Vutrisiran: A Review in Polyneuropathy of Hereditary Transthyretin-Mediated Amyloidosis.

Silencing the transthyretin (TTR) gene is an effective strategy in the treatment of hereditary transthyretin-mediated (hATTR) amyloidosis. Vutrisiran (Amvuttra®), an RNA interference (RNAi) therapeutic targeting TTR mRNA, is approved in the USA and EU for the treatment of adults with polyneuropathy of hATTR amyloidosis. N-acetylgalactosamine conjugation and enhanced stabilisation chemistry are utilised to target vutrisiran to the liver and increase stability, respectively, allowing for subcutaneous administration once every 3 months. In a pivotal phase 3 study in patients with hATTR amyloidosis with polyneuropathy, subcutaneous vutrisiran 25 mg every 3 months significantly reduced neuropathy impairment versus external placebo. Vutrisiran was also associated with significant improvements in neuropathy-specific quality of life, gait speed, nutritional status and disability scores. Vutrisiran was generally well tolerated; the only common adverse events to occur at a greater incidence than with external placebo were pain in extremity and arthralgia. Vutrisiran reduces serum vitamin A levels and vitamin A supplementation is recommended. In conclusion, vutrisiran is an efficacious and generally well-tolerated alternative option for the treatment of polyneuropathy of hATTR amyloidosis, which has the potential advantage of infrequent subcutaneous dosage.

Hereditary transthyretin-mediated (hATTR) amyloidosis is a progressive and disabling disease caused by variants in the transthyretin (TTR) gene, which cause destabilisation and misfolding of the TTR protein. Deposition of misfolded TTR protein (amyloid) around nerves causes a range of neuropathic symptoms. Vutrisiran (Amvuttra^®) silences the TTR gene via RNA interference (RNAi). Vutrisiran, administered subcutaneously once every 3 months, is approved in the USA and EU for the treatment of polyneuropathy of hATTR amyloidosis in adults. In a phase 3 study in patients with hATTR amyloidosis with polyneuropathy, vutrisiran significantly reduced neuropathy impairment and improved other disease-related outcomes versus external placebo. Vutrisiran was generally well tolerated, with most adverse events being mild or moderate in severity. As vutrisiran decreases vitamin A levels, patients undergoing vutrisiran treatment should supplement with vitamin A. In conclusion, vutrisiran is an efficacious and generally well-tolerated alternative option for the treatment of polyneuropathy of hATTR amyloidosis, with a convenient dosage regimen.

Bilateral Striatal Necrosis with Polyneuropathy with a Novel SLC25A19 (Mitochondrial Thiamine Pyrophosphate Carrier OMIMI*606521) Mutation: Treatable Thiamine Metabolic Disorder-A Report of Two Indian Cases.

BACKGROUND: SLC25A19 gene mutations cause Amish congenital lethal microcephaly and bilateral striatal necrosis with polyneuropathy. We are reporting two cases of bilateral striatal necrosis with polyneuropathy due to SLC25A19 gene mutations. METHODS: A 36-month-old boy and a 5-year-old girl, unrelated, presented with recurrent episodes of flaccid paralysis and encephalopathy following nonspecific febrile illness. Examination showed dystonia and absent deep tendon reflexes. RESULTS: Nerve conduction studies showed an axonal polyneuropathy. Magnetic resonance imaging (MRI) of the brain in both cases showed signal changes in the basal ganglia. Next-generation sequencing revealed a novel homozygous missense variation c.910G>A (p.Glu304Lys) in the SLC25A19 gene in the boy and a homozygous mutation c.869T > A (p. Leu290Gln) in the SLC25A19 gene in the girl. Mutations were validated by Sanger sequencing, and carrier statuses of parents of both children were confirmed. Both children improved with thiamine supplementation. CONCLUSION: If any child presents with recurrent encephalopathy with flaccid paralysis, dystonia, and neuropathy, a diagnosis of bilateral striatal necrosis with polyneuropathy due to SLC25A19 mutations should be considered and thiamine should be initiated.

