Norepinephrine transporter defects lead to sympathetic hyperactivity in stem cell and mouse models of Familial Dysautonomia

Familial dysautonomia (FD) is a rare neurodevelopmental and neurodegenerative disorder that affects the sympathetic nervous system. Patients harbor a mutation in ELP1 yet, how loss of Elp1 affects the function of symNs remains unresolved. Such an understanding is critical since the most debilitating hallmarks of the disease include cardiovascular instability, dysautonomic crises and renal failure, which all result from dysregulated sympathetic activity. Here, we employ the human pluripotent stem cell (hPSC) technology as a modeling system to understand human, sympathetic neuron (symN)-specific disease mechanisms and provide a platform for drug testing and discovery. We show that FD symNs are intrinsically hyperactive in vitro, in co-cultures with cardiomyocyte target tissue and in FD animal models. We show that ELP1-rescued isogenic lines remain hyperactive, suggesting a different/additional disease mechanism. Accordingly, we report decreased intracellular norepinephrine (NE) levels, decreased NE re-uptake via NET and excessive extracellular NE in FD symNs. Finally, we performed a mini drug screen showing that current and new candidate drugs were able to lower hyperactivity. These findings may have implications for other peripheral nervous system disorders. Our drug screening platform may allow future drug testing and discovery for such disorders.

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

ABSTRACT 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.

Case report: Perioperative management of a patient with familial dysautonomia

Familial dysautonomia is a rare autosomal recessive neurodegenerative disease affecting cells of the autonomic nervous system. Patients with this disease are insensitive to pain but their autonomic nervous system is still activated with noxious stimuli. This report details a case of a patient with familial dysautonomia who underwent right ankle open reduction and internal fixation for a bimalleolar right ankle fracture. The patients preoperative and intraoperative course were uneventful but shortly after handoff to the intensive care unit, the patient experienced an autonomic crisis. Management of these patients is complex, requiring maintenance of physiologic homeostasis as well as preventing hemodynamic instability caused by noxious stimuli. Any deviations from baseline may cause an autonomic crisis, as happened in our patient. Herein, we detail the perioperative management of a patient with familial dysautonomia in further detail.

Therapeutic manipulation of IKBKAP mis-splicing with a small molecule to cure familial dysautonomia

Approximately half of genetic disease-associated mutations cause aberrant splicing. However, a widely applicable therapeutic strategy to splicing diseases is yet to be developed. Here, we analyze the mechanism whereby IKBKAP-familial dysautonomia (FD) exon 20 inclusion is specifically promoted by a small molecule splice modulator, RECTAS, even though IKBKAP-FD exon 20 has a suboptimal 5′ splice site due to the IVS20 + 6 T > C mutation. Knockdown experiments reveal that exon 20 inclusion is suppressed in the absence of serine/arginine-rich splicing factor 6 (SRSF6) binding to an intronic splicing enhancer in intron 20. We show that RECTAS directly interacts with CDC-like kinases (CLKs) and enhances SRSF6 phosphorylation. Consistently, exon 20 splicing is bidirectionally manipulated by targeting cellular CLK activity with RECTAS versus CLK inhibitors. The therapeutic potential of RECTAS is validated in multiple FD disease models. Our study indicates that small synthetic molecules affecting phosphorylation state of SRSFs is available as a new therapeutic modality for mechanism-oriented precision medicine of splicing diseases.

The case of a patient with MIRAGE syndrome with familial dysautonomia-like symptoms

We describe a case of posthumously diagnosed MIRAGE syndrome (Myelodysplasia, Infection, Restriction of growth, Adrenal hypoplasia, Genital problems, and Enteropathy) in a girl with a new pathogenic SAMD9 variant (p.F437S), who was initially considered to have familial dysautonomia (FD)-like disease due to increased levels of catecholamine metabolites. Functional analyses of F437S-SAMD9 were performed, showing characteristics of disease-causing variants. This new SAMD9 variant (p.F437S) also causes MIRAGE syndrome.

Loss of Elp1 disrupts trigeminal ganglion neurodevelopment in a model of Familial Dysautonomia

Familial Dysautonomia (FD) is a sensory and autonomic neuropathy caused by a mutation in Elongator complex protein 1 (ELP1). FD patients have small trigeminal nerves and impaired perception of facial pain and temperature. These signals are relayed by nociceptive neurons in the trigeminal ganglion, a structure comprised of both neural crest- and placode-derived cells. Mice lacking Elp1 in neural crest derivatives (Elp1 CKO) are born with smaller trigeminal ganglia, suggesting Elp1 is important for trigeminal ganglion development, yet the function of Elp1 in this context is unknown. We demonstrate Elp1 expression in both neural crest- and placode-derived trigeminal neurons, which our data suggest give rise to primarily TrkA- and TrkB/C-expressing neurons, respectively. While Elp1 is not required for initial trigeminal ganglion formation, Elp1 CKO trigeminal neurons exhibit abnormal axon outgrowth and decreased target innervation. Developing nociceptors that express the receptor TrkA are especially vulnerable to Elp1 loss. TrkA expression is decreased in Elp1 CKO trigeminal nerve endings, coinciding with increased cell death. Subsequently, fewer TrkA neurons are present in the Elp1 CKO trigeminal ganglion, indicating Elp1 supports the target innervation and survival of trigeminal nociceptors. These findings explain the loss of facial pain and temperature sensation in FD.

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 aspect 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; Elp1Δ20/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.