularly with more severe symptoms. Regrettably, there are currently no treatments available to halt or reverse the pathological progression of hereditary spastic paraplegia. This review aims to explore the metabolic mechanisms underlying the pathophysiology of hereditary spastic paraplegia, emphasising the interactions of various genes identified in recent network studies. By comprehending these associations, targeted molecular therapies that address these biochemical processes can be developed to enhance treatment strategies for hereditary spastic paraplegia and guide clinical practice effectively.

Clinical genetics of Charcot-Marie-Tooth disease

Recent research in the field of inherited peripheral neuropathies (IPNs) such as Charcot-Marie-Tooth (CMT) disease has helped identify the causative genes provided better understanding of the pathogenesis, and unraveled potential novel therapeutic targets. Several reports have described the epidemiology, clinical characteristics, molecular pathogenesis, and novel causative genes for CMT/IPNs in Japan. Based on the functions of the causative genes identified so far, the following molecular and cellular mechanisms are believed to be involved in the causation of CMTs/IPNs: myelin assembly, cytoskeletal structure, myelin-specific transcription factor, nuclear related, endosomal sorting and cell signaling, proteasome and protein aggregation, mitochondria-related, motor proteins and axonal transport, tRNA synthetases and RNA metabolism, and ion channel-related mechanisms. In this article, we review the epidemiology, genetic diagnosis, and clinicogenetic characteristics of CMT in Japan. In addition, we discuss the newly identified novel causative genes for CMT/IPNs in Japan, namely MME and COA7. Identification of the new causes of CMT will facilitate in-depth characterization of the underlying molecular mechanisms of CMT, leading to the establishment of therapeutic approaches such as drug development and gene therapy.

Role of Seipin in Human Diseases and Experimental Animal Models

Seipin, a protein encoded by the Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) gene, is famous for its key role in the biogenesis of lipid droplets and type 2 congenital generalised lipodystrophy (CGL2). BSCL2 gene mutations result in genetic diseases including CGL2, progressive encephalopathy with or without lipodystrophy (also called Celia’s encephalopathy), and BSCL2-associated motor neuron diseases. Abnormal expression of seipin has also been found in hepatic steatosis, neurodegenerative diseases, glioblastoma stroke, cardiac hypertrophy, and other diseases. In the current study, we comprehensively summarise phenotypes, underlying mechanisms, and treatment of human diseases caused by BSCL2 gene mutations, paralleled by animal studies including systemic or specific Bscl2 gene knockout, or Bscl2 gene overexpression. In various animal models representing diseases that are not related to Bscl2 mutations, differential expression patterns and functional roles of seipin are also described. Furthermore, we highlight the potential therapeutic approaches by targeting seipin or its upstream and downstream signalling pathways. Taken together, restoring adipose tissue function and targeting seipin-related pathways are effective strategies for CGL2 treatment. Meanwhile, seipin-related pathways are also considered to have potential therapeutic value in diseases that are not caused by BSCL2 gene mutations.

Rare among Rare: Phenotypes of Uncommon CMT Genotypes

(1) Background: Charcot–Marie–Tooth disease (CMT) is the most frequent form of inherited chronic motor and sensory polyneuropathy. Over 100 CMT causative genes have been identified. Previous reports found PMP22, GJB1, MPZ, and MFN2 as the most frequently involved genes. Other genes, such as BSCL2, MORC2, HINT1, LITAF, GARS, and autosomal dominant GDAP1 are responsible for only a minority of CMT cases. (2) Methods: we present here our records of CMT patients harboring a mutation in one of these rare genes (BSCL2, MORC2, HINT1, LITAF, GARS, autosomal dominant GDAP1). We studied 17 patients from 8 unrelated families. All subjects underwent neurologic evaluation and genetic testing by next-generation sequencing on an Ion Torrent PGM (Thermo Fischer) with a 44-gene custom panel. (3) Results: the following variants were found: BSCL2 c.263A > G p.Asn88Ser (eight subjects), MORC2 c.1503A > T p.Gln501His (one subject), HINT1 c.110G > C p.Arg37Pro (one subject), LITAF c.404C > G p.Pro135Arg (two subjects), GARS c.1660G > A p.Asp554Asn (three subjects), GDAP1 c.374G > A p.Arg125Gln (two subjects). (4) Expanding the spectrum of CMT phenotypes is of high relevance, especially for less common variants that have a higher risk of remaining undiagnosed. The necessity of reaching a genetic definition for most patients is great, potentially making them eligible for future experimentations.

A De Novo BSCL2 Gene S90L Mutation in a Progressive Tetraparesis with Urinary Dysfunction and Corpus Callosum Involvement

A Silver syndrome is a rare autosomal dominant spastic paraparesis in which spasticity of the lower limbs is accompanied by amyotrophy of the small hand muscles. The causative gene is the Berardinelli-Seip congenital lipodystrophy 2 ( BSCL2) , which is related to a spectrum of neurological phenotypes. In the current study, we presented a 14-year-old male with a slowly progressive spastic paraparesis with urinary incontinence that later on exhibited atrophy and weakness in the thenar and dorsal interosseous muscles. Magnetic resonance imaging (MRI) revealed discrete atrophy of the corpus callosum isthmus and an extended next-generation sequencing panel identified a de novo heterozygous mutation in BSCL2 gene, c.269C > T p.(S90L). Various clinical expression and incomplete penetrance of BSCL2 gene mutations complicate the establishment of a genetic etiology for these cases. Therefore, Silver syndrome should be included in the differential diagnosis if the initial presentation is a spastic paraparesis by urinary involvement with childhood-onset, even with MRI atypical findings. This report described the first Iberian Silver syndrome case carrying a de novo c.269C > T p. (S90L) BSCL2 gene mutation.

[Autosomal dominant spastic paraplegias]

OBJECTIVE

To estimate the proportion and spectrum of infrequent autosomal dominant spastic paraplegias in a group of families with DNA-confirmed diagnosis and to investigate their molecular and clinical characteristics.

MATERIAL AND METHODS

Ten families with 6 AD-SPG: SPG6 (n=1), SPG8 (n=2), SPG9A (n=1), SPG12 (n=1), SPG17 (n=3), SPG31 (n=2) were studied using clinical, genealogical, molecular-genetic (massive parallel sequencing, spastic paraplegia panel, whole-exome sequencing, multiplex ligation-dependent amplification, Sanger sequencing) and bioinformatic methods.

RESULTS AND CONCLUSION

Nine heterozygous mutations were detected in 6 genes, including the common de novo mutation p.Gly106Arg in NIPA1 (SPG6), the earlier reported mutation p.Val626Phe in WASHC5 (SPG8) in isolated case and the novel p.Val695Ala in WASHC5 (SPG8) in a family with 4 patients, the novel mutation p.Thr301Arg in RTN2 (SPG12) in a family with 2 patients, the novel mutation c.105+4A>G in REEP1 (SPG31) in a family with 4 patients and the reported earlier p.Lys101Lys in REEP1 (SPG31) in a family with 3 patients, the known de novo mutation p.Arg252Gln in ALDH18A1 (SPG9A) in two monozygous twins; the common mutation p.Ser90Leu in BSCL2 (SPG17) in a family with 3 patients and in isolated case, reported mutation p.Leu363Pro in a family with 2 patients. SPG6, SPG8, SPG12 and SPG31 presented 'pure' phenotypes, SPG31 had most benign course. Age of onset varied in SPG31 family and was atypically early in SPG6 case. Patients with SPG9A and SPG17 had 'complicated' paraplegias; amyotrophy of hands typical for SPG17 was absent in a child and in an adolescent from 2 families, but may develop later.