Excitatory Dysfunction Drives Network and Calcium Handling Deficits in 16p11.2 Duplication Schizophrenia Induced Pluripotent Stem Cell-Derived Neurons

BACKGROUND

Schizophrenia (SCZ) is a debilitating psychiatric disorder with a large genetic contribution; however, its neurodevelopmental substrates remain largely unknown. Modeling pathogenic processes in SCZ using human induced pluripotent stem cell-derived neurons (iNs) has emerged as a promising strategy. Copy number variants confer high genetic risk for SCZ, with duplication of the 16p11.2 locus increasing the risk 14.5-fold.

METHODS

To dissect the contribution of induced excitatory neurons (iENs) versus GABAergic (gamma-aminobutyric acidergic) neurons (iGNs) to SCZ pathophysiology, we induced iNs from CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 isogenic and SCZ patient-derived induced pluripotent stem cells and analyzed SCZ-related phenotypes in iEN monocultures and iEN/iGN cocultures.

RESULTS

In iEN/iGN cocultures, neuronal firing and synchrony were reduced at later, but not earlier, stages of in vitro development. These were fully recapitulated in iEN monocultures, indicating a primary role for iENs. Moreover, isogenic iENs showed reduced dendrite length and deficits in calcium handling. iENs from 16p11.2 duplication-carrying patients with SCZ displayed overlapping deficits in network synchrony, dendrite outgrowth, and calcium handling. Transcriptomic analysis of both iEN cohorts revealed molecular markers of disease related to the glutamatergic synapse, neuroarchitecture, and calcium regulation.

CONCLUSIONS

Our results indicate the presence of 16p11.2 duplication-dependent alterations in SCZ patient-derived iENs. Transcriptomics and cellular phenotyping reveal overlap between isogenic and patient-derived iENs, suggesting a central role of glutamatergic, morphological, and calcium dysregulation in 16p11.2 duplication-mediated pathogenesis. Moreover, excitatory dysfunction during early neurodevelopment is implicated as the basis of SCZ pathogenesis in 16p11.2 duplication carriers. Our results support network synchrony and calcium handling as outcomes directly linked to this genetic risk variant.

Monoallelic and biallelic mutations in RELN underlie a graded series of neurodevelopmental disorders

Reelin, a large extracellular protein, plays several critical roles in brain development and function. It is encoded by RELN, first identified as the gene disrupted in the reeler mouse, a classic neurological mutant exhibiting ataxia, tremors and a 'reeling' gait. In humans, biallelic variants in RELN have been associated with a recessive lissencephaly variant with cerebellar hypoplasia, which matches well with the homozygous mouse mutant that has abnormal cortical structure, small hippocampi and severe cerebellar hypoplasia. Despite the large size of the gene, only 11 individuals with RELN-related lissencephaly with cerebellar hypoplasia from six families have previously been reported. Heterozygous carriers in these families were briefly reported as unaffected, although putative loss-of-function variants are practically absent in the population (probability of loss of function intolerance = 1). Here we present data on seven individuals from four families with biallelic and 13 individuals from seven families with monoallelic (heterozygous) variants of RELN and frontotemporal or temporal-predominant lissencephaly variant. Some individuals with monoallelic variants have moderate frontotemporal lissencephaly, but with normal cerebellar structure and intellectual disability with severe behavioural dysfunction. However, one adult had abnormal MRI with normal intelligence and neurological profile. Thorough literature analysis supports a causal role for monoallelic RELN variants in four seemingly distinct phenotypes including frontotemporal lissencephaly, epilepsy, autism and probably schizophrenia. Notably, we observed a significantly higher proportion of loss-of-function variants in the biallelic compared to the monoallelic cohort, where the variant spectrum included missense and splice-site variants. We assessed the impact of two canonical splice-site variants observed as biallelic or monoallelic variants in individuals with moderately affected or normal cerebellum and demonstrated exon skipping causing in-frame loss of 46 or 52 amino acids in the central RELN domain. Previously reported functional studies demonstrated severe reduction in overall RELN secretion caused by heterozygous missense variants p.Cys539Arg and p.Arg3207Cys associated with lissencephaly suggesting a dominant-negative effect. We conclude that biallelic variants resulting in complete absence of RELN expression are associated with a consistent and severe phenotype that includes cerebellar hypoplasia. However, reduced expression of RELN remains sufficient to maintain nearly normal cerebellar structure. Monoallelic variants are associated with incomplete penetrance and variable expressivity even within the same family and may have dominant-negative effects. Reduced RELN secretion in heterozygous individuals affects only cortical structure whereas the cerebellum remains intact. Our data expand the spectrum of RELN-related neurodevelopmental disorders ranging from lethal brain malformations to adult phenotypes with normal brain imaging.

