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New model for arrhythmias It is difficult to model cardiac arrhythmias in mice and other genetically tractable animals because the mechanisms of cardiomyocyte contraction in these animals are unlike those in humans. A new model for studying these conditions is reported, in the form of cardiomyocytes produced from induced pluripotent stem cells derived by reprogramming fibroblasts from two patients with Timothy syndrome, a disorder characterized by autism, immune deficiency and cardiac arrhythmias. The abnormal electrical and calcium-signalling properties of these patients' cells were restored by a drug, roscovitine, known to increase voltage-dependent inactivation of Ca V 1.2, a calcium channel that is defective in patients with Timothy syndrome. A mutation in the gene CACNA1C , encoding the L-type calcium channel Ca V 1.2 in humans, causes Timothy syndrome, a disorder characterized by autism, syndactyly, immune deficiency and cardiac arrhythmias. This study generated induced pluripotent stem cells from the fibroblasts of two patients with Timothy syndrome and converted them into cardiac cells. The patient cells displayed abnormal electrical and calcium signalling properties, which were restored by a drug, roscovitine, known to increase the voltage-dependent inactivation of Ca V 1.2. Individuals with congenital or acquired prolongation of the QT interval, or long QT syndrome (LQTS), are at risk of life-threatening ventricular arrhythmia 1 , 2 . LQTS is commonly genetic in origin but can also be caused or exacerbated by environmental factors 1 , 3 . A missense mutation in the L-type calcium channel Ca V 1.2 leads to LQTS in patients with Timothy syndrome 4 , 5 . To explore the effect of the Timothy syndrome mutation on the electrical activity and contraction of human cardiomyocytes, we reprogrammed human skin cells from Timothy syndrome patients to generate induced pluripotent stem cells, and differentiated these cells into cardiomyocytes. Electrophysiological recording and calcium (Ca 2+ ) imaging studies of these cells revealed irregular contraction, excess Ca 2+ influx, prolonged action potentials, irregular electrical activity and abnormal calcium transients in ventricular-like cells. We found that roscovitine, a compound that increases the voltage-dependent inactivation of Ca V 1.2 (refs 6–8 ), restored the electrical and Ca 2+ signalling properties of cardiomyocytes from Timothy syndrome patients. This study provides new opportunities for studying the molecular and cellular mechanisms of cardiac arrhythmias in humans, and provides a robust assay for developing new drugs to treat these diseases.

Timothy syndrome (TS) is a severe, multisystem disorder characterized by autism, epilepsy, long-QT syndrome and other neuropsychiatric conditions 1 . TS type 1 (TS1) is caused by a gain-of-function variant in the alternatively spliced and developmentally enriched CACNA1C exon 8A, as opposed to its counterpart exon 8. We previously uncovered several phenotypes in neurons derived from patients with TS1, including delayed channel inactivation, prolonged depolarization-induced calcium rise, impaired interneuron migration, activity-dependent dendrite retraction and an unanticipated persistent expression of exon 8A 2 – 6 . We reasoned that switching CACNA1C exon utilization from 8A to 8 would represent a potential therapeutic strategy. Here we developed antisense oligonucleotides (ASOs) to effectively decrease the inclusion of exon 8A in human cells both in vitro and, following transplantation, in vivo. We discovered that the ASO-mediated switch from exon 8A to 8 robustly rescued defects in patient-derived cortical organoids and migration in forebrain assembloids. Leveraging a transplantation platform previously developed 7 , we found that a single intrathecal ASO administration rescued calcium changes and in vivo dendrite retraction of patient neurons, suggesting that suppression of CACNA1C exon 8A expression is a potential treatment for TS1. Broadly, these experiments illustrate how a multilevel, in vivo and in vitro stem cell model-based approach can identify strategies to reverse disease-relevant neural pathophysiology. Antisense oligonucleotides effectively decrease the inclusion of exon  8A of CACNA1C in human cells both in vitro and in rodents transplanted with human brain organoids, and a single intrathecal administration rescued both calcium changes and in vivo dendrite morphology of patient neurons.

