Aminoglycoside drugs induce efficient read-through of CDKL5 nonsense mutations, slightly restoring its kinase activity

GM1 ganglioside protects against LPS-induced neuroinflammatory and oxidative responses by inhibiting the activation of Akt, TAK1 and NADPH oxidase in MG6 microglial cells

GM1 is a major brain ganglioside that exerts neurotrophic, neuroprotective and antineuroinflammatory effects. The aim of this study was to obtain insights into the antineuroinflammatory mechanisms of exogenous GM1 in lipopolysaccharide (LPS)-stimulated MG6 mouse transformed microglial cell line. First, we found that GM1 prevented the LPS-induced transformation of microglia into an amoeboid-like shape. GM1 treatment inhibited LPS-induced expression of inducible nitric oxide synthase, cyclooxygenase-2 (COX-2), and proinflammatory cytokines such as TNF-α, IL-1β and IL-6 in MG6 cells. In LPS-treated mice, GM1 also reduced striatal microglia activation and attenuated COX-2 expression. Subsequent mechanistic studies showed that GM1 suppressed LPS-induced nuclear translocation of nuclear factor κB (NF-κB) and activator protein-1 (AP-1), two critical transcription factors responsible for the production of proinflammatory mediators. GM1 exhibited antineuroinflammatory properties by suppressing Akt/NF-κB signaling and the activation of mitogen-activated protein kinases (MAPKs), including p38 MAPK, extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK). Furthermore, GM1 suppressed LPS-induced activation of transforming growth factor-β-activated kinase 1 (TAK1) and NADPH oxidase 2 (NOX2), upstream regulators of the IκBα/NF-κB and MAPK/AP-1 signaling pathways. GM1 also inhibited NOX-mediated reactive oxygen species (ROS) production and protected against LPS-induced MG6 cell death, suggesting an antioxidant role of GM1. In conclusion, GM1 exerts both antineuroinflammatory and antioxidative effects by inhibiting Akt, TAK1 and NOX2 activation.

Pharmaceuticals Promoting Premature Termination Codon Readthrough: Progress in Development

Around 11% of all known gene lesions causing human genetic diseases are nonsense mutations that introduce a premature stop codon (PTC) into the protein-coding gene sequence. Drug-induced PTC readthrough is a promising therapeutic strategy for treating hereditary diseases caused by nonsense mutations. To date, it has been found that more than 50 small-molecular compounds can promote PTC readthrough, known as translational readthrough-inducing drugs (TRIDs), and can be divided into two major categories: aminoglycosides and non-aminoglycosides. This review summarizes the pharmacodynamics and clinical application potential of the main TRIDs discovered so far, especially some newly discovered TRIDs in the past decade. The discovery of these TRIDs brings hope for treating nonsense mutations in various genetic diseases. Further research is still needed to deeply understand the mechanism of eukaryotic cell termination and drug-induced PTC readthrough so that patients can achieve the greatest benefit from the various TRID treatments.

Advanced genetic therapies for the treatment of Rett syndrome: state of the art and future perspectives

Loss and gain of functions mutations in the X-linked MECP2 (methyl-CpG-binding protein 2) gene are responsible for a set of generally severe neurological disorders that can affect both genders. In particular, Mecp2 deficiency is mainly associated with Rett syndrome (RTT) in girls, while duplication of the MECP2 gene leads, mainly in boys, to the MECP2 duplication syndrome (MDS). No cure is currently available for MECP2 related disorders. However, several studies have reported that by re-expressing the wild-type gene is possible to restore defective phenotypes of Mecp2 null animals. This proof of principle endorsed many laboratories to search for novel therapeutic strategies to cure RTT. Besides pharmacological approaches aimed at modulating MeCP2-downstream pathways, genetic targeting of MECP2 or its transcript have been largely proposed. Remarkably, two studies focused on augmentative gene therapy were recently approved for clinical trials. Both use molecular strategies to well-control gene dosage. Notably, the recent development of genome editing technologies has opened an alternative way to specifically target MECP2 without altering its physiological levels. Other attractive approaches exclusively applicable for nonsense mutations are the translational read-through (TR) and t-RNA suppressor therapy. Reactivation of the MECP2 locus on the silent X chromosome represents another valid choice for the disease. In this article, we intend to review the most recent genetic interventions for the treatment of RTT, describing the current state of the art, and the related advantages and concerns. We will also discuss the possible application of other advanced therapies, based on molecular delivery through nanoparticles, already proposed for other neurological disorders but still not tested in RTT.

