Generation of 3 patient induced Pluripotent stem cell lines containing SORD mutations linked to a recessive neuropathy.

The SORD neuropathy has been identified as the most common autosomal recessive inherited neuropathy, occurring in thousands of patients worldwide. Fibroblast lines from 3 different patients containing the c.753delG; p.Ala253GlnfsTer27 SORD mutations were reprogrammed into induced Pluripotent Stem Cell (iPSC) lines. These iPSC lines demonstrate an apparent normal karyotype and have positive expression of pluripotency markers. These iPSC lines also stain positively for Ectoderm, Endoderm and Mesoderm markers following Embryoid body differentiation. These lines pose to serve as a valuable disease modeling resource for studying the SORD neuropathy, including studying disease phenotype and treatment efficacy.

A study concept of expeditious clinical enrollment for genetic modifier studies in Charcot-Marie-Tooth neuropathy 1A.

BACKGROUND: Caused by duplications of the gene encoding peripheral myelin protein 22 (PMP22), Charcot-Marie-Tooth disease type 1A (CMT1A) is the most common hereditary neuropathy. Despite this shared genetic origin, there is considerable variability in clinical severity. It is hypothesized that genetic modifiers contribute to this heterogeneity, the identification of which may reveal novel therapeutic targets. In this study, we present a comprehensive analysis of clinical examination results from 1564 CMT1A patients sourced from a prospective natural history study conducted by the RDCRN-INC (Inherited Neuropathy Consortium). Our primary objective is to delineate extreme phenotype profiles (mild and severe) within this patient cohort, thereby enhancing our ability to detect genetic modifiers with large effects. METHODS: We have conducted large-scale statistical analyses of the RDCRN-INC database to characterize CMT1A severity across multiple metrics. RESULTS: We defined patients below the 10th (mild) and above the 90th (severe) percentiles of age-normalized disease severity based on the CMT Examination Score V2 and foot dorsiflexion strength (MRC scale). Based on extreme phenotype categories, we defined a statistically justified recruitment strategy, which we propose to use in future modifier studies. INTERPRETATION: Leveraging whole genome sequencing with base pair resolution, a future genetic modifier evaluation will include single nucleotide association, gene burden tests, and structural variant analysis. The present work not only provides insight into the severity and course of CMT1A, but also elucidates the statistical foundation and practical considerations for a cost-efficient and straightforward patient enrollment strategy that we intend to conduct on additional patients recruited globally.

Quantitative Foot Muscle Magnetic Resonance Imaging Reliably Measures Disease Progression in Children and Adolescents with Charcot-Marie-Tooth Disease Type 1A.

Quantitative muscle fat fraction (FF) responsiveness is lower in younger Charcot-Marie-Tooth disease type 1A (CMT1A) patients with lower baseline calf-level FF. We investigated the practicality, validity, and responsiveness of foot-level FF in this cohort involving 22 CMT1A patients and 14 controls. The mean baseline foot-level FF was 25.9 ± 20.3% in CMT1A patients, and the 365-day FF (n = 15) increased by 2.0 ± 2.4% (p < 0.001 vs controls). Intrinsic foot-level FF demonstrated large responsiveness (12-month standardized response mean (SRM) of 0.86) and correlated with the CMT examination score (ρ = 0.58, P = 0.01). Intrinsic foot-level FF has the potential to be used as a biomarker in future clinical trials involving younger CMT1A patients. ANN NEUROL 2024.

Activation of XBP1s attenuates disease severity in models of proteotoxic Charcot-Marie-Tooth type 1B.

