IKAP/Elp1 is required in vivo for neurogenesis and neuronal survival, but not for neural crest migration

Genetic modeling of ELP1-associated Sonic hedgehog medulloblastoma identifies MDM2 as a selective therapeutic target

Germline loss-of-function (LOF) variants in Elongator acetyltransferase complex subunit 1 (ELP1) are the most prevalent predisposing genetic events in childhood medulloblastoma (MB), accounting for ∼30% of the Sonic hedgehog (SHH) 3 subtype. The mechanism(s) by which germline ELP1 deficiency provokes SHH-MB pathogenesis remain unknown. Genetically engineered mice mimicking heterozygous Elp1 LOF (Elp1HET) seen in affected germline carriers exhibit hallmark features of premalignancy in cerebellar granule neuron progenitors (GNPs), including increased DNA replication stress, genomic instability, accelerated cell cycle, and stalled differentiation. Orthotopic transplantation of Elp1HET GNPs harboring somatic Ptch1 inactivation yields SHH-MB-like tumors with compromised p53 signaling, providing a plausible explanation for the exclusivity of ELP1-associated MBs in the SHH-3 subtype. Preclinical treatment of ELP1-mutant patient-derived xenografts with an FDA-approved MDM2 inhibitor reactivates p53-dependent apoptosis and extends survival. Our findings functionally substantiate the role of ELP1 deficiency in SHH-MB predisposition and nominate therapeutics targeting MDM2 as a rational treatment option.

Elp1 function in placode-derived neurons is critical for proper trigeminal ganglion development

BACKGROUND

The trigeminal nerve is the largest cranial nerve and functions in somatosensation. Cell bodies of this nerve are positioned in the trigeminal ganglion, which arises from the coalescence of neural crest and placode cells. While this dual cellular origin has been known for decades, the molecular mechanisms controlling trigeminal ganglion development remain obscure. We performed RNA sequencing on the forming chick trigeminal ganglion and identified Elongator acetyltransferase complex subunit 1 (Elp1) for further study. Mutations in ELP1 cause familial dysautonomia (FD), a fatal disorder characterized by the presence of smaller trigeminal nerves and sensory deficits. While Elp1 has established roles in neurogenesis, its function in placode cells during trigeminal gangliogenesis has not been investigated.

RESULTS

To this end, we used morpholinos to deplete Elp1 from chick trigeminal placode cells. Elp1 knockdown decreased trigeminal ganglion size and led to aberrant innervation of the eye by placode-derived neurons. Trigeminal nerve branches also appeared to exhibit reduced axon outgrowth to target tissues.

CONCLUSIONS

These findings reveal a new role for Elp1 in placode-derived neurons during chick trigeminal ganglion development. These results have potential high significance to provide new insights into trigeminal ganglion development and the etiology of FD.

ELP1, the Gene Mutated in Familial Dysautonomia, Is Required for Normal Enteric Nervous System Development and Maintenance and for Gut Epithelium Homeostasis

Familial dysautonomia (FD) is a rare sensory and autonomic neuropathy that results from a mutation in the ELP1 gene. Virtually all patients report gastrointestinal (GI) dysfunction and we have recently shown that FD patients have a dysbiotic gut microbiome and altered metabolome. These findings were recapitulated in an FD mouse model and moreover, the FD mice had reduced intestinal motility, as did patients. To understand the cellular basis for impaired GI function in FD, the enteric nervous system (ENS; both female and male mice) from FD mouse models was analyzed during embryonic development and adulthood. We show here that not only is Elp1 required for the normal formation of the ENS, but it is also required in adulthood for the regulation of both neuronal and non-neuronal cells and for target innervation in both the mucosa and in intestinal smooth muscle. In particular, CGRP innervation was significantly reduced as was the number of dopaminergic neurons. Examination of an FD patient's gastric biopsy also revealed reduced and disoriented axons in the mucosa. Finally, using an FD mouse model in which Elp1 was deleted exclusively from neurons, we found significant changes to the colon epithelium including reduced E-cadherin expression, perturbed mucus layer organization, and infiltration of bacteria into the mucosa. The fact that deletion of Elp1 exclusively in neurons is sufficient to alter the intestinal epithelium and perturb the intestinal epithelial barrier highlights a critical role for neurons in regulating GI epithelium homeostasis.

Elp1 function in placode-derived neurons is critical for proper trigeminal ganglion development

Background

The trigeminal nerve is the largest cranial nerve and functions in somatosensation. Cell bodies of this nerve are positioned in the trigeminal ganglion, which arises from the coalescence of neural crest and placode cells. While this dual cellular origin has been known for decades, the molecular mechanisms controlling trigeminal ganglion development remain obscure. We performed RNAsequencing on the forming chick trigeminal ganglion and identified Elongator acetyltransferase complex subunit 1 ( Elp1 ) for further study. Mutations in ELP1 cause familial dysautonomia (FD), a fatal disorder characterized by the presence of smaller trigeminal nerves and sensory deficits. While Elp1 has established roles in neurogenesis, its functions in placode cells during trigeminal gangliogenesis have not been investigated.

Results

To this end, we used morpholinos to deplete Elp1 from chick trigeminal placode cells. Elp1 knockdown decreased trigeminal ganglion size and led to aberrant innervation of the eye by placode-derived neurons. Trigeminal nerve branches exhibited fewer axons, and abnormal interactions between placode-derived neurons and neural crest cells were observed.

Conclusions

These findings reveal a new role for Elp1 in chick placode-derived neurons during trigeminal ganglion development. These results have potential high significance to provide new insights into trigeminal ganglion development and the etiology of FD.

Bullet points

Elp1 is expressed in undifferentiated neural crest cells and placode-derived neurons contributing to the trigeminal ganglion.Elp1 knockdown in trigeminal placode cells reduces trigeminal ganglion size.Elp1 depletion from trigeminal placode cells leads to aberrant target tissue innervation and disrupts proper neural crest-placodal neuron interactions in the trigeminal ganglion.

Grant sponsor and number

NIH R01DE024217 and NIH R03HD108480.

