Human Alpha-fetoprotein Gene Research Articles

Relationship between expression of the human alpha-fetoprotein gene and DNA methylation status of the promoter region

OBJECTIVE DNA methylation has been regarded as an important epigenetic signature reflecting the transcription state of DNA in cells. This study was to conducted to assess the relationship between human alpha-fetoprotein (AFP) gene expression and the DNA methylation status of the promoter region in three different cells, namely two human hepatocellular carcinoma (HCC) cell lines and normal human fibroblasts.

Subcloning and expression of human alpha-fetoprotein gene in Pichia pastoris

Alpha-fetoprotein protein (AFP) is naturally found in fetal serum while production and appearance after birth is indicative of presence of malignant tumors. Therefore, by measuring this protein during fetal period and after birth, it is possible to diagnose abnormalities and tumors in fetus or the newborn. The objective of this study was to produce and purify AFP protein using DNA recombinant technology to apply in diagnostic kit preparations. In this study Pichia pastoris as metylotrophic yeast was used for AFP production. After construction of recombinant plasmid, pS1-AFP, electroporation and lithium chloride techniques were used for transferring to susceptible cells. The quantity and quality of the produced protein were checked by SDS-PAGE and ELISA methods. Selection of transformed mutant strains of Mut s and culturing them in glycerol media (YPG) up to OD600=6 and their transfer to methanol media (YPM) with augmentation of methanol to 1% final concentration resulted in inducing protein production in auxotrophic media lacking histidine. This protein could be useful in monoclonal antibody production and in diagnostic kit preparations.

Regulation of alpha-fetoprotein expression by Nkx2.8.

The alpha-fetoprotein (AFP) gene is an important model of developmental gene silencing and neoplastic gene reactivation. Nkx2.8 is a divergent homeodomain factor originally cloned through its binding to the promoter-coupling element (PCE), a regulatory region upstream of the AFP promoter that mediates stimulation by distant enhancers. Nkx2.8 is the only developmentally regulated factor that has been associated with AFP gene expression. Fetoprotein transcription factor, an orphan nuclear receptor, has also been shown to bind the PCE but is not developmentally regulated. The binding specificities of both families of transcription factor were determined, and overlapping sites for each were defined in the PCE. After modification of nuclear extract and gel shift analysis procedures, Nkx2.8 was identified in six AFP-positive cell lines. Transient-transfection analysis did not show transcriptional stimulation by Nkx2.8 or other active NK2 factors, which only interfered with gene expression. However, two sets of analysis demonstrated the relationship of Nkx2.8 to AFP expression: chromatin immunoprecipitation demonstrated that Nkx2.8 bound to the active AFP promoter, and antisense inhibition of Nkx2.8 mRNA translation selectively reduced expression of both the endogenous human AFP gene and transfected reporters containing the rat AFP promoter.

DETECTION OF β-HUMAN CHORIONIC GONADOTROPIN EXPRESSING CELLS BY NESTED REVERSE TRANSCRIPTASE-POLYMERASE CHAIN REACTION IN THE PERIPHERAL BLOOD STEM CELLS OF PATIENTS WITH ADVANCED GERM CELL TUMOR

We investigated whether peripheral blood stem cells (PBSCs) were contaminated by tumor cells. A total of 13 patients with advanced testicular cancer underwent PBSC transplantation at our institute. Nested reverse transcriptase-polymerase chain reaction using primers specific for beta-human chorionic gonadotropin (beta-HCG) or alpha-fetoprotein (AFP) detected 1 beta-HCG or AFP producing cell in 1 x 10(6) PBSCs. Although AFP messenger (m) RNA was not identified in any of the 13 patients, beta-HCG mRNA was detected in 7. All patients with beta-HCG mRNA in PBSCs had elevated serum beta-HCG before treatment. Three patients with a normal beta-HCG range before treatment did not have beta-HCG mRNA in the PBSCs. The serum beta-HCG level in the patients whose PBSCs were contaminated with tumor cells was significantly higher than in patients whose PBSCs contained no tumor cells. Four of the 7 patients with beta-HCG mRNA in PBSCs eventually died of cancer, whereas those whose PBSCs were not contaminated with tumor cells survived without disease. Patients with elevated serum beta-HCG levels are likely to have PBSCs contaminated by tumor cells. Moreover, the prognosis of patients with tumor cells in the PBSCs is poor.