Loss-of-function variants in TIAM1 are associated with developmental delay, intellectual disability, and seizures

TIAM Rac1-associated GEF 1 (TIAM1) regulates RAC1 signaling pathways that affect the control of neuronal morphogenesis and neurite outgrowth by modulating the actin cytoskeletal network. To date, TIAM1 has not been associated with a Mendelian disorder. Here, we describe five individuals with bi-allelic TIAM1 missense variants who have developmental delay, intellectual disability, speech delay, and seizures. Bioinformatic analyses demonstrate that these variants are rare and likely pathogenic. We found that the Drosophila ortholog of TIAM1, still life (sif), is expressed in larval and adult central nervous system (CNS) and is mainly expressed in a subset of neurons, but not in glia. Loss of sif reduces the survival rate, and the surviving adults exhibit climbing defects, are prone to severe seizures, and have a short lifespan. The TIAM1 reference (Ref) cDNA partially rescues the sif loss-of-function (LoF) phenotypes. We also assessed the function associated with three TIAM1 variants carried by two of the probands and compared them to the TIAM1 Ref cDNA function in vivo. TIAM1 p.Arg23Cys has reduced rescue ability when compared to TIAM1 Ref, suggesting that it is a partial LoF variant. In ectopic expression studies, both wild-type sif and TIAM1 Ref are toxic, whereas the three variants (p.Leu862Phe, p.Arg23Cys, and p.Gly328Val) show reduced toxicity, suggesting that they are partial LoF variants. In summary, we provide evidence that sif is important for appropriate neural function and that TIAM1 variants observed in the probands are disruptive, thus implicating loss of TIAM1 in neurological phenotypes in humans.

KNa1.1 gain-of-function preferentially dampens excitability of murine parvalbumin-positive interneurons

KCNT1 encodes the sodium-activated potassium channel K_Na1.1, expressed preferentially in the frontal cortex, hippocampus, cerebellum, and brainstem. Pathogenic missense variants in KCNT1 are associated with intractable epilepsy, namely epilepsy of infancy with migrating focal seizures (EIMFS), and sleep-related hypermotor epilepsy (SHE). In vitro studies of pathogenic KCNT1 variants support predominantly a gain-of-function molecular mechanism, yet how these variants behave in a neuron or ultimately drive formation of an epileptogenic circuit is an important and timely question. Using CRISPR/Cas9 gene editing, we introduced a gain-of-function variant into the endogenous mouse Kcnt1 gene. Compared to wild-type (WT) littermates, heterozygous and homozygous knock-in mice displayed greater seizure susceptibility to the chemoconvulsants kainate and pentylenetetrazole (PTZ), but not to flurothyl. Using acute slice electrophysiology in heterozygous and homozygous Kcnt1 knock-in and WT littermates, we demonstrated that CA1 hippocampal pyramidal neurons exhibit greater amplitude of miniature inhibitory postsynaptic currents in mutant mice with no difference in frequency, suggesting greater inhibitory tone associated with the Kcnt1 mutation. To address alterations in GABAergic signaling, we bred Kcnt1 knock-in mice to a parvalbumin-tdTomato reporter line, and found that parvalbumin-expressing (PV+) interneurons failed to fire repetitively with large amplitude current injections and were more prone to depolarization block. These alterations in firing can be recapitulated by direct application of the K_Na1.1 channel activator loxapine in WT but are occluded in knock-in littermates, supporting a direct channel gain-of-function mechanism. Taken together, these results suggest that K_Na1.1 gain-of-function dampens interneuron excitability to a greater extent than it impacts pyramidal neuron excitability, driving seizure susceptibility in a mouse model of KCNT1-associated epilepsy.