Genetic predisposition to ocular surface disorders and opportunities for gene-based therapies

The ocular surface, comprised of the corneal and conjunctival epithelium, innervation system, immune components, and tear-film apparatus, plays a key role in ocular integrity as well as comfort and vision. Gene defects may result in congenital ocular or systemic disorders with prominent ocular surface involvement. Examples include epithelial corneal dystrophies, aniridia, ectrodactyly-ectodermal dysplasia-clefting (EEC) syndrome, xeroderma pigmentosum (XP), and hereditary sensory and autonomic neuropathy. In addition, genetic factors may interact with environmental risk factors in the development of several multifactorial ocular surface disorders (OSDs) such as autoimmune disorders, allergies, neoplasms, and dry eye disease. Advanced gene-based technologies have already been introduced in disease modelling and proof-of-concept gene therapies for monogenic OSDs. For instance, patient-derived induced pluripotent stem cells have been used for modelling aniridia-associated keratopathy (AAK), XP, and EEC syndrome. Moreover, CRISPR/Cas9 genome editing has been used for disease modelling and/or gene therapy for AAK and Meesmann's epithelial corneal dystrophy. A better understanding of the role of genetic factors in OSDs may be helpful in designing personalized disease models and treatment approaches. Gene-based approaches in monogenic OSDs and genetic predisposition to multifactorial OSDs such as immune-mediated disorders and neoplasms with known or possible genetic risk factors has been seldom reviewed. In this narrative review, we discuss the role of genetic factors in monogenic and multifactorial OSDs and potential opportunities for gene therapy.

Development of an oral treatment that rescues gait ataxia and retinal degeneration in a phenotypic mouse model of familial dysautonomia

Familial dysautonomia (FD) is a rare neurodegenerative disease caused by a splicing mutation in elongator acetyltransferase complex subunit 1 (ELP1). This mutation leads to the skipping of exon 20 and a tissue-specific reduction of ELP1, mainly in the central and peripheral nervous systems. FD is a complex neurological disorder accompanied by severe gait ataxia and retinal degeneration. There is currently no effective treatment to restore ELP1 production in individuals with FD, and the disease is ultimately fatal. After identifying kinetin as a small molecule able to correct the ELP1 splicing defect, we worked on its optimization to generate novel splicing modulator compounds (SMCs) that can be used in individuals with FD. Here, we optimize the potency, efficacy, and bio-distribution of second-generation kinetin derivatives to develop an oral treatment for FD that can efficiently pass the blood-brain barrier and correct the ELP1 splicing defect in the nervous system. We demonstrate that the novel compound PTC258 efficiently restores correct ELP1 splicing in mouse tissues, including brain, and most importantly, prevents the progressive neuronal degeneration that is characteristic of FD. Postnatal oral administration of PTC258 to the phenotypic mouse model TgFD9;Elp1Δ20/flox increases full-length ELP1 transcript in a dose-dependent manner and leads to a 2-fold increase in functional ELP1 in the brain. Remarkably, PTC258 treatment improves survival, gait ataxia, and retinal degeneration in the phenotypic FD mice. Our findings highlight the great therapeutic potential of this novel class of small molecules as an oral treatment for FD.

Norepinephrine transporter defects lead to sympathetic hyperactivity in Familial Dysautonomia models

Familial dysautonomia (FD), a rare neurodevelopmental and neurodegenerative disorder affects the sympathetic and sensory nervous system. Although almost all patients harbor a mutation in ELP1, it remains unresolved exactly how function of sympathetic neurons (symNs) is affected; knowledge critical for understanding debilitating disease hallmarks, including cardiovascular instability or dysautonomic crises, that result from dysregulated sympathetic activity. Here, we employ the human pluripotent stem cell (hPSC) system to understand symN disease mechanisms and test candidate drugs. FD symNs are intrinsically hyperactive in vitro, in cardiomyocyte co-cultures, and in animal models. We report reduced norepinephrine transporter expression, decreased intracellular norepinephrine (NE), decreased NE re-uptake, and excessive extracellular NE in FD symNs. SymN hyperactivity is not a direct ELP1 mutation result, but may connect to NET via RAB proteins. We found that candidate drugs lowered hyperactivity independent of ELP1 modulation. Our findings may have implications for other symN disorders and may allow future drug testing and discovery.

Elongator stabilizes microtubules to control central spindle asymmetry and polarized trafficking of cell fate determinants

Asymmetric cell division gives rise to two daughter cells that inherit different determinants, thereby acquiring different fates. Polarized trafficking of endosomes containing fate determinants recently emerged as an evolutionarily conserved feature of asymmetric cell division to enhance the robustness of asymmetric cell fate determination in flies, fish and mammals. In particular, polarized sorting of signalling endosomes by an asymmetric central spindle contributes to asymmetric cell division in Drosophila melanogaster. However, how central spindle asymmetry arises remains elusive. Here we identify a moonlighting function of the Elongator complex-an established protein acetylase and tRNA methylase involved in the fidelity of protein translation-as a key factor for central spindle asymmetry. Elongator controls spindle asymmetry by stabilizing microtubules differentially on the anterior side of the central spindle. Accordingly, lowering the activity of Elongator on the anterior side using nanobodies mistargets endosomes to the wrong cell. Molecularly, Elongator regulates microtubule dynamics independently of its acetylation and methylation enzymatic activities. Instead, Elongator directly binds to microtubules and increases their polymerization speed while decreasing their catastrophe frequency. Our data establish a non-canonical role of Elongator at the core of cytoskeleton polarity and asymmetric signalling.

Selective retinal ganglion cell loss and optic neuropathy in a humanized mouse model of familial dysautonomia

Familial dysautonomia (FD) is an autosomal recessive neurodegenerative disease caused by a splicing mutation in the gene encoding Elongator complex protein 1 (ELP1, also known as IKBKAP). This mutation results in tissue-specific skipping of exon 20 with a corresponding reduction of ELP1 protein, predominantly in the central and peripheral nervous system. Although FD patients have a complex neurological phenotype caused by continuous depletion of sensory and autonomic neurons, progressive visual decline leading to blindness is one of the most problematic aspects of the disease, as it severely affects their quality of life. To better understand the disease mechanism as well as to test the in vivo efficacy of targeted therapies for FD, we have recently generated a novel phenotypic mouse model, TgFD9; IkbkapΔ20/flox. This mouse exhibits most of the clinical features of the disease and accurately recapitulates the tissue-specific splicing defect observed in FD patients. Driven by the dire need to develop therapies targeting retinal degeneration in FD, herein, we comprehensively characterized the progression of the retinal phenotype in this mouse, and we demonstrated that it is possible to correct ELP1 splicing defect in the retina using the splicing modulator compound (SMC) BPN-15477.