Gene therapy vectors harboring AFP regulatory sequences. Preparation of an adenoviral vector.

Gene therapy for hepatocellular carcinoma (HCC) may be achieved by introducing a therapeutic gene under the control of transcriptional regulatory sequences of the alpha-fetoprotein (AFP) gene. Transcription of the human AFP gene is controlled positively by the promoter and the enhancer and negatively by the silencer. The AFP promoter is a 200-bp region immediately upstream of the AFP gene, and the enhancer is present between 3 and 4.9 kb upstream of the transcription initiation site. Two silencer regions have been identified upstream of the gene, one at -0.31 kb and the other at -1.75 kb. To achieve specific killing of HCC, adenoviral vectors carrying AFP regulatory sequences have been constructed. In this article, we describe the details of the preparation of an adenoviral vector designed to express the herpes simplex virus thymidine kinase gene under the control of the 4.9-kb AFP 5'-regulatory sequence. Treatment with this viral vector followed by ganciclovir resulted in specific killing of AFP-positive HCC transplanted in nude mice. Other viral vectors containing AFP-regulatory sequences are also discussed.

Selective inhibition of hepatoma cells using diphtheria toxin A under the control of the promoter/enhancer region of the human alpha-fetoprotein gene.

We constructed a plasmid containing human α‐fetoprotein (AFP) promoter/enhancer to direct the cell type‐specific expression of diphtheria toxin fragment A (DTA), designated as pAF‐DTA, to AFP‐producing hepatocellular carcinoma cells. The transfection was carried out with cationic liposomes (DMRIE‐C) and the expression of the DTA gene was confirmed by a northern blot analysis. When pAF‐DTA was transfected, the growth of AFP‐positive HuH‐7 cells was inhibited, whereas growth inhibition was not observed in AFP‐negative MKN45 cells. In this experiment, the secretion of AFP was similarly suppressed, but the secretion of carcinoembryonic antigen from MKN45 was not altered. pAF‐DTA could also exert its growth inhibitory effect on PLC, a cell line with a low level of AFP. However, no inhibitory effect of pAF‐DTA was observed on the proliferation of primary hepatocyte cells. Furthermore, transfection experiments in which HuH‐7 and splenic stromal cells were co‐cultured revealed the growth inhibition by pAF‐DTA to be selective in HuH‐7 cells. Finally, the growth of HuH‐7 transplanted on BALB/c nu/nu mice was inhibited by the direct injection of pAF‐DTA/liposome complex into a tumor mass. These results suggest that use of pAF‐DTA may be potentially useful as a novel approach for the selective treatment of tumor cells producing AFP even at low levels, without affecting other types of cells.

Cell-specific expression of the diphtheria toxin A-chain coding sequence under the control of the upstream region of the human alpha-fetoprotein gene.

Development of the system to express a suicide gene selectively in tumor cells is essential for gene therapy. We constructed a plasmid containing the diphtheria toxin A (DTA) fragment linked to human alpha-fetoprotein (AFP) promoter and enhancer, and tested whether it can exert its cytocidal effect selectively on AFP-producing cells. The chloramphenical acetyltransferase (CAT) reporter gene or DTA gene was linked to the 5' upstream region of the AFP gene. The plasmids were transfected into AFP-producing or non-producing cells by the lipopolyamine-coated DNA method. Expression of CAT activity and effects on cell growth of transfected cells were assessed. When the AFP-producing cells HuH-7 or HepG2 were cotransfected with CAT reporter plasmid and pAF5.1DTA plasmid, the CAT activity was greatly suppressed. In contrast, cotransfection with pAF5.1DTA-R, the inversely inserted DTA gene, did not inhibit CAT activity. Furthermore, cell growth of HuH-7 cells transfected with pAF5.1DTA plasmid was significantly inhibited compared with HuH-7 cells transfected with DTA-R plasmid. Our results indicate that selective killing of AFP-producing cells will be attained by introducing the DTA gene linked to the promoter and enhancer region of AFP.

