Alternative titles; symbols
HGNC Approved Gene Symbol: EPHX1
Cytogenetic location: 1q42.12 Genomic coordinates (GRCh38): 1:225,810,124-225,845,563 (from NCBI)
Microsomal epoxide hydrolase (EPHX1; EC 3.3.2.3) is a bifunctional protein expressed in 2 distinct topologic orientations. The type I form plays a central role in hepatic metabolism of xenobiotics, whereas the type II form is targeted to plasma membrane, where it mediates sodium-dependent transport of bile acids (Peng et al., 2015).
Jackson et al. (1987) reported the nucleotide sequence of EPOX. The deduced 455-amino protein is 82% homologous to rat microsomal epoxide hydrolase.
Skoda et al. (1988) isolated cDNA clones for human microsomal epoxide hydrolase and determined the nucleotide sequence. The deduced amino acid sequence of the human enzyme was found to be 80% similar to the previously reported rabbit enzyme and 84% similar to the deduced rat protein sequence. The N-terminal amino acids deduced from the human cDNA were identical to the published 19 N-terminal amino acids of the purified human enzyme. Northern blot analysis showed a single mRNA band of 1.8 kilobases. Southern blot analysis indicated that there is only 1 copy of the gene per haploid genome. Several restriction fragment length polymorphisms were observed with the human EPOX cDNA.
Hassett et al. (1994) isolated and sequenced clones that encoded the entire human EPHX1 gene. The primary nuclear transcript, extending from the start of transcription to the site of poly(A) addition, is 20,271 nucleotides long.
Using 5-prime RACE with human liver, kidney, and small intestine RNA, Liang et al. (2005) identified transcripts using 2 alternative EPHX1 first exons, exons E1 and E1b. Northern blot analysis detected expression of E1 transcripts in adult and fetal liver only, whereas E1b transcripts were expressed in all adult and fetal tissues examined. Similarly, E1 transcripts were expressed in hepatoma cell lines only, and E1b transcripts were expressed in a variety of cell lines. Slot blot analysis of 25 human liver samples revealed highly variable expression of both E1 and E1b transcripts.
Using RACE with human lung cancer and hepatoma cell line RNA, Nguyen et al. (2013) identified an EPHX1 transcript containing an alternative first exon, E1b-prime. E1b-prime includes 2 upstream ORFs (uORFs): uORF1, which encodes a highly basic 26-residue peptide that is in-frame with the main ORF, and uORF2, which encodes a highly basic 17-residue peptide that is out-of frame with and overlaps the main ORF by 10 nucleotides. Real-time PCR detected variable expression of total EPHX1 in all 20 human tissues examined, with highest expression in ovary, liver, and kidney. E1b-prime transcripts were variably expressed in all tissues examined, with high content in ovary.
Hassett et al. (1994) determined that the EPHX1 gene contains 9 exons. The introns vary in size from 335 to 6,696 basepairs and contain numerous repetitive DNA elements, including 18 Alu sequences (each more than 100 nucleotides long) within 4 of the introns.
Liang et al. (2005) reported that the EPHX1 gene contains 2 alternative first exons, E1 and E1b, both of which are associated with promoter regions. E1 is located 3.2 kb upstream of exon 2, which contains the translation start site, whereas E1b is located approximately 18.5 kb upstream of exon 2. Liang et al. (2005) identified various transcription factor-binding sites upstream of E1, including GATA, HNF3 (see 602294), CCAAT, and GC box elements.
Nguyen et al. (2013) identified an alternative first exon of EPHX1, E1b-prime, that lies just 3-prime to E1b. E1b and E1b-prime share a primate-specific promoter. E1b-prime has a GC-rich leader sequence, followed by 2 uORFs, and has the potential to adopt a stable 9-hairpin structure.