Mutations in myelin protein zero (MPZ) are generally associated with Charcot-Marie-Tooth type 1B (CMT1B) disease, one of the most common forms of demyelinating neuropathy. Pathogenesis of some MPZ mutants, such as S63del and R98C, involves the misfolding and retention of MPZ in the endoplasmic reticulum (ER) of myelinating Schwann cells. To cope with proteotoxic ER-stress, Schwann cells mount an unfolded protein response (UPR) characterized by activation of the PERK, ATF6 and IRE1α/XBP1 pathways. Previous results showed that targeting the PERK UPR pathway mitigates neuropathy in mouse models of CMT1B; however, the contributions of other UPR pathways in disease pathogenesis remains poorly understood. Here, we probe the importance of the IRE1α/XBP1 signalling during normal myelination and in CMT1B. In response to ER stress, IRE1α is activated to stimulate the non-canonical splicing of Xbp1 mRNA to generate spliced Xbp1 (Xbp1s). This results in the increased expression of the adaptive transcription factor XBP1s, which regulates the expression of genes involved in diverse pathways including ER proteostasis. We generated mouse models where Xbp1 is deleted specifically in Schwann cells, preventing XBP1s activation in these cells. We observed that Xbp1 is dispensable for normal developmental myelination, myelin maintenance and remyelination after injury. However, Xbp1 deletion dramatically worsens the hypomyelination and the electrophysiological and locomotor parameters observed in young and adult CMT1B neuropathic animals. RNAseq analysis suggested that XBP1s exerts its adaptive function in CMT1B mouse models in large part via the induction of ER proteostasis genes. Accordingly, the exacerbation of the neuropathy in Xbp1 deficient mice was accompanied by upregulation of ER-stress pathways and of IRE1-mediated RIDD signaling in Schwann cells, suggesting that the activation of XBP1s via IRE1 plays a critical role in limiting mutant protein toxicity and that this toxicity cannot be compensated by other stress responses. Schwann cell specific overexpression of XBP1s partially re-established Schwann cell proteostasis and attenuated CMT1B severity in both the S63del and R98C mouse models. In addition, the selective, pharmacologic activation of IRE1α/XBP1 signaling ameliorated myelination in S63del dorsal root ganglia explants. Collectively, these data show that XBP1 has an essential adaptive role in different models of proteotoxic CMT1B neuropathy and suggest that activation of the IRE1α/XBP1 pathway may represent a therapeutic avenue in CMT1B and possibly for other neuropathies characterized by UPR activation.

Lower limb muscle MRI fat fraction is a responsive outcome measure in CMT X1, 1B and 2A.

OBJECTIVE: With potential therapies for many forms of Charcot-Marie-Tooth disease (CMT), responsive outcome measures are urgently needed for clinical trials. Quantitative lower limb MRI demonstrated progressive calf intramuscular fat accumulation in the commonest form, CMT1A with large responsiveness. In this study, we evaluated the responsiveness and validity in the three other common forms, due to variants in GJB1 (CMTX1), MPZ (CMT1B) and MFN2 (CMT2A). METHODS: 22 CMTX1, 21 CMT1B and 21 CMT2A patients and matched controls were assessed at a 1-year interval. Intramuscular fat fraction (FF) was evaluated using three-point Dixon MRI at thigh and calf level along with clinical measures including CMT examination score, clinical strength assessment, CMT-HI and plasma neurofilament light chain. RESULTS: All patient groups had elevated muscle fat fraction at thigh and calf levels, with highest thigh FF and atrophy in CMT2A. There was moderate correlation between calf muscle FF and clinical measures (CMTESv2 rho = 0.405; p = 0.001, ankle MRC strength rho = -0.481; p < 0.001). Significant annualised progression in calf muscle FF was seen in all patient groups (CMTX1 2.0 ± 2.0%, p < 0.001, CMT1B 1.6 ± 2.1% p = 0.004 and CMT2A 1.6 ± 2.1% p = 0.002). Greatest increase was seen in patients with 10-70% FF at baseline (calf 2.7 ± 2.3%, p < 0.0001 and thigh 1.7 ± 2.1%, p = 0.01). INTERPRETATION: Our results confirm that calf muscle FF is highly responsive over 12 months in three additional common forms of CMT which together with CMT1A account for 90% of genetically confirmed cases. Calf muscle MRI FF should be a valuable outcome measure in upcoming CMT clinical trials.

Multicenter Validation of the Charcot-Marie-Tooth Functional Outcome Measure.