Hypoglycemia in Infants and Effect on Neurodevelopment

In a prospective, randomized treatment trial, investigators from multiple institutions in the HypoEXIT Study Group investigated the developmental outcomes after neonatal hypoglycemia, comparing the traditional glucose threshold 47 mg/dL vs. 36 mg/dL.

A single-center, retrospective analysis of genotype-phenotype correlations in children with Dravet syndrome

PURPOSE

Dravet syndrome is an early-onset epileptic encephalopathy caused most often by loss-of-function SCN1A variants. Following recognition of its genetic basis and unique clinical features, Dravet syndrome has become one of the most well-studied genetic epilepsies. We sought to evaluate the genetic diversity and correlative seizure phenotype, comorbidities, and response to antiepileptic therapies of patients with clinically-diagnosed Dravet syndrome seen in a tertiary care center. The goal of this study was to examine genotype-phenotype correlations and to ascertain if specific antiepileptic therapies may be more effective on the basis of genetic test result alone.

METHOD

Retrospective chart review of demographics, comorbidities, seizure types, and responses to antiepileptic therapies of all patients (n = 137) with a clinical diagnosis of Dravet syndrome seen at Lurie Children's Hospital of Chicago from 2008 to 2016.

RESULTS

Of the 96% of Dravet syndrome patients with pathogenic SCN1A variants subdivided by missense or truncating variant, there was no difference in clinical presentation. Response to antiepileptic therapies did not differ by genotype with regard to medication class.

CONCLUSIONS

This is the largest cohort of Dravet patients from within the US to report medication response with respect to genotype. Missense variants in SCN1A were most common in the voltage-sensor and pore domains. All patients were most likely to respond to the recommended medication triad compared to other antiepileptic therapies.

Functional consequences of a KCNT1 variant associated with status dystonicus and early-onset infantile encephalopathy

Objective

We identified a novel de novo KCNT1 variant in a patient with early‐infantile epileptic encephalopathy (EIEE) and status dystonicus, a life‐threatening movement disorder. We determined the functional consequences of this variant on the encoded K_Na1.1 channel to investigate the molecular mechanisms responsible for this disorder.

Methods

A retrospective case review of the proband is presented. We performed manual and automated electrophysiologic analyses of the KCNT1‐L437F variant expressed heterologously in Chinese hamster ovary (CHO) cells in the presence of channel activators/blockers.

Results

The KCNT1‐L437F variant, identified in a patient with refractory EIEE and status dystonicus, confers a gain‐of‐function channel phenotype characterized by instantaneous, voltage‐dependent activation. Channel openers do not further increase L437F channel function, suggesting maximal activation, whereas channel blockers similarly block wild‐type and variant channels. We further demonstrated that KCNT1 current can be measured on a high‐throughput automated electrophysiology platform with potential value for future screening of novel and repurposed pharmacotherapies.

Interpretation

A novel pathogenic variant in KCNT1 associated with early‐onset, medication‐refractory epilepsy and dystonia causes gain‐of‐function with rapid activation kinetics. Our findings extend the genotype–phenotype relationships of KCNT1 variants to include severe dystonia.

