Get to know a gene: UGT1A4

What is UGT1A4?
The UGT1A4 gene encodes an enzyme of the glucuronidation pathway, a phase II metabolism process that transforms small lipophilic molecules into water-soluble excretable metabolites. The UGT1A4 enzyme mediates the metabolism of important psychotropic medications, including some tricyclic antidepressants, antipsychotics, and mood stabilizers^1–20. Therefore, genetic polymorphisms in this gene may have pharmacological importance.

More than 100 UGT1A4 polymorphisms have been identified. Two of the best studied polymorphisms are UGT1A4*2, which results in a proline to threonine amino acid change at codon 24 (P24T) caused by a single C>A substitution, and UGT1A4*3, which results in a leucine to valine amino acid change at codon 48 (L48V) caused by a single T>G substitution²¹. Both of these variants are located in the first exon of the UGT1A4 gene and may result in altered enzyme activity, with UGT1A4*3 increasing glucuronidation and UGT1A4*2 decreasing glucuronidation.

Is there a connection between UGT1A4*3 rapid metabolism and the efficacy of psychotropic medications?
The UGT1A4*3 variant has been shown to have an effect on the metabolism of lamotrigine. Three in vivo studies (n=413) investigated the effect of UGT1A4*3 on lamotrigine metabolism and demonstrated that carriers of the G allele (TG + GG) had significantly reduced lamotrigine concentrations compared to wild-type (TT)^3,22,23. One of these studies specifically examined the effect of genotype on lamotrigine efficacy by comparing seizure frequency before and after lamotrigine therapy. This study found that wild-type (TT) subjects exhibited significantly better therapeutic efficacy compared to carriers of the G allele²². While a fourth in vivo study (n=75) failed to find an association between UGT1A4*3 and lamotrigine clearance, the authors acknowledge several limitations to the study, which may cause this finding to be a false null association²⁴. Contrary to the in vivo results, one in vitro study reported a decrease in the rate of lamotrigine glucuronidation in cells expressing the UGT1A4*3 variant. However, the authors give potential explanations for these contradictory findings, such as differences in experimental design and stability of enzyme preparations²⁵.

The UGT1A4*3 variant has also been shown to affect the metabolism of olanzapine. Two in vitro studies investigated the effect of UGT1A4*3 on olanzapine metabolism and found that UGT1A4*3 causes increased enzyme activity^9,26. Multivariate analyses indicated that Caucasian subjects (n=129) who were either heterozygous or homozygous for the UGT1A4*3 allelic variant exhibited a significant increase in olanzapine glucuronidation over patients with homozygous wild type genotypes (38% and 246% higher in *1/*3 and *3/*3 patients, respectively). Although serum concentrations of the parent drug were not affected by UGT1A4 genotype in this study, an increase in the glucuronidation rate supports the ultrarapid phenotype of this polymorphism²⁶. Additionally, three in vivo studies (n=247) showed clear trends towards lower olanzapine concentrations in *3 allele carriers^27–29. While another in vivo study (n=47) found a trend towards higher olanzapine concentrations in UGT1A4*3 carriers³⁰, it has been shown that this contradictory finding may be attributed to erroneous genotyping methods ²⁹.

Is there a connection between UGT1A4*2 reduced metabolism and the efficacy of psychotropic medications?
The overall literature on UGT1A4*2 is quite limited. Olanzapine, clozapine, and lamotrigine have been individually evaluated in three separate in vitro studies, each examining the effect of UGT1A4*2 on metabolism. All three studies found that UGT1A4*2 may cause lower enzyme activity^9,20,25. However, three in vivo studies (n=415) examined the effect of UGT1A4*2 on lamotrigine metabolism, and found that the variant allele frequency was either too low to determine the effect of this polymorphism or the results were not significant^3,22,23.