BACKGROUND AND OBJECTIVES: Charcot-Marie-Tooth disease type 1A (CMT1A), caused by a duplication of PMP22, is the most common hereditary peripheral neuropathy. For participants with CMT1A, few clinical trials have been performed; however, multiple therapies have reached an advanced stage of preclinical development. In preparation for imminent clinical trials in participants with CMT1A, we have produced a Clinical Outcome Assessment (COA), known as the CMT-Functional Outcome Measure (CMT-FOM), in accordance with the FDA Roadmap to Patient-Focused Outcome Measurement to capture the key clinical end point of function. METHODS: Participants were recruited through CMT clinics in the United States (n = 130), the United Kingdom (n = 52), and Italy (n = 32). To derive the most accurate signal with the fewest items to identify a therapeutic response, a series of validation studies were conducted including item and factor analysis, Rasch model analysis and testing of interrater reliability, discriminative ability, and convergent validity. RESULTS: A total of 214 participants aged 18-75 years with CMT1A (58% female) were included in this study. Item, factor, and Rasch analysis supported the viability of the 12-item CMT-FOM as a unidimensional interval scale of function in adults with CMT1A. The CMT-FOM covers strength, upper and lower limb function, balance, and mobility. The 0-100 point scoring system showed good overall model fit, no evidence of misfitting items, and no person misfit, and it was well targeted for adults with CMT1A exhibiting high inter-rater reliability across a range of clinical settings and evaluators. The CMT-FOM was significantly correlated with the CMT Examination Score (r = 0.643; p < 0.001) and the Overall Neuropathy Limitation Scale (r = 0.516; p < 0.001). Significantly higher CMT-FOM total scores were observed in participants self-reporting daily trips and falls, unsteady ankles, hand tremor, and hand weakness (p < 0.05). DISCUSSION: The CMT-FOM is a psychometrically robust multi-item, unidimensional, disease-specific COA covering strength, upper and lower limb function, balance, and mobility to capture how participants with CMT1A function to identify therapeutic efficacy.

Homomeric interactions of the MPZ Ig domain and their relation to Charcot-Marie-Tooth disease.

Mutations in MPZ (myelin protein zero) can cause demyelinating early-onset Charcot-Marie-Tooth type 1B disease or later onset type 2I/J disease characterized by axonal degeneration, reflecting the diverse roles of MPZ in Schwann cells. MPZ holds apposing membranes of the myelin sheath together, with the adhesion role fulfilled by its extracellular immunoglobulin-like domain (IgMPZ), which oligomerizes. Models for how the IgMPZ might form oligomeric assemblies has been extrapolated from a protein crystal structure in which individual rat IgMPZ subunits are packed together under artificial conditions, forming three weak interfaces. One interface organizes the IgMPZ into tetramers, a second 'dimer' interface links tetramers together across the intraperiod line, and a third hydrophobic interface that mediates binding to lipid bilayers or the same hydrophobic surface on another IgMPZ domain. Presently, there are no data confirming whether the proposed IgMPZ interfaces actually mediate oligomerization in solution, whether they are required for the adhesion activity of MPZ, whether they are important for myelination, or whether their loss results in disease. We performed nuclear magnetic resonance spectroscopy and small angle X-ray scattering analysis of wild-type IgMPZ as well as mutant forms with amino acid substitutions designed to interrupt its presumptive oligomerization interfaces. Here, we confirm the interface that mediates IgMPZ tetramerization, but find that dimerization is mediated by a distinct interface that has yet to be identified. We next correlated different types of Charcot-Marie-Tooth disease symptoms to subregions within IgMPZ tetramers. Variants causing axonal late-onset disease (CMT2I/J) map to surface residues of IgMPZ proximal to the transmembrane domain. Variants causing early-onset demyelinating disease (CMT1B) segregate into two groups: one is described by variants that disrupt the stability of the Ig-fold itself and are largely located within the core of the IgMPZ domain; whereas another describes a region on the surface of IgMPZ tetramers, accessible to protein interactions. Computational docking studies predict that this latter disease-relevant subregion may potentially mediate dimerization of IgMPZ tetramers.

Hereditary neuropathy.

The hereditary neuropathies, collectively referred as Charcot-Marie-Tooth disease (CMT) and related disorders, are heterogeneous genetic peripheral nerve disorders that collectively comprise the commonest inherited neurological disease with an estimated prevalence of 1:2500 individuals. The field of hereditary neuropathies has made significant progress in recent years with respect to both gene discovery and treatment as a result of next-generation sequencing (NGS) approach. These investigations which have identified over 100 causative genes and new mutations have made the classification of CMT even more challenging. Despite so many different mutated genes, the majority of CMT forms share a similar clinical phenotype, and due to this phenotypic homogeneity, genetic testing in CMT is increasingly being performed through the use of NGS panels. The majority of patients still have a mutation in one the four most common genes (PMP22 duplication-CMT1A, MPZ-CMT1B, GJB1-CMTX1, and MFN2-CMT2A). This chapter focuses primarily on these four forms and their potential therapeutic approaches.