Loss of Elp1 disrupts trigeminal ganglion neurodevelopment in a model of familial dysautonomia

Familial dysautonomia (FD) is a sensory and autonomic neuropathy caused by mutations in elongator complex protein 1 (ELP1). FD patients have small trigeminal nerves and impaired facial pain and temperature perception. These signals are relayed by nociceptive neurons in the trigeminal ganglion, a structure that is composed of both neural crest- and placode-derived cells. Mice lacking Elp1 in neural crest derivatives ('Elp1 CKO') are born with small trigeminal ganglia, suggesting Elp1 is important for trigeminal ganglion development, yet the function of Elp1 in this context is unknown. We demonstrate that Elp1, expressed in both neural crest- and placode-derived neurons, is not required for initial trigeminal ganglion formation. However, Elp1 CKO trigeminal neurons exhibit abnormal axon outgrowth and deficient target innervation. Developing nociceptors expressing the receptor TrkA undergo early apoptosis in Elp1 CKO, while TrkB- and TrkC-expressing neurons are spared, indicating Elp1 supports the target innervation and survival of trigeminal nociceptors. Furthermore, we demonstrate that specific TrkA deficits in the Elp1 CKO trigeminal ganglion reflect the neural crest lineage of most TrkA neurons versus the placodal lineage of most TrkB and TrkC neurons. Altogether, these findings explain defects in cranial gangliogenesis that may lead to loss of facial pain and temperature sensation in FD.

Loss of Elp1 perturbs histone H2A.Z and the Notch signaling pathway

Elongator dysfunction is increasingly recognized as a contributor to multiple neurodevelopmental and neurodegenerative disorders including familial dysautonomia, intellectual disability, amyotrophic lateral sclerosis, and autism spectrum disorder. Although numerous cellular processes are perturbed in the context of Elongator loss, converging evidence from multiple studies has resolved Elongator's primary function in the cell to the modification of tRNA wobble uridines and the translational regulation of codon-biased genes. Here we characterize H2a.z, encoding the variant H2a histone H2A.Z, as an indirect Elongator target. We further show that canonical Notch signaling, a pathway directed by H2A.Z, is perturbed as a consequence of Elp1 loss. Finally, we demonstrate that hyperacetylation of H2A.Z and other histones via exposure to the histone deacetylase inhibitor Trichostatin A during neurogenesis corrects the expression of Notch3 and rescues the development of sensory neurons in embryos lacking the Elp1 Elongator subunit.

Summary: The maldevelopment of sensory neurons in Elongator knockout embryos is associated with elevated H2A.Z and perturbed Notch signaling that can be rescued by Trichostatin A.

Biological Role of MYCN in Medulloblastoma: Novel Therapeutic Opportunities and Challenges Ahead

The constitutive and dysregulated expression of the transcription factor MYCN has a central role in the pathogenesis of the paediatric brain tumour medulloblastoma, with an increased expression of this oncogene correlating with a worse prognosis. Consequently, the genomic and functional alterations of MYCN represent a major therapeutic target to attenuate tumour growth in medulloblastoma. This review will provide a comprehensive synopsis of the biological role of MYCN and its family components, their interaction with distinct signalling pathways, and the implications of this network in medulloblastoma development. We will then summarise the current toolbox for targeting MYCN and highlight novel therapeutic avenues that have the potential to results in better-tailored clinical treatments.

ELP1 Splicing Correction Reverses Proprioceptive Sensory Loss in Familial Dysautonomia

Familial dysautonomia (FD) is a recessive neurodegenerative disease caused by a splice mutation in Elongator complex protein 1 (ELP1, also known as IKBKAP); this mutation leads to variable skipping of exon 20 and to a drastic reduction of ELP1 in the nervous system. Clinically, many of the debilitating aspects of the disease are related to a progressive loss of proprioception; this loss leads to severe gait ataxia, spinal deformities, and respiratory insufficiency due to neuromuscular incoordination. There is currently no effective treatment for FD, and the disease is ultimately fatal. The development of a drug that targets the underlying molecular defect provides hope that the drastic peripheral neurodegeneration characteristic of FD can be halted. We demonstrate herein that the FD mouse TgFD9;Ikbkap^Δ20/flox recapitulates the proprioceptive impairment observed in individuals with FD, and we provide the in vivo evidence that postnatal correction, promoted by the small molecule kinetin, of the mutant ELP1 splicing can rescue neurological phenotypes in FD. Daily administration of kinetin starting at birth improves sensory-motor coordination and prevents the onset of spinal abnormalities by stopping the loss of proprioceptive neurons. These phenotypic improvements correlate with increased amounts of full-length ELP1 mRNA and protein in multiple tissues, including in the peripheral nervous system (PNS). Our results show that postnatal correction of the underlying ELP1 splicing defect can rescue devastating disease phenotypes and is therefore a viable therapeutic approach for persons with FD.

Combinatorial treatment increases IKAP levels in human cells generated from Familial Dysautonomia patients

Familial Dysautonomia (FD) is an autosomal recessive congenital neuropathy that results from a point mutation at the 5' splice site of intron 20 in the IKBKAP gene. This mutation decreases production of the IKAP protein, and treatments that increase the level of the full-length IKBKAP transcript are likely to be of therapeutic value. We previously found that phosphatidylserine (PS), an FDA-approved food supplement, elevates IKAP levels in cells generated from FD patients. Here we demonstrate that combined treatment of cells generated from FD patients with PS and kinetin or PS and the histone deacetylase inhibitor trichostatin A (TSA) resulted in an additive elevation of IKAP compared to each drug alone. This indicates that the compounds influence different pathways. We also found that pridopidine enhances production of IKAP in cells generated from FD patients. Pridopidine has an additive effect on IKAP levels when used in combination with kinetin or TSA, but not with PS; suggesting that PS and pridopidine influence IKBKAP levels through the same mechanism. Indeed, we demonstrate that the effect of PS and pridopidine is through sigma-1 receptor-mediated activation of the BDNF signaling pathway. A combination treatment with any of these drugs with different mechanisms has potential to benefit FD patients.