Conclusions
Multiple clinical studies have found UGT1A4*3 to be associated with higher enzyme activity^{3,22,23,26–29}. This could result in clinically relevant reductions in exposure of drugs for which UGT1A4 is the major elimination route. Furthermore, it has also been shown that carriers of the UGT1A4*3 variant exhibited reduced therapeutic efficacy²². Thus, higher than average doses of UGT1A4 substrates may be required for patients carrying the UGT1A4*3 variant.

Due to the lack of in vivo evidence, more data is needed before UGT1A4*2 can be recommended for use in treatment selection.

Reference
1. Chen, H., Yang, K., Choi, S., Fischer, J. H. & Jeong, H. Up-regulation of UDP-glucuronosyltransferase (UGT) 1A4 by 17β-estradiol: A potential mechanism of increased lamotrigine elimination in pregnancy. Drug Metab. Dispos. 37, 1841–1847 (2009).

2. Christensen, J. et al. Oral contraceptives induce lamotrigine metabolism: Evidence from a double-blind, placebo-controlled trial. Epilepsia 48, 484–489 (2007).

3. Gulcebi, M. I. et al. The relationship between UGT1A4 polymorphism and serum concentration of lamotrigine in patients with epilepsy. Epilepsy Res. 95, 1–8 (2011).

4. Johannessen, S. I. & Landmark, C. J. Antiepileptic drug interactions – principles and clinical implications. Curr Neuropharmacol. 8, 254–267 (2010).

5. Rowland, A., Elliot, D.J., Williams, A., Mackenzie, P.I., Dickinson, R.G., Miners, J. O. IN VITRO CHARACTERIZATION OF LAMOTRIGINE N 2-GLUCURONIDATION AND THE LAMOTRIGINE-VALPROIC ACID INTERACTION Andrew Rowland , David J . Elliot , J . Andrew Williams , Peter I . Mackenzie , Ronald G . Dickinson , ABSTRACT : Drug Metab. Dispos. 34, 1055–1062 (2006).

6. Linnet, K. Glucuronidation of olanzapine by cDNA-expressed human UDP-glucuronosyltransferases and human liver microsomes. Hum. Psychopharmacol. 17, 233–238 (2002).

7. Argikar, U. a & Remmel, R. P. Variation in glucuronidation of lamotrigine in human liver microsomes. Xenobiotica. 39, 355–363 (2009).

8. Kato, Y. et al. Human UDP-glucuronosyltransferase (ugt) 2b10 in drug n-glucuronidation: Substrate screening and comparison with UGT1A3 and UGT1A4. Drug Metab. Dispos. 41, 1389–1397 (2013).

9. Erickson-Ridout, K. K., Zhu, J. & Lazarus, P. Olanzapine metabolism and the significance of UGT1A448V and UGT2B1067Y variants. Pharmacogenet. Genomics 21, 539–551 (2011).

10. Green, M. D., King, C. D., Mojarrabi, B., Mackenzie, P. I. & Tephly, T. R. Glucuronidation of amines and other xenobiotics catalyzed by expressed human UDP-glucuronosyltransferase 1A3. Drug Metab. Dispos. 26, 507–512 (1998).

11. Green, M., Bishop, W. & Tephly, T. Expressed human UGT1.4 protein catalyzes the formation of quaternary amonium-linked glucuronides. Drug Metab. Dispos. 23, 299–302 (1995).

12. Breyer-Pfaff, U., Mey, U., Green, M. & Tephly, T. Comparative N-Glucuronidation Kinetics of Ketotifen and Amitriptyline by Expressed Human UDP-Glucuronosyltransferases and Liver Microsomes. Drug Metab. Dispos. 28, 869–872 (2000).

13. Zhou, D., Guo, J., Linnenbach, A. J., Booth-genthe, C. L. & Grimm, S. W. Role of Human UGT2B10 in N-Glucuronidation of Tricyclic and Trimipramine ABSTRACT : Pharmacology 38, 863–870 (2010).

14. Kato, Y., Nakajima, M., Oda, S., Fukami, T. & Yokoi, T. Human UDP-glucuronosyltransferase isoforms involved in haloperidol glucuronidation and quantitative estimation of their contribution. Drug Metab. Dispos. 40, 240–248 (2012).