Treatment Responsiveness in KCNT1-Related Epilepsy

Pathogenic variants in KCNT1 represent an important cause of treatment-resistant epilepsy, for which an effective therapy has been elusive. Reports about the effectiveness of quinidine, a candidate precision therapy, have been mixed. We sought to evaluate the treatment responsiveness of patients with KCNT1-related epilepsy. We performed an observational study of 43 patients using a collaborative KCNT1 patient registry. We assessed treatment efficacy based upon clinical seizure reduction, side effects of quinidine therapy, and variant-specific responsiveness to treatment. Quinidine treatment resulted in a > 50% seizure reduction in 20% of patients, with rare patients achieving transient seizure freedom. Multiple other therapies demonstrated some success in reducing seizure frequency, including the ketogenic diet and vigabatrin, the latter particularly in patients with epileptic spasms. Patients with the best quinidine response had variants that clustered distal to the NADP domain within the RCK2 domain of the protein. Half of patients did not receive a quinidine trial. In those who did, nearly half did not achieve therapeutic blood levels. More favorable response to quinidine in patients with KCNT1 variants distal to the NADP domain within the RCK2 domain may suggest a variant-specific response.

Screening of conventional anticonvulsants in a genetic mouse model of epilepsy

OBJECTIVE

Epilepsy is a common neurological disorder that affects 1% of the population. Approximately, 30% of individuals with epilepsy are refractory to treatment, highlighting the need for novel therapies. Conventional anticonvulsant screening relies predominantly on induced seizure models. However, these models may not be etiologically relevant for genetic epilepsies. Mutations in SCN1A are a common cause of Dravet Syndrome, a severe epileptic encephalopathy. Dravet syndrome typically begins in infancy with seizures provoked by fever and then progresses to include afebrile pleomorphic seizure types. Affected children respond poorly to available anticonvulsants. Scn1a^+/- heterozygous knockout mice recapitulate features of Dravet syndrome and provide a potential screening platform to investigate novel therapeutics. In this study, we conducted a screening of conventional anticonvulsants in Scn1a^+/- mice to establish assays that most closely correlate with human response data.

METHODS

On the basis of clinical response data from a large, single center, retrospective survey of Dravet syndrome case records, we selected nine drugs for screening in Scn1a^+/- mice to determine which phenotypic measures correlate best with human therapeutic response. We evaluated several screening paradigms and incorporated pharmacokinetic monitoring to establish drug exposure levels.

RESULTS

Scn1a^+/- mice exhibited responses to anticonvulsant treatment similar to those observed clinically. Sodium channel blockers were not effective or exacerbated seizures in Scn1a^+/- mice. Overall, clobazam was the most effective anticonvulsant in Scn1a^+/- mice, consistent with its effect in Dravet syndrome.

INTERPRETATION

Genetic models of spontaneous epilepsy provide alternative screening platforms and may augment the AED development process. In this study, we established an effective screening platform that pharmacologically validated Scn1a^+/- mice for preclinical screening of potential Dravet syndrome therapeutics.

Imaging of Glutamate Concentration in Sturge-Weber Syndrome

Investigators from Wayne State University studied a cohort of children with Sturge-Weber syndrome (SWS) and epilepsy using both glucose-based positron emission tomography (FDG-PET) to evaluate metabolic activity and proton magnetic resonance spectroscopic imaging (MRSI) to evaluate glutamate turnover.

Visual Processing in Infants with Tuberous Sclerosis Complex

Investigators from Harvard University and UCLA have reported that despite evidence of structural abnormalities in the visual pathway of animal models and children with tuberous sclerosis complex (TSC), visual evoked potentials (VEPs) in 12-month old children with TSC compared to an age-matched control group are not significantly altered.

Prognosis with Incidental Rolandic spikes

Investigators from Johns Hopkins University reported a cohort of 27 patients with incidentally-noted rolandic spikes (RS) on EEG.

