Benjamin Davies

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and is promoted by atrial fibrosis, a form of structural remodelling for which effective therapies are lacking. MicroRNA-31-5p (miR-31-5p), an aging-associated, atrial-enriched microRNA, has been linked to AF, but its role in atrial fibrosis is unknown. Here we show that atrial fibroblasts from patients with persistent AF exhibit increased miR-31-5p expression and reduced expression of the calcitonin receptor (CTR, encoded by CALCR ). MiR-31-5p directly represses CTR and promotes pro-fibrotic fibroblast responses, whereas its inhibition restores CTR expression. Fibroblast-specific overexpression of miR-31 in mice reduces atrial CTR expression, increases atrial fibrosis and enhances arrhythmia susceptibility. Selective disruption of miR-31-5p− CALCR binding using a locked nucleic acid target site blocker restores atrial CTR expression, prevents fibrosis progression and reduces arrhythmia burden. In mice with established atrial remodeling, this strategy promotes fibrosis regression, identifying the miR-31-5p−CTR axis as a potential therapeutic target in AF.

Background: KCNV2‐associated retinopathy is a rare inherited retinal dystrophy caused by variants in the KCNV2 gene, leading to disrupted photoreceptor behaviour and progressive deterioration of vision. Patients have characteristic electroretinography abnormalities, including reduced cone response, delayed and reduced rod response to low light flashes and paradoxically large rod‐driven response to bright flashes of light. To model this condition, we have generated a Kcnv2 E151X mouse line and assessed its structural and functional retinal features. Methods: We have employed CRISPR/Cas 9 gene editing technology to generate a mouse line with an early stop mutation in position E151—orthologous to the commonly encountered E143X mutation in humans—and performed a combination of immunohistochemistry and Western blot to confirm the absence of the full‐length KCNV2—encoded protein, Kv8.2. Next, to assess how closely it models the human disease, we have characterised the KCNV2 mutant mouse line at histological and functional levels, via immunohistochemistry and electroretinography experiments, respectively. Results: Kcnv2 mutant mice showed markedly reduced photopic responses and reproduced the supernormal rod phenotype described in affected individuals. In the morphological context, mutant retinas demonstrated strong glial fibrillary acidic protein upregulation together with reduced cone arrestin positive cell counts and photoreceptor layers, indicating photoreceptor loss. Conclusions: The Kcnv2 mutant mouse line replicates key functional and structural hallmarks of KCNV2‐associated retinopathy. This model provides a relevant platform for mechanistic studies and preclinical evaluation of gene‐based or pharmacological therapies targeting cone and rod photoreceptor dysfunction.

Aims: Titin truncating variants (TTNtv) are a major genetic cause of dilated cardiomyopathy (DCM), accounting for approximately 25% of familial cases. Atrial fibrillation (AF) frequently occurs in DCM patients carrying TTNtv and may precede overt ventricular dysfunction, suggesting an atrial-specific disease mechanism. How TTNtv increase susceptibility to AF, particularly in the absence of established DCM, remains incompletely understood. This study aimed to define the cellular and molecular mechanisms by which a clinically relevant TTNtv predisposes to atrial arrhythmogenesis. Methods and results: We introduced a patient-associated TTNtv (TTN c.59926+1G>A) into human induced pluripotent stem cell-derived atrial cardiomyocytes (hiPSC-CMs). TTNtv hiPSC-CMs exhibited proarrhythmic electrophysiological alterations, including increased spontaneous beating frequency, abnormal sodium channel kinetics, and heightened sensitivity to cholinergic agonists. In silico simulations demonstrated that heightened cholinergic sensitivity was sufficient to trigger AF under conditions of sinus tachycardia. RNA sequencing revealed dysregulation of sarcomere assembly and extracellular matrix pathways, and TTNtv hiPSC-CMs showed structurally shortened sarcomeres. Engineered heart tissues composed of TTNtv hiPSC-CMs co-cultured with cardiac fibroblasts demonstrated reduced contractile force and increased secretion of collagen, fibronectin-1 and TGF-β1, consistent with activation of profibrotic signalling. Together, these findings indicate that a TTNtv can cause intrinsic atrial electrical instability and promote pro-fibrotic signalling. Conclusion: Our results identify atrial electrophysiological abnormalities and profibrotic remodelling as key mechanisms by which TTNtv increase AF risk, even in the absence of overt DCM. These findings support a primary atrial contribution to TTNtv-associated arrhythmogenesis and provide mechanistic insight into AF as an early clinical manifestation in carriers.

