Stanford Advisors

All Publications

  • Generation and characterization of induced pluripotent stem cells from breast cancer patients carrying ATM mutations. Stem cell research Zhang, M., Venkateshappa, R., Li, A., Fowler, M. B., Telli, M. L., Wu, J. C. 2023; 73: 103246


    We generated two induced pluripotent stem cell (iPSC) lines from peripheral blood mononuclear cells (PBMCs) of breast cancer patients carrying germline ATM mutations, a gene associated with a 7% prevalence in breast cancer. These iPSC lines displayed typical morphology, expressed pluripotency markers, maintained a stable karyotype, and retained the ability to differentiate into the three germ layers. These patient-specific iPSC lines hold great potential for mechanistic investigations and the development of drug screening strategies aimed at addressing ATM-related cancer.

    View details for DOI 10.1016/j.scr.2023.103246

    View details for PubMedID 37951143

  • Targeted activation of hERG channels rescues electrical instability induced by the hERG R56Q+/- Long QT Syndrome variant. Cardiovascular research Venkateshappa, R., Hunter, D. V., Muralidharan, P., Nagalingam, R. S., Huen, G., Faizi, S., Luthra, S., Lin, E., Cheng, Y. M., Hughes, J., Khelifi, R., Dhunna, P., Johal, R., Sergeev, V., Shafaattalab, S., Julian, L. M., Poburko, D. T., Laksman, Z., Tibbits, G. F., Claydon, T. W. 2023


    Long QT Syndrome Type 2 (LQTS2) is associated with inherited variants in the cardiac hERG K+ channel. However, the pathogenicity of hERG channel gene variants is often uncertain. Using CRISPR-Cas9 gene-edited hiPSC-derived cardiomyocytes (hiPSC-CMs), we investigated the pathogenic mechanism underlying the LQTS-associated hERG R56Q variant, and its phenotypic rescue by the type 1 hERG activator, RPR260243.These approaches enable characterization of the unclear causative mechanism of arrhythmia in the R56Q variant (an N-terminal PAS domain mutation that primarily accelerates channel deactivation) and translational investigation of the potential for targeted pharmacologic manipulation of hERG deactivation. Using perforated patch clamp electrophysiology of single hiPSC-CMs, programmed electrical stimulation showed that the hERG R56Q variant does not significantly alter the mean APD90. However, the R56Q variant increases the beat-to-beat variability in APD90 during pacing at constant cycle lengths, enhances the variance of action potential duration (APD90) during rate transitions, and increases the incidence of 2:1 block. During paired S1-S2 stimulations measuring electrical restitution properties, the R56Q variant was also found to increase the variability in rise time and duration of the response to premature stimulations. Application of the hERG channel activator, RPR260243, reduces the APD variance in hERG R56Q hiPSC-CMs, reduces the variability in responses to premature stimulations, and increases the post-repolarization refractoriness.Based on our findings, we propose that the hERG R56Q variant leads to heterogeneous APD dynamics, which could result in spatial dispersion of repolarization and increased risk for re-entry without significantly affecting the average APD90. Furthermore, our data highlight the antiarrhythmic potential of targeted slowing of hERG deactivation gating, which we demonstrate increases protection against premature action potentials and reduces electrical heterogeneity in hiPSC-CMs.

    View details for DOI 10.1093/cvr/cvad155

    View details for PubMedID 37739930

  • CRISPR-Cas9-mediated Precise Knock-in Edits in Zebrafish Hearts. Journal of visualized experiments : JoVE Simpson, K. E., Faizi, S., Venkateshappa, R., Yip, M., Johal, R., Poburko, D., Cheng, Y. M., Hunter, D., Lin, E., Tibbits, G. F., Claydon, T. W. 2022


    Clustered regularly interspaced short palindromic repeats (CRISPR) in animal models enable precise genetic manipulation for the study of physiological phenomena. Zebrafish have been used as an effective genetic model to study numerous questions related to heritable disease, development, and toxicology at the whole-organ and -organism level. Due to the well-annotated and mapped zebrafish genome, numerous tools for gene editing have been developed. However, the efficacy of generating and ease of detecting precise knock-in edits using CRISPR is a limiting factor. Described here is a CRISPR-Cas9-based knock-in approach with the simple detection of precise edits in a gene responsible for cardiac repolarization and associated with the electrical disorder, Long QT Syndrome (LQTS). This two-single-guide RNA (sgRNA) approach excises and replaces the target sequence and links a genetically encoded reporter gene. The utility of this approach is demonstrated by describing non-invasive phenotypic measurements of cardiac electrical function in wild-type and gene-edited zebrafish larvae. This approach enables the efficient study of disease-associated variants in a whole organism. Furthermore, this strategy offers possibilities for the insertion of exogenous sequences of choice, such as reporter genes, orthologs, or gene editors.

    View details for DOI 10.3791/64209

    View details for PubMedID 36190280

  • Ion channel model reduction using manifold boundaries. Journal of the Royal Society, Interface Whittaker, D. G., Wang, J., Shuttleworth, J. G., Venkateshappa, R., Kemp, J. M., Claydon, T. W., Mirams, G. R. 2022; 19 (193): 20220193


    Mathematical models of voltage-gated ion channels are used in basic research, industrial and clinical settings. These models range in complexity, but typically contain numerous variables representing the proportion of channels in a given state, and parameters describing the voltage-dependent rates of transition between states. An open problem is selecting the appropriate degree of complexity and structure for an ion channel model given data availability. Here, we simplify a model of the cardiac human Ether-à-go-go related gene (hERG) potassium ion channel, which carries cardiac IKr, using the manifold boundary approximation method (MBAM). The MBAM approximates high-dimensional model-output manifolds by reduced models describing their boundaries, resulting in models with fewer parameters (and often variables). We produced a series of models of reducing complexity starting from an established five-state hERG model with 15 parameters. Models with up to three fewer states and eight fewer parameters were shown to retain much of the predictive capability of the full model and were validated using experimental hERG1a data collected in HEK293 cells at 37°C. The method provides a way to simplify complex models of ion channels that improves parameter identifiability and will aid in future model development.

