Genetics of inherited peripheral neuropathies and the next frontier: looking backwards to progress forwards.

Inherited peripheral neuropathies (IPNs) encompass a clinically and genetically heterogeneous group of disorders causing length-dependent degeneration of peripheral autonomic, motor and/or sensory nerves. Despite gold-standard diagnostic testing for pathogenic variants in over 100 known associated genes, many patients with IPN remain genetically unsolved. Providing patients with a diagnosis is critical for reducing their 'diagnostic odyssey', improving clinical care, and for informed genetic counselling. The last decade of massively parallel sequencing technologies has seen a rapid increase in the number of newly described IPN-associated gene variants contributing to IPN pathogenesis. However, the scarcity of additional families and functional data supporting variants in potential novel genes is prolonging patient diagnostic uncertainty and contributing to the missing heritability of IPNs. We review the last decade of IPN disease gene discovery to highlight novel genes, structural variation and short tandem repeat expansions contributing to IPN pathogenesis. From the lessons learnt, we provide our vision for IPN research as we anticipate the future, providing examples of emerging technologies, resources and tools that we propose that will expedite the genetic diagnosis of unsolved IPN families.

PPP2R5D heterozygous pathogenic variant causes early-onset parkinsonism and treatment implications: A case report.

Neurodevelopmental disorders with early-onset parkinsonism have diverse genetic aetiologies and can mimic Parkinson's disease. We report the clinical evaluation and neuroimaging studies of a woman with intellectual disability and levodopa-responsive akinetic rigid parkinsonism. Whole-genome sequencing of family trio identified a de novo missense variant in PPP2R5D in the proband.

Bridging Genetic Insights with Neuroimaging in Autism Spectrum Disorder-A Systematic Review.

Autism Spectrum Disorder (ASD) is an early onset neurodevelopmental disorder characterized by impaired social interaction and communication, and repetitive patterns of behavior. Family studies show that ASD is highly heritable, and hundreds of genes have previously been implicated in the disorder; however, the etiology is still not fully clear. Brain imaging and electroencephalography (EEG) are key techniques that study alterations in brain structure and function. Combined with genetic analysis, these techniques have the potential to help in the clarification of the neurobiological mechanisms contributing to ASD and help in defining novel therapeutic targets. To further understand what is known today regarding the impact of genetic variants in the brain alterations observed in individuals with ASD, a systematic review was carried out using Pubmed and EBSCO databases and following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. This review shows that specific genetic variants and altered patterns of gene expression in individuals with ASD may have an effect on brain circuits associated with face processing and social cognition, and contribute to excitation-inhibition imbalances and to anomalies in brain volumes.

The regulation of enteric neuron connectivity by semaphorin 5A is affected by the autism-associated S956G missense mutation.

The neural network of the enteric nervous system (ENS) underlies gastrointestinal functions. However, the molecular mechanisms involved in enteric neuronal connectivity are poorly characterized. Here, we studied the role of semaphorin 5A (Sema5A), previously characterized in the central nervous system, on ENS neuronal connectivity. Sema5A is linked to autism spectrum disorder (ASD), a neurodevelopmental disorder frequently associated with gastrointestinal comorbidities, and potentially associated with ENS impairments. This study investigated in rat enteric neuron cultures and gut explants the role of Sema5A on enteric neuron connectivity and the impact of ASD-associated mutations on Sema5A activity. Our findings demonstrated that Sema5A promoted axonal complexity and reduced functional connectivity in enteric neurons. Strikingly, the ASD-associated mutation S956G in Sema5A strongly affected these activities. This study identifies a critical role of Sema5A in the ENS as a regulator of neuronal connectivity that might be compromised in ASD.

Characteristic Fetal Brain MRI Abnormalities in Pyruvate Dehydrogenase Complex Deficiency.