Brown and Chalmers (1986) measured microsomal epoxide hydrolase activity in human/mouse hybrid cells prepared from human cells expressing 6 to 7 times the activity of the mouse cells. Of 25 clones examined by antihuman and antimouse antisera raised in the rabbit, none expressed human enzyme. This correlated with the loss of human chromosome 6 from each cell line. Brown and Chalmers (1986) concluded that the human gene for epoxide hydrolase may be on chromosome 6. Certain observations in hybrid cells suggested that other gene products can affect the level of activity expressed by the cell. Brown and Chalmers (1986) recognized that assignment of genes to chromosomes on the basis of negative data is not completely satisfactory. They also observed that other chromosomes, particularly chromosome 19, seemed to affect expression. Jackson et al. (1987) assigned the gene to chromosome 1 by somatic cell hybridization. Analysis of 2 hybrids containing spontaneous breaks permitted regional localization of the gene to 1q or proximal to NRAS (164790) on 1p.
By fluorescence in situ hybridization, Hartsfield et al. (1998) mapped the EPOX gene to 1q42.1 The mouse equivalent of EPOX, symbolized Eph-1, is located on chromosome 1 (Nadeau, 1988).
Using supershift assays, Liang et al. (2005) confirmed involvement of GATA4 (600576), GATA6 (601656), HNF3-alpha (FOXA1; 602294), and HNF3-beta (FOXA2; 600288) in binding to regulator elements upstream of exon E1 of the EPHX1 gene. In human hepatoma cells, HNF3 negatively regulated transactivation of the E1 promoter by GATA4.
Using an in vitro translation system and transfected 293A cells, Nguyen et al. (2013) found that EPHX1 transcripts containing E1b-prime showed impaired expression compared with those that included E1 or E1b. Mutation of the first AUG in uORF1 or uORF2 enhanced EPHX1 expression, and mutation of both further enhanced EPHX1 expression. Addition of E1b-prime constructs to reaction mixtures suppressed translation of both E1- and E1b-containing transcripts. Synthetic 26- and 17-residue peptides based on the E1b-prime uORFs also inhibited generation of EPHX1 protein in a sequence-specific and dose-dependent manner in vitro or following expression in A549 human lung carcinoma cells. Since the uORF1 and uORF2 peptides are highly basic, Nguyen et al. (2013) hypothesized that they may impede translation in a manner similar to other highly basic peptides.
Using EMSA, Peng et al. (2015) found that PARP1 (173870) bound to the proximal promoter region of EPHX1 and induced EPHX1 expression. Mass spectrometric, coimmunoprecipitation, and chromatin immunoprecipitation analyses revealed that linker histone H1.2 (HIST1H1C; 142710) and ALY (ALYREF; 604171) bound to intron 1 of the EPHX1 gene. Binding of H1.2 to intron 1 inhibited PARP1-dependent induction of EPHX1 expression. ALY alone had little effect on PARP1-dependent EPHX1 expression, but it enhanced the inhibitory activity of H1.2 when cotransfected with H1.2. Knockdown of H1.2 via short hairpin RNA significantly increased EPHX1 promoter activity.
Associations Pending Confirmation
For a possible association beteen variation in the EPHX1 gene and hypercholanemia (see 607748), see 132810.0003.
Miyata et al. (1999) determined that Ephx1-null mice were fertile and had no phenotypic abnormalities. Ephx1-null embryonic fibroblasts were unable to produce the carcinogenic metabolite of 7,12-dimethylbenz(alpha)anthracene (DMBA), an experimental prototype for the polycyclic aromatic hydrocarbon class of chemical carcinogens. These mice were resistant to DMBA-mediated toxicity and DMBA-induced carcinogenesis.
This variant, formerly titled LYMPHOPROLIFERATIVE DISORDERS, SUSCEPTIBILITY TO, with the following included titles, PREECLAMPSIA, SUSCEPTIBILITY TO; EMPHYSEMA, SUSCEPTIBILITY TO; and PULMONARY DISEASE, CHRONIC OBSTRUCTIVE, SUSCEPTIBILITY TO, has been reclassified as a polymorphism because in the ExAC database (September 20, 2017) the variant was present in 38,033 of 121,362 alleles, including 6,357 homozygotes (Hamosh, 2017).