Chemoreflex failure and sleep-disordered breathing in familial dysautonomia: Implications for sudden death during sleep

Familial dysautonomia (Riley-Day syndrome, hereditary sensory and autonomic neuropathy type III) is a rare autosomal recessive disease characterized by impaired development of primary sensory and autonomic neurons resulting in a severe neurological phenotype, which includes arterial baroreflex and chemoreflex failure with high frequency of sleep-disordered breathing and sudden death during sleep. Although a rare disease, familial dysautonomia represents a unique template to study the interactions between sleep-disordered breathing and abnormal chemo- and baroreflex function. In patients with familial dysautonomia, ventilatory responses to hypercapnia are reduced, and to hypoxia are almost absent. In response to hypoxia, these patients develop paradoxical hypoventilation, hypotension, bradycardia, and potentially, death. Impaired ventilatory control due to chemoreflex failure acquires special relevance during sleep when conscious control of respiration withdraws. Overall, almost all adult (85%) and pediatric (95%) patients have some degree of sleep-disordered breathing. Obstructive apnea events are more frequent in adults, whereas central apnea events are more severe and frequent in children. The annual incidence rate of sudden death during sleep in patients with familial dysautonomia is 3.4 per 1000 person-year, compared to 0.5-1 per 1000 person-year of sudden unexpected death in epilepsy. This review summarizes recent developments in the understanding of sleep-disordered breathing in patients with familial dysautonomia, the risk factors for sudden death during sleep, and the specific interventions that could prevent it.

Retina-specific loss of Ikbkap/Elp1 causes mitochondrial dysfunction that leads to selective retinal ganglion cell degeneration in a mouse model of familial dysautonomia

Familial dysautonomia (FD) is an autosomal recessive disorder marked by developmental and progressive neuropathies. It is caused by an intronic point-mutation in the IKBKAP/ELP1 gene, which encodes the inhibitor of κB kinase complex-associated protein (IKAP, also called ELP1), a component of the elongator complex. Owing to variation in tissue-specific splicing, the mutation primarily affects the nervous system. One of the most debilitating hallmarks of FD that affects patients' quality of life is progressive blindness. To determine the pathophysiological mechanisms that are triggered by the absence of IKAP in the retina, we generated retina-specific Ikbkap conditional knockout (CKO) mice using Pax6-Cre, which abolished Ikbkap expression in all cell types of the retina. Although sensory and autonomic neuropathies in FD are known to be developmental in origin, the loss of IKAP in the retina did not affect its development, demonstrating that IKAP is not required for retinal development. The loss of IKAP caused progressive degeneration of retinal ganglion cells (RGCs) by 1 month of age. Mitochondrial membrane integrity was breached in RGCs, and later in other retinal neurons. In Ikbkap CKO retinas, mitochondria were depolarized, and complex I function and ATP were significantly reduced. Although mitochondrial impairment was detected in all Ikbkap-deficient retinal neurons, RGCs were the only cell type to degenerate; the survival of other retinal neurons was unaffected. This retina-specific FD model is a useful in vivo model for testing potential therapeutics for mitigating blindness in FD. Moreover, our data indicate that RGCs and mitochondria are promising targets.

Summary: The elongator subunit IKBKAP/ELP1 is not required for development, but is essential for maintaining mitochondrial function and retina morphology. Loss of this subunit causes progressive, selective degeneration of retinal ganglion cells.

Sudden Unexpected Death During Sleep in Familial Dysautonomia: A Case-Control Study

Study Objectives

Sudden unexpected death during sleep (SUDS) is the most common cause of death in patients with familial dysautonomia (FD), an autosomal recessive disease characterized by sensory and autonomic dysfunction. It remains unknown what causes SUDS in these patients and who is at highest risk. We tested the hypothesis that SUDS in FD is linked to sleep-disordered breathing.

Methods

We retrospectively identified patients with FD who died suddenly and unexpectedly during sleep and had undergone polysomnography within the 18-month period before death. For each case, we sampled one age-matched surviving subject with FD that had also undergone polysomnography within the 18-month period before study. Data on polysomnography, EKG, ambulatory blood pressure monitoring, arterial blood gases, blood count, and metabolic panel were analyzed.

Results

Thirty-two deceased cases and 31 surviving controls were included. Autopsy was available in six cases. Compared with controls, participants with SUDS were more likely to be receiving treatment with fludrocortisone (odds ratio [OR]; 95% confidence interval) (OR 29.7; 4.1-213.4), have untreated obstructive sleep apnea (OR 17.4; 1.5-193), and plasma potassium levels <4 mEq/L (OR 19.5; 2.36-161) but less likely to use noninvasive ventilation at night (OR 0.19; 0.06-0.61).

Conclusions

Initiation of noninvasive ventilation when required and discontinuation of fludrocortisone treatment may reduce the high incidence rate of SUDS in patients with FD. Our findings contribute to the understanding of the link between autonomic, cardiovascular, and respiratory risk factors in SUDS.

Animal and cellular models of familial dysautonomia

Since Riley and Day first described the clinical phenotype of patients with familial dysautonomia (FD) over 60 years ago, the field has made considerable progress clinically, scientifically, and translationally in treating and understanding the etiology of FD. FD is classified as a hereditary sensory and autonomic neuropathy (HSAN type III) and is both a developmental and a progressive neurodegenerative condition that results from an autosomal recessive mutation in the gene IKBKAP, also known as ELP1. FD primarily impacts the peripheral nervous system but also manifests in central nervous system disruption, especially in the retina and optic nerve. While the disease is rare, the rapid progress being made in elucidating the molecular and cellular mechanisms mediating the demise of neurons in FD should provide insight into degenerative pathways common to many neurological disorders. Interestingly, the protein encoded by IKBKAP/ELP1, IKAP or ELP1, is a key scaffolding subunit of the six-subunit Elongator complex, and variants in other Elongator genes are associated with amyotrophic lateral sclerosis (ALS), intellectual disability, and Rolandic epilepsy. Here we review the recent model systems that are revealing the molecular and cellular pathophysiological mechanisms mediating FD. These powerful model systems can now be used to test targeted therapeutics for mitigating neuronal loss in FD and potentially other disorders.

