Many pharmaceuticals and other xenobiotics (such as drugs, environmental and industrial chemicals) and endobiotics (such as bilirubin, bile acids, fatty acids, 20-hydroxyeicosatetraenoic acid, thyroid hormones, and steroids) are non-polar, lipid-soluble substances. Their phase II metabolism (also known as conjugation reaction) yields polar and hydrophilic compounds by adding an endogenous polar group (e.g., glucuronic acid, sulphate, glutathione, or acetyl) to a lipophilic substrate. This enhances their clearance with urine and bile and works as a detoxification mechanism (1, 2, 3). In humans, the most common conjugation pathway is glucuronidation due to a wide range of potential substrates and high availability of glucuronic acid, an endogenous chemical derived from cofactor uridine diphosphate glucuronic acid (UDP glucuronic acid), which covalently binds to a nucleophilic substrate to form a water-soluble conjugate and uridine diphosphate. Glucuronidation is mediated by uridine diphosphate glucuronosyltransferases (UDP glucuronosyltransferases, UGTs), enzymes present in many tissues (mainly in the liver, gut, and kidneys) and localised in the endoplasmic reticulum, which implies lipophilic properties of their substrates (2, 3, 4). Furthermore, glucuronidation is essential for the clearance of drugs such as analgesics, nonsteroidal anti-inflammatory drugs (NSAIDs), antineoplastics, antiepileptics, and benzodiazepines (5).
Most glucuronides are less active than their parent substances, but there are exceptions, such as morphine-6-glucuronide, a strong μ-opioid receptor agonist, whose activity is even higher than that of morphine (6). According to their amino acid sequence, UGTs in humans generally belong to four families: UGT1, UGT2, UGT3, and UGT8. The most significant drug-conjugating UGTs are members of the UGT1A and UGT2B subfamilies. Their isoforms are extensively expressed in the intestine and the gut, where they play a crucial role in facilitating the first-pass metabolism of various pharmaceutical and biological phenolic substances. Some like UGT1A1, 1A2, 1A3, 1A6, 1A9, 2B7, and 2B15 are clinically the most important (4).
UGTs have different selectivity for specific substrates, but it occasionally overlaps, as several isoforms can often participate in glucuronidation of the same substrate. From a toxicological standpoint, this feature is beneficial, because if one isoform malfunctions, it does not always entail lower clearance and detoxification (7).
Considering that there are many drug substrates (Table 1) (3, 8), UGTs are involved in various drug-drug interactions of which the inhibitory ones are a key source of adverse reactions to drugs. A number of medicines inhibit UGTs
Enzymes | Substrates |
---|---|
atazanavir, R-carvedilol, etoposide, β-oestradiol, ezetimibe, SN-38 (active metabolite of irinotecan) | |
ezetimibe, telmisartan | |
amitriptyline, lamotrigine, 1-OH midazolam, olanzapine, trifluoperazine | |
deferiprone, paracetamol, serotonin | |
edaravone, entacapone, indomethacin, mycophenolic acid, R-oxazepam, paracetamol, propofol, sorafenib | |
aldosterone, chloramphenicol, codeine, diclofenac, efavirenz, epirubicin, fenofibrate, flurbiprofen, morphine, naloxone, naproxen, zidovudine | |
dabigatran, lorazepam, R-methadone, S-oxazepam | |
testosterone, vorinostat |
The UGT2B7 enzyme is encoded by
Genetic association studies indicate that the
Considering that
For this reason, several studies have investigated the distribution of different polymorphisms of this gene in different populations, but no such study has been carried out in Croatia, save for a very limited study on
The aim of this study was to complement our earlier genotyping research of the
The study included 501 Caucasian participants from different parts of Croatia, 252 men and 249 women (median age 34 years; range 2–77 years), who make a good sample of mixed Croatian population. All participants were recruited at the University Hospital Centre Zagreb to which they were referred for regular pharmacogenetic testing with different diagnoses and pharmacotherapy. The study includes pharmacogenetic data collected from 2016 to 2022.
For comparison, we relied on the Genome Aggregation Database (gnomAD) v.2.1.1 (33, 34) and 1000 Genomes database (35, 36) as sources of allele population frequencies worldwide.
For genotyping, 3 mL of blood samples were collected into BD Vacutainer™ K3EDTA tubes (Becton, Dickinson and Company, Franklin Lakes, NJ, USA). Genomic DNA was extracted from whole blood using the QIAamp® kit (Qiagen, Hilden, Germany). For genotyping we relied on the TaqMan® Drug Metabolism Genotyping Assay for
Allele and genotype frequencies were counted directly and data entered into Microsoft Excel 2016 (Microsoft Corporation, Redmond, WA, USA). Testing for Hardy-Weinberg equilibrium (HWE) was performed with online HWE calculator Gene Calc (37).
The distribution of
Gene - allele | Genotype | Croatian populationa | European population | CEUb | FINb | GBRb | IBSb | TSIb |
---|---|---|---|---|---|---|---|---|
0.250 | 0.232 | 0.333 | 0.220 | 0.271 | 0.196 | |||
0.529 | 0.576 | 0.434 | 0.505 | 0.551 | 0.570 | |||
0.221 | 0.192 | 0.232 | 0.275 | 0.178 | 0.234 |
a Frequencies determined in this study. b Allele frequencies from the 1000 Genomes database: CEU – Utah residents with Northern and Western European ancestry from the CEPH collection; FIN – Finnish in Finland; GBR – British in England; IBS – Iberian population; TSI – Tuscans in Italy
NCBI dbSNP ID | Alleles | Croatian populationa | EURb | FINb | AFRb | EASb | SASc | AMRb | AJb | Otherb |
---|---|---|---|---|---|---|---|---|---|---|
0.4624 | 0.5599 | 0.7078 | 0.7057 | 0.6010 | 0.6896 | 0.5625 | 0.5221 | |||
0.5376 | 0.4401 | 0.2922 | 0.2943 | 0.3990 | 0.3104 | 0.4375 | 0.4779 |
a Frequencies determined in this study. b Data from the Genome Aggregation Database (gnomAD): EUR – non-Finnish Europeans; FIN – Finnish; AFR – African/African American; EAS – East Asian; AMR – Latino/Admixed American; AJ – Ashkenazi Jewish. c Data from the 1000 Genomes database: SAS – South Asia
Our results in 500 participants are in line with the Croatian study in adult epilepsy patients reporting of
Considering that this is not a genetic association study, our study is limited to genotyping only one rs7668258 variant and to determining the