Hassett et al. (1994) described a correlation between mutant alleles of EPHX and diminished enzymatic activity. They demonstrated by in vitro expression studies of cDNA that substitution of his113 for the more commonly occurring tyr113 residue in exon 3 decreased EPHX activity approximately 40%. Variation of this type may be responsible for genetic susceptibility to the environmental carcinogen aflatoxin B1 (McGlynn et al., 1995), and explained variation in the frequency of hepatocellular carcinoma (HCC; 114550).
Sarmanova et al. (2001) determined the frequency of polymorphisms in several biotransformation enzymes in patients with morbus Hodgkin and non-Hodgkin lymphomas (NHL; 605027) and age- and sex-matched healthy individuals. The distribution of genotypes in CYP2E1-intron 6 (124040.0002) was significantly different between the control group and all lymphomas (P = 0.03), patients with NHL (P = 0.024), and especially aggressive diffuse NHL (P = 0.007). The EPHX-exon 3 genotype distribution was significantly different between control males and males with all lymphomas (P = 0.01) or with NHL (P = 0.019). The authors suggested that genetic polymorphisms of biotransformation enzymes may play a significant role in the development of lymphoid malignancies.
Zusterzeel et al. (2001) studied genetic variability of the EPHX1 gene in women with a history of preeclampsia (189800). They found a significantly higher frequency of the high activity tyr113/tyr113 genotype (odds ratio 2.0, 95% C.I. 1.2-3.7) in women with a history of preeclampsia compared to controls.
Smith and Harrison (1997) studied EPHX1 polymorphisms in patients with various pulmonary diseases and found that the very slow phenotype (his113) was 4 to 5 times more common in patients with COPD or emphysema compared to controls.
Laasanen et al. (2002) studied an A-G polymorphism (his139 to arg) in exon 4 of the EPHX1 gene in 133 Finnish preeclamptic and 115 healthy control women with at least 2 normal pregnancies. Genotype and allele distributions did not reveal statistically significant single-point association with preeclampsia (189800). However, haplotype analysis using this polymorphism and the exon 3 T-C polymorphism (tyr113 to his; 132810.0001) showed that the high activity haplotype T/A (tyr113/his139) was significantly overrepresented in the preeclampsia group (P = 0.01; odds ratio 1.61, 95% CI, 1.12-2.32).
Hamosh (2017) noted that the H239R variant in the EPHX1 gene was present in 23,810 of 121,306 alleles, including 2,660 homozygotes, in the ExAC database (September 20, 2017).
This variant, formerly titled HYPERCHOLANEMIA, FAMILIAL, has been reclassified because the pathogenicity of the variant has not been confirmed.
In a patient with hypercholanemia (see 607748) in the absence of observable hepatocellular injury, suggesting a defect in bile acid uptake, Zhu et al. (2003) identified compound heterozygosity for mutations in the EPHX1 gene: a -4238T-A transversion in the 5-prime region at a putative HNF3 site inherited from the father, and a 2557C-G transversion in intron 1 (132810.0004). Testing of the mother was not mentioned. Both mutations significantly decreased EPHX1 promoter activity. EPHX1 protein and mRNA levels were reduced by approximately 95% and 85%, respectively. The existence of both mutant alleles appeared to be necessary for the observed hypercholanemia, since the father, who possessed only the -4238T-A transversion, had normal serum glycocholate levels. The patient had extremely elevated serum bile salt levels in the absence of observable hepatocellular injury, suggesting a defect in bile acid uptake. EPHX1 protein and mRNA levels were reduced by approximately 95% and 85%, respectively, whereas the expression and amino acid sequence of another bile acid transport protein, sodium/taurocholate cotransporting polypeptide (NTCP1; 182396), was unaffected.
This variant, formerly titled HYPERCHOLANEMIA, FAMILIAL, has been reclassified because the pathogenicity of the variant has not been confirmed.
For discussion of the 2557C-G transversion in intron 1 of the EPHX1 gene that was found in compound heterozygous state in a patient with hypercholanemia (see 607748) by Zhu et al. (2003), see 132810.0003.
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