Pyruvate dehydrogenase complex deficiency (PDCD) is a disorder of mitochondrial metabolism that is caused by pathogenic variants in multiple genes, including PDHA1. Typical neonatal brain imaging findings in PDCD have been described, with a focus on malformative features and chronic encephaloclastic changes. However, fetal brain MRI imaging in confirmed PDCD has not been comprehensively described. We sought to demonstrate the prenatal neurological and systemic manifestations of PDCD determined by comprehensive fetal imaging and genomic sequencing. All fetuses with a diagnosis of genetic PDCD who had undergone fetal MRI were included in the study. Medical records, imaging data, and genetic testing results were reviewed and reported descriptively. Ten patients with diagnosis of PDCD were included. Most patients had corpus callosum dysgenesis, abnormal gyration pattern, reduced brain volumes, and periventricular cystic lesions. One patient had associated intraventricular hemorrhages. One patient had a midbrain malformation with aqueductal stenosis and severe hydrocephalus. Fetuses imaged in the second trimester were found to have enlargement of the ganglionic eminences with cystic cavitations, while those imaged in the third trimester had germinolytic cysts. Fetuses with PDCD have similar brain MRI findings to neonates described in the literature, although some of these findings may be subtle early in pregnancy. Additional features, such as cystic cavitations of the ganglionic eminences, are noted in the second trimester in fetuses with PDCD, and these may represent a novel early diagnostic marker for PDCD. Using fetal MRI to identify these radiological hallmarks to inform prenatal diagnosis of PDCD may guide genetic counseling, pregnancy decision-making, and neonatal care planning.

Spinocerebellar ataxia 27B (SCA27B), a frequent late-onset cerebellar ataxia.

Genetic cerebellar ataxias are still a diagnostic challenge, and yet not all of them have been identified. Very recently, in early 2023, a new cause of late-onset cerebellar ataxia (LOCA) was identified, spinocerebellar ataxia 27B (SCA27B). This is an autosomal dominant ataxia due to a GAA expansion in intron 1 of the FGF14 gene. Thanks to the many studies carried out since its discovery, it is now possible to define the clinical phenotype, its particularities, and the progression of SCA27B. It has also been established that it is one of the most frequent causes of LOCA. The core phenotype of the disease consists of slowly progressive late-onset ataxia with cerebellar syndrome, oculomotor disorders including downbeat nystagmus, and episodic symptoms such as diplopia. Therapeutic approaches have been proposed, including acetazolamide, and 4-aminopyridine, the latter with a better benefit/tolerance profile.

Lessons and pitfalls of whole genome sequencing.

Whole-genome sequencing (WGS) has recently become the first-line genetic investigation for many suspected genetic neurological disorders. While its diagnostic capabilities are innumerable, as with any test, it has its limitations. Clinicians should be aware of where WGS is extremely reliable (detecting single-nucleotide variants), where its reliability is much improved (detecting copy number variants and small repeat expansions) and where it may miss/misinterpret a variant (large repeat expansions, balanced structural variants or low heteroplasmy mitochondrial DNA variants). Bioinformatic technology and virtual gene panels are constantly evolving, and it is important to know what genes and what types of variant are being tested; the current National Health Service Genomic Medicine Service WGS offers more than early iterations of the 100 000 Genomes Project analysis. Close communication between clinician and laboratory, ideally through a multidisciplinary team meeting, is encouraged where there is diagnostic uncertainty.

C. elegans males optimize mate-preference decisions via sex-specific responses to multimodal sensory cues.

For sexually reproducing animals, selecting optimal mates is important for maximizing reproductive fitness. In the nematode C. elegans, populations reproduce largely by hermaphrodite self-fertilization, but the cross-fertilization of hermaphrodites by males also occurs. Males' ability to recognize hermaphrodites involves several sensory cues, but an integrated view of the ways males use these cues in their native context to assess characteristics of potential mates has been elusive. Here, we examine the mate-preference behavior of C. elegans males evoked by natively produced cues. We find that males use a combination of volatile sex pheromones (VSPs), ascaroside sex pheromones, surface-associated cues, and other signals to assess multiple features of potential mates. Specific aspects of mate preference are communicated by distinct signals: developmental stage and sex are signaled by ascaroside pheromones and surface cues, whereas the presence of a self-sperm-depleted hermaphrodite is likely signaled by VSPs. Furthermore, males prefer to interact with virgin over mated, and well-fed over food-deprived, hermaphrodites; these preferences are likely adaptive and are also mediated by ascarosides and other cues. Sex-typical mate-preference behavior depends on the sexual state of the nervous system, such that pan-neuronal genetic masculinization in hermaphrodites generates male-typical social behavior. We also identify an unexpected role for the sex-shared ASH sensory neurons in male attraction to ascaroside sex pheromones. Our findings lead to an integrated view in which the distinct physical properties of various mate-preference cues guide a flexible, stepwise behavioral program by which males assess multiple features of potential mates to optimize mate preference.