A novel liver-directed gene delivery system using an autonomously replicating vector specifically expressed in AFP positive hepatoma cells

A composite EBV-based expression vector, pEBAF, was constructed with the 5'-flanking sequence of human alpha-fetoprotein gene as a promoter. Complexed to galactosylated histone, this vector with a foreign DNA insertion could be transferred into hepatic cells via the asialoglycoprotein receptor mediated endocytosis pathway. It was replicated as an episome, and its expression was restricted to the AFP positive hepatoma cells. This new gene delivery method has the following advantages: (i) absence of a potential toxicity is related to viral vectors; (ii) the targeting is specific to AFP positive hepatoma cells; (iii) the introduction of the EBV replicon into this system results in the high level expression and long-term maintenance of the transferred gene. This novel gene delivery system has potential applications in gene therapy for the treatment of hepatocellular carcinoma.

ATBF1, a multiple-homeodomain zinc finger protein, selectively down-regulates AT-rich elements of the human alpha-fetoprotein gene.

ATBF1 is a 306-kDa protein containing four homeodomains, 17 zinc finger motifs, and several segments potentially involved in transcriptional regulation (T. Morinaga, H. Yasuda, T. Hashimoto, K. Higashio, and T. Tamaoki, Mol. Cell. Biol. 11:6041-6049, 1991). At least one of the homeodomains of ATBF1 binds to an AT-rich element in the human alpha-fetoprotein (AFP) enhancer (enhancer AT motif). In the present work, we analyzed the transcriptional regulatory activity of ATBF1 with respect to the enhancer AT motif and similar AT-rich elements in the human AFP promoter and the human albumin promoter and enhancer. Gel retardation assays showed that ATBF1 binds to the AFP enhancer AT motif efficiently; however, it binds weakly or not at all to other AT-rich elements in the AFP and albumin regulatory regions studied. Alterations of the enhancer AT motif by site-specific mutagenesis resulted in the loss of binding of ATBF1. Cotransfection experiments with an ATBF1 expression plasmid and the chloramphenicol acetyltransferase (CAT) gene fused to AFP promoter or enhancer fragments showed that ATBF1 suppressed the activity of AFP enhancer and promoter regions containing AT-rich elements. This suppression was reduced when the mutated AT motifs with low affinity to ATBF1 were linked to the CAT gene. The ATBF1 suppression of AFP promoter and enhancer activities appeared to be due, at least in part, to competition between ATBF1 and HNF1 for the same binding site. In contrast to the AFP promoter and enhancer, the albumin promoter and enhancer were not affected by ATBF1, although they contain homologous AT-rich elements. These results show that ATBF1 is able to distinguish AFP and albumin AT-rich elements, leading to selective suppression of the AFP promoter and enhancer activities.

Nucleosome interactions with a human Alu element. Transcriptional repression and effects of template methylation.

Alu interspersed repetitive elements possess internal RNA polymerase III promoters which are strongly transcribed in vitro, yet these elements are nearly silent in somatic cells. To examine whether repression by chromatin proteins could contribute to the low level of Alu expression, a conserved Alu element from the fourth intron of the human alpha-fetoprotein gene was reconstituted with purified octamer or tetramer particles. Analysis of reconstitutes revealed that this Alu element directed translational and rotational positioning of octamers as well as tetramers. In vitro transcription experiments with reconstituted templates demonstrated that RNA polymerase III-dependent transcription of the Alu element was profoundly repressed by positioned octamer particles. Furthermore, complete CpG methylation of this template enhanced the capacity of tetramers to repress transcription.

A G-->A substitution in an HNF I binding site in the human alpha-fetoprotein gene is associated with hereditary persistence of alpha-fetoprotein (HPAFP).