Thirty-Year Follow-Up of Early Onset Amyotrophic Lateral Sclerosis with a Pathogenic Variant in SPTLC1.

Dominant mutations in serine palmitoyltransferase long chain base subunit 1 (SPTLC1), a known cause of hereditary sensory autonomic neuropathy type 1 (HSAN1), are a recently identified cause of juvenile amyotrophic lateral sclerosis (JALS) with slow progression. We present a case of SPTLC1-associated JALS followed for 30 years. She was initially evaluated at age 22 years for upper extremity weakness. She experienced gradual decline in muscle strength with development of weakness and hyperreflexia in lower extremities and diffuse fasciculations in the upper extremities at 26 years. She lost independent ambulation at age 45 years. Pulmonary function declined from a forced vital capacity of 94% predicted at 27 years to 49% predicted at 47 years, and she was hospitalized twice for respiratory failure. To our knowledge, this is the longest documented follow-up period of JALS caused by a de novo pathogenic variant in SPTLC1.

Association of Body Mass Index With Disease Progression in Children With Charcot-Marie-Tooth Disease.

BACKGROUND AND OBJECTIVES: The aim of this study was to evaluate the impact of body mass index (BMI) on disease progression over 2 years in children with Charcot-Marie-Tooth disease (CMT). METHODS: BMI was classified in 242 participants aged 3-20 years with CMT enrolled in the Inherited Neuropathy Consortium, using the International Obesity Task Force (based on adult BMI values, kg/m2) criteria. Groups were categorized as severely underweight (BMI <17 kg/m2), underweight (BMI ≥17 to <18.5 kg/m2), healthy weight (BMI ≥18.5 to <25 kg/m2), overweight (BMI ≥25 to <30 kg/m2), and obese (BMI ≥30 kg/m2). Disease severity was assessed using the CMT Pediatric Scale (CMTPedS), a clinical outcome assessment of disability (0-44 points, mild to severe). RESULTS: At baseline, compared with individuals being of a healthy weight (mean CMTPedS 15.48, SD 9.22), children who were severely underweight (mean CMTPedS difference 9.03, 95% CI 0.94-17.12; p = 0.02), underweight (mean CMTPedS difference 5.97, 95% CI 0.62-11.31; p = 0.02), or obese (mean CMTPedS difference 7.96, 95% CI 1.03-14.88; p = 0.015) exhibited greater disability. At 2 years, compared with individuals being of a healthy weight (mean CMTPedS 17.53, SD 9.41), children who were severely underweight exhibited greater disability (mean CMTPedS difference 9.27, 95% CI 0.90-17.64; p = 0.02). Over the 2-year periods, the mean CMTPedS for the whole sample deteriorated by 1.72 points (95% CI 1.09-2.38; p < 0.001), with severely underweight children progressing at the fastest rate (mean CMTPedS change of 2.3, 95% CI 1.53-6.13; p = 0.21). In children who did not have a change in BMI categories over 2 years (69% of sample), CMTPedS scores deteriorated faster in those who were severely underweight (mean CMTPedS change 6.40 points, 95% CI 2.42-10.38; p = 0.01) than those of healthy weight (mean CMTPedS change 1.79 points, 95% CI 0.93-2.69; p < 0.001). For children who changed BMI categories (31% of sample), CMTPedS scores deteriorated faster in children who became overweight/obese (mean CMTPedS change 2.76 points, 95% CI 0.11-5.41; p = 0.031). DISCUSSION: Children with CMT who were severely underweight, underweight, or obese exhibited greater disability at baseline. Over the 2-year period in those whose BMI remained stable, severely underweight children deteriorated at the fastest rate. For children who changed BMI categories over the 2 years, CMTPedS scores deteriorated faster in children who became overweight/obese. Interventions that maintain or improve BMI toward healthy weight may reduce disability in children with CMT.

Genetic analysis and natural history of Charcot-Marie-Tooth disease CMTX1 due to GJB1 variants.