Translation termination codons in protein synthesis and disease

Fidelity of protein synthesis, a process shaped by several mechanisms involving specialized ribosome regions and external factors, ensures the precise reading of sense as well as stop codons (UGA, UAG, UAA), which are usually localized at the 3' of mRNA and drive the release of the polypeptide chain. However, either natural (NTCs) or premature (PTCs) termination codons, the latter arising from nucleotide changes, can undergo a recoding process named ribosome or translational readthrough, which insert specific amino acids (NTCs) or subset(s) depending on the stop codon type (PTCs). This process is particularly relevant for nonsense mutations, a relatively frequent cause of genetic disorders, which impair gene expression at different levels by potentially leading to mRNA degradation and/or synthesis of truncated proteins. As a matter of fact, many efforts have been made to develop efficient and safe readthrough-inducing compounds, which have been challenged in several models of human disease to provide with a therapy. In this view, the dissection of the molecular determinants shaping the outcome of readthrough, namely nucleotide and protein contexts as well as their interplay and impact on protein structure/function, is crucial to identify responsive nonsense mutations resulting in functional full-length proteins. The interpretation of experimental and mechanistic findings is also important to define a possibly clear picture of potential readthrough-favorable features useful to achieve rescue profiles compatible with therapeutic thresholds typical of each targeted disorder, which is of primary importance for the potential translatability of readthrough into a personalized and mutation-specific, and thus patient-oriented, therapeutic strategy.

CDKL5 deficiency disorder: clinical features, diagnosis, and management

CDKL5 deficiency disorder (CDD) was first identified as a cause of human disease in 2004. Although initially considered a variant of Rett syndrome, CDD is now recognised as an independent disorder and classified as a developmental epileptic encephalopathy. It is characterised by early-onset (generally within the first 2 months of life) seizures that are usually refractory to polypharmacy. Development is severely impaired in patients with CDD, with only a quarter of girls and a smaller proportion of boys achieving independent walking; however, there is clinical variability, which is probably genetically determined. Gastrointestinal, sleep, and musculoskeletal problems are common in CDD, as in other developmental epileptic encephalopathies, but the prevalence of cerebral visual impairment appears higher in CDD. Clinicians diagnosing infants with CDD need to be familiar with the complexities of this disorder to provide appropriate counselling to the patients' families. Despite some benefit from ketogenic diets and vagal nerve stimulation, there has been little evidence that conventional antiseizure medications or their combinations are helpful in CDD, but further treatment trials are finally underway.

Anti-Inflammatory Effects of GM1 Ganglioside on Endotoxin-Induced Uveitis in Rats

Exogenous ganglioside GM1 has been reported to exert an immunomodulatory effect. We investigated the anti-inflammatory effect of GM1 ganglioside on endotoxin-induced uveitis (EIU) in rats and RAW 264.7 macrophages. Methods: EIU was induced in Lewis rats by administering a subcutaneous injection of lipopolysaccharide (LPS). GM1 was injected intraperitoneally for three consecutive days prior to the LPS injection. Twenty-four hours after the LPS injection, the integrity of the blood-aqueous barrier was evaluated by determining the protein concentration and number of infiltrating cells in the aqueous humor (AqH). Immunohistochemical and Western blot analyses of the iris-ciliary body (ICB) were performed to evaluate the effect of GM1 on the LPS-induced expression of cyclooxygenase-2 (COX-2) and intercellular adhesion molecule-1 (ICAM-1). The effect of GM1 on proinflammatory mediators and signaling cascades was examined in LPS-stimulated RAW 264.7 cells using Western blotting and immunofluorescence staining to further clarify the underlying anti-inflammatory mechanism. Results: GM1 significantly reduced the protein concentration and number of infiltrating cells in the AqH of rats with EIU. GM1 also decreased the LPS-induced expression of the ICAM-1 and COX-2 proteins in the ICB. In RAW 264.7 cells, GM1 inhibited the proinflammatory mediators induced by LPS, including inducible nitric oxide synthase (iNOS), COX-2, tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6), and this inhibitory effect was potentially mediated by suppressing transforming growth factor-β-activated kinase 1 (TAK1) and reactive oxygen species (ROS)-mediated activation of nuclear factor-κB (NF-κB) and mitogen-activated protein kinases (MAPKs). Conclusions: Based on this study, GM1 may be a potential anti-inflammatory agent for ocular inflammatory diseases.