A family displaying hereditary persistence of alpha-fetoprotein (HPAFP) in adult life was detected in an antenatal screening programme for spina bifida. RFLP linkage analysis shows that the trait is linked with the albumin-AFP locus. The molecular mechanism responsible for the post-natal repression of the AFP gene is unknown. We wished to determine the molecular mechanism underlying HPAFP in this family. Sequence analysis of the 5'-flanking sequences of their gene revealed a GA substitution at position -119 associated with the trait. This substitution occurs in a potential HNF I binding site, and increases the similarity of the sequence to a consensus HNF I recognition site. In a competitive gel retardation assay the mutant sequence binds HNF I alpha more tightly than the wild type sequence. Furthermore, 5'-flanking sequences of the human AFP gene containing the G-->A substitution direct a higher level of CAT expression in transfected human hepatoma cells than the wild type sequences. We conclude that the G-->A substitution at position -119 of the AFP gene is the mutation causing HPAFP in this family. These results highlight the importance of this HNF I binding site in the developmental regulation of the AFP gene.

A new restriction fragment length polymorphism of the human alpha-fetoprotein gene in Japanese individuals: detection of loss of heterozygosity in human hepatocellular carcinoma.

By using the cDNA probe for the human alpha-fetoprotein (AFP) gene, one restriction fragment length polymorphism (RFLP) was discovered in Japanese patients. A variety of combinations of restriction enzymes and probes were evaluated in DNA samples from 25 Japanese individuals who were genetically unrelated. As a result, AvaII identified one invariant band of 8 kb and polymorphic bands at either 2.8 or 2.0 kb with the 5'-region probe from a cDNA clone, pHAF7. With the AvaII/pHAF7 combination, we examined the loss of heterozygosity in 14 patients with hepatocellular carcinoma (HCC). Of 6 informative specimens, 2 showed loss of heterozygosity. These data suggest that the AFP cDNA probe could be useful for analyzing loss of heterozygosity on chromosome 4q in human HCC.

A Human α-Fetoprotein Enhancer-Binding Protein, ATBF1, Contains Four Homeodomains and Seventeen Zinc Fingers

We have isolated a full-length cDNA encoding a protein (ATBF1) that binds to an AT-rich motif in the human alpha-fetoprotein gene enhancer. The amino acid sequence deduced from the cDNA revealed that this is the largest DNA-binding protein (306 kDa) known to date, containing four homeodomains, 17 zinc finger motifs, and a number of segments potentially involved in transcriptional regulation. Although the exact function of this protein has not been determined, these structural features suggest that ATBF1 plays a transcriptional regulatory role.

A human alpha-fetoprotein enhancer-binding protein, ATBF1, contains four homeodomains and seventeen zinc fingers.

We have isolated a full-length cDNA encoding a protein (ATBF1) that binds to an AT-rich motif in the human alpha-fetoprotein gene enhancer. The amino acid sequence deduced from the cDNA revealed that this is the largest DNA-binding protein (306 kDa) known to date, containing four homeodomains, 17 zinc finger motifs, and a number of segments potentially involved in transcriptional regulation. Although the exact function of this protein has not been determined, these structural features suggest that ATBF1 plays a transcriptional regulatory role.

Newly arisen DNA repeats in primate phylogeny.