Hereditary motor and sensory neuropathy Okinawa type mimicking proximal myopathy.

Hereditary motor and sensory neuropathy with proximal dominant involvement (HMSN-P), or, Okinawa type, is a rare neuromuscular disorder characterized by proximal dominant neurogenic atrophy and distal sensory alterations with an autosomal dominant inheritance pattern. We present a case of a Brazilian woman of Okinawan ancestry, with symmetrical proximal weakness, fasciculations, absent patellar reflexes and positive familial history for the same symptoms. These findings led to genetic testing, which identified a variant in the TFG gene (c.854 C>T;p.(Pro285Leu), confirming the diagnosis of HMSN-P. HMSN-P seemed to be restricted to populations in Okinawa, however, other HMSN-P cases were described in several parts of the world, especially in South America. This case report emphasizes the importance of considering HMSN-P in patients presenting with clinical features resembling proximal myopathy, especially in individuals with Okinawan ancestry.

The Use of CGH Arrays for Identifying Copy Number Variations in Children with Autism Spectrum Disorder.

Autism spectrum disorders (ASDs) encompass a broad group of neurodevelopmental disorders with varied clinical symptoms, all being characterized by deficits in social communication and repetitive behavior. Although the etiology of ASD is heterogeneous, with many genes involved, a crucial role is believed to be played by copy number variants (CNVs). The present study examines the role of copy number variation in the development of isolated ASD, or ASD with additional clinical features, among a group of 180 patients ranging in age from two years and four months to 17 years and nine months. Samples were taken and subjected to array-based comparative genomic hybridization (aCGH), the gold standard in detecting gains or losses in the genome, using a 4 × 180 CytoSure Autism Research Array, with a resolution of around 75 kb. The results indicated the presence of nine pathogenic and six likely pathogenic imbalances, and 20 variants of uncertain significance (VUSs) among the group. Relevant variants were more prevalent in patients with ASD and additional clinical features. Twelve of the detected variants, four of which were probably pathogenic, would not have been identified using the routine 8 × 60 k microarray. These results confirm the value of microarrays in ASD diagnostics and highlight the need for dedicated tools.

The future of neuroscience in developmental psychopathology.

Developmental psychopathology started as an intersection of fields and is now a field itself. As we contemplate the future of this field, we consider the ways in which a newer, interdisciplinary field - human developmental neuroscience - can inform, and be informed by, developmental psychopathology. To do so, we outline principles of developmental psychopathology and how they are and/or can be implemented in developmental neuroscience. In turn, we highlight how the collaboration between these fields can lead to richer models and more impactful translation. In doing so, we describe the ways in which models from developmental psychopathology can enrich developmental neuroscience and future directions for developmental psychopathology.

Kennedy's disease.

A 57-year-old man developed worsening early morning headaches, muscle cramps and falls over 12 months. He had widespread fasciculation and was diagnosed with motor neurone disease, and treated with nocturnal hypoventilation. Based on this diagnosis, he made significant personal and financial decisions including retiring and selling his house. He subsequently developed a lump in his right breast and was found to have gynaecomastia. This triggered genetic testing for Kennedy's disease leading to the correct diagnosis. This case highlights an unusual presentation of a rare disease leading to misdiagnosis and major repercussions for the patient. Recent genetic analysis from the 100 000 genome project suggests Kennedy's disease may be four times more prevalent in the population than previously thought, highlighting the need to consider genetic testing, especially if there is a suggestion of multisystem disease.

Whole genome sequencing increases the diagnostic rate in Charcot-Marie-Tooth disease.

