Analysis of polymorphisms of canine Cytochrome P 450-CYP2D15

A variety of cytochrome P450 enzymes and flavin-containing monooxygenases in dogs and pigs commonly used as preclinical animal models

Drug oxygenation is mainly mediated by cytochromes P450 (P450s, CYPs) and flavin-containing monooxygenases (FMOs). Polymorphic variants of P450s and FMOs are known to influence drug metabolism.　Species differences exist in terms of drug metabolism and can be important when determining the contributions of individual enzymes. The success of research into drug-metabolizing enzymes and their impacts on drug discovery and development has been remarkable. Dogs and pigs are often used as preclinical animal models. This research update provides information on P450 and FMO enzymes in dogs and pigs and makes comparisons with their human enzymes. Newly identified dog CYP3A98, a testosterone 6β- and estradiol 16α-hydroxylase, is abundantly expressed in small intestine and is likely the major CYP3A enzyme in small intestine, whereas dog CYP3A12 is the major CYP3A enzyme in liver. The roles of recently identified dog CYP2J2 and pig CYP2J33/34/35 were investigated. FMOs have been characterized in humans and several other species including dogs and pigs. P450 and FMO family members have been characterized also in cynomolgus macaques and common marmosets. P450s have industrial applications and have been the focus of attention of many pharmaceutical companies. The techniques used to investigate the roles of P450/FMO enzymes in drug oxidation and clinical treatments have not yet reached maturity and require further development. The findings summarized here provide a foundation for understanding individual pharmacokinetic and toxicological results in dogs and pigs as preclinical models and will help to further support understanding of the molecular mechanisms of human P450/FMO functionality.

Functional and molecular characterization of the Atlantic salmon gill epithelium cell line ASG-10; a tool for in vitro gill research

Fish gills are not only the respiratory organ, but also essential for ion-regulation, acid-base control, detoxification, waste excretion and host defense. Multifactorial gill diseases are common in farmed Atlantic salmon, and still poorly understood. Understanding gill pathophysiology is of paramount importance, but the sacrifice of large numbers of experimental animals for this purpose should be avoided. Therefore, in vitro models, such as cell lines, are urgently required to replace fish trials. An Atlantic salmon gill epithelial cell line, ASG-10, was established at the Norwegian Veterinary institute in 2018. This cell line forms a monolayer expressing cytokeratin, e-cadherin and desmosomes, hallmarks of a functional epithelial barrier. To determine the value of ASG-10 for comparative studies of gill functions, the characterization of ASG-10 was taken one step further by performing functional assays and comparing the cell proteome and transcriptome with those of gills from juvenile freshwater Atlantic salmon. The ASG-10 cell line appear to be a homogenous cell line consisting of epithelial cells, which express tight junction proteins. We demonstrated that ASG-10 forms a barrier, both alone and in co-culture with the Atlantic salmon gill fibroblast cell line ASG-13. ASG-10 cells can phagocytose and express several ATP-binding cassette transport proteins. Additionally, ASG-10 expresses genes involved in biotransformation of xenobiotics and immune responses. Taken together, this study provides an overview of functions that can be studied using ASG-10, which will be an important contribution to in vitro gill epithelial research of Atlantic salmon.

Citalopram in vitro metabolism in a Beagle dog: A role for CYP2D15 in the production of toxic didesmethylcitalopram?

After administration of the serotonergic antidepressant citalopram (CIT) to Beagle dogs, the dogs may experience severe convulsive attacks in relation to the considerably higher plasma concentrations of the metabolite didesmethyl-CIT (DDCIT), when compared to those in humans medicated with CIT. This pilot study aimed at determining the role of cytochrome P-450 (CYP450) isozymes in the in vitro metabolism of CIT to desmethyl-CIT (DCIT), and of DCIT to DDCIT in the liver microsomes of a single Beagle dog. Incubations with racemic CIT or DCIT reveal a high-affinity enzyme with Km between 0.3 μM and 1.4 μM for S- and R-DCIT and S- and R-DDCIT productions, respectively. In comparison to human enzymes, the intrinsic clearance values of this high-affinity enzyme are between 15 μl/(min × mg of protein) and 52 μl/(min × mg of protein), i.e., very high. In vitro experiments with inhibitors suggest that CYP2D15, which shows an analogy with human CYP2D6, is by far the main CYP450 isozyme involved in the production of DCIT and DDCIT, whereas CYP3A12 and CYP2C21/41 showed a weak implication. These observations partly explain why, in humans, the plasma concentrations of the toxic DDCIT are considerably lower than those observed in dogs, after administration of CIT.