We discovered the presence of an Alu and an Xba repetitive DNA element within introns 4 and 7, respectively, of the human alpha-fetoprotein (AFP) gene; these elements are absent from the same gene in the gorilla. The Alu element is flanked by 12-base-pair direct repeats, AGGATGTTGTGG ... (Alu) ... AGGATGTTGTGG, which presumably arose by way of duplication of the intronic target site AGGATGTTGTGG at the time of the Alu insertion. In the gorilla, only a single copy of the unoccupied target site is present, which is identical to the terminal repeat flanking the human Alu element. There are two copies of an Xba repeat in the human AFP gene, apparently the only two in the genome. Xba1 and Xba2, located within introns 8 and 7, respectively, differ from each other at 3 of 303 positions. Xba1 is referred to as the old (ancestral) repeat because it lacks direct repeats. The new (derived) Xba2 is flanked by direct repeats, TTTCTTTTT ... (Xba) ... TTTCTTCTT, and is thought to have arisen as a result of transposition of Xba1. The ancestral Xba1 and a single copy of the Xba2 target site are present at orthologous positions in the gorilla, but the new Xba2 is absent. We conclude that the Alu and Xba DNA repeats emerged in the human genome at a time postdating the human-gorilla divergence and became established as genetic novelties in the human lineage. We submit that the chronology of divergence of primate lines of evolution can be correlated with the timing of insertion of new DNA repeats into the genomes of those primates.

Transcriptional regulation of α-fetoprotein expression by dexamethasone in human hepatoma cells

The level of alpha-fetoprotein (AFP) mRNA in HuH-7 human hepatoma cells is elevated by the addition of dexamethasone to the culture medium. To locate the DNA region involved in hormonal regulation of the AFP gene, we constructed recombinant plasmids in which various lengths of the 5'-flanking sequence of the human AFP gene were fused to the CAT gene. Various cell lines were transfected with the recombinant plasmids, incubated with or without 3 x 10(-6) M dexamethasone, and then assayed for chloramphenicol acetyltransferase expression. In hepatoma cells that produce AFP, the dexamethasone treatment resulted in the stimulated chloramphenicol acetyltransferase expression when the transfected plasmids contained 169 base pairs (bp) or longer AFP 5'-flanking sequence. No dexamethasone effect was observed when the 5'-flanking sequence was less than 98 bp long. The dexamethasone stimulation was effectively suppressed by the glucocorticoid antagonist RU486, indicating that this effect is mediated by glucocorticoid receptors. The 71-bp region between positions -169 and -98 contains a nucleotide stretch which is similar to the consensus sequence of the glucocorticoid responsive element (GRE). Partial alterations of this sequence resulted in decreased dexamethasone response. The GRE-containing region stimulated heterologous (SV40) promoter activity in response to dexamethasone treatment in an orientation- and position-independent manner. The GRE and the upstream AFP enhancer function independently from each other.

Cell-specific enhancer activity in a far upstream region of the human alpha-fetoprotein gene.

We describe experiments showing that the 5'-flanking region of the human alpha-fetoprotein (AFP) gene contains transcription control elements with characteristics of enhancers. The enhancer activity was detected and characterized by the ability to direct the expression of a linked chloramphenicol acetyltransferase (CAT) gene in transfected AFP-producing hepatoma cells in culture. The enhancer activity is cell-specific in that it occurs in hepatoma cells producing AFP, but not in non-AFP-producing hepatoma or nonhepatic cells. The active elements can direct CAT expression in conjunction with the SV40 promoter in an orientation- and position-independent manner. The sequences important for enhancer activity reside in the 400-base pair region between 3.3 and 3.7 kilobase pairs upstream of the AFP gene. This and proximal upstream regions contain multiple enhancer "core"-like sequences and other stretches of potential biological significance.

Structure, polymorphism and novel repeated DNA elements revealed by a complete sequence of the human .alpha.-fetoprotein gene