Charcot-Marie-Tooth disease (CMT) is one of the most common and genetically heterogeneous inherited neurological diseases, with more than 130 disease-causing genes. Whole genome sequencing (WGS) has improved diagnosis across genetic diseases, but the diagnostic impact in CMT is yet to be fully reported. We present the diagnostic results from a single specialist inherited neuropathy centre, including the impact of WGS diagnostic testing. Patients were assessed at our specialist inherited neuropathy centre from 2009-2023. Genetic testing was performed using single gene testing, next-generation sequencing targeted panels, research whole exome and whole genome sequencing (WGS), and latterly WGS through the UK National Health Service. Variants were assessed using the American College of Medical Genetics and Genomics and Association for Clinical Genomic Science criteria. Excluding patients with hereditary ATTR amyloidosis, 1515 patients with a clinical diagnosis of CMT and related disorders were recruited. 621 patients had CMT1 (41.0%), 294 CMT2 (19.4%), 205 intermediate CMT (CMTi, 13.5%), 139 hereditary motor neuropathy (HMN, 9.2%), 93 hereditary sensory neuropathy (HSN, 6.1%), 38 sensory ataxic neuropathy (2.5%), 72 hereditary neuropathy with liability to pressure palsies (HNPP, 4.8%) and 53 'complex' neuropathy (3.5%). Overall, a genetic diagnosis was reached in 76.9% (1165/1515). A diagnosis was most likely in CMT1 (96.8%, 601/621), followed by CMTi (81.0%, 166/205) and then HSN (69.9%, 65/93). Diagnostic rates remained less than 50% in CMT2, HMN and complex neuropathies. The most common genetic diagnosis was PMP22 duplication (CMT1A; 505/1165, 43.3%), then GJB1 (CMTX1; 151/1165, 13.0%), PMP22 deletion (HNPP; 72/1165, 6.2%) and MFN2 (CMT2A; 46/1165, 3.9%). We recruited 233 cases to the UK 100,000 Genomes Project (100KGP), of which 74 (31.8%) achieved a diagnosis; 28 had been otherwise diagnosed since recruitment leaving a true diagnostic rate of WGS through the 100KGP of 19.7% (46/233). However, almost half of the solved cases (35/74) received a negative report from the study, and the diagnosis was made through our research access to the WGS data. The overall diagnostic uplift of WGS for the entire cohort was 3.5%. Our diagnostic rate is the highest reported from a single centre, and has benefitted from the use of WGS, particularly access to the raw data. However, almost one quarter of all cases remain unsolved, and a new reference genome and novel technologies will be important to narrow the 'diagnostic gap'.

Bi-allelic variants in SNF8 cause a disease spectrum ranging from severe developmental and epileptic encephalopathy to syndromic optic atrophy.

The endosomal sorting complex required for transport (ESCRT) machinery is essential for membrane remodeling and autophagy and it comprises three multi-subunit complexes (ESCRT I-III). We report nine individuals from six families presenting with a spectrum of neurodevelopmental/neurodegenerative features caused by bi-allelic variants in SNF8 (GenBank: NM_007241.4), encoding the ESCRT-II subunit SNF8. The phenotypic spectrum included four individuals with severe developmental and epileptic encephalopathy, massive reduction of white matter, hypo-/aplasia of the corpus callosum, neurodevelopmental arrest, and early death. A second cohort shows a milder phenotype with intellectual disability, childhood-onset optic atrophy, or ataxia. All mildly affected individuals shared the same hypomorphic variant, c.304G>A (p.Val102Ile). In patient-derived fibroblasts, bi-allelic SNF8 variants cause loss of ESCRT-II subunits. Snf8 loss of function in zebrafish results in global developmental delay and altered embryo morphology, impaired optic nerve development, and reduced forebrain size. In vivo experiments corroborated the pathogenicity of the tested SNF8 variants and their variable impact on embryo development, validating the observed clinical heterogeneity. Taken together, we conclude that loss of ESCRT-II due to bi-allelic SNF8 variants is associated with a spectrum of neurodevelopmental/neurodegenerative phenotypes mediated likely via impairment of the autophagic flux.

The Importance of Offering Exome or Genome Sequencing in Adult Neuromuscular Clinics.

Advances in gene-specific therapeutics for patients with neuromuscular disorders (NMDs) have brought increased attention to the importance of genetic diagnosis. Genetic testing practices vary among adult neuromuscular clinics, with multi-gene panel testing currently being the most common approach; follow-up testing using broad-based methods, such as exome or genome sequencing, is less consistently offered. Here, we use five case examples to illustrate the unique ability of broad-based testing to improve diagnostic yield, resulting in identification of SORD-neuropathy, HADHB-related disease, ATXN2-ALS, MECP2 related progressive gait decline and spasticity, and DNMT1-related cerebellar ataxia, deafness, narcolepsy, and hereditary sensory neuropathy type 1E. We describe in each case the technological advantages that enabled identification of the causal gene, and the resultant clinical and personal implications for the patient, demonstrating the importance of offering exome or genome sequencing to adults with NMDs.

