Mutant Hydrocarbon Receptor

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Summary:  An Aryl Hydrocarbon (AHR) null rat model has been generated with the CRISPR/Cas9 editing system. The rat model may be used to study how exposure to environmental pollutants (especially dioxins), acting through AHR, shapes placental development. Overview:  TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) is the most potent member of a class of chlorinated hydrocarbons that interact with the aryl hydrocarbon receptor. The AHR is a ligand-activated transcription factor which plays a role in the development of multiple tissues and is activated by many ligands, including TCDD. To examine the role of the AHR in both normal biological development and response to environmental chemicals, an AHR knockout rat has been created. Applications:  Dioxins produce developmental effects, chloracne, and an increase in all cancers and may also alter immune and endocrine function. Comparisons of biochemical changes show humans and rat models have similar degrees of sensitivity to dioxin-induced effects. The information gained from rat models is important for developing mechanistic models of dioxin toxicity and critical for assessing the risks to human populations under different circumstances of exposure. How it works:  AHR binds to the ligands with varying affinity depending upon species and type of ligand. Upon ligand binding, AHR translocates into the nucleus and heterodimerizes with AHR nuclear translocator (ARNT) where it regulates target genes, including those encoding enzymes that are important in drug metabolism and detoxification of environmental pollutants. CRISPR/Cas9 genome editing was used to delete exon 2 of the AHR gene, which harbours the DNA binding bHLH domain. TCDD was used as the model compound. Benefits:  By using a rat model with a null mutation at the AHR locus, generated with CRISPR/Cas9 technology, a unique examination of the mechanism of action of drugs and toxic compounds may be achieved. The AHR null rat model can potentially save millions of dollars in development costs for potential drug candidates by providing a more human-like model, and at the same time significantly decrease time to market for therapeutics. Why it is better:  Two other widely used gene editing technologies, TALEN and ZFN, work through protein DNA recognition, while CRISPR/Cas9 recognizes DNA through an RNA sequence that base pairs with a target DNA. As specificity is dictated by DNA complementarity (without the need for multistep protein engineering), the CRISPR/Cas9 technology is the faster, more straightforward, and affordable way for genome-editing in comparison to traditional ZFN and TALENs approaches. Furthermore, there are no licensing restrictions with this AHR knockout rat. Other Applications:  The rat model with a null mutation at the AHR locus is useful for studying metabolism of xenobiotic compounds and hepatotoxicity. This model may be adaptable to examine the mechanism of action of a broader group of drugs and toxic chemicals.