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Understanding the genetic mutations that cause hereditary sensory and autonomic neuropathies (HSANs) is crucial to identify new therapeutic targets for patients with these neurodegenerative diseases. Rotthier et al . review the currently known genetics of the HSANs, discussing the new findings that provide insights into the mechanisms of disease and highlighting how these discoveries could improve treatment for patients with these diseases. Hereditary sensory and autonomic neuropathies (HSANs) are a clinically and genetically heterogeneous group of disorders of the PNS. Progressive degeneration, predominantly of sensory and autonomic neurons, is the main pathological feature in patients with HSAN, and causes prominent sensory loss and ulcerative mutilations in combination with variable autonomic and motor disturbances. Advances in molecular genetics have enabled identification of disease-causing mutations in 12 genes, and studies on the functional effects of these mutations are underway. Although some of the affected proteins—such as nerve growth factor and its receptor—have obvious nerve-specific roles, others are ubiquitously expressed proteins that are involved in sphingolipid metabolism, vesicular transport, transcription regulation and structural integrity. An important challenge in the future will be to understand the common molecular pathways that result in HSANs. Unraveling the mechanisms that underlie sensory and autonomic neurodegeneration could assist in identifying targets for future therapeutic strategies in patients with HSAN. This Review highlights key advances in the understanding of HSANs, including insights into the molecular mechanisms of disease, derived from genetic studies of patients with these disorders. Key Points Hereditary sensory and autonomic neuropathies (HSANs) are a diverse group of diseases of the PNS, characterized by profound distal sensory loss, acral mutilations and variable autonomic disturbances The genetic spectrum of HSANs encompasses autosomal dominant and autosomal recessive forms, with causative mutations identified in 12 genes The genetic cause of disease remains unresolved in at least two-thirds of patients with HSAN The molecular mechanisms that underlie HSANs are incompletely understood, but emerging evidence suggests that axonal transport, control of neuronal membrane excitability and neuronal development might be affected Supportive care is the only therapy available for patients with HSAN; future studies should aim to identify common mechanisms of disease that could be targets for therapeutic strategies

[...]the variant filtering strategy used with a Minor Allele Frequency (MAF) cut-off of 1% in the healthy population is not in accordance with the severity and age-of-onset of the described phenotypes (Gudmundsson et al., 2022). The authors propose a haploinsufficiency mechanism leading to pathogenicity for the p.Tyr311Ter mutation in family C. Although this specific mutation only has a GnomAD allele count of 2, loss-of-function (LoF) variants in SPTBN5 overall are highly frequent in the population, with as many as 190 homozygous LoF variants and more than 9000 heterozygous LoF variants. [...]in the discussion the authors only briefly mention cerebral MR imaging in their cohort and do not mention presence (or absence) of corpus callosum atrophy, other morphological changes or any other (acquired) cerebral pathologies that could be pertinent to the description of these cases.

Hereditary spastic paraplegia type 5 (SPG5) is an autosomal recessively inherited movement disorder characterized by progressive spastic gait disturbance and afferent ataxia. SPG5 is caused by bi-allelic loss of function mutations in CYP7B1 resulting in accumulation of the oxysterols 25-hydroxycholesterol and 27-hydroxycholesterol in serum and cerebrospinal fluid of SPG5 patients. An effect of 27- hydroxycholesterol via the estrogen and liver X receptors was previously shown on bone homeostasis. This study analyzed bone homeostasis and osteopenia in 14 SPG5 patients as a non-motor feature leading to a potential increased risk for bone fractures. T-Scores in CT bone density measurements were reduced, indicating osteopenia in SPG5 patients. Further, we analyzed various metabolites of bone homeostasis by ELISA in serum samples of these patients. We identified a lack of vitamin D 3 metabolites (Calcidiol and Calcitriol), an increase in Sclerostin as a bone formation/mineralization inhibiting factor, and a decrease in cross-linked N-telopeptide of type I collagen (NTX), a marker indicating reduced bone resorption. As statin treatment has been found to lower oxysterol levels, we evaluated its effect in samples of the STOP-SPG5 trial and found atorvastatin to normalize the increased sclerostin levels. In summary, our study identified osteopenia as a non-motor feature in SPG5 and suggests the need for vitamin D 3 substitution in SPG5 patients. Sclerostin may be considered a therapeutic target and biomarker in upcoming therapeutical trials in SPG5.

Ingo Kurth and Christian Hübner report the identification of loss-of-function mutations in FAM134B , which encodes a novel cis -Golgi protein, in hereditary sensory and autonomic neuropathy type II. Hereditary sensory and autonomic neuropathy type II (HSAN II) leads to severe mutilations because of impaired nociception and autonomic dysfunction. Here we show that loss-of-function mutations in FAM134B , encoding a newly identified cis -Golgi protein, cause HSAN II. Fam134b knockdown results in structural alterations of the cis -Golgi compartment and induces apoptosis in some primary dorsal root ganglion neurons. This implicates FAM134B as critical in long-term survival of nociceptive and autonomic ganglion neurons.