The human alpha-fetoprotein gene spans 19,489 base pairs from the putative "Cap" site to the polyadenylation site. It is composed of 15 exons separated by 14 introns, which are symmetrically placed within the three domains of alpha-fetoprotein. In the 5' region, a putative TATAAA box is at position -21, and a variant sequence, CCAAC, of the common CAT box is at -65. Enhancer core sequences GTGGTTTAAAG are found in introns 3 and 4, and several copies of glucocorticoid response sequences AGATACAGTA are found on the template strand of the gene. There are six polymorphic sites within 4690 base pairs of contiguous DNA derived from two allelic alpha-fetoprotein genes. This amounts to a measured polymorphic frequency of 0.13%, or 6.4 X 10(-4)/site, which is about 5-10 times lower than values estimated from studies on polymorphic restriction sites in other regions of the human genome. There are four types of repetitive sequence elements in the introns and flanking regions of the human alpha-fetoprotein gene. At least one of these is apparently a novel structure (designated Xba) and is found as a pair of direct repeats, with one copy in intron 7 and the other in intron 8. It is conceivable that within the last 2 million years the copy in intron 8 gave rise to the repeat in intron 7. Their present location on both sides of exon 8 gives these sequences a potential for disrupting the functional integrity of the gene in the event of an unequal crossover between them. There are three Alu elements, one of which is in intron 4; the others are located in the 3' flanking region. A solitary Kpn repeat is found in intron 3. The Xba and Kpn repeats were only detected by complete sequencing of the introns. Neither X, Xba, nor Kpn elements are present in the related human albumin gene, whereas Alu's are present in different positions. From phylogenetic evidence, it appears that Alu elements were inserted into the alpha-fetoprotein gene at some time postdating the mammalian radiation 85 million years ago.

Invasion of the human albumin-alpha-fetoprotein gene family by Alu, Kpn, and two novel repetitive DNA elements.

The human albumin-alpha-fetoprotein genomic domain contains 13 repetitive DNA elements randomly distributed throughout the symmetrical structures of these genes. These repeated sequences are located at different sites within the two genes. The human albumin gene contains five Alu elements within four of its 14 intervening sequences. Two of these repeats are located in intron 2, and the remaining three are located in introns 7, 8, and 11. The human alpha-fetoprotein gene contains three of these Alu elements, one in intron 4 and the remaining two in the 3'-untranslated region. In addition, the human alpha-fetoprotein gene contains a Kpn repeat and two classes of novel repeats that are absent from the human albumin gene. Six of the Alu elements within the two genes are bound by short direct repeats that harbor five base substitutions in 120 possible positions (60 bp times 2 termini). The absence of Alu repeats from analogous positions in rodents indicates that these repeats invaded the albumin-alpha-fetoprotein domain less than 85 Myr ago (the time of mammalian radiation). Furthermore, considering the conservation of terminal repeats flanking the Alu sequences of the albumin-alpha-fetoprotein domain (0.042 changes per site), we submit that the average time of Alu insertion into this gene family could have been as recently as 15-30 Myr ago.

The human albumin gene. Characterization of the 5‘ and 3‘ flanking regions and the polymorphic gene transcripts.

We have characterized the 5' and 3' regions of the human albumin gene with respect to nucleotide sequences, mRNA transcription initiation and polyadenylation. There are at least two transcription initiation sites and two polyadenylation sites in the human albumin gene. The multiple transcription initiation sites are utilized almost equally. The two polyadenylation sites are used differentially, with the site proximal to the protein termination codon dominating. The "TATA" and the "CCAAT" boxes are present at about 30 and 90 nucleotides upstream of the putative cap sites. About 250 nucleotides immediately 5' to the transcription initiation sites are highly conserved among the human, rat, and chicken. Within the 1-kilobase pair 5' flanking region of the human albumin gene there are 8- and 35-base pair alternating purine/pyrimidine sequences, primarily consisting of dG and dT, and three nucleotide segments exhibiting homology to the proposed progesterone receptor-binding site. In these and other characteristics, the 5' flanking region of the human albumin gene is distinct from the corresponding region of the human alpha-fetoprotein gene. Each of the two polyadenylation sites of the human albumin gene is preceded by a AATAAA hexamer. In addition, the first polyadenylation site has several conserved sequences believed to play a role in the 3'-end formation. The second polyadenylation site lacks these signals which may explain why it is used less frequently. In spite of these differences the two polyadenylation sites show a high overall sequence similarity, suggesting that they arose by duplication. There is no major difference between the normal liver and hepatoma cells in the relative utilization of multiple sites for transcriptional initiation and polyadenylation of the human albumin gene, suggesting that the albumin mRNA polymorphism has no direct relevance to hepatocarcinogenesis.