PDXK mutations cause polyneuropathy responsive to pyridoxal 5'-phosphate supplementation.

OBJECTIVE: To identify disease-causing variants in autosomal recessive axonal polyneuropathy with optic atrophy and provide targeted replacement therapy. METHODS: We performed genome-wide sequencing, homozygosity mapping, and segregation analysis for novel disease-causing gene discovery. We used circular dichroism to show secondary structure changes and isothermal titration calorimetry to investigate the impact of variants on adenosine triphosphate (ATP) binding. Pathogenicity was further supported by enzymatic assays and mass spectroscopy on recombinant protein, patient-derived fibroblasts, plasma, and erythrocytes. Response to supplementation was measured with clinical validated rating scales, electrophysiology, and biochemical quantification. RESULTS: We identified biallelic mutations in PDXK in 5 individuals from 2 unrelated families with primary axonal polyneuropathy and optic atrophy. The natural history of this disorder suggests that untreated, affected individuals become wheelchair-bound and blind. We identified conformational rearrangement in the mutant enzyme around the ATP-binding pocket. Low PDXK ATP binding resulted in decreased erythrocyte PDXK activity and low pyridoxal 5'-phosphate (PLP) concentrations. We rescued the clinical and biochemical profile with PLP supplementation in 1 family, improvement in power, pain, and fatigue contributing to patients regaining their ability to walk independently during the first year of PLP normalization. INTERPRETATION: We show that mutations in PDXK cause autosomal recessive axonal peripheral polyneuropathy leading to disease via reduced PDXK enzymatic activity and low PLP. We show that the biochemical profile can be rescued with PLP supplementation associated with clinical improvement. As B6 is a cofactor in diverse essential biological pathways, our findings may have direct implications for neuropathies of unknown etiology characterized by reduced PLP levels. ANN NEUROL 2019;86:225-240.

The shifting paradigm of Charcot-Marie-Tooth disease.

Molecular studies have created a paradigm shift in our perception of Charcot-Marie-Tooth disease (CMT). Indeed, CMT has evolved from the concept of a rather homogeneous hereditary disease exclusively involving peripheral nerves to the concept of a highly heterogeneous clinical and genetic syndrome mainly - but sometimes not exclusively - involving the peripheral nervous system. The phenotypic spectrum of CMT overlaps with other inherited neuropathies such as distal hereditary motor neuropathy (dHMN), hereditary sensory and autonomic neuropathy (HSAN), spinal muscular atrophy (SMA) subtypes, and the neuropathies of mitochondrial disorders. At a molecular level, mutations in one given gene may alternatively provoke CMT, HSAN, dHMN or SMA variants. Over the last years, there have been dramatic advances in deciphering the molecular basis for many CMT subtypes and more than 900 different mutations in more than 60 causative genes are now described. However, as 75% of CMT causative genes apparently remain unknown and as disease-specific therapies are not available, major advances are yet to come in the field of CMT.

Haploinsufficiency of Dmxl2, encoding a synaptic protein, causes infertility associated with a loss of GnRH neurons in mouse.