Charcot-Marie-Tooth disease (CMT) due to GJB1 variants (CMTX1) is the second most common form of CMT. It is an X-linked disorder characterized by progressive sensory and motor neuropathy with males affected more severely than females. Many reported GJB1 variants remain classified as variants of uncertain significance (VUS). In this large, international, multicentre study we prospectively collected demographic, clinical and genetic data on patients with CMT associated with GJB1 variants. Pathogenicity for each variant was defined using adapted American College of Medical Genetics criteria. Baseline and longitudinal analyses were conducted to study genotype-phenotype correlations, to calculate longitudinal change using the CMT Examination Score (CMTES), to compare males versus females, and pathogenic/likely pathogenic (P/LP) variants versus VUS. We present 387 patients from 295 families harbouring 154 variants in GJB1. Of these, 319 patients (82.4%) were deemed to have P/LP variants, 65 had VUS (16.8%) and three benign variants (0.8%; excluded from analysis); an increased proportion of patients with P/LP variants compared with using ClinVar's classification (74.6%). Male patients (166/319, 52.0%, P/LP only) were more severely affected at baseline. Baseline measures in patients with P/LP variants and VUS showed no significant differences, and regression analysis suggested the disease groups were near identical at baseline. Genotype-phenotype analysis suggested c.-17G>A produces the most severe phenotype of the five most common variants, and missense variants in the intracellular domain are less severe than other domains. Progression of disease was seen with increasing CMTES over time up to 8 years follow-up. Standard response mean (SRM), a measure of outcome responsiveness, peaked at 3 years with moderate responsiveness [change in CMTES (ΔCMTES) = 1.3 ± 2.6, P = 0.00016, SRM = 0.50]. Males and females progressed similarly up to 8 years, but baseline regression analysis suggested that over a longer period, females progress more slowly. Progression was most pronounced for mild phenotypes (CMTES = 0-7; 3-year ΔCMTES = 2.3 ± 2.5, P = 0.001, SRM = 0.90). Enhanced variant interpretation has yielded an increased proportion of GJB1 variants classified as P/LP and will aid future variant interpretation in this gene. Baseline and longitudinal analysis of this large cohort of CMTX1 patients describes the natural history of the disease including the rate of progression; CMTES showed moderate responsiveness for the whole group at 3 years and higher responsiveness for the mild group at 3, 4 and 5 years. These results have implications for patient selection for upcoming clinical trials.

Validation of the parent-proxy version of the pediatric Charcot-Marie-Tooth disease quality of life instrument for children aged 0-7 years.

OBJECTIVE: To evaluate the parent-proxy version of the pediatric Charcot Marie Tooth specific quality of life (pCMT-QOL) outcome instrument for children aged 7 or younger with CMT. We have previously developed and validated the direct-report pCMT-QOL for children aged 8-18 years and a parent proxy version of the instrument for children 8-18 years old. There is currently no CMT-QOL outcome measure for children aged 0-7 years old. METHODS: Testing was conducted in parents or caregivers of children aged 0-7 years old with CMT evaluated at participating INC sites from the USA, United Kingdom, and Australia. The development of the instrument was iterative, involving identification of relevant domains, item pool generation, prospective pilot testing and clinical assessments, structured focus group interviews, and psychometric testing. The parent-proxy instrument was validated rigorously by examining previously identified domains and undergoing psychometric tests for children aged 0-7. RESULTS: The parent-proxy pCMT-QOL working versions were administered to 128 parents/caregivers of children aged 0-7 years old between 2010 and 2016. The resulting data underwent rigorous psychometric analysis, including factor analysis, internal consistency, and convergent validity, and longitudinal analysis to develop the final parent-proxy version of the pCMT-QOL outcome measure for children aged 0-7 years old. CONCLUSIONS: The parent-proxy version of the pCMT-QOL outcome measure, known as the pCMT-QOL (0-7 years parent-proxy) is a valid and sensitive proxy measure of health-related QOL for children aged 0-7 years with CMT.

Biallelic variants in COQ7 cause distal hereditary motor neuropathy with upper motor neuron signs.