Not Just Loss-of-Function Variations: Identification of a Hypermorphic Variant in a Patient With a CDKL5 Missense Substitution

Background and Objectives

CDKL5 deficiency disorder (CDD) is a neurodevelopmental encephalopathy characterized by early-onset epilepsy and impaired psychomotor development. Variations in the X-linked CDKL5 gene coding for a kinase cause CDD. Molecular genetics has proved that almost all pathogenic missense substitutions localize in the N-terminal catalytic domain, therefore underlining the importance for brain development and functioning of the kinase activity. CDKL5 also features a long C-terminal domain that acts as negative regulator of the enzymatic activity and modulates its subcellular distribution. CDD is generally attributed to loss-of-function variations, whereas the clinical consequences of increased CDKL5 activity remain uncertain. We have identified a female patient characterized by mild epilepsy and neurologic symptoms, harboring a novel c.2873C>G nucleotide substitution, leading to the missense variant p.(Thr958Arg). To increase our comprehension of genetic variants in CDKL5-associated neurologic disorders, we have characterized the molecular consequences of the identified substitution.

Methods

MRI and video EEG telemetry were used to describe brain activity and capture seizure. The Bayley III test was used to evaluate the patient development. Reverse transcriptase PCR was used to analyze whether the identified nucleotide variant affects messenger RNA stability and/or splicing. The X chromosome inactivation pattern was analyzed determining the DNA methylation status of the androgen receptor (AR) gene and by sequencing of expressed alleles. Western blotting was used to investigate whether the novel Thr958Arg substitution affects the stability and/or enzymatic activity of CDKL5. Immunofluorescence was used to define whether CDKL5 subcellular distribution is affected by the Thr958Arg substitution.

Results

Our data suggested that the proband tends toward a skewed X chromosome inactivation pattern in favor of the novel variant. The molecular investigation revealed that the p.(Thr958Arg) substitution leads to a significant increase in the autophosphorylation of both the TEY motif and residue Tyr¹⁷¹ of CDKL5, as well as in the phosphorylation of the target protein MAP1S, indicating an hyperactivation of CDKL5. This occurs without evidently affecting the kinase subcellular distribution.

Discussion

Our data provide a strong indication that the c.2873C>G nucleotide substitution represents an hypermorphic pathogenic variation of CDKL5, therefore highlighting the importance of a tight control of CDKL5 activity in the brain.

Nonsense Suppression Therapy: An Emerging Treatment for Hereditary Skin Diseases

Nonsense mutations cause the premature termination of protein translation via premature termination codons (PTCs), leading to the synthesis of incomplete functional proteins and causing large numbers of genetic disorders. The emergence of nonsense suppression therapy is considered to be an effective method for the treatment of hereditary diseases, but its application in hereditary skin diseases is relatively limited. This review summarizes the current research status of nonsense suppression therapy for hereditary skin diseases, and discusses the potential opportunities and challenges of applying new technologies related to nonsense suppression therapy to dermatology. Further research is needed into the possible use of nonsense suppression therapy as a strategy for the safer and specific treatment of hereditary skin diseases.

Current neurologic treatment and emerging therapies in CDKL5 deficiency disorder

Background

CDKL5 deficiency disorder (CDD) is associated with refractory infantile onset epilepsy, global developmental delay, and variable features that include sleep, behavioral disturbances, and movement disorders. Current treatment is primarily symptom-based and informed by experience in caring for this population.

Methods

We describe medication and non-medication approaches to treatment of epilepsy and additional key neurologic symptoms (sleep disturbances, behavioral issues, movement disorders, and swallowing dysfunction) in a cohort of 177 individuals meeting criteria for CDD, 154 evaluated at 4 CDKL5 Centers of Excellence in the USA and 40 identified through the NIH Natural History Study of Rett and Related Disorders.

Results

The four most frequently prescribed anti-seizure medications were broad spectrum, prescribed in over 50% of individuals. While the goal was not to ascertain efficacy, we obtained data from 86 individuals regarding response to treatment, with 2-week response achieved in 14–48% and sustained 3-month response in 5–36%, of those with known response. Additional treatments for seizures included cannabis derivatives, tried in over one-third of individuals, and clinical trial medications. In combination with pharmacological treatment, 50% of individuals were treated with ketogenic diet for attempted seizure control. Surgical approaches included vagus nerve stimulators, functional hemispherectomy, and corpus callosotomy, but numbers were too limited to assess response. Nearly one-third of individuals received pharmacologic treatment for sleep disturbances, 13% for behavioral dysregulation and movement disorders, and 43% had gastrostomy tubes.