15. FDA Label. Saphris (asenapine) sublingual tablets. 2009.

16. U.S. Food and Drug Administration Briefing Book: Saphris (asenapine) Sublingual Tablets. 2009.

17. Argikar, U. & Remmel, R. Effect of aging on glucuronidation of valproic acid in human liver microsomes and the role of UDP-glucuronosyltransferase UGT1A4, UGT1A8, and UGT1A10. Drug Metab. Dispos. 37, 229–236 (2009).

18. Green, M. D. & Tephly, T. R. Glucuronidation of amines and hydroxylated xenobiotics and endobiotics catalyzed by expressed human UGT1.4 protein. Drug Metab. Dispos. 24, 356–63 (1996).

19. Mori, A., Maruo, Y., Iwai, M., Sato, H. & Takeuchi, Y. UDP-glucuronosyltransferase 1A4 polymorphisms in a Japanese population and kinetics of clozapine glucuronidation. Drug Metab. Dispos. 33, 672–675 (2005).

20. Erickson-Ridout, K. K., Sun, D. & Lazarus, P. Glucuronidation of the second-generation antipsychotic clozapine and its active metabolite N-desmethylclozapine. Potential importance of the UGT1A1 A(TA)7TAA and UGT1A4 L48V polymorphisms. Pharmacogenet. Genomics 22, 561–576 (2012).

21. Ehmer, U. et al. Variation of hepatic glucuronidation: Novel functional polymorphisms of the UDP-glucuronosyltransferase UGT1A4. Hepatology 39, 970–7 (2004).

22. Chang, Y., Yang, L., Zhang, M. & Liu, S.-Y. Correlation of the UGT1A4 gene polymorphism with serum concentration and therapeutic efficacy of lamotrigine in Han Chinese of Northern China. Eur. J. Clin. Pharmacol. 941–946 (2014). doi:10.1007/s00228-014-1690-1

23. Reimers, A., Sjursen, W. & Helde, G. Frequencies of UGT1A4 * 2 ( P24T ) and * 3 ( L48V ) and their effects on serum concentrations of lamotrigine. Eur J Drug Metab Pharmacokinet 2, (2014).

24. Singkham, N., Towanabut, S., Lertkachatarn, S. & Punyawudho, B. Influence of the UGT2B7 -161C>T polymorphism on the population pharmacokinetics of lamotrigine in Thai patients. Eur. J. Clin. Pharmacol. 69, 1285–1291 (2013).

25. Zhou, J., Argikar, U. & Remmel, R. P. Functional analysis of UGT1A4 P24T and UGT1A4 L48V variant enzymes. Pharmacogenomics 12, 1671–1679 (2011).

26. Haslemo, T. et al. UGT1A4*3 encodes significantly increased glucuronidation of olanzapine in patients on maintenance treatment and in recombinant systems. Clin. Pharmacol. Ther. 92, 221–7 (2012).

27. Mao, M., Skogh, E., Scordo, M. & Dahl, M. Interindividual Variation in Olanzapine Concentration Influenced by UGT1A4 L48V Polymorphism in Serum and Upstream FMO Polymorphisms in Cerebrospinal Fluid. J Clin. Psychopharmacol. 32, 287–289 (2012).

28. Czerwensky, F., Leucht, S. & Steimer, W. CYP1A2*1D and *1F Polymorphisms Have a Significant Impact on Olanzapine Serum Concentrations. Therapeutic Drug Monitoring 37, (2015).

29. Ghotbi, R. et al. Carriers of the UGT1A4 142T>G gene variant are predisposed to reduced olanzapine exposure – An impact similar to male gender or smoking in schizophrenic patients. Eur. J. Clin. Pharmacol. 66, 465–474 (2010).

30. Nozawa, M. et al. The relationship between the response of clinical symptoms and plasma olanzapine concentration, based on pharmacogenetics: Juntendo University Schizophrenia Projects (JUSP). Ther. Drug Monit. 30, 35–40 (2008)