1 Department of Obstetrics and Gynecology, University of Tartu and 2 Department of Biotechnology, Institute of Molecular and Cell Biology, University of Tartu, Tartu, Estonia
3 To whom correspondence should be addressed: Department of Obstetrics and Gynecology, University of Tartu, Lossi 36, Tartu, 51003 Estonia. E-mail: Kristiina.Rull{at}kliinikum.ee
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Abstract |
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Key words: CGB gene expression/ectopic pregnancy/HCG/implantation/recurrent miscarriage
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Introduction |
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HCG, like other gonadotrophic glycoproteins, is composed of two subunits: a common - and a hormone-specific
-subunit. A single gene located on chromosome 6q12-q21 encodes the
-subunit; a cluster of genes localized on chromosome 19q13.3 determines the
-subunit of HCG. The whole cluster consists of seven homologous genes: one luteinizing hormone beta (LHB) gene and six chorionic gonadotrophin beta (CGB) genes that all originate from LHB gene as a result of duplication during primate evolution (Talmadge et al., 1984
). The
-subunits of HCG (163 amino acid protein) coding CGB, CGB5, CGB7 and CGB8 share 9799% DNA sequence identity; and shares 9293% with the functionally distinct LHB gene.
Although CGB1 and CGB2 genes are similar to the other genes in the cluster (85% identity), they encode a novel hypothetical protein (132 amino acids in length) that does not have any homology with the functional -subunit; neither corresponds to any known protein GenBank database. This change has been caused by an inserted DNA fragment within the 5' untranslated region (UTR) of CGB1 and CGB2 providing a novel exon 1 (Figure 1), and one base pair (bp) shifted open reading frame for exons 2 and 3. Earlier studies have detected mRNA of CGB1 and CGB2 in the placenta in the pituitary also, predicting the functionality of these genes (Bo and Boime, 1992
; Dirnhofer et al., 1996
). Previously, two reports (Bo and Boime, 1992
; Miller-Lindholm et al., 1997
) have studied the expression pattern of individual HCG
-subunit-determining genes (CGB, CGB5, CGB7 and CGB8) in first trimester placentas, single cases of complicated pregnancies (spontaneous abortion, blighted ovum, hydatidiform mole), and cultured JAR choriocarcinoma cells.
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In the study reported here, we aimed to examine mRNA expression of all the six CGB genes, and the single LHB in parallel with HCG concentration during the course of normal pregnancy. As the changes in mRNA transcription of CGB genes may play a role in implantation failure, we also investigated the tissue samples from patients with recurrent (3) spontaneous or missed abortion, and patients with ectopic pregnancy.
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Materials and methods |
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Serum and chorionic villi/placental samples were obtained from females who underwent elective therapeutic abortion during first trimester of pregnancy (412 weeks of gestational age, n = 11); therapeutic abortion during second trimester due to medical risks of pregnancy, no fetal anomalies being detected (1721 weeks of gestational age, n = 8); normal delivery at term resulting from uncomplicated pregnancy (3842 weeks of gestational age, n = 12); surgical treatment of ectopic pregnancy (614 weeks of gestational age, n = 6); cervical dilatation and uterine curettage because of recurrent (at least the third case in a row) incomplete or missed abortion (69 weeks of gestational age, n = 7).
As chromosomal aberrations of an incidental origin contribute to first trimester miscarriage in nearly 70% of cases (Fritz et al., 2001), we included only the cases of recurrent miscarriages (having at least two spontaneous abortions preceeding current event) when the alternative mechanisms leading to pregnancy loss are more relevant. All couples had a normal karyotype and as a result of clinical evaluation (anatomical, immunological, hormonal, blood coagulation assessment) were diagnosed with unexplained cause for miscarriage.
The blood samples were taken on the day of abortion, delivery or surgery and frozen at 20°C. Total HCG in the serum was measured by chemiluminescent enzyme immunometric assay (Immulite; United Laboratories, Tartu University Clinics, Estonia). Chorionic villi/placental samples were snap-frozen in liquid nitrogen and stored at80°C until RNA isolation.
RNA extraction and cDNA synthesis
Total RNA from 901000 mg of tissue was extracted using TRIzol® reagent (Invitrogen, Carlsbad, CA, USA). For purification of RNA, NucleoSpin® II (MachereyNagel) Isolation Kit was used. According to the protocol, RNAse-free DNAse I was used to remove contamination with genomic DNA. The products were quantified by measuring absorption ratios at 260/280 nm (Heios Alpha&Beta Spectrophotometer; Thermo Spectronic, Cambridge, UK).