Characterization of the genetic defects causing gonadotropic deficiency has made a major contribution to elucidation of the fundamental role of Kisspeptins and Neurokinin B in puberty onset and reproduction. The absence of puberty may also reveal neurodevelopmental disorders caused by molecular defects in various cellular pathways. Investigations of these neurodevelopmental disorders may provide information about the neuronal processes controlling puberty onset and reproductive capacity. We describe here a new syndrome observed in three brothers, which involves gonadotropic axis deficiency, central hypothyroidism, peripheral demyelinating sensorimotor polyneuropathy, mental retardation, and profound hypoglycemia, progressing to nonautoimmune insulin-dependent diabetes mellitus. High-throughput sequencing revealed a homozygous in-frame deletion of 15 nucleotides in DMXL2 in all three affected patients. This homozygous deletion was associated with lower DMXL2 mRNA levels in the blood lymphocytes of the patients. DMXL2 encodes the synaptic protein rabconnectin-3α, which has been identified as a putative scaffold protein for Rab3-GAP and Rab3-GEP, two regulators of the GTPase Rab3a. We found that rabconnectin-3α was expressed in exocytosis vesicles in gonadotropin-releasing hormone (GnRH) axonal extremities in the median eminence of the hypothalamus. It was also specifically expressed in cells expressing luteinizing hormone (LH) and follicle-stimulating hormone (FSH) within the pituitary. The conditional heterozygous deletion of Dmxl2 from mouse neurons delayed puberty and resulted in very low fertility. This reproductive phenotype was associated with a lower number of GnRH neurons in the hypothalamus of adult mice. Finally, Dmxl2 knockdown in an insulin-secreting cell line showed that rabconnectin-3α controlled the constitutive and glucose-induced secretion of insulin. In conclusion, this study shows that low levels of DMXL2 expression cause a complex neurological phenotype, with abnormal glucose metabolism and gonadotropic axis deficiency due to a loss of GnRH neurons. Our findings identify rabconectin-3α as a key controller of neuronal and endocrine homeostatic processes.

Choice of oils for essential fat supplements can enhance production of abnormal metabolites in fat oxidation disorders.

Patients with mitochondrial long-chain fat oxidation deficiencies are usually treated with diets containing reduced fat and increased carbohydrate, at times via gastrostomy feeding. To ensure adequate intake of essential fatty acids, supplements are provided to their diets using commercially available oils. These oils contain large quantities of non-essential fats that are preferentially oxidized and produce disease-specific metabolites (acyl-CoA intermediates) due to the genetic defect. This study describes the concentrations of these intermediates as reflected by acylcarnitines as well as the % contribution from each of four fatty acids: palmitate, oleate, linoleate, and alpha-linolenate when incubated with fibroblasts from patients with VLCAD, LCHAD, and trifunctional protein (TFP) deficiencies. Palmitate and oleate produce the majority of disease-specific acylcarnitines with these defective cell lines (79-94%) whereas linoleate and linolenate produced less (6-21%). On average, the amount of acylcarnitines decreased with increasing unsaturation (C18:1>C18:2>C18:3:34%>11%>3%, respectively. This relationship may reflect the "gatekeeper" role of carnitine palmitoyltransferase I (CPT I). A diet comparison between Canola and a combination of Flax/Walnut oils revealed that the latter, containing the least amount of non-essential fats, reduced blood acylcarnitine levels by 33-36%. The etiology of the severe peripheral neuropathy of TFP deficiency may result from the unique metabolite, 3-keto-acyl-CoA, after conversion to a methylketone via spontaneous decarboxylation. Essential fatty acid supplementation with oils should consider these findings to decrease production of disease-specific acyl-CoA intermediates.

Mutations in the gene encoding peroxisomal sterol carrier protein X (SCPx) cause leukencephalopathy with dystonia and motor neuropathy.

In this report, we describe the first known patient with a deficiency of sterol carrier protein X (SCPx), a peroxisomal enzyme with thiolase activity, which is required for the breakdown of branched-chain fatty acids. The patient presented with torticollis and dystonic head tremor as well as slight cerebellar signs with intention tremor, nystagmus, hyposmia, and azoospermia. Magnetic resonance imaging showed leukencephalopathy and involvement of the thalamus and pons. Metabolite analyses of plasma revealed an accumulation of the branched-chain fatty acid pristanic acid, and abnormal bile alcohol glucuronides were excreted in urine. In cultured skin fibroblasts, the thiolytic activity of SCPx was deficient, and no SCPx protein could be detected by western blotting. Mutation analysis revealed a homozygous 1-nucleotide insertion, 545_546insA, leading to a frameshift and premature stop codon (I184fsX7).