COQ7 encodes a hydroxylase responsible for the penultimate step of coenzyme Q10 (CoQ10) biosynthesis in mitochondria. CoQ10 is essential for multiple cellular functions, including mitochondrial oxidative phosphorylation, lipid metabolism, and reactive oxygen species homeostasis. Mutations in COQ7 have been previously associated with primary CoQ10 deficiency, a clinically heterogeneous multisystemic mitochondrial disorder. We identified COQ7 biallelic variants in nine families diagnosed with distal hereditary motor neuropathy with upper neuron involvement, expending the clinical phenotype associated with defects in this gene. A recurrent p.Met1? change was identified in five families from Brazil with evidence of a founder effect. Fibroblasts isolated from patients revealed a substantial depletion of COQ7 protein levels, indicating protein instability leading to loss of enzyme function. High-performance liquid chromatography assay showed that fibroblasts from patients had reduced levels of CoQ10, and abnormal accumulation of the biosynthetic precursor DMQ10. Accordingly, fibroblasts from patients displayed significantly decreased oxygen consumption rates in patients, suggesting mitochondrial respiration deficiency. Induced pluripotent stem cell-derived motor neurons from patient fibroblasts showed significantly increased levels of extracellular neurofilament light protein, indicating axonal degeneration. Our findings indicate a molecular pathway involving CoQ10 biosynthesis deficiency and mitochondrial dysfunction in patients with distal hereditary motor neuropathy. Further studies will be important to evaluate the potential benefits of CoQ10 supplementation in the clinical outcome of the disease.

Sorbitol reduction via govorestat ameliorates synaptic dysfunction and neurodegeneration in sorbitol dehydrogenase deficiency.

Sorbitol dehydrogenase (SORD) deficiency has been identified as the most frequent autosomal recessive form of hereditary neuropathy. Loss of SORD causes high sorbitol levels in tissues due to the inability to convert sorbitol to fructose in the 2-step polyol pathway, leading to degenerative neuropathy. The underlying mechanisms of sorbitol-induced degeneration have not been fully elucidated, and no current FDA-approved therapeutic options are available to reduce sorbitol levels in the nervous system. Here, in a Drosophila model of SORD deficiency, we showed synaptic degeneration in the brain, neurotransmission defect, locomotor impairment, and structural abnormalities in the neuromuscular junctions. In addition, we found reduced ATP production in the brain and ROS accumulation in the CNS and muscle, indicating mitochondrial dysfunction. Applied Therapeutics has developed a CNS-penetrant next-generation aldose reductase inhibitor (ARI), AT-007 (govorestat), which inhibits the conversion of glucose to sorbitol. AT-007 significantly reduced sorbitol levels in patient-derived fibroblasts, induced pluripotent stem cell-derived (iPSC-derived) motor neurons, and Drosophila brains. AT-007 feeding in Sord-deficient Drosophila mitigated synaptic degeneration and significantly improved synaptic transduction, locomotor activity, and mitochondrial function. Moreover, AT-007 treatment significantly reduced ROS accumulation in Drosophila CNS, muscle, and patient-derived fibroblasts. These findings uncover the molecular and cellular pathophysiology of SORD neuropathy and provide a potential treatment strategy for patients with SORD deficiency.

Advances in diagnosis and management of distal sensory polyneuropathies.

Distal sensory polyneuropathy (DSP) is characterised by length-dependent, sensory-predominant symptoms and signs, including potentially disabling symmetric chronic pain, tingling and poor balance. Some patients also have or develop dysautonomia or motor involvement depending on whether large myelinated or small fibres are predominantly affected. Although highly prevalent, diagnosis and management can be challenging. While classic diabetes and toxic causes are well-recognised, there are increasingly diverse associations, including with dysimmune, rheumatological and neurodegenerative conditions. Approximately half of cases are initially considered idiopathic despite thorough evaluation, but often, the causes emerge later as new symptoms develop or testing advances, for instance with genetic approaches. Improving and standardising DSP metrics, as already accomplished for motor neuropathies, would permit in-clinic longitudinal tracking of natural history and treatment responses. Standardising phenotyping could advance research and facilitate trials of potential therapies, which lag so far. This review updates on recent advances and summarises current evidence for specific treatments.

Trials for Slowly Progressive Neurogenetic Diseases Need Surrogate Endpoints.