Impaired TrkB receptor signaling underlies corticostriatal dysfunction in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder. The debilitating choreic movements that plague HD patients have been attributed to striatal degeneration induced by the loss of cortically supplied brain-derived neurotrophic factor (BDNF). Here, we show that in mouse models of early symptomatic HD, BDNF delivery to the striatum and its activation of tyrosine-related kinase B (TrkB) receptors were normal. However, in striatal neurons responsible for movement suppression, TrkB receptors failed to properly engage postsynaptic signaling mechanisms controlling the induction of potentiation at corticostriatal synapses. Plasticity was rescued by inhibiting p75 neurotrophin receptor (p75NTR) signaling or its downstream target phosphatase-and-tensin-homolog-deleted-on-chromosome-10 (PTEN). Thus, corticostriatal synaptic dysfunction early in HD is attributable to a correctable defect in the response to BDNF, not its delivery.

Strain-specific regulation of striatal phenotype in Drd2-eGFP BAC transgenic mice

Mice carrying bacterial artificial chromosome (BAC) transgenes have become important tools for neuroscientists, providing a powerful means of dissecting complex neural circuits in the brain. Recently, it was reported that one popular line of these mice--mice possessing a BAC transgene with a D(2) dopamine receptor (Drd2) promoter construct coupled to an enhanced green fluorescent protein (eGFP) reporter--had abnormal striatal gene expression, physiology, and motor behavior. Unlike most of the work using BAC mice, this interesting study relied upon mice backcrossed on the outbred Swiss Webster (SW) strain that were homozygous for the Drd2-eGFP BAC transgene. The experiments reported here were conducted to determine whether mouse strain or zygosity was a factor in the reported abnormalities. As reported, SW mice were very sensitive to transgene expression. However, in more commonly used inbred strains of mice (C57BL/6, FVB/N) that were hemizygous for the transgene, the Drd2-eGFP BAC transgene did not alter striatal gene expression, physiology, or motor behavior. Thus, the use of inbred strains of mice that are hemizygous for the Drd2 BAC transgene provides a reliable tool for studying basal ganglia function.

A molecular basis for the increased vulnerability of substantia nigra dopamine neurons in aging and Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder of unknown etiology. There is no cure or proven strategy for slowing the progression of the disease. Although there are signs of pathology in many brain regions, the core symptoms of PD are attributable to the selective degeneration of dopaminergic neurons in the substantia nigra pars compacta. A potential clue to the vulnerability of these neurons is an increasing reliance with age upon L-type Ca(2+) channels with a pore-forming Cav1.3 subunit to support autonomous activity. This reliance could pose a sustained stress on mitochondrial ATP generating oxidative phosphorylation, accelerating cellular aging and death. Systemic administration of isradipine, a dihydropyridine blocker of these channels, forces dopaminergic neurons in rodents to revert to a juvenile, L-type Ca(2+) channel independent mechanism to generate autonomous activity. This "rejuvenation" confers protection against toxins that produce experimental Parkinsonism, pointing to a potential neuroprotective strategy for PD. Their decades-long track record of safe use in the treatment of hypertension makes dihydropyridines particularly attractive as a therapeutic tool in PD.

Calcium homeostasis, selective vulnerability and Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder of which the core motor symptoms are attributable to the degeneration of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Recent work has revealed that the engagement of L-type Ca(2+) channels during autonomous pacemaking renders SNc DA neurons susceptible to mitochondrial toxins used to create animal models of PD, indicating that homeostatic Ca(2+) stress could be a determinant of their selective vulnerability. This view is buttressed by the central role of mitochondria and the endoplasmic reticulum (linchpins of current theories about the origins of PD) in Ca(2+) homeostasis. Here, we summarize this evidence and suggest the dual roles had by these organelles could compromise their function, leading to accelerated aging of SNc DA neurons, particularly in the face of genetic or environmental stress. We conclude with a discussion of potential therapeutic strategies for slowing the progression of PD.