BGP-15 prevents the death of neurons in a mouse model of familial dysautonomia

Hereditary sensory and autonomic neuropathy type III, or familial dysautonomia [FD; Online Mendelian Inheritance in Man (OMIM) 223900], affects the development and long-term viability of neurons in the peripheral nervous system (PNS) and retina. FD is caused by a point mutation in the gene IKBKAP/ELP1 that results in a tissue-specific reduction of the IKAP/ELP1 protein, a subunit of the Elongator complex. Hallmarks of the disease include vasomotor and cardiovascular instability and diminished pain and temperature sensation caused by reductions in sensory and autonomic neurons. It has been suggested but not demonstrated that mitochondrial function may be abnormal in FD. We previously generated an Ikbkap/Elp1 conditional-knockout mouse model that recapitulates the selective death of sensory (dorsal root ganglia) and autonomic neurons observed in FD. We now show that in these mice neuronal mitochondria have abnormal membrane potentials, produce elevated levels of reactive oxygen species, are fragmented, and do not aggregate normally at axonal branch points. The small hydroxylamine compound BGP-15 improved mitochondrial function, protecting neurons from dying in vitro and in vivo, and promoted cardiac innervation in vivo. Given that impairment of mitochondrial function is a common pathological component of neurodegenerative diseases such as amyotrophic lateral sclerosis and Alzheimer's, Parkinson's, and Huntington's diseases, our findings identify a therapeutic approach that may have efficacy in multiple degenerative conditions.

Proteasome inhibitors to alleviate aberrant IKBKAP mRNA splicing and low IKAP/hELP1 synthesis in familial dysautonomia

FD is a rare neurodegenerative disorder caused by a mutation of the IKBKAP gene, which induces low expression levels of the Elongator subunit IKAP/hELP1 protein. A rational strategy for FD treatment could be to identify drugs increasing IKAP/hELP1 expression levels by blocking protein degradation pathways such as the 26S proteasome. Proteasome inhibitors are promising molecules emerging in cancer treatment and could thus constitute an enticing pharmaceutical strategy for FD treatment. Therefore, we tested three proteasome inhibitors on FD human olfactory ecto-mesenchymal stem cells (hOE-MSCs): two approved by the Food and Drug Administration (FDA) and European Medicines Agency (EMA), bortezomib and carfilzomib, as well as epoxomicin. Although all 3 inhibitors demonstrated activity in correcting IKBKAP mRNA aberrant splicing, carfilzomib was superior in enhancing IKAP/hELP1 quantity. Moreover, we observed a synergistic effect of suboptimal doses of carfilzomib on kinetin in improving IKBKAP isoforms ratio and IKAP/hELP1 expression levels allowing to counterbalance carfilzomib toxicity. Finally, we identified several dysregulated miRNAs after carfilzomib treatment that target proteasome-associated mRNAs and determined that IKAP/hELP1 deficiency in FD pathology is correlated to an overactivity of the 26S proteasome. Altogether, these results reinforce the rationale for using chemical compounds inhibiting the 26S proteasome as an innovative option for FD and a promising therapeutic pathway for many other neurodegenerative diseases.

Pathological Confirmation of Optic Neuropathy in Familial Dysautonomia

Clinical data suggest that optic neuropathy and retinal ganglion cell loss are the main cause of visual decline in patients with familial dysautonomia, but this has not previously been confirmed by pathological analyses. We studied retinas and optic nerves in 6 eyes from 3 affected patients obtained at autopsy. Analyses included routine neurohistology and immunohistochemistry for neurofilaments, cytochrome c oxidase (COX), and melanopsin-containing ganglion cells. We observed profound axon loss in the temporal portions of optic nerves with relative preservation in the nasal portions; this correlated with clinical and optical coherence tomography findings in 1 patient. Retinal ganglion cell layers were markedly reduced in the central retina, whereas melanopsin-containing ganglion cells were relatively spared. COX staining was reduced in the temporal portions of the optic nerve indicating reduced mitochondrial density. Axonal swelling with degenerating lysosomes and mitochondria were observed by electron microscopy. These findings support the concept that there is a specific optic neuropathy and retinopathy in patients with familial dysautonomia similar to that seen in other optic neuropathies with mitochondrial dysfunction. This raises the possibility that defective expression of the IkB kinase complex-associated protein (IKAP) resulting from mutations in IKBKAP affects mitochondrial function in the metabolism-dependent retinal parvocellular ganglion cells in this condition.

Phosphatidylserine Ameliorates Neurodegenerative Symptoms and Enhances Axonal Transport in a Mouse Model of Familial Dysautonomia

Familial Dysautonomia (FD) is a neurodegenerative disease in which aberrant tissue-specific splicing of IKBKAP exon 20 leads to reduction of IKAP protein levels in neuronal tissues. Here we generated a conditional knockout (CKO) mouse in which exon 20 of IKBKAP is deleted in the nervous system. The CKO FD mice exhibit developmental delays, sensory abnormalities, and less organized dorsal root ganglia (DRGs) with attenuated axons compared to wild-type mice. Furthermore, the CKO FD DRGs show elevated HDAC6 levels, reduced acetylated α-tubulin, unstable microtubules, and impairment of axonal retrograde transport of nerve growth factor (NGF). These abnormalities in DRG properties underlie neuronal degeneration and FD symptoms. Phosphatidylserine treatment decreased HDAC6 levels and thus increased acetylation of α-tubulin. Further PS treatment resulted in recovery of axonal outgrowth and enhanced retrograde axonal transport by decreasing histone deacetylase 6 (HDAC6) levels and thus increasing acetylation of α-tubulin levels. Thus, we have identified the molecular pathway that leads to neurodegeneration in FD and have demonstrated that phosphatidylserine treatment has the potential to slow progression of neurodegeneration.

We create a novel FD mouse model, in which exon 20 of IKBKAP was deleted in the nervous system, to study the role of IKAP in the neurodegeneration process. The lack of IKBKAP exon 20 impaired retrograde nerve growth factor (NGF) transport and axonal outgrowth. Reduction of IKAP levels resulted in elevated HDAC6 levels and thus reduced acetylated α-tubulin levels. Phosphatidylserine down-regulated HDAC6 levels, furthermore phosphatidylserine treatment facilitated axonal transport and stabilized microtubules. In brief: Naftelberg et al. identify the molecular pathway leading to neurodegeneration using a mouse model of familial dysautonomia and suggest that phosphatidylserine acts as an HDAC6 inhibitor to improve neurologic function.