The development of the nervous system involves a coordinated succession of events including the migration of GABAergic (γ-aminobutyric-acid-releasing) neurons from ventral to dorsal forebrain and their integration into cortical circuits. However, these interregional interactions have not yet been modelled with human cells. Here we generate three-dimensional spheroids from human pluripotent stem cells that resemble either the dorsal or ventral forebrain and contain cortical glutamatergic or GABAergic neurons. These subdomain-specific forebrain spheroids can be assembled in vitro to recapitulate the saltatory migration of interneurons observed in the fetal forebrain. Using this system, we find that in Timothy syndrome—a neurodevelopmental disorder that is caused by mutations in the Ca V 1.2 calcium channel—interneurons display abnormal migratory saltations. We also show that after migration, interneurons functionally integrate with glutamatergic neurons to form a microphysiological system. We anticipate that this approach will be useful for studying neural development and disease, and for deriving spheroids that resemble other brain regions to assemble circuits in vitro . Human pluripotent stem cells were used to develop dorsal and ventral forebrain 3D spheroids, which can be assembled to study interneuron migration and to derive a functionally integrated forebrain system with cortical interneurons and glutamatergic neurons. Modelling forebrains in a dish GABAergic neurons play important roles in brain function and are implicated in numerous psychiatric disorders. They migrate long distances from the ventral to the dorsal forebrain before integrating to cortical circuits. In vitro modelling of GABAergic neuronal differentiation during this interaction would allow us to investigate the cause of human brain disorders associated with defects in neuronal migration, but this has so far been difficult. Sergiu Paşca and colleagues have developed an approach for generating neural three-dimensional spheroids resembling either the ventral or dorsal forebrain. They show that assembling the two types of spheroids separately in vitro allows the saltatory migration of human interneurons into the cortex, as seen in human development, and the formation of functional synapses with the dorsally derived cortical glutamatergic neurons. In this context, they find that interneurons from Timothy syndrome patients exhibit perturbation in migration patterns. Elsewhere in this issue, Paola Arlotta and colleagues carried out single cell expression analysis on cells from human brain organoids to investigate the nature of cells generated by these three-dimensional models.

Human limbs emerge during the fourth post-conception week as mesenchymal buds, which develop into fully formed limbs over the subsequent months 1 . This process is orchestrated by numerous temporally and spatially restricted gene expression programmes, making congenital alterations in phenotype common 2 . Decades of work with model organisms have defined the fundamental mechanisms underlying vertebrate limb development, but an in-depth characterization of this process in humans has yet to be performed. Here we detail human embryonic limb development across space and time using single-cell and spatial transcriptomics. We demonstrate extensive diversification of cells from a few multipotent progenitors to myriad differentiated cell states, including several novel cell populations. We uncover two waves of human muscle development, each characterized by different cell states regulated by separate gene expression programmes, and identify musculin (MSC) as a key transcriptional repressor maintaining muscle stem cell identity. Through assembly of multiple anatomically continuous spatial transcriptomic samples using VisiumStitcher, we map cells across a sagittal section of a whole fetal hindlimb. We reveal a clear anatomical segregation between genes linked to brachydactyly and polysyndactyly, and uncover transcriptionally and spatially distinct populations of the mesenchyme in the autopod. Finally, we perform single-cell RNA sequencing on mouse embryonic limbs to facilitate cross-species developmental comparison, finding substantial homology between the two species. Using single-cell and spatial transcriptomics, human embryonic limb development across space and time and the diversification and cross-species conservation of cells are demonstrated.

BackgroundAarskog-Scott syndrome (AAS) is a rare condition with multiple congenital anomalies, caused by hemizygote variants in the FGD1 gene. Its description was based mostly on old case reports, in whom a molecular diagnosis was not always available, or on small series. The aim of this study was to better delineate the phenotype and the natural history of AAS and to provide clues for the diagnosis and the management of the patients.MethodsPhenotypic characterisation of the largest reported AAS cohort, comprising 111 male patients with proven causative variants in FGD1, through comprehensive analyses of clinical data including congenital anomalies, growth and neurodevelopment. Review of photographs and radiographs by experts in dysmorphology and skeletal disorders.ResultsThis study refines the phenotypic spectrum of AAS, with the description of new morphological and radiological features, and refines the prevalence of the features. Short stature is less frequent than previously reported and has a prenatal onset in more than half of the patients. The growth has a specific course with a catch-up during the first decade often leading to low-normal stature in adulthood. Whereas intellectual disability is rare, patients with AAS have a high prevalence of specific learning difficulties and attention hyperactivity disorder. In light of this better knowledge of AAS, we provide management recommendations.ConclusionA better knowledge of the natural history and phenotypic spectrum of AAS will be helpful for the clinical diagnosis and for the interpretation of FGD1 variants using a retrophenotyping strategy, which is becoming the most common way of diagnosis nowadays. Recommendations for care will improve the management of the patients.

Timothy Syndrome (TS) is a multisystem disorder, prominently featuring cardiac action potential prolongation with paroxysms of life-threatening arrhythmias. The underlying defect is a single de novo missense mutation in CaV1.2 channels, either G406R or G402S. Notably, these mutations are often viewed as equivalent, as they produce comparable defects in voltage-dependent inactivation and cause similar manifestations in patients. Yet, their effects on calcium-dependent inactivation (CDI) have remained uncertain. Here, we find a significant defect in CDI in TS channels, and uncover a remarkable divergence in the underlying mechanism for G406R versus G402S variants. Moreover, expression of these TS channels in cultured adult guinea pig myocytes, combined with a quantitative ventricular myocyte model, reveals a threshold behaviour in the induction of arrhythmias due to TS channel expression, suggesting an important therapeutic principle: a small shift in the complement of mutant versus wild-type channels may confer significant clinical improvement.

