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Dravet syndrome is a genetic encephalopathy characterized by severe epilepsy combined with motor, cognitive, and behavioral abnormalities. Current antiepileptic drugs achieve only partial control of seizures and provide little benefit on the patient's neurological development. In >80% of cases, the disease is caused by haploinsufficiency of the SCN1A gene, which encodes the alpha subunit of the Nav1.1 voltage-gated sodium channel. Novel therapies aim to restore SCN1A expression in order to address all disease manifestations. We provide evidence that a high-capacity adenoviral vector harboring the 6-kb SCN1A cDNA is feasible and able to express functional Nav1.1 in neurons. In vivo , the best biodistribution was observed after intracerebral injection in basal ganglia, cerebellum, and prefrontal cortex. SCN1A A1783V knockin mice received the vector at 5 weeks of age, when most neurological alterations were present. Animals were protected from sudden death, and the epileptic phenotype was attenuated. Improvement of motor performance and interaction with the environment was observed. In contrast, hyperactivity persisted, and the impact on cognitive tests was variable (success in novel object recognition and failure in Morris water maze tests). These results provide proof of concept for gene supplementation in Dravet syndrome and indicate new directions for improvement., Competing Interests: The authors declare no competing interests., (© 2021 The Author(s).)

Cerebrotendinous xanthomatosis (CTX) is an autosomal recessive disease caused by mutations in the CYP27A1 gene, encoding the sterol 27-hydroxylase. Disruption of the bile acid biosynthesis pathway and accumulation of toxic precursors such as cholestanol cause chronic diarrhea, bilateral juvenile cataracts, tissue deposition of cholestanol and cholesterol (xanthomas), and progressive motor/neuropsychiatric alterations. We have evaluated the therapeutic potential of adeno-associated virus (AAV) vectors expressing CYP27A1 in a CTX mouse model. We found that a vector equipped with a strong liver-specific promoter (albumin enhancer fused with the α1 anti-trypsin promoter) is well tolerated and shows therapeutic effect at relatively low doses (1.5 × 10 12 viral genomes [vg]/kg), when less than 20% of hepatocytes overexpress the transgene. This vector restored bile acid metabolism and normalized the concentration of most bile acids in plasma. By contrast, standard treatment (oral chenodeoxycholic acid [CDCA]), while reducing cholestanol, did not normalize bile acid composition in plasma and resulted in supra-physiological levels of CDCA and its derivatives. At the transcriptional level, only the vector was able to avoid the induction of xenobiotic-induced pathways in mouse liver. In conclusion, the overexpression of CYP27A1 in a fraction of hepatocytes using AAV vectors is well tolerated and provides full metabolic restoration in Cyp27a1 -/- mice. These features make gene therapy a feasible option for the etiological treatment of CTX patients., Competing Interests: The authors declare no competing interests., (© 2021 The Author(s).)

The adaptation of adenoviruses as gene delivery tools has resulted in the development of high-capacity adenoviral vectors (HC-AdVs), also known, helper-dependent or "gutless". Compared with earlier generations (E1/E3-deleted vectors), HC-AdVs retain relevant features such as genetic stability, remarkable efficacy of in vivo transduction, and production at high titers. More importantly, the lack of viral coding sequences in the genomes of HC-AdVs extends the cloning capacity up to 37 Kb, and allows long-term episomal persistence of transgenes in non-dividing cells. These properties open a wide repertoire of therapeutic opportunities in the fields of gene supplementation and gene correction, which have been explored at the preclinical level over the past two decades. During this time, production methods have been optimized to obtain the yield, purity, and reliability required for clinical implementation. Better understanding of inflammatory responses and the implementation of methods to control them have increased the safety of these vectors. We will review the most significant achievements that are turning an interesting research tool into a sound vector platform, which could contribute to overcome current limitations in the gene therapy field., Competing Interests: The authors declare no conflict of interest.

Dravet Syndrome (DS) is an encephalopathy with epilepsy associated with multiple neuropsychiatric comorbidities. In up to 90% of cases, it is caused by functional happloinsufficiency of the SCN1A gene, which encodes the alpha subunit of a voltage-dependent sodium channel (Nav1.1). Preclinical development of new targeted therapies requires accessible animal models which recapitulate the disease at the genetic and clinical levels. Here we describe that a C57BL/6 J knock-in mouse strain carrying a heterozygous, clinically relevant SCN1A mutation (A1783V) presents a full spectrum of DS manifestations. This includes 70% mortality rate during the first 8 weeks of age, reduced threshold for heat-induced seizures (4.7 °C lower compared with control littermates), cognitive impairment, motor disturbances, anxiety, hyperactive behavior and defects in the interaction with the environment. In contrast, sociability was relatively preserved. Electrophysiological studies showed spontaneous interictal epileptiform discharges, which increased in a temperature-dependent manner. Seizures were multifocal, with different origins within and across individuals. They showed intra/inter-hemispheric propagation and often resulted in generalized tonic-clonic seizures. 18 F-labelled flourodeoxyglucose positron emission tomography (FDG-PET) revealed a global increase in glucose uptake in the brain of Scn1a WT/A1783V mice. We conclude that the Scn1a WT/A1783V model is a robust research platform for the evaluation of new therapies against DS.