Loss of Ikbkap Causes Slow, Progressive Retinal Degeneration in a Mouse Model of Familial Dysautonomia

Familial dysautonomia (FD) is an autosomal recessive congenital neuropathy that is caused by a mutation in the gene for inhibitor of kappa B kinase complex-associated protein (IKBKAP). Although FD patients suffer from multiple neuropathies, a major debilitation that affects their quality of life is progressive blindness. To determine the requirement for Ikbkap in the developing and adult retina, we generated Ikbkap conditional knockout (CKO) mice using a TUBA1a promoter-Cre (Tα1-Cre). In the retina, Tα1-Cre expression is detected predominantly in retinal ganglion cells (RGCs). At 6 months, significant loss of RGCs had occurred in the CKO retinas, with the greatest loss in the temporal retina, which is the same spatial phenotype observed in FD, Leber hereditary optic neuropathy, and dominant optic atrophy. Interestingly, the melanopsin-positive RGCs were resistant to degeneration. By 9 months, signs of photoreceptor degeneration were observed, which later progressed to panretinal degeneration, including RGC and photoreceptor loss, optic nerve thinning, Müller glial activation, and disruption of layers. Taking these results together, we conclude that although Ikbkap is not required for normal development of RGCs, its loss causes a slow, progressive RGC degeneration most severely in the temporal retina, which is later followed by indirect photoreceptor loss and complete retinal disorganization. This mouse model of FD is not only useful for identifying the mechanisms mediating retinal degeneration, but also provides a model system in which to attempt to test therapeutics that may mitigate the loss of vision in FD patients.

Familial Dysautonomia: Mechanisms and Models

Hereditary Sensory and Autonomic Neuropathies (HSANs) compose a heterogeneous group of genetic disorders characterized by sensory and autonomic dysfunctions. Familial Dysautonomia (FD), also known as HSAN III, is an autosomal recessive disorder that affects 1/3,600 live births in the Ashkenazi Jewish population. The major features of the disease are already present at birth and are attributed to abnormal development and progressive degeneration of the sensory and autonomic nervous systems. Despite clinical interventions, the disease is inevitably fatal. FD is caused by a point mutation in intron 20 of the IKBKAP gene that results in severe reduction in expression of IKAP, its encoded protein. In vitro and in vivo studies have shown that IKAP is involved in multiple intracellular processes, and suggest that failed target innervation and/or impaired neurotrophic retrograde transport are the primary causes of neuronal cell death in FD. However, FD is far more complex, and appears to affect several other organs and systems in addition to the peripheral nervous system. With the recent generation of mouse models that recapitulate the molecular and pathological features of the disease, it is now possible to further investigate the mechanisms underlying different aspects of the disorder, and to test novel therapeutic strategies.

MicroRNA screening identifies a link between NOVA1 expression and a low level of IKAP in familial dysautonomia

Familial dysautonomia (FD) is a rare neurodegenerative disease caused by a mutation in intron 20 of the IKBKAP gene (c.2204+6T>C), leading to tissue-specific skipping of exon 20 and a decrease in the synthesis of the encoded protein IKAP (also known as ELP1). Small non-coding RNAs known as microRNAs (miRNAs) are important post-transcriptional regulators of gene expression and play an essential role in the nervous system development and function. To better understand the neuronal specificity of IKAP loss, we examined expression of miRNAs in human olfactory ecto-mesenchymal stem cells (hOE-MSCs) from five control individuals and five FD patients. We profiled the expression of 373 miRNAs using microfluidics and reverse transcription coupled to quantitative PCR (RT-qPCR) on two biological replicate series of hOE-MSC cultures from healthy controls and FD patients. This led to the total identification of 26 dysregulated miRNAs in FD, validating the existence of a miRNA signature in FD. We then selected the nine most discriminant miRNAs for further analysis. The signaling pathways affected by these dysregulated miRNAs were largely within the nervous system. In addition, many targets of these dysregulated miRNAs had been previously demonstrated to be affected in FD models. Moreover, we found that four of our nine candidate miRNAs target the neuron-specific splicing factor NOVA1. We demonstrated that overexpression of miR-203a-3p leads to a decrease of NOVA1, counter-balanced by an increase of IKAP, supporting a potential interaction between NOVA1 and IKAP. Taken together, these results reinforce the choice of miRNAs as potential therapeutic targets and suggest that NOVA1 could be a regulator of FD pathophysiology.

Summary: A miRNA screening conducted in olfactory stem cells from patients links the neuron-specific splicing factor NOVA1 to neurodegeneration in familial dysautonomia.

Phosphatidylserine enhances IKBKAP transcription by activating the MAPK/ERK signaling pathway

Familial dysautonomia (FD) is a genetic disorder manifested due to abnormal development and progressive degeneration of the sensory and autonomic nervous system. FD is caused by a point mutation in the IKBKAP gene encoding the IKAP protein, resulting in decreased protein levels. A promising potential treatment for FD is phosphatidylserine (PS); however, the manner by which PS elevates IKAP levels has yet to be identified. Analysis of ChIP-seq results of the IKBKAP promoter region revealed binding of the transcription factors CREB and ELK1, which are regulated by the mitogen-activated protein kinase (MAPK)/extracellular-regulated kinase (ERK) signaling pathway. We show that PS treatment enhanced ERK phosphorylation in cells derived from FD patients. ERK activation resulted in elevated IKBKAP transcription and IKAP protein levels, whereas pretreatment with the MAPK inhibitor U0126 blocked elevation of the IKAP protein level. Overexpression of either ELK1 or CREB activated the IKBKAP promoter, whereas downregulation of these transcription factors resulted in a decrease of the IKAP protein. Additionally, we show that PS improves cell migration, known to be enhanced by MAPK/ERK activation and abrogated in FD cells. In conclusion, our results demonstrate that PS activates the MAPK/ERK signaling pathway, resulting in activation of transcription factors that bind the promoter region of IKBKAP and thus enhancing its transcription. Therefore, compounds that activate the MAPK/ERK signaling pathway could constitute potential treatments for FD.