Genotype-specific spinal cord damage in spinocerebellar ataxias: an ENIGMA-Ataxia study.

BACKGROUND: Spinal cord damage is a feature of many spinocerebellar ataxias (SCAs), but well-powered in vivo studies are lacking and links with disease severity and progression remain unclear. Here we characterise cervical spinal cord morphometric abnormalities in SCA1, SCA2, SCA3 and SCA6 using a large multisite MRI dataset. METHODS: Upper spinal cord (vertebrae C1-C4) cross-sectional area (CSA) and eccentricity (flattening) were assessed using MRI data from nine sites within the ENIGMA-Ataxia consortium, including 364 people with ataxic SCA, 56 individuals with preataxic SCA and 394 nonataxic controls. Correlations and subgroup analyses within the SCA cohorts were undertaken based on disease duration and ataxia severity. RESULTS: Individuals in the ataxic stage of SCA1, SCA2 and SCA3, relative to non-ataxic controls, had significantly reduced CSA and increased eccentricity at all examined levels. CSA showed large effect sizes (d>2.0) and correlated with ataxia severity (r<-0.43) and disease duration (r<-0.21). Eccentricity correlated only with ataxia severity in SCA2 (r=0.28). No significant spinal cord differences were evident in SCA6. In preataxic individuals, CSA was significantly reduced in SCA2 (d=1.6) and SCA3 (d=1.7), and the SCA2 group also showed increased eccentricity (d=1.1) relative to nonataxic controls. Subgroup analyses confirmed that CSA and eccentricity are abnormal in early disease stages in SCA1, SCA2 and SCA3. CSA declined with disease progression in all, whereas eccentricity progressed only in SCA2. CONCLUSIONS: Spinal cord abnormalities are an early and progressive feature of SCA1, SCA2 and SCA3, but not SCA6, which can be captured using quantitative MRI.

One Train May Hide Another: Two Cases of Co-Occurring Primary Familial Brain Calcification and Alzheimer's Disease.

Primary familial brain calcification (PFBC) is a rare disorder that can manifest with a wide spectrum of motor, cognitive, and psychiatric symptoms or even remain asymptomatic. Alzheimer disease (AD) is a common condition that typically starts as a progressive amnestic disorder and progresses to major cognitive impairment. Accurately attributing an etiology to cognitive impairment can sometimes be challenging, especially when multiple pathologies with potentially overlapping symptomatology contribute to the clinical phenotype. Here, we present the case of two patients with autosomal dominant PFBC and non-monogenic AD. Cerebrospinal fluid (CSF) biomarker analysis combined with genetic testing permitted the dual diagnosis. We emphasize the importance of thoroughly characterizing the patient's phenotype at onset and during the follow-up. Particular attention is placed on psychiatric symptoms given that both patients had a history of mood disorder, a frequent condition in the general population and in neurological diseases. We also discuss and challenge the paradigm of seeking a single diagnosis explaining all symptoms, remembering the possibility of a rare disease co-occurring with a common one.

A structurally precise mechanism links an epilepsy-associated KCNC2 potassium channel mutation to interneuron dysfunction.

De novo heterozygous variants in KCNC2 encoding the voltage-gated potassium (K+) channel subunit Kv3.2 are a recently described cause of developmental and epileptic encephalopathy (DEE). A de novo variant in KCNC2 c.374G > A (p.Cys125Tyr) was identified via exome sequencing in a patient with DEE. Relative to wild-type Kv3.2, Kv3.2-p.Cys125Tyr induces K+ currents exhibiting a large hyperpolarizing shift in the voltage dependence of activation, accelerated activation, and delayed deactivation consistent with a relative stabilization of the open conformation, along with increased current density. Leveraging the cryogenic electron microscopy (cryo-EM) structure of Kv3.1, molecular dynamic simulations suggest that a strong π-π stacking interaction between the variant Tyr125 and Tyr156 in the α-6 helix of the T1 domain promotes a relative stabilization of the open conformation of the channel, which underlies the observed gain of function. A multicompartment computational model of a Kv3-expressing parvalbumin-positive cerebral cortex fast-spiking γ-aminobutyric acidergic (GABAergic) interneuron (PV-IN) demonstrates how the Kv3.2-Cys125Tyr variant impairs neuronal excitability and dysregulates inhibition in cerebral cortex circuits to explain the resulting epilepsy.