Association of cytochrome P450 2D15 (CYP2D15) nonsynonymous polymorphisms and exon 3 deleted RNA splice variant with CYP2D15 protein content and enzyme function in dog liver microsomes

CYP2D15 is a major drug metabolizing P450 in canine liver. Like the human orthologue (CYP2D6), this enzyme is highly polymorphic with at least five common nonsynonymous variants reported that result in amino acid changes, including p.Ile109Val, p.Leu115Phe, p.Gly186Ser, p.Ile250Phe and p.Ile307Val. Furthermore, a mRNA splice variant of CYP2D15 has been found in canine liver that lacks the exon 3 gene region resulting in an inactive enzyme. The objective of this study was to evaluate whether any of these amino acid variants or the exon 3 deletion mRNA variant (exon3-delta) was associated with differences in CYP2D15-selective activities or protein content in a bank of canine livers. Livers were obtained from 25 Beagles and 34 dogs of various other breeds. CYP2D15-selective activities measured included dextromethorphan o-demethylation and tramadol o-demethylation. Reverse transcription PCR showed that 76% of livers (44/58) expressed both exon3-delta and normally spliced CYP2D15 RNA, while the remaining 24% (14/58) expressed only normally spliced RNA. The presence of exon3-delta was not correlated with CYP2D15 activities or protein content. Compared with wild-type livers, Beagle dog livers heterozygous for the p.Ile109Val and p.Gly186Ser variants showed from 40 to 50% reductions in median enzyme activities, while heterozygous p.Gly186Ser livers were associated with a 41% reduction in median CYP2D15 protein content (p < .05; Dunn's test). In the entire liver bank, livers homozygous for p.Ile109Val were also associated with a 40% reduction in median dextromethorphan O-demethylation activities versus wild-type livers (p < .05). These results identify several nonsynonymous CYP2D15 gene variants associated with variable CYP2D15 metabolism in canine liver.

Validation of a Chromosome 14 Risk Haplotype for Idiopathic Epilepsy in the Belgian Shepherd Dog Found to Be Associated with an Insertion in the RAPGEF5 Gene

An idiopathic epilepsy (IE) risk haplotype on canine chromosome (CFA) 14 has been reported to interact with the CFA37 common risk haplotype in the Belgian shepherd (BS). Additional IE cases and control dogs were genotyped for the risk haplotypes to validate these previous findings. In the new cohort, the interaction between the two regions significantly elevated IE risk. When the haplotypes were analyzed individually, particular haplotypes on both CFA14 (ACTG) and 37 (GG) were associated with elevated IE risk, though only the CFA37 AA was significantly associated (p < 0.003) with reduced risk in the new cohort. However, the CFA14 ACTG risk was statistically significant when the new and previous cohort data were combined. The frequency of the ACTG haplotype was four-fold higher in BS dogs than in other breeds. Whole genome sequence analysis revealed that a 3-base pair predicted disruptive insertion in the RAPGEF5 gene, which is adjacent to the CFA14 risk haplotype. RAPGEF5 is involved in the Wnt-β-catenin signaling pathway that is crucial for normal brain function. Although this risk variant does not fully predict the likelihood of a BS developing IE, the association with a variant in a candidate gene may provide insight into the genetic control of canine IE.

Analysis of polymorphisms of canine Cytochrome P 450-CYP2D15

Cytochrome P450 (CYP) proteins constitute a large ancient family of oxidative enzymes essential for the efficient elimination of a wide variety of clinically used drugs. Polymorphic variants of human CYP2D6 are associated with the conversion rate and efficacy of several drugs such as antidepressants. Polymorphisms of the canine orthologue CYP2D15 are of interest because these antidepressants are also used in dogs with behavioral problems and the outcome of the treatment is variable. However, the annotated CYP2D15 gene is incomplete and inaccurately assembled in CanFam3.1, hampering DNA sequence analysis of the gene in individual dogs. We elucidated the complete exon-intron structure of CYP2D15 to enable comprehensive genotyping of the gene using genomic DNA. We surveyed variations of the gene in four diverse dog breeds and identified novel polymorphisms in exon 2 in border collies. Further investigation to establish the impact of these canine CYP2D15 polymorphisms on interindividual variability in expression and function of this metabolizing enzyme is now feasible. Further knowledge of CYP pharmacogenetics will help individualize therapy and thereby increase therapeutic efficacy, especially in the use of antidepressants in veterinary behavioral medicine.