Sensory and autonomic deficits in a new humanized mouse model of familial dysautonomia

Familial dysautonomia (FD) is an autosomal recessive neurodegenerative disease that affects the development and survival of sensory and autonomic neurons. FD is caused by an mRNA splicing mutation in intron 20 of the IKBKAP gene that results in a tissue-specific skipping of exon 20 and a corresponding reduction of the inhibitor of kappaB kinase complex-associated protein (IKAP), also known as Elongator complex protein 1. To date, several promising therapeutic candidates for FD have been identified that target the underlying mRNA splicing defect, and increase functional IKAP protein. Despite these remarkable advances in drug discovery for FD, we lacked a phenotypic mouse model in which we could manipulate IKBKAP mRNA splicing to evaluate potential efficacy. We have, therefore, engineered a new mouse model that, for the first time, will permit to evaluate the phenotypic effects of splicing modulators and provide a crucial platform for preclinical testing of new therapies. This new mouse model, TgFD9; Ikbkap(Δ20/flox) was created by introducing the complete human IKBKAP transgene with the major FD splice mutation (TgFD9) into a mouse that expresses extremely low levels of endogenous Ikbkap (Ikbkap(Δ20/flox)). The TgFD9; Ikbkap(Δ20/flox) mouse recapitulates many phenotypic features of the human disease, including reduced growth rate, reduced number of fungiform papillae, spinal abnormalities, and sensory and sympathetic impairments, and recreates the same tissue-specific mis-splicing defect seen in FD patients. This is the first mouse model that can be used to evaluate in vivo the therapeutic effect of increasing IKAP levels by correcting the underlying FD splicing defect.

Familial Dysautonomia (FD) Human Embryonic Stem Cell Derived PNS Neurons Reveal that Synaptic Vesicular and Neuronal Transport Genes Are Directly or Indirectly Affected by IKBKAP Downregulation

A splicing mutation in the IKBKAP gene causes Familial Dysautonomia (FD), affecting the IKAP protein expression levels and proper development and function of the peripheral nervous system (PNS). Here we found new molecular insights for the IKAP role and the impact of the FD mutation in the human PNS lineage by using a novel and unique human embryonic stem cell (hESC) line homozygous to the FD mutation originated by pre implantation genetic diagnosis (PGD) analysis. We found that IKBKAP downregulation during PNS differentiation affects normal migration in FD-hESC derived neural crest cells (NCC) while at later stages the PNS neurons show reduced intracellular colocalization between vesicular proteins and IKAP. Comparative wide transcriptome analysis of FD and WT hESC-derived neurons together with the analysis of human brains from FD and WT 12 weeks old embryos and experimental validation of the results confirmed that synaptic vesicular and neuronal transport genes are directly or indirectly affected by IKBKAP downregulation in FD neurons. Moreover we show that kinetin (a drug that corrects IKBKAP alternative splicing) promotes the recovery of IKAP expression and these IKAP functional associated genes identified in the study. Altogether, these results support the view that IKAP might be a vesicular like protein that might be involved in neuronal transport in hESC derived PNS neurons. This function seems to be mostly affected in FD-hESC derived PNS neurons probably reflecting some PNS neuronal dysfunction observed in FD.

Depletion of the IKBKAP ortholog in zebrafish leads to hirschsprung disease-like phenotype

AIM: To investigate the role of IKBKAP (inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase complex-associated protein) in the development of enteric nervous system (ENS) and Hirschsprung disease (HSCR).

METHODS: In this study, we injected a morpholino that blocked the translation of ikbkap protein to 1-cell stage zebrafish embryos. The phenotype in the ENS was analysed by antibody staining of the pan-neuronal marker HuC/D followed by enteric neuron counting. The mean numbers of enteric neurons were compared between the morphant and the control. We also studied the expressions of ret and phox2bb, which are involved in ENS development, in the ikbkap morpholino injected embryos by quantitative reverse transcriptase polymerase chain reaction and compared them with the control.

RESULTS: We observed aganglionosis (χ², P < 0.01) and a reduced number of enteric neurons (38.8 ± 9.9 vs 50.2 ± 17.3, P < 0.05) in the zebrafish embryos injected with ikbkap translation-blocking morpholino (morphant) when compared with the control embryos. Specificity of the morpholino was confirmed by similar results obtained using a second non-overlapping morpholino that blocked the translation of ikbkap. We further studied the morphant by analysing the expression levels of genes involved in ENS development such as ret, phox2bb and sox10, and found that phox2bb, the ortholog of human PHOX2B, was significantly down-regulated (0.51 ± 0.15 vs 1.00 ± 0, P < 0.05). Although we also observed a reduction in the expression of ret, the difference was not significant.

CONCLUSION: Loss of IKBKAP contributed to HSCR as demonstrated by functional analysis in zebrafish embryos.

Vascular endothelial function and blood pressure regulation in afferent autonomic failure

BACKGROUND

Familial dysautonomia (FD) is a rare hereditary disease characterized by loss of afferent autonomic neural fiber signaling and consequent profound impairment of arterial baroreflex function and blood pressure regulation. Whether vascular endothelial dysfunction contributes to defective vasomotor control in this form of afferent autonomic failure is not known.

METHODS

We assessed blood pressure response to orthostatic stress and vascular endothelial function with brachial artery reactivity testing in 34 FD subjects with afferent autonomic failure and 34 healthy control subjects.

RESULTS

Forty-four percent of the afferent autonomic failure subjects had uncontrolled hypertension at supine rest (median systolic blood pressure = 148mm Hg, interquartile range (IQR) = 144-155mm Hg; median diastolic blood pressure = 83mm Hg, IQR = 78-105mm Hg), and 88% had abnormal response to orthostatic stress (median decrease in systolic blood pressure after upright tilt = 48mm Hg, IQR = 29-61mm Hg). Flow-mediated brachial artery reactivity did not differ in subjects with afferent autonomic failure vs. healthy control subjects (median = 6.00%, IQR = 1.86-11.77%; vs. median = 6.27%, IQR = 4.65-9.34%; P = 0.75). In afferent autonomic failure subjects, brachial artery reactivity was not associated with resting blood pressure or the magnitude of orthostatic hypotension but was decreased in association with reduced glomerular filtration rate (r = 0.62; P < 0.001).

CONCLUSIONS

Brachial artery reactivity was preserved in subjects with afferent autonomic failure despite the presence of marked blood pressure dysregulation. Comorbid renal dysfunction was associated with reduced brachial artery reactivity.

