Very central to the topic of gene-environment interactions are endogenous and exogenous “signals,” recognized by the basic-helix/loop/helix (bHLH) transcription factor, aryl hydrocarbon receptor (AHH) — followed by activation of numerous genetic and biochemical pathways that represent “responses” to those signals. One of the responses is the up-regulation of cytochromes P450 family 1 (CYP1A1, CYP1A2, CYP1B1), three enzymes that metabolize numerous lipid mediators (e.g. downstream products of arachidonic acid, eicosapenatenoic acid, and docosahexaenoic acid) as well as many polycyclic aromatic hydrocarbons (PAHs; e.g. benzo[a]pyrene, arylamines, and other products of combustion) and halogenated hydrocarbons (e.g. polychlorinated biphenyls, commonly found in toxic waste dump sites).
How many genes are required for AHR-dependent induction of CYP1A? Ligand-bound AHR translocates from cytoplasm to nucleus, where it dimerizes with the aryl hydrocarbon receptor nuclear translocator (ARNT) protein. The AHR/ARNT dimer is then able to bind to enhancer regions of responsive genes to activate transcription. AHR participates in chemical carcinogenesis caused by PAHs — when the three CYP1 enzymes, that are induced by activated AHR in cell-type-specific fashion, generate carcinogenic metabolites of PAHs and arylamines; these induced enzymes also participate in detoxication of carcinogenic and toxic PAHs and arylamines, thus representing a double-edged sword that can be either beneficial or detrimental — depending on genotype, route-of-exposure, size of the exposure dose, time over which the exposure occurs, and organ- and cell-type specificity.
Authors [see attached article] used a particular mouse genome-wide CRISPR/Cas9 library to identify novel genes in the AHR pathway — by taking advantage of a benzo[a][pyrene-selection assay that this lab had originally used (40 years ago..!!) to identify fundamental AHR-pathway genes in mouse hepatoma cells. In addition to the mouse Ahr, Arnt, and Cyp1a1 genes [Cyp1a1 & Cyp1b1 are not expressed in these hepatoma cells] — authors identified additional putative AHR-pathway genes; these included P450 oxidoreductase (Por) and five genes in the heme biosynthesis pathway: 5’-aminolevulinate synthase-1 (Alas1), hydroxymethylbilane synthase (Hmbs), uroporphyrinogen decarboxylase (Urod), coproporphyrinogen oxidase (Cpox), and ferrochelatase (Fech). These last five genes are credible candidates, because heme is an essential prosthetic group of all cytochrome P450 proteins.
These experiments demonstrate the power of high-sensitivity CRISPR/Cas9 library genome-wide genetic screening. This paper also shows a clever approach for identifying genes in any pathway that includes a “sensor” gene which detects an endogenous or exogenous signal, and subsequently all downstream genes that participate in the response to that signal. 😊
Toxicol Sci. 2019 Aug 1;170(2):310-319