BackgroundCharcot-Marie-Tooth type 2 (CMT2) neuropathy is characterised by a vast clinical and genetic heterogeneity complicating its diagnosis and therapeutic intervention. Identification of molecular signatures that are common to multiple CMT2 subtypes can aid in developing therapeutic strategies and measuring disease outcomes.MethodsA proteomics-based approach was performed on lymphoblasts from CMT2 patients genetically diagnosed with different gene mutations to identify differentially regulated proteins. The candidate proteins were validated through real-time quantitative PCR and western blotting on lymphoblast samples of patients and controls, motor neurons differentiated from patient-derived induced pluripotent stem cells (iPSCs) and sciatic nerves of CMT2 mouse models.ResultsProteomic profiling of patient lymphoblasts resulted in the identification of profilin 2 (PFN2) and guanidinoacetate methyltransferase (GAMT) as commonly downregulated proteins in different genotypes compared with healthy controls. This decrease was also observed at the transcriptional level on screening 43 CMT2 patients and 22 controls, respectively. A progressive decrease in PFN2 expression with age was observed in patients, while in healthy controls its expression increased with age. Reduced PFN2 expression was also observed in motor neurons differentiated from CMT2 patient-derived iPSCs and sciatic nerves of CMT2 mice when compared with controls. However, no change in GAMT levels was observed in motor neurons and CMT2 mouse-derived sciatic nerves.ConclusionsWe unveil PFN2 and GAMT as molecular determinants of CMT2 with possible indications of the role of PFN2 in the pathogenesis and disease progression. This is the first study describing biomarkers that can boost the development of therapeutic strategies targeting a wider spectrum of CMT2 patients.

Background Recessive loss-of-function variations in HINT1 cause a peculiar subtype of Charcot-Marie-Tooth disease: neuromyotonia and axonal neuropathy (NMAN; OMIM[#137200]). With 25 causal variants identified worldwide, HINT1 mutations are among the most common causes of recessive neuropathy. The majority of patients are compound heterozygous or homozygous for a Slavic founder variant (c.110G>C, p.Arg37Pro) that has spread throughout Eurasia and America. Results In a cohort of 46 genetically unresolved Lithuanian patients with suspected inherited neuropathy, we identified eight families with HINT1 biallelic variations. Most patients displayed sensorimotor or motor-predominant axonal polyneuropathy and were homozygous for the p.Arg37Pro variant. However, in three families we identified a novel variant (c.299A>G, p.Glu100Gly). The same variant was also found in an American patient with distal hereditary motor neuropathy in compound heterozygous state (p.Arg37Pro/p.Glu100Gly). Haplotype analysis demonstrated a shared chromosomal region of 1.9 Mb between all p.Glu100Gly carriers, suggesting a founder effect. Functional characterization showed that the p.Glu100Gly variant renders a catalytically active enzyme, yet highly unstable in patient cells, thus supporting a loss-of-function mechanism. Conclusion Our findings broaden NMAN’s genetic epidemiology and have implications for the molecular diagnostics of inherited neuropathies in the Baltic region and beyond. Moreover, we provide mechanistic insights allowing patient stratification for future treatment strategies.

BackgroundMutations in the gene coding for protein O-mannosyl-transferase 2 (POMT2) are known to cause severe congenital muscular dystrophy, and recently, mutations in POMT2 have also been linked to a milder limb-girdle muscular dystrophy (LGMD) phenotype, named LGMD type 2N (LGMD2N). Only four cases have been reported so far.ClinicalTrials.gov ID: NCT02759302MethodsWe report 12 new cases of LGMD2N, aged 18–63 years. Muscle involvement was assessed by MRI, muscle strength testing and muscle biopsy analysis. Other clinical features were also recorded.ResultsPresenting symptoms were difficulties in walking, pain during exercise, delayed motor milestones and learning disabilities at school. All had some degree of cognitive impairment. Brain MRIs were abnormal in 3 of 10 patients, showing ventricular enlargement in one, periventricular hyperintensities in another and frontal atrophy of the left hemisphere in a third patient. Most affected muscle groups were hip and knee flexors and extensors on strength testing. On MRI, most affected muscles were hamstrings followed by paraspinal and gluteal muscles. The 12 patients in our cohort carried 11 alleles with known mutations, whereas 11 novel mutations accounted for the remaining 13 alleles.ConclusionWe describe the first cohort of patients with LGMD2N and show that unlike other LGMD types, all patients had cognitive impairment. Primary muscle involvement was found in hamstring, paraspinal and gluteal muscles on MRI, which correlated well with reduced muscle strength in hip and knee flexors and extensors. The study expands the mutational spectrum for LGMD2N, with the description of 11 novel POMT2 mutations in the association with LGMD2N.Clinical trial registrationNCT02759302.