RNA was reverse-transcribed in a final volume of 20 µl containing total 1 µg of RNA, reaction buffer [50 mmol/l TrisHCl (pH 8.3 at 25°C), 50 mmol/l KCl, 4 mmol/l MgCl2, 10 mmol/l DTT], 20 IU of RiboLockTM Ribonuclease Inhibitor, 0.5 mmol/l of each dNTP, 0.5 µg of oligo(dT)18 primer, 40 IU M-MuLV Reverse Transcriptase (First Strand cDNA Synthesis Kit, Fermentas Life Sciences). The samples were initially incubated at 70°C for 5 min, the reverse transcription reaction was performed at 37°C for 60 min, and stopped by heating at 70°C for 10 min to inactivate the reverse transcriptase.
Oligonucleotide primer design
RTPCR primers (Table I) were designed taking into account polymorphic positions in CGB genes defined by recent resequencing study (95 worldwide individuals) in our laboratory (P.Hallast, L.Nagirnaja, T.Margus and M.Laan, unpublished data). The primers for co-amplification of the HCG -subunit-determining genes CGB, CGB5, CGB7 and CGB8 were positioned to the different exons with the criteria to include the restriction sites specific to each gene (Lazar et al., 1995
; Miller-Lindholm et al., 1997
) (Figure 1). For co-amplification of CGB1and CGB2 we used the mixture of two reverse primers differing in 1 bp (Giovangrandi et al., 2001
), and designed a forward primer to obtain the PCR product covering the CGB2-specific BcnI restriction site. For normalization we amplified a widely used reference gene of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), with previously reported primers (Miller-Lindholm et al., 1997
). An alternative reference gene, largest subunit of RNA polymerase II (RPII) was selected based on a recent methodological report comparing reference genes for expression studies (Radonic et al., 2004
). For RPII amplification, we used the reported reverse primer (Radonic et al., 2004
) coupled with a novel forward primer, in order to obtain a PCR product sized to be distinguishable during the GeneScan analysis. The RTPCR of LHB was attempted with three primer pairs, including one reported (Giovangrandi et al., 2001
) and two designed in this study to obtain a larger PCR product that could be distinguishable better during the GeneScan analysis.
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The 5'-ends of reverse primers were labelled with an ABI-compatible fluorescent dye (6-FAM, TET, HEX; Metabion International AG, Germany). All primer sequences are indicated in Table I.
RTPCR amplification
The RTPCR mixture in total volume of 25 µl consisted of 1 µl first-strand reaction product, 1 IU Smart-Taq Hot DNA Polymerase (AppliChem GmbH), 2.5 mmol/l MgCl2, 0.2 mmol/l of each dNTP (Fermentas Life Sciences), 2.5 µl 10 x PCR reaction buffer containing (NH4)2 SO4 and 400 nmol of forward and reverse primers. Each cDNA sample was also the amplification target for the two reference genes GAPDH and RPII under the same conditions. Amplification was attained by GeneAmp PCR System 2700 (Applied Biosystems) thermal cycling for 1 cycle of 95°C for 15 min to denature, then 10 cycles of 95°C for 20 s to denature, and from 61 to 52°C for 30 s (touch-down) to anneal, and 72°C for 2 min to extend, followed by 20 cycles (GAPDH and CGB, CGB5, CGB7, CGB8) or 25 cycles (RPII) or 30 cycles (CGB1/2 and LHB) of 95°C for 20 s, 51°C for 30 s and 72°C for 2 min. The reaction was ended by final incubation of 72°C for 10 min. The number of cycles needed for amplification for each assay was optimized to ensure that PCR remained in the logarithmic phase of amplification (data not shown).
Restriction and GeneScan analysis
The experimental scheme for discrimination of the RTPCR products of CGB, CGB5, CGB7 and CGB8 was originally described by Lazar et al. (1995). Five microlitres of RTPCR product containing a mixture of CGB, CGB5, CGB7 and CGB8 was digested with 1 IU HhaI (Fermentas Life Sciences) and 1 IU DraI (Fermentas Life Sciences), which discriminate divergence in the 5'UTR resulting in the gene-specific fluorescent-labelled products of 273 bp for CGB8, 380 bp for CGB5, 401 bp for CGB and 431 bp for CGB7. Discrimination of CGB1 and CGB2 was solved by a similar approach. The 5 µl of the combined product of CGB1/2 was treated with 1 IU BcnI (Fermentas Life Sciences) resulting in fluorescent fragments of 207 bp for CGB1, and restricted 144 bp for CGB2. The restriction analysis was repeated twice in independent reactions to confirm the full digestion.