The authors report that calcium channels with a mutation associated with Timothy syndrome cause activity-dependent dendrite retraction in rodent neurons and in induced pluripotent stem cell–derived neurons from individuals with Timothy syndrome. This retraction was independent of Ca permeation but was associated with activation of RhoA signaling. L-type voltage gated calcium channels have an important role in neuronal development by promoting dendritic growth and arborization. A point mutation in the gene encoding Ca V 1.2 causes Timothy syndrome, a neurodevelopmental disorder associated with autism spectrum disorders (ASDs). We report that channels with the Timothy syndrome alteration cause activity-dependent dendrite retraction in rat and mouse neurons and in induced pluripotent stem cell (iPSC)-derived neurons from individuals with Timothy syndrome. Dendrite retraction was independent of calcium permeation through the mutant channel, was associated with ectopic activation of RhoA and was inhibited by overexpression of the channel-associated GTPase Gem. These results suggest that Ca V 1.2 can activate RhoA signaling independently of Ca 2+ and provide insights into the cellular basis of Timothy syndrome and other ASDs.

Syndactyly is one of the most common hereditary limb malformations depicting the fusion of certain fingers and/or toes. It may occur as an isolated entity or a component of more than 300 syndromic anomalies. Syndactylies exhibit great inter- and intra-familial clinical variability. Even within a subject, phenotype can be unilateral or bilateral and symmetrical or asymmetrical. At least nine non-syndromic syndactylies with additional sub-types have been characterized. Most of the syndactyly types are inherited as autosomal dominant but two autosomal recessive and an X-linked recessive entity have also been described. Whereas the underlying genes/mutations for types II-1, III, IV, V, and VII have been worked out, the etiology and molecular basis of the other syndactyly types remain unknown. In this communication, based on an overview of well-characterized isolated syndactylies, their cardinal phenotypes, inheritance patterns, and clinical and genetic heterogeneities, a 'current classification scheme' is presented. Despite considerable progress in the understanding of syndactyly at clinical and molecular levels, fundamental questions regarding the disturbed developmental mechanisms leading to fused digits, remain to be answered.

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are a powerful platform for modeling inherited arrhythmias, yet current in silico representations face limitations in Ca 2+ handling. Here, we present a novel ventricular hiPSC-CM ionic model incorporating a Markovian formulation of the L-type Ca 2+ current (I ), tailored to better recapitulate Ca dynamics and voltage-dependent inactivation. The model was calibrated against experimental data from hiPSC-CMs derived from a healthy individual and validated through a series of simulations relevant to both physiological and pathological conditions. These included pharmacological inhibition of I with nifedipine, Ca overload and DAD-mediated triggered activity, and the interplay between intracellular Ca cycling and membrane mechanisms in driving automaticity. Sensitivity analysis was used to generate a population of models capturing intercellular variability. In addition, the model was able to reproduce the effects of genetic mutations in the L-type Ca channel, including those associated with Timothy Syndrome, providing an additional layer of validation. Overall, this computational framework offers a flexible and physiologically grounded tool for investigating the mechanisms of arrhythmogenesis in hiPSC-CMs and for supporting personalized medicine applications.

The canonical G406R mutation that increases Ca 2+ influx through the CACNA1C -encoded Ca V 1.2 Ca 2+ channel underlies the multisystem disorder Timothy syndrome (TS), characterized by life-threatening arrhythmias. Severe episodic hypoglycemia is among the poorly characterized non-cardiac TS pathologies. While hypothesized from increased Ca 2+ influx in pancreatic beta cells and consequent hyperinsulinism, this hypoglycemia mechanism is undemonstrated because of limited clinical data and lack of animal models. We generated a Ca V 1.2 G406R knockin mouse model that recapitulates key TS features, including hypoglycemia. Unexpectedly, these mice do not show hyperactive beta cells or hyperinsulinism in the setting of normal intrinsic beta cell function, suggesting dysregulated glucose homeostasis. Patient data confirm the absence of hyperinsulinism. We discover multiple alternative contributors, including perturbed counterregulatory hormone responses with defects in glucagon secretion and abnormal hypothalamic control of glucose homeostasis. These data provide new insights into contributions of Ca V 1.2 channels and reveal integrated consequences of the mutant channels driving life-threatening events in TS. Gain of function mutant CaV1.2 channels drive life-threatening hypoglycemia in the multisystem disorder Timothy syndrome, but the underlying mechanisms are unknown. Here the authors show the mutant channels have adverse effects on counterregulatory hormones and CNS control of glucose homeostasis.