Heritable neurological disorders provide insights into disease mechanisms that permit development of novel therapeutic approaches including antisense oligonucleotides, RNA interference, and gene replacement. Many neurogenetic diseases are rare and slowly progressive making it challenging to measure disease progression within short time frames. We share our experience developing clinical outcome assessments and disease biomarkers in the inherited peripheral neuropathies. We posit that carefully developed biomarkers from imaging, plasma, or skin can predict meaningful progression in functional and patient reported outcome assessments such that clinical trials of less than 2 years will be feasible for these rare and ultra-rare disorders. ANN NEUROL 2023;93:906-910.

Neuropathy due to bi-allelic SH3TC2 variants: genotype-phenotype correlation and natural history.

Recessive SH3TC2 variants cause Charcot-Marie-Tooth disease type 4C (CMT4C). CMT4C is typically a sensorimotor demyelinating polyneuropathy, marked by early onset spinal deformities, but its clinical characteristics and severity are quite variable. Clear relationships between pathogenic variants and the spectrum of disease manifestations are to date lacking. Gene replacement therapy has been shown to ameliorate the phenotype in a mouse model of CMT4C, emphasizing the need for natural history studies to inform clinical trial readiness. Data, including both genetic information and clinical characteristics, were compiled from the longitudinal, prospective dataset of the Inherited Neuropathy Consortium, a member of the Rare Diseases Clinical Research Network (INC-RDCRN). The Charcot Marie Tooth Neuropathy Score (CMTNS), Examination Score (CMTES) and the Rasch-weighted CMTES (CMTES-R) were used to describe symptoms, neurological examinations and neurophysiological characteristics. Standardized response means were calculated at yearly intervals and a mixed model for repeated measures was used to estimate the change in CMTES and CMTES-R over time. Fifty-six individuals (59% female), median age 27 years (range 2-67 years) with homozygous or compound heterozygous variants in SH3TC2 were identified, including 34 unique variants, 14 of which have not previously been published. Twenty-eight participants had longitudinal data available. While there was no significant difference in the CMTES in those with protein truncating versus non-protein truncating variants, there were significant differences in the mean ulnar nerve compound muscle action potential amplitude, the mean radial sensory nerve action potential amplitude, and in the prevalence of scoliosis, suggesting the possibility of a milder phenotype in individuals with one or two non-protein-truncating variants. Overall, the mean value of the CMTES was 13, reflecting moderate clinical severity. There was a high rate of scoliosis (81%), scoliosis surgery (36%), and walking difficulty (94%) among study participants. The CMTES and CMTES-R appeared moderately responsive to change over extended follow-up, demonstrating a standardized response mean of 0.81 standard deviation units or 0.71 standard deviation units, respectively, over 3 years. Our analysis represents the largest cross-sectional and only longitudinal study to date, of the clinical phenotype of both adults and children with CMT4C. With the promise of upcoming genetic treatments, these data will further define the natural history of the disease and inform study design in preparation for clinical trials.

Biallelic pathogenic variants in the mitochondrial form of phosphoenolpyruvate carboxykinase cause peripheral neuropathy.

Phosphoenolpyruvate carboxykinase (PCK) plays a critical role in cytosolic gluconeogenesis, and defects in PCK1 cause a fasting-aggravated metabolic disease with hypoglycemia and lactic acidosis. However, there are two genes encoding PCK, and the role of the mitochondrial resident PCK (encoded by PCK2) is unclear, since gluconeogenesis is cytosolic. We identified three patients in two families with biallelic variants in PCK2. One has compound heterozygous variants (p.Ser23Ter/p.Pro170Leu), and the other two (siblings) have homozygous p.Arg193Ter variation. All three patients have weakness and abnormal gait, an absence of PCK2 protein, and profound reduction in PCK2 activity in fibroblasts, but no obvious metabolic phenotype. Nerve conduction studies showed reduced conduction velocities with temporal dispersion and conduction block compatible with a demyelinating peripheral neuropathy. To validate the association between PCK2 variants and clinical disease, we generated a mouse knockout model of PCK2 deficiency. The animals present abnormal nerve conduction studies and peripheral nerve pathology, corroborating the human phenotype. In total, we conclude that biallelic variants in PCK2 cause a neurogenetic disorder featuring abnormal gait and peripheral neuropathy.