The mixture of digested CGB genes and reference gene products (GAPDH and RPII) in equal volumes was combined with formamide containing fluorescently labelled GeneScan-500 TAMRA internal size standard (Applied BiosystemsPerkin Elmer), and electrophoresed through a denaturing 6% polyacrylamide gel in 0.5 x TBE buffer in an Applied BiosystemsTM 373 DNA Sequencer.
The gel was analysed by GeneScan 2.1 software that allows determining the length of the fragment in the each lane. The intensity of the fluorescent dye derived from labelled primers and expressed the absorbance intensity in relative fluorescence units. The results on an electrophoretogram show both height and area of the peaks corresponding to the gene-specific PCR product.
Sample target evaluation and statistical analysis
All samples were amplified in triplicate, and each PCR product combination (reference and target genes, internal markers) was electrophoresed at least twice in order to minimize the effect of the researchers errors in manual loading of the sample, gel preparation or gel electrophoresis. As the correlation between values of the peak height and area (both in relative fluorescent units) from all experiments was high (r = 0.82, P < 0.0001), only values of peak height were used in analyses. The relative expression level of each gene was calculated as follows: the value of the peak height (in relative fluorescent units) of each target gene was divided with the value of peak height of the reference gene (GAPDH or RPII) from the same lane of the gel.
For statistical analysis a Pearson correlation test and a random effects linear regression model, allowing a patient-level random intercept, were used. All statistics were performed using R 2.0.1, a free software environment for statistical computing and graphics (http://www.r-project.org/).
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Results |
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The summarized expression patterns of CGB, CGB5, CGB7 and CGB8 during normal pregnancy and two patient groups (ectopic pregnancy, EP; recurrent miscarriages, RM) are shown in Figure 3A and B and by each individual gene in Table II. mRNA of CGB, CGB5 and CGB8 were present in placental material of all studied individuals, whereas CGB7 expression was below detection limit in 28 cases of the total 210 of observations. None of the studied groups stood out dominantly in lacking CGB7 expression; the contribution of CGB7 to the HCG -subunit determining transcription product varied 4.78.1% across study groups (Table II). In contrast to an extreme peak of HCG hormone production during the first trimester (Figure 3C), the
-subunit-coding genes are expressed continuously at the same level during the whole pregnancy, with only a small decrease in the second trimester. Thus, the expression level of
-subunit-determining genes in placenta showed a moderate but significant correlation to HCG concentration in the serum (r = 0.44, P< 0.0001). Consistent with a previous report (Miller-Lindholm et al., 1997
), the variation between individuals within the same study group was large for mRNA levels of all HCG
-subunit-coding genes (Figure 3A, B). In cases of RM, the expression levels of CGB, CGB5, CGB7 and CGB8 were significantly lower than expected for the normal first trimester placenta, being adjusted to gestational age and HCG concentration (P= 0.017) (Figure 3A, B). Notwithstanding the low values of HCG in the serum (Figure 3C), the summarized level of mRNA of HCG
-subunit-coding genes in cases of EP was comparable with the expression during normal implantation to uterus. The most prevalent contribution pattern of individual genes to HCG
-subunit production, for both normal and complicated cases, was CGB8 > CGB5
CGB >> CGB7 (Table II). For the third trimester placentas, the contribution pattern of individual genes was slightly altered, CGB8
CGB5 > CGB >> CGB7 (Table II). Compared to the first trimester, the expression level of CGB5 was increased (P< 0.05) and of CGB decreased (P< 0.005).
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The expression of CGB1 and CGB2 genes
The mRNA expression of CGB1 and CGB2 was low (Figure 4AD). A reliable peak above non-specific background for CGB1 was detectable in 80% of specimens, and for only in 50% of observationsfor CGB2. Although we did not succeed in demonstrating the statistically significant differences between the study groups, the results differed from the expression pattern of CGB, CGB5, CGB7 and CGB8. A clear expression peak was visible during the first trimester of normal pregnancy as well as for EP (Figure 4AD). For RM the levels of CGB1 and 2 were at the borderline of detection. In addition, for all study groups we detected an extra peak at 254 bp that corresponds to an alternatively spliced product of CGB1, and similarly for RM it was close to the detection limit (Figures 1, 2, 4E, F). The expression of this alternative product correlated moderately with the major CGB1 product (r = 0.44, P < 0.0001), and in most cases the transcription level of the alternative product exceeded the level of original CGB1. We could not detect an alternative splicing of CGB2 (expected fragment size 191 bp in length on GeneScan analysis).
