Circulation: Tell us about the key findings from your recent article in Circulation.
Dr. Delmar: This work relates to two inherited arrhythmogenic disorders: Brugada syndrome (BrS) and arrhythmogenic cardiomyopathy (AC; also called “ARVC” or “ARVD”). Brugada syndrome (BrS) primarily associates with loss of sodium channel function, whereas AC is most commonly associated with mutations in proteins of the desmosome and among them, with a particular protein called “plakophilin-2” (PKP2). In previous studies, my collaborators and I showed that loss of expression of PKP2 leads to reduced sodium current amplitude. Here we asked whether patients with clinical diagnosis of BrS could actually have mutations in PKP2, a gene normally associated with AC. We searched for PKP2 variants in genomic DNA of 200 patients with BrS diagnosis, no signs of AC, and no mutations in BrS-related genes, and found five cases of single amino acid substitutions in PKP2. Through a series of experimental models, including induced human pluripotent stem cell-derived cardiac myocytes (hIPSC-CMs) from a patient with a PKP2 deficit, we confirmed that PKP2 variants that reduce INa can yield a BrS phenotype, even without overt structural features.
Circulation: What are the major implications of this work?
Dr. Delmar: This is the first systematic retrospective analysis of a patient group to define the co-existence of a sodium channelopathy and genetic PKP2 variations. Our findings suggest that PKP2 mutations may be a molecular substrate leading to the diagnosis not only of AC, but of BrS as well. In our opinion, the inclusion of PKP2 as part of routine BrS genetic testing remains premature; yet, the possibility that some patients showing signs of BrS may harbor PKP2 variants should be considered when the genotype is negative for other genes associated with BrS.
Circulation: How did you get the idea to do this study?
Dr. Delmar: When the finding that AC is linked to mutations in desmosomal proteins was first published, I was puzzled by the notion that a structure that is supposed to just be acting as the glue between cells (a desmosome), was associated with lethal arrhythmias. At that time, I decided to tackle the question from the other end: What if molecules “of the desmosome” are not only there to make desmosomes? As a test, I decided to dissociate a heart into single myocytes (therefore lacking desmosomes), silence expression of a “desmosomal protein” (PKP2) and test the properties of the sodium current. To my surprise, the data showed that sodium current was reduced in half and the voltage dependence of the current was also shifted. Those findings, first published in 2009, were followed by a series of other published studies and most recently, by the present paper.
Circulation: What was your biggest obstacle in completing this study?
Dr. Delmar: Closing “the translational gap.” We had the sequencing results showing PKP2 mutations in the BrS patients, and we had a system of HL1 cells in culture to look for sodium current, but the reviewers made an excellent point: the HL1 cells are derived from a mouse atrial tumor and as such, not the ideal (or the only) surrogate for studying the disease. Here, the collaboration with Vincent Chen and with Dan Judge was critical. They had previously characterized an hIPSC-CM cell line from a patient with a PKP2 deficiency. Working in collaboration, we were able to show that those cells had low sodium current, which could be rescued by re-introducing the wild-type PKP2 gene, but not a BrS-related PKP2 mutant form. Those experiments “nailed” the paper!
Circulation: What was your most unexpected finding?
Dr. Delmar: I guess the most unexpected finding was the data showing a drastic separation of the microtubule plus-end tracking protein EB1 from the cell end in PKP2-deficient myocytes. Part of the unexpected was how beautifully clear EB1 showed by super-resolution fluorescence microscopy. The system allowed us to see fluorescently labeled proteins with a spatial resolution previously reserved only to electron microscopy studies! Those experiments were possible thanks to the very enjoyable collaboration that I have with Dr. Eli Rothenberg, here at NYU.
Circulation: What do you plan to do next, based on these current findings?
Dr. Delmar: One of the best parts of carrying out this project was that I was able to establish wonderful collaborations. This allowed us to attack the problem from multiple directions, using complementary methods. I want to build on those collaborations and further develop the idea of combining super-resolution imaging, hIPSC-CMs and HL1 cells to study the molecular organization of complexes relevant to sudden cardiac death in the young. I think that visual proteomics methods can shed new light on the organization of these molecules and one day, help us in the process of risk stratification of patients in the pre-clinical stage.
Circulation: What do you like to do in your free time?
Dr. Delmar: My wife and I enjoy everything that New York City has to offer: good restaurants, great museums, terrific opera, ballet, shows, parks, shops, people. Running in Central Park is one of my favorite activities. Taking long walks with my wife through the parks and streets of the city is also a favorite for both of us.
Circulation: What is your favorite sports team or musical group?
Dr. Delmar: The sports event that I follow more or less closely is the football soccer World Cup. (And of course my favorite team is Italy!) As for musical groups, I grew up hearing a bit of everything, but my favorite albums growing up were “The Dark Side of the Moon,” “Wish you were Here” and of course “The Wall” so I guess the top spot could still go to Pink Floyd. Recently though I mostly hear Opera, which I have also loved since I was young.