Conclusions

Treatment for neurologic features of CDD is currently symptom-based and empiric rather than CDD-specific, though clinical trials for CDD are emerging. Epilepsy in this population is highly refractory, and no specific anti-seizure medication was associated with improved seizure control. Ketogenic diet is commonly used in patients with CDD. While behavioral interventions are commonly instituted, information on the use of medications for sleep, behavioral management, and movement disorders is sparse and would benefit from further characterization and optimization of treatment approaches. The heterogeneity in treatment approaches highlights the need for systematic review and guidelines for CDD. Additional disease-specific and disease-modifying treatments are in development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11689-021-09384-z.

Ataluren-Promising Therapeutic Premature Termination Codon Readthrough Frontrunner

Around 12% of hereditary disease-causing mutations are in-frame nonsense mutations. The expression of genes containing nonsense mutations potentially leads to the production of truncated proteins with residual or virtually no function. However, the translation of transcripts containing premature stop codons resulting in full-length protein expression can be achieved using readthrough agents. Among them, only ataluren was approved in several countries to treat nonsense mutation Duchenne muscular dystrophy (DMD) patients. This review summarizes ataluren’s journey from its identification, via first in vitro activity experiments, to clinical trials in DMD, cystic fibrosis, and aniridia. Additionally, data on its pharmacokinetics and mechanism of action are presented. The range of diseases with underlying nonsense mutations is described for which ataluren therapy seems to be promising. What is more, experiments in which ataluren did not show its readthrough activity are also included, and reasons for their failures are discussed.

Nonsense suppression therapies in human genetic diseases

About 11% of all human disease-associated gene lesions are nonsense mutations, resulting in the introduction of an in-frame premature translation-termination codon (PTC) into the protein-coding gene sequence. When translated, PTC-containing mRNAs originate truncated and often dysfunctional proteins that might be non-functional or have gain-of-function or dominant-negative effects. Therapeutic strategies aimed at suppressing PTCs to restore deficient protein function-the so-called nonsense suppression (or PTC readthrough) therapies-have the potential to provide a therapeutic benefit for many patients and in a broad range of genetic disorders, including cancer. These therapeutic approaches comprise the use of translational readthrough-inducing compounds that make the translational machinery recode an in-frame PTC into a sense codon. However, most of the mRNAs carrying a PTC can be rapidly degraded by the surveillance mechanism of nonsense-mediated decay (NMD), thus decreasing the levels of PTC-containing mRNAs in the cell and their availability for PTC readthrough. Accordingly, the use of NMD inhibitors, or readthrough-compound potentiators, may enhance the efficiency of PTC suppression. Here, we review the mechanisms of PTC readthrough and their regulation, as well as the recent advances in the development of novel approaches for PTC suppression, and their role in personalized medicine.

Nonsense Suppression Therapy: New Hypothesis for the Treatment of Inherited Bone Marrow Failure Syndromes

Inherited bone marrow failure syndromes (IBMFS) are a group of cancer-prone genetic diseases characterized by hypocellular bone marrow with impairment in one or more hematopoietic lineages. The pathogenesis of IBMFS involves mutations in several genes which encode for proteins involved in DNA repair, telomere biology and ribosome biogenesis. The classical IBMFS include Shwachman–Diamond syndrome (SDS), Diamond–Blackfan anemia (DBA), Fanconi anemia (FA), dyskeratosis congenita (DC), and severe congenital neutropenia (SCN). IBMFS are associated with high risk of myelodysplastic syndrome (MDS), acute myeloid leukemia (AML), and solid tumors. Unfortunately, no specific pharmacological therapies have been highly effective for IBMFS. Hematopoietic stem cell transplantation provides a cure for aplastic or myeloid neoplastic complications. However, it does not affect the risk of solid tumors. Since approximately 28% of FA, 24% of SCN, 21% of DBA, 20% of SDS, and 17% of DC patients harbor nonsense mutations in the respective IBMFS-related genes, we discuss the use of the nonsense suppression therapy in these diseases. We recently described the beneficial effect of ataluren, a nonsense suppressor drug, in SDS bone marrow hematopoietic cells ex vivo. A similar approach could be therefore designed for treating other IBMFS. In this review we explain in detail the new generation of nonsense suppressor molecules and their mechanistic roles. Furthermore, we will discuss strengths and limitations of these molecules which are emerging from preclinical and clinical studies. Finally we discuss the state-of-the-art of preclinical and clinical therapeutic studies carried out for IBMFS.