    View details for DOI 10.1098/rsif.2022.0193

    View details for PubMedID 35946166

    View details for PubMedCentralID PMC9363999

  • Electrophysiological characterization of the hERG R56Q LQTS variant and targeted rescue by the activator RPR260243. The Journal of general physiology Kemp, J. M., Whittaker, D. G., Venkateshappa, R., Pang, Z., Johal, R., Sergeev, V., Tibbits, G. F., Mirams, G. R., Claydon, T. W. 2021; 153 (10)


    Human Ether-à-go-go (hERG) channels contribute to cardiac repolarization, and inherited variants or drug block are associated with long QT syndrome type 2 (LQTS2) and arrhythmia. Therefore, hERG activator compounds present a therapeutic opportunity for targeted treatment of LQTS. However, a limiting concern is over-activation of hERG resurgent current during the action potential and abbreviated repolarization. Activators that slow deactivation gating (type I), such as RPR260243, may enhance repolarizing hERG current during the refractory period, thus ameliorating arrhythmogenicity with reduced early repolarization risk. Here, we show that, at physiological temperature, RPR260243 enhances hERG channel repolarizing currents conducted in the refractory period in response to premature depolarizations. This occurs with little effect on the resurgent hERG current during the action potential. The effects of RPR260243 were particularly evident in LQTS2-associated R56Q mutant channels, whereby RPR260243 restored WT-like repolarizing drive in the early refractory period and diastolic interval, combating attenuated protective currents. In silico kinetic modeling of channel gating predicted little effect of the R56Q mutation on hERG current conducted during the action potential and a reduced repolarizing protection against afterdepolarizations in the refractory period and diastolic interval, particularly at higher pacing rates. These simulations predicted partial rescue from the arrhythmic effects of R56Q by RPR260243 without risk of early repolarization. Our findings demonstrate that the pathogenicity of some hERG variants may result from reduced repolarizing protection during the refractory period and diastolic interval with limited effect on action potential duration, and that the hERG channel activator RPR260243 may provide targeted antiarrhythmic potential in these cases.

    View details for DOI 10.1085/jgp.202112923

    View details for PubMedID 34398210

    View details for PubMedCentralID PMC8493834

  • The hERG channel activator, RPR260243, enhances protective IKr current early in the refractory period reducing arrhythmogenicity in zebrafish hearts. American journal of physiology. Heart and circulatory physiology Shi, Y. P., Pang, Z., Venkateshappa, R., Gunawan, M., Kemp, J., Truong, E., Chang, C., Lin, E., Shafaattalab, S., Faizi, S., Rayani, K., Tibbits, G. F., Claydon, V. E., Claydon, T. W. 2020; 319 (2): H251-H261


    Human ether-à-go-go related gene (hERG) K+ channels are important in cardiac repolarization, and their dysfunction causes prolongation of the ventricular action potential, long QT syndrome, and arrhythmia. As such, approaches to augment hERG channel function, such as activator compounds, have been of significant interest due to their marked therapeutic potential. Activator compounds that hinder channel inactivation abbreviate action potential duration (APD) but carry risk of overcorrection leading to short QT syndrome. Enhanced risk by overcorrection of the APD may be tempered by activator-induced increased refractoriness; however, investigation of the cumulative effect of hERG activator compounds on the balance of these effects in whole organ systems is lacking. Here, we have investigated the antiarrhythmic capability of a hERG activator, RPR260243, which primarily augments channel function by slowing deactivation kinetics in ex vivo zebrafish whole hearts. We show that RPR260243 abbreviates the ventricular APD, reduces triangulation, and steepens the slope of the electrical restitution curve. In addition, RPR260243 increases the post-repolarization refractory period. We provide evidence that this latter effect arises from RPR260243-induced enhancement of hERG channel-protective currents flowing early in the refractory period. Finally, the cumulative effect of RPR260243 on arrhythmogenicity in whole organ zebrafish hearts is demonstrated by the restoration of normal rhythm in hearts presenting dofetilide-induced arrhythmia. These findings in a whole organ model demonstrate the antiarrhythmic benefit of hERG activator compounds that modify both APD and refractoriness. Furthermore, our results demonstrate that targeted slowing of hERG channel deactivation and enhancement of protective currents may provide an effective antiarrhythmic approach.NEW & NOTEWORTHY hERG channel dysfunction causes long QT syndrome and arrhythmia. Activator compounds have been of significant interest due to their therapeutic potential. We used the whole organ zebrafish heart model to demonstrate the antiarrhythmic benefit of the hERG activator, RPR260243. The activator abbreviated APD and increased refractoriness, the combined effect of which rescued induced ventricular arrhythmia. Our findings show that the targeted slowing of hERG channel deactivation and enhancement of protective currents caused by the RPR260243 activator may provide an effective antiarrhythmic approach.

    View details for DOI 10.1152/ajpheart.00038.2020

    View details for PubMedID 32559136