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The expression of LHB gene
We failed to demonstrate the presence of LHB during normal pregnancy as well as in complicated cases (EP, RM) using three alternative primer pairs. Therefore we conclude that the expression of LHB gene is fully down-regulated during pregnancy, or remained undetectable by the semiquantitative approach used in this study.
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Discussion |
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Despite high DNA sequence similarity between individual HCG -subunit coding genes, our study demonstrated high variation in expression levels, both between the genes themselves as well as between the individuals. The expression variation between CGB, CGB5, CGB7 and CGB8 could result from a few divergent nucleotides located in the 5'-UTR or upstream region of the genes that serve as regulatory variants influencing the gene expression. The most divergent from the other genes, and also with the lowest expression level, is CGB7, with
10 gene-discriminating nucleotides 330 bp upstream of exon 1 as well as three amino acid changes. The contribution of individual genes to HCG
-subunit production differed to some extent from previous studies (Bo and Boime, 1992
; Miller-Lindholm et al., 1997
), which reported CGB5 as the most actively transcribed gene in contrast to CGB8 in our study. The between-individual variation could be caused by either polymorphic variation of the studied genes and/or an individuals differences in the process of trophoblast differentiation. A resequencing study of LHB/CGB genes has identified extremely high diversity level of these genes, most probably favoured by the spread of polymorphisms by gene conversion between highly similar genes (P.Hallast, L.Nagirnaja, T.Margus and M.Laan, unpublished data. Thus, there is an abundance of CGB, CGB5, CGB7 and CGB8 gene variants and their combinations carried by individuals in the population. On the other hand, the expression of HCG hormone
-subunit is associated with the fusion of cytotrophoblasts into a multinuclear syncytium (Maruo et al., 1992
). During syncytialization, selective promoter factors (Sp) and activating protein 2 (AP2) located in the proximal promoter region interact in increasing amounts with transcription factors Sp1, Sp3 and AP-2
, which enhance transcription of HCG
-subunit genes in differentiating term trophoblasts (Maruo et al., 1992
; Knofler et al., 2004
). In early placentas a different combination of factors may control
-subunit expression.
We detected an expression peak of both CGB1 and CGB2 during first trimester of normal pregnancy and EP, but not in placentas of the second, third trimester and RM. Thus, the pattern differed from what we found in expression of HCG -subunit-coding genes CGB, CGB5, CGB7 and CGB8. At the beginning of implantation and placentation, the invasion of the placental bed occurs in the tube in the same manner as it does in the uterus, suggesting that the ability to implant and form the normal placenta is determined by trophoblastic tissue of fetal origin rather than by maternal tissues (Randall et al., 1987
). In contrast, miscarried pregnancies are related to poor trophoblast invasion (Meegdes et al., 1988
; Lyall, 2002
). By the second and third trimester the process of implantation has been replaced by the growth and maturation of the placenta. Our data on expression patterns of CGB1 and CGB2 suggest that these genes might have a role in implantation and placentation. As the protein product of CGB1 and CGB2 is still not characterized, further studies are needed to understand the function of these genes.
For CGBI we detected an alternative spliced product, which arises when the splicing occurs at the consensus splice donor site of LHB, CGB and CGB5, CGB7 and CGB8 genes (Figure 1). The extra 47 bp long sequence from the first intron is added into the transcript. In previous studies, the presence of alternatively spliced transcript of either CGB1 or CGB2 has been demonstrated in placenta (Bo and Boime, 1992) but not in pituitaries (Dirnhofer et al., 1996).
Is the expression profile of LHB/CGB genes prognostic to pregnancy outcome? Indeed, recurrent miscarriages were characterized by a low level of mRNA of HCG -subunit-coding genes, and almost lacking CGB1 and CGB2 expression, supporting the hypothesis that expression failure of CGB genes could contribute to miscarriages. Multiple scenarios for the expression failure have been proposed such as polymorphisms affecting hormone