Cyclin-Dependent Kinase-Like 5 (CDKL5): Possible Cellular Signalling Targets and Involvement in CDKL5 Deficiency Disorder

Cyclin-dependent kinase-like 5 (CDKL5, also known as STK9) is a serine/threonine protein kinase originally identified in 1998 during a transcriptional mapping project of the human X chromosome. Thereafter, a mutation in CDKL5 was reported in individuals with the atypical Rett syndrome, a neurodevelopmental disorder, suggesting that CDKL5 plays an important regulatory role in neuronal function. The disease associated with CDKL5 mutation has recently been recognised as CDKL5 deficiency disorder (CDD) and has been distinguished from the Rett syndrome owing to its symptomatic manifestation. Because CDKL5 mutations identified in patients with CDD cause enzymatic loss of function, CDKL5 catalytic activity is likely strongly associated with the disease. Consequently, the exploration of CDKL5 substrate characteristics and regulatory mechanisms of its catalytic activity are important for identifying therapeutic target molecules and developing new treatment. In this review, we summarise recent findings on the phosphorylation of CDKL5 substrates and the mechanisms of CDKL5 phosphorylation and dephosphorylation. We also discuss the relationship between changes in the phosphorylation signalling pathways and the Cdkl5 knockout mouse phenotype and consider future prospects for the treatment of mental and neurological disease associated with CDKL5 mutations.

The oligosaccharide portion of ganglioside GM1 regulates mitochondrial function in neuroblastoma cells

The crucial role of ganglioside GM1 in the regulation of neural homeostasis has been assessed by several studies. Recently we shed new light on the molecular basis underlying GM1 effects demonstrating that GM1 oligosaccharide directly binds TrkA receptor and triggers MAPK pathway activation leading to neuronal differentiation and protection. Following its exogenous administration, proteomic analysis revealed an increased expression of proteins involved in several biochemical mechanisms, including mitochondrial bioenergetics. Based on these data, we investigated the possible effect of GM1 oligosaccharide administration on mitochondrial function. We show that wild-type Neuro2a cells exposed to GM1 oligosaccharide displayed an increased mitochondrial density and an enhanced mitochondrial activity together with reduced reactive oxygen species levels. Interestingly, using a Neuro2a model of mitochondrial dysfunction, we found an increased mitochondrial oxygen consumption rate as well as increased complex I and II activities upon GM1 oligosaccharide administration. Taken together, our data identify GM1 oligosaccharide as a mitochondrial regulator that by acting at the plasma membrane level triggers biochemical signaling pathway inducing mitochondriogenesis and increasing mitochondrial activity. Although further studies are necessary, the capability to enhance the function of impaired mitochondria points to the therapeutic potential of the GM1 oligosaccharide for the treatment of pathologies where these organelles are compromised, including Parkinson's disease.

Gangliosides in the differentiation process of primary neurons: the specific role of GM1-oligosaccharide

It has been recently reported by our group that GM1-oligosaccharide added to neuroblastoma cells or administered to mouse experimental model mimics the neurotrophic and neuroprotective properties of GM1 ganglioside. In addition to this, differently from GM1, GM1-oligosaccharide is not taken up by the cells, remaining solubilized into the extracellular environment interacting with cell surface proteins. Those characteristics make GM1-oligosaccharide a good tool to study the properties of the endogenous GM1, avoiding to interfere with the ganglioside natural metabolic pathway. In this study, we show that GM1-oligosaccharide administered to mice cerebellar granule neurons by interacting with cell surface induces TrkA-MAP kinase pathway activation enhancing neuron clustering, arborization and networking. Accordingly, in the presence of GM1-oligosaccharide, neurons show a higher phosphorylation rate of FAK and Src proteins, the intracellular key regulators of neuronal motility. Moreover, treated cells express increased level of specific neuronal markers, suggesting an advanced stage of maturation compared to controls. In parallel, we found that in the presence of GM1-oligosaccharide, neurons accelerate the expression of complex gangliosides and reduce the level of the simplest ones, displaying the typical ganglioside pattern of mature neurons. Our data confirms the specific role of GM1 in neuronal differentiation and maturation, determined by its oligosaccharide portion. GM1-oligosacchairide interaction with cell surface receptors triggers the activation of intracellular biochemical pathways responsible for neuronal migration, dendrites emission and axon growth.

Nonsense suppression induced readthrough of a novel PAX6 mutation in patient-derived cells of congenital aniridia

Background

Congenital aniridia is a severe ocular abnormality characterized by incomplete formation of the iris and many other ocular complications. Most cases are caused by the paired box 6 (PAX6) gene mutations generating premature termination codons (PTCs).