Background Inclusion body myositis (IBM) is the most common acquired myopathy in adults over the age of 50 years, characterised by inflammatory and degenerative features that lead to progressive muscle weakness and physical disability for lack of effective therapies. The complex interplay between inflammatory and degenerative processes, occurring seemingly simultaneously, presents a challenge to systematically dissect disease pathology and discover novel therapeutic targets. Methods To identify proteomic IBM disease signatures and upstream regulators of disease processes in an unbiased manner, we performed high‐resolution iTRAQ‐labelled mass spectrometry on whole muscle lysates of 28 IBM patients and 28 control individuals. Validation experiments were carried out by conducting immunohistochemical (IHC) stainings on KDM5A and myogenin using control and IBM patient muscle tissue sections. Human myoblasts were used to study involvement of KDM5A, a selected candidate‐upstream regulator, in IBM pathomechanisms in vitro. Results A total of 627 significantly differentially expressed proteins were found in IBM patients compared to control individuals. The proteomics dataset strongly reflected inflammatory signatures, dysregulations in cellular energy metabolism and altered myogenesis in IBM muscle. Identification of upstream regulators of IBM pathology yielded KDM5A as the top activated and RB1 as the top inhibited upstream regulator. KDM5A, a histone demethylase involved in transcription regulation and (myogenic) differentiation, interacts with RB1 and interconnects core IBM disease signatures in patient muscle tissue. IHC stainings on muscle tissue showed increased presence of myogenin‐positive myonuclei (p < 0.0001). KDM5A levels were increased in these myogenin‐positive myonuclei in IBM patient muscle tissue compared to healthy controls (p < 0.0001). In vitro differentiation of myoblasts showed gradual KDM5A downregulation throughout myogenic differentiation, confirming presence in immature myoblasts and low levels in more mature myotubes. Proof‐of‐concept pharmacological inhibition of KDM5A with ryuvidine showed a significant effect on amyloid precursor protein (APP) abundance (p = 0.0003) and aggregation (p = 0.0132) in a conditional IBM‐mimicking inflammatory model. Conclusions This unbiased proteomics study reflects known core features of IBM pathomechanisms while simultaneously providing novel insights into the proteomic landscape of IBM, most notably dysregulation of metabolic pathways and failure of myogenesis. The identification and exploration of KDM5A as a potential upstream driver of disease pathology could interconnect failure of myogenic differentiation with (known) disease processes in IBM and provides a target for future study and therapy.

Some degree of exertional rhabdomyolysis (ER), striated muscle breakdown associated with strenuous exercise, is a well-known phenomenon associated with endurance sports. However in rare cases, severe and/or recurrent ER is a manifestation of an underlying condition, which puts patients at risk for significant morbidity and mortality. Selecting the patients that need a diagnostic work up of an acute rhabdomyolysis episode is an important task. Based on the diagnostic work up of three illustrative patients treated in our hospital, retrospectively using the 'RHABDO' screening tool, we discuss the clinical and biochemical clues that should trigger further investigation for an underlying condition. Finally, we describe the most common genetic causes of this clinical syndrome.

Autosomal recessive forms of Charcot-Marie-Tooth disease (ARCMT) are rare but severe disorders of the peripheral nervous system. Their molecular basis is poorly understood due to the extensive genetic and clinical heterogeneity, posing considerable challenges for patients, physicians, and researchers. We report on the genetic findings from a systematic study of a large collection of 174 independent ARCMT families. Initial sequencing of the three most common ARCMT genes ( ganglioside-induced differentiation protein 1 — GDAP1 , SH3 domain and tetratricopeptide repeats-containing protein 2 — SH3TC2 , histidine-triad nucleotide binding protein 1 — HINT1 ) identified pathogenic mutations in 41 patients. Subsequently, 87 selected nuclear families underwent single nucleotide polymorphism (SNP) genotyping and homozygosity mapping, followed by targeted screening of known ARCMT genes. This strategy provided molecular diagnosis to 22 % of the families. Altogether, our unbiased genetic approach identified pathogenic mutations in ten ARCMT genes in a total of 41.3 % patients. Apart from a newly described founder mutation in GDAP1 , the majority of variants constitute private molecular defects. Since the gene testing was independent of the clinical phenotype of the patients, we identified mutations in patients with unusual or additional clinical features, extending the phenotypic spectrum of the SH3TC2 gene. Our study provides an overview of the ARCMT genetic landscape and proposes guidelines for tackling the genetic heterogeneity of this group of hereditary neuropathies.