Involvement of IKAP in peripheral target innervation and in specific JNK and NGF signaling in developing PNS neurons

A splicing mutation in the ikbkap gene causes Familial Dysautonomia (FD), affecting the IKAP protein expression levels and proper development and function of the peripheral nervous system (PNS). Here we attempted to elucidate the role of IKAP in PNS development in the chick embryo and found that IKAP is required for proper axonal outgrowth, branching, and peripheral target innervation. Moreover, we demonstrate that IKAP colocalizes with activated JNK (pJNK), dynein, and β-tubulin at the axon terminals of dorsal root ganglia (DRG) neurons, and may be involved in transport of specific target derived signals required for transcription of JNK and NGF responsive genes in the nucleus. These results suggest the novel role of IKAP in neuronal transport and specific signaling mediated transcription, and provide, for the first time, the basis for a molecular mechanism behind the FD phenotype.

A neuron autonomous role for the familial dysautonomia gene ELP1 in sympathetic and sensory target tissue innervation

Familial dysautonomia (FD) is characterized by severe and progressive sympathetic and sensory neuron loss caused by a highly conserved germline point mutation of the human ELP1/IKBKAP gene. Elp1 is a subunit of the hetero-hexameric transcriptional elongator complex, but how it functions in disease-vulnerable neurons is unknown. Conditional knockout mice were generated to characterize the role of Elp1 in migration, differentiation and survival of migratory neural crest (NC) progenitors that give rise to sympathetic and sensory neurons. Loss of Elp1 in NC progenitors did not impair their migration, proliferation or survival, but there was a significant impact on post-migratory sensory and sympathetic neuron survival and target tissue innervation. Ablation of Elp1 in post-migratory sympathetic neurons caused highly abnormal target tissue innervation that was correlated with abnormal neurite outgrowth/branching and abnormal cellular distribution of soluble tyrosinated α-tubulin in Elp1-deficient primary sympathetic and sensory neurons. These results indicate that neuron loss and physiologic impairment in FD is not a consequence of abnormal neuron progenitor migration, differentiation or survival. Rather, loss of Elp1 leads to neuron death as a consequence of failed target tissue innervation associated with impairments in cytoskeletal regulation.

IKAP deficiency in an FD mouse model and in oligodendrocyte precursor cells results in downregulation of genes involved in oligodendrocyte differentiation and myelin formation

The splice site mutation in the IKBKAP gene coding for IKAP protein leads to the tissue-specific skipping of exon 20, with concomitant reduction in IKAP protein production. This causes the neurodevelopmental, autosomal-recessive genetic disorder - Familial Dysautonomia (FD). The molecular hallmark of FD is the severe reduction of IKAP protein in the nervous system that is believed to be the main reason for the devastating symptoms of this disease. Our recent studies showed that in the brain of two FD patients, genes linked to oligodendrocyte differentiation and/or myelin formation are significantly downregulated, implicating IKAP in the process of myelination. However, due to the scarcity of FD patient tissues, these results awaited further validation in other models. Recently, two FD mouse models that faithfully recapitulate FD were generated, with two types of mutations resulting in severely low levels of IKAP expression. Here we demonstrate that IKAP deficiency in these FD mouse models affects a similar set of genes as in FD patients' brains. In addition, we identified two new IKAP target genes involved in oligodendrocyte cells differentiation and myelination, further underscoring the essential role of IKAP in this process. We also provide proof that IKAP expression is needed cell-autonomously for the regulation of expression of genes involved in myelin formation since knockdown of IKAP in the Oli-neu oligodendrocyte precursor cell line results in similar deficiencies. Further analyses of these two experimental models will compensate for the lack of human postmortem tissues and will advance our understanding of the role of IKAP in myelination and the disease pathology.

Familial dysautonomia model reveals Ikbkap deletion causes apoptosis of Pax3+ progenitors and peripheral neurons

Familial dysautonomia (FD) is a devastating developmental and progressive peripheral neuropathy caused by a mutation in the gene inhibitor of kappa B kinase complex-associated protein (IKBKAP). To identify the cellular and molecular mechanisms that cause FD, we generated mice in which Ikbkap expression is ablated in the peripheral nervous system and identify the steps in peripheral nervous system development that are Ikbkap-dependent. We show that Ikbkap is not required for trunk neural crest migration or pathfinding, nor for the formation of dorsal root or sympathetic ganglia, or the adrenal medulla. Instead, Ikbkap is essential for the second wave of neurogenesis during which the majority of tropomyosin-related kinase A (TrkA(+)) nociceptors and thermoreceptors arise. In its absence, approximately half the normal complement of TrkA(+) neurons are lost, which we show is partly due to p53-mediated premature differentiation and death of mitotically-active progenitors that express the paired-box gene Pax3 and give rise to the majority of TrkA(+) neurons. By the end of sensory development, the number of TrkC neurons is significantly increased, which may result from an increase in Runx3(+) cells. Furthermore, our data demonstrate that TrkA(+) (but not TrkC(+)) sensory and sympathetic neurons undergo exacerbated Caspase 3-mediated programmed cell death in the absence of Ikbkap and that this death is not due to a reduction in nerve growth factor synthesis. In summary, these data suggest that FD does not result from a failure in trunk neural crest migration, but rather from a critical function for Ikbkap in TrkA progenitors and TrkA(+) neurons.

IKAP expression levels modulate disease severity in a mouse model of familial dysautonomia

Hereditary sensory and autonomic neuropathies (HSANs) encompass a group of genetically inherited disorders characterized by sensory and autonomic dysfunctions. Familial dysautonomia (FD), also known as HSAN type III, is an autosomal recessive disorder that affects 1/3600 live births in the Ashkenazi Jewish population. The disease is caused by abnormal development and progressive degeneration of the sensory and autonomic nervous systems and is inevitably fatal, with only 50% of patients reaching the age of 40. FD is caused by a mutation in intron 20 of the Ikbkap gene that results in severe reduction in the expression of its encoded protein, inhibitor of kappaB kinase complex-associated protein (IKAP). Although the mutation that causes FD was identified in 2001, so far there is no appropriate animal model that recapitulates the disorder. Here, we report the generation and characterization of the first mouse models for FD that recapitulate the molecular and pathological features of the disease. Important for therapeutic interventions is also our finding that a slight increase in IKAP levels is enough to ameliorate the phenotype and increase the life span. Understanding the mechanisms underlying FD will provide insights for potential new therapeutic interventions not only for FD, but also for other peripheral neuropathies.