Methods

Ophthalmic examination was performed on a Chinese pedigree with congenital aniridia. The mutation was identified by targeted next‐generation sequencing. Nonsense suppression therapy was applied on patient‐derived lymphocytes. The PAX6 expression was assayed by real‐time polymerase chain reaction and Western blot.

Results

Complete aniridia was complicated with horizontal nystagmus, contract, foveal hypoplasia, and microphthalmia. A novel heterozygous c.702_703delinsAT (p.Tyr234*) mutation was found in exon 9 of PAX6, generating a PTC at the homeodomain. There were about 50% reductions of both full‐length PAX6 protein and PAX6 mRNA in patient‐derived lymphocytes, indicating haploinsufficiency due to nonsense‐mediated mRNA decay. Ataluren (PTC124) and geneticin (G418) could induce about 30%–40% translational readthrough. Nonsense suppression therapy restored PAX6 protein to about 65%–70% of unaffected family controls.

Conclusion

Our data expanded the genetic and phenotypic variations of congenital aniridia, and showed the therapeutic effect of nonsense suppression on this disease using patient‐derived cells.

Translational readthrough of GLA nonsense mutations suggests dominant-negative effects exerted by the interaction of wild-type and missense variants

Nonsense mutations are relatively frequent in the rare X-linked lysosomal α-galactosidase A (α-Gal) deficiency (Fabry disease; FD), but have been poorly investigated. Here, we evaluated the responsiveness of a wide panel (n = 14) of GLA premature termination codons (PTCs) to the RNA-based approach of drug-induced readthrough through expression of recombinant α-Gal (rGal) nonsense and missense variants.We identified four high-responders to the readthrough-inducing aminoglycoside G418 in terms of full-length protein (C56X/W209X, ≥10% of wild-type rGal) and/or activity (Q119X/W209X/Q321X, ~5-7%), resulting in normal (Q119X/Q321X) or reduced (C56X, 0.27 ± 0.11; W209X, 0.35 ± 0.1) specific activity.To provide mechanistic insights we investigated the predicted amino acid substitutions mediated by readthrough (W209C/R, C56W/R), which resulted in correct lysosomal localization and appreciable protein/activity levels for the W209C/R variants. Differently, the C56W/R variants, albeit appreciably produced and localized into lysosomes, were inactive, thus indicating detrimental effects of substitutions at this position.Noticeably, when co-expressed with the functional W209C or W209R variants, the wild-type rGal displayed a reduced specific activity (0.5 ± 0.2 and 0.6 ± 0.2, respectively) that, considering the dimeric features of the α-Gal enzyme, suggested dominant-negative effects of missense variants through their interaction with the wild-type.Overall, we provide a novel mechanism through which amino acids inserted during readthrough might impact on the functional protein output. Our findings may also have implications for the interpretation of pathological phenotypes in heterozygous FD females, and for other human disorders involving dimeric or oligomeric proteins.

Splicing Mutations Impairing CDKL5 Expression and Activity Can be Efficiently Rescued by U1snRNA-Based Therapy

Mutations in the CDKL5 gene lead to an incurable rare neurological condition characterized by the onset of seizures in the first weeks of life and severe intellectual disability. Replacement gene or protein therapies could represent intriguing options, however, their application may be inhibited by the recent demonstration that CDKL5 is dosage sensitive. Conversely, correction approaches acting on pre-mRNA splicing would preserve CDKL5 physiological regulation. Since ~15% of CDKL5 pathogenic mutations are candidates to affect splicing, we evaluated the capability of variants of the spliceosomal U1 small nuclear RNA (U1snRNA) to correct mutations affecting +1 and +5 nucleotides at the 5′ donor splice site and predicted to cause exon skipping. Our results show that CDKL5 minigene variants expressed in mammalian cells are a valid approach to assess CDKL5 splicing pattern. The expression of engineered U1snRNA effectively rescued mutations at +5 but not at the +1 nucleotides. Importantly, we proved that U1snRNA-mediated splicing correction fully restores CDKL5 protein synthesis, subcellular distribution and kinase activity. Eventually, by correcting aberrant splicing of an exogenously expressed splicing-competent CDKL5 transgene, we provided insights on the morphological rescue of CDKL5 null neurons, reporting the first proof-of-concept of the therapeutic value of U1snRNA-mediated CDKL5 splicing correction.