Rodent genetic models of Ah receptor signaling.

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that is a member of the PER-ARNT-SIM superfamily of environmental sensors. This receptor has been a molecule of interest for many years in the field of toxicology, as it was originally discovered to mediate the toxic effects of certain environmental pollutants like benzo(a)pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. While all animals express this protein, there is naturally occurring variability in receptor size and responsiveness to ligand. This naturally occurring variation, particularly in mice, has been an essential tool in the discovery and early characterization of the AHR. Genetic models including congenic mice and induced mutations at the Ahr locus have proven invaluable in further understanding the role of the AHR in adaptive metabolism and TCDD-induced toxicity. The creation and examination of Ahr null mice revealed an important physiological role for the AHR in vascular and hepatic development and mediation of the immune system. In this review, we attempt to provide an overview to many of the AHR models that have aided in the understanding of AHR biology thus far. We describe the naturally occurring polymorphisms, congenic models, induced mutations at the Ahr locus and at the binding partner Ah Receptor Nuclear Translocator and chaperone, Ah receptor associated 9 loci in mice, with a brief description of naturally occurring and induced mutations in rats.

The Ah Receptor: Adaptive Metabolism, Ligand Diversity, and the Xenokine Model.

The Ah receptor (AHR) has been studied for almost five decades. Yet, we still have many important questions about its role in normal physiology and development. Moreover, we still do not fully understand how this protein mediates the adverse effects of a variety of environmental pollutants, such as the polycyclic aromatic hydrocarbons (PAHs), the chlorinated dibenzo-p-dioxins ("dioxins"), and many polyhalogenated biphenyls. To provide a platform for future research, we provide the historical underpinnings of our current state of knowledge about AHR signal transduction, identify a few areas of needed research, and then develop concepts such as adaptive metabolism, ligand structural diversity, and the importance of proligands in receptor activation. We finish with a discussion of the cognate physiological role of the AHR, our perspective on why this receptor is so highly conserved, and how we might think about its cognate ligands in the future.

Regional Variation in Cardiovascular Genes Enables a Tractable Genome Editing Strategy.

BACKGROUND: To realize the potential of genome engineering therapeutics, tractable strategies must be identified that balance personalized therapy with the need for off-the-shelf availability. We hypothesized that regional clustering of pathogenic variants can inform the design of rational prime editing therapeutics to treat the majority of genetic cardiovascular diseases with a limited number of reagents. METHODS: We collated 2435 high-confidence pathogenic/likely pathogenic (P/LP) variants in 82 cardiovascular disease genes from ClinVar. We assessed the regional density of these variants by defining a regional clustering index. We then combined a highly active base editor with prime editing to demonstrate the feasibility of a P/LP hotspot-directed genome engineering therapeutic strategy in vitro. RESULTS: P/LP variants in cardiovascular disease genes display higher regional density than rare variants found in the general population. P/LP missense variants displayed higher average regional density than P/LP truncating variants. Following hypermutagenesis at a pathogenic hotspot, mean prime editing efficiency across introduced variants was 57±27%. CONCLUSIONS: Designing therapeutics that target pathogenic hotspots will not only address known missense P/LP variants but also novel P/LP variants identified in these hotspots as well. Moreover, the clustering of P/LP missense rather than truncating variants in these hotspots suggests that prime editing technology is particularly valuable for dominant negative disease. Although prime editing technology in relation to cardiac health continues to improve, this study presents an approach to targeting the most impactful regions of the genome for inherited cardiovascular disease.

Generation of an Allelic Series at the Ahr Locus Using an Edited Recombinant Approach.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor and a member of the PER-ARNT-SIM (PAS) superfamily of environmental sensors. The AHR is involved in a series of biological processes including adaptive metabolism of xenobiotics, toxicity of certain environmental pollutants, vascular development, fertility, and immune function. Mouse models, including the Ahr null and Ahr conditional null (Ahrfx) mice, are widely used for the study of AHR-mediated biology and toxicity. The Ahr conditional null mouse harbors the low-affinity Ahrd allele that exhibits approximately a 10-fold lower binding affinity for certain xenobiotic AHR ligands than the widely used C57BL/6 mouse that harbors the higher affinity Ahrb1 allele. Here, we report a novel mouse model that introduces a V375A polymorphism that converts the low-affinity allele into a high-affinity allele, offering a more sensitive conditional model. In the generation of this novel conditional allele, two additional mutants arose, including a 3-bp deletion in the PAS-B domain (AhrNG367R) and an early termination codon in the PAS-B domain (AhrTer383). The AhrNG367R allele presents as a phenocopy of the null and the AhrTer383 allele presents as an antimorph when assessing for the ductus venosus and liver lobe weight endpoints. These new models represent a series of tools that will be useful in further characterizing AHR biology.