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Optimal cardiac function requires appropriate contractile proteins in each heart chamber. Atria require slow myosins to act as variable reservoirs, while ventricles demand fast myosin for swift pumping functions. Hence, myosin is under chamber-biased cis-regulatory control to achieve this functional distribution. Failure in proper regulation of myosin genes can lead to severe congenital heart dysfunction. The precise regulatory input leading to cardiac chamber-biased expression remains uncharted. To address this, we computationally and molecularly dissected the quail Slow Myosin Heavy Chain III (SMyHC III) promoter that drives specific gene expression to the atria to uncover the regulatory information leading to chamber expression and understand their evolutionary origins. We show that SMyHC III gene states are autonomously orchestrated by a complex nuclear receptor cis-regulatory element (cNRE), a 32- bp sequence with hexanucleotide binding repeats. Using in vivo transgenic assays in zebrafish and mouse models, we demonstrate that preferential atrial expression is achieved by the combinatorial regulatory input composed of atrial activation motifs and ventricular repression motifs. Through comparative genomics, we provide evidence that the cNRE emerged from an endogenous viral element, most likely through infection of an ancestral host germline. Our study reveals an evolutionary pathway to cardiac chamber-specific expression.

The human PDZK1 gene is located in a genomic susceptibility region for neurodevelopmental disorders. A genome-wide association study (GWAS) identified links between PDZK1 polymorphisms and altered visual contrast sensitivity, an endophenotype for schizophrenia and autism spectrum disorder. The PDZK1 protein is implicated in neurological functioning, interacting with synaptic molecules including post-synaptic density 95 (PSD-95), N-methyl-D-aspartate receptors (NMDAR), corticotropin-releasing factor receptor 1 (CRFR1) and serotonin 2A receptors. To elucidate the role of PDZK1, we generated pdzk1-knockout (pdzk1-KO) zebrafish using CRISPR/Cas-9 genome editing. Visual function of 7-day-old fish was assessed at behavioural and functional levels using the optomotor response (OMR) and scotopic electroretinogram (ERG). We also quantified retinal morphology and densities of PSD-95, NMDAR1, CRFR1 and serotonin in the synaptic inner plexiform layer at 7 days, 4 weeks and 8 weeks of age. Relative to wild-type, pdzk1-KO larvae showed spatial-frequency tuning functions with increased amplitude (likely due to abnormal gain control) and reduced ERG b-waves (suggestive of inner retinal dysfunction). However, these functional differences were not associated with gross synaptic or morphological retinal phenotypes. The findings corroborate a role for pdzk1 in visual function, and our model system provides a platform for investigating other genes associated with abnormal visual behaviour.