Enhancers and their target promoters often come into close physical proximity when activated. This may be explained by a variety of mechanisms, including cohesin-mediated chromatin loop extrusion. However, acute depletion of cohesin does not cause widespread changes in gene expression. We have tested the role of cohesin-mediated loop extrusion in gene expression at the mouse alpha-globin locus during erythropoiesis. Acute depletion of cohesin disrupts alpha-globin expression at early but not late stages of differentiation. Furthermore, when single or multiple CTCF sites, known to block cohesin, are placed between the alpha-globin enhancers and promoters, alpha-gene expression is disrupted. Importantly, the CTCF site’s orientation is critical, suggesting that within this activated domain, in definitive erythroid cells, cohesin predominantly but not exclusively, translocates from the enhancers to the promoters. We find that loop extrusion does play an important role in establishing enhancer-promoter proximity and consequent expression of inducible genes during differentiation.

The mammalian genome is organised into large topologically associating domains (TADs) and smaller sub-TADs or enhancer-promoter loops, which may contribute to the regulation of gene expression. These dynamic structures arise, at least partly, via cohesin-mediated loop extrusion delimited by insulator elements. By studying the structure and function of the alpha-globin locus during erythroid differentiation, we have previously shown that the juxtaposition of the enhancers and promoters during this process partly depends on cohesin-mediated loop extrusion, which appears to be delimited by 12 largely convergently orientated CTCF boundary elements. To define the downstream boundary of the sub-TAD, we removed four CTCF sites in informative combinations. This showed that rather than CTCF insulators, it is the transcriptionally active alpha-globin gene that defines the downstream boundary of the sub-TAD. Further, insertion of actively transcribed fragments of the α-globin gene between the enhancers and native genes leads to a reduction in native α-globin expression and accumulation of cohesin at the insertion site. This highlights an overlap in the functional role of the fundamental elements of the genome.

Summary: RNA-binding proteins (RBPs) are key regulators of gene expression and often contain intrinsically disordered regions that drive biomolecular condensation. Yet, how condensation affects RBP specificity and function remains poorly defined. Here, we use strategically designed point mutations to selectively impair canonical RNA-binding and condensation of the amyotrophic lateral sclerosis (ALS)-linked RBP Fused in Sarcoma (FUS). Using automated high-content imaging, we show that both properties shape nuclear ribonucleoprotein condensates and govern distinct aspects of FUS function in the DNA damage response. Transcriptome-wide mapping of FUS-RNA interactions reveals that the canonical RNA-binding domains recognise G-rich and C-rich motifs, whereas condensation selectively enhances binding to G-rich and structured sequences, often acting in concert with canonical RNA-binding to control transcriptional programs and splicing decisions. These findings provide a mechanistic framework for how FUS integrates condensation with sequence-specific RNA recognition to orchestrate nuclear organisation, genome stability and RNA metabolism. Furthermore, our rigorously validated mutants offer a platform for future mechanistic dissection of ALS pathogenesis and the development of targeted therapeutic strategies. Highlights: FUS condensation and RNA-binding can be functionally uncoupled with targeted point mutations Condensation drives FUS recruitment and assembly of nuclear RNP condensates FUS condensation and RNA-binding play distinct roles in the DNA damage response The structured RNA-binding domains confer specificity to G-rich and C-rich motifs Condensation enhances FUS binding to G-rich, structured RNA elements FUS condensation and canonical RNA-binding synergistically regulate gene expression

Background: KCNV2-associated retinopathy is a rare inherited retinal dystrophy caused by variants in the KCNV2 gene, leading to disrupted photoreceptor function and slowly progressive vision loss. Patients have characteristic electroretinography abnormalities, including reduced cone responses, delayed and reduced rod responses to low light flashes and paradoxically large rod-driven responses to bright flashes of light. To model this condition, we generated the Kcnv2 E151X mouse line and assessed its structural and functional retinal features. Methods: We have employed CRISPR/Cas 9 gene editing technology to generate a mouse line with an early stop mutation in position E151 - orthologous to the commonly encountered E143X mutation in humans - and performed a combination of immunohistochemistry and Western Blot to confirm the absence of the full-length KCNV2-encoded protein, Kv8.2. To assess how closely it models the human disease, we have characterised the KCNV2 mutant mouse line at histological and functional levels employing immunohistochemistry and electroretinography, respectively. Results: Kcnv2 mutant mice showed markedly reduced photopic responses and reproduced the supernormal rod phenotype as described in affected individuals. In the morphological context, mutant retinas demonstrated strong and early glial fibrillary acidic protein upregulation together with reduced counts of cone arrestin positive cells as well as photoreceptors in general. Conclusions: The Kcnv2 mutant mouse line replicates key functional and structural hallmarks of KCNV2-associated retinopathy. This model provides a relevant platform for mechanistic studies and preclinical evaluation of gene based or pharmacological therapies targeting cone and rod photoreceptor dysfunction.

ERCC6L2 disease is a recessive bone marrow failure (BMF) syndrome caused by mutations in the SNF2-like putative DNA helicase ERCC6L2. While implicated in DNA replication, double strand break (DSB) repair via non-homologous end joining (NHEJ), and interstrand crosslink (ICL) repair, how ERCC6L2 supports haematopoietic longevity remains unclear. Investigating this in vivo , we find that an Ercc6l2- deficient haematopoietic stem and progenitor cell (HSPC) compartment in mice is unexpectedly resilient. Ercc6l2 loss was also tolerated in mice co-deficient for endogenous formaldehyde detoxification, which precipitates early-onset BMF in models of Fanconi anaemia. Instead, Ercc6l2 -deficient mice display a mild immunodeficiency, arising from defects in immunoglobulin class-switch recombination (CSR), that synergise with shieldin-deficiency, implicating ERCC6L2 and shieldin in distinct repair mechanisms. Furthermore, we demonstrate that ERRC6L2 stimulates chromosome fusions in the context of staggered, but not blunt dysfunctional telomeres. We reconcile ERCC6L2’s NHEJ function through proteomic elucidation of its endogenous interactome and AlphaFold structural modelling to reveal a complex formed of ERCC6L2 and KU that is bridged by the NHEJ accessory factor MRI/CYREN. Consequently, ERCC6L2-MRI inter-dependence characterises CSR. Together, our findings implicate the ERCC6L2-MRI complex as a KU-regulatory DNA translocase coordinating classical-NHEJ at staggered-end DSBs. We suggest that similar staggered-end breaks represent the pathological substrates driving haematopoietic failure in ERCC6L2 disease.

Cryopreservation methods for archiving and distributing mouse strains mostly focus on freezing embryos or sperm. Although protocols for the cryopreservation of wild-type mouse oocytes are available, these methods have not been widely adopted in large biomedical research facilities. This is partly due to a lack of validation of the available methods on a large scale using a range of genetically modified oocytes. Furthermore, some of the existing methods report a relatively low rate of fertilization, requiring either the zona pellucida to be physically breached or the inclusion of cumulus cells to enable efficient fertilization, interventions which might be incompatible with maintaining hygiene barriers. Existing methods also often use cryovials rather than straws or slimline vitrification devices, which are more practical for storage and handling. Here, we present a robust vitrification protocol for large-scale oocyte cryopreservation, achieving high viability and fertilization rates comparable to fresh oocytes. We have extensively tested the protocol for in vitro fertilization of many genetically altered strains, using both genetically altered and wild-type C57BL/6J oocytes and sperm. Providing an archive of cryopreserved oocytes harboring genetically modified alleles separately from archives of cryopreserved sperm allows multiple allele combinations to be generated during the rederivation process. This reduces subsequent breeding steps and the need to maintain live stocks of mice. Furthermore, cryopreserving oocytes for later use enables them to be obtained from females at the optimal age, thereby reducing the number of mice required and providing greater scheduling flexibility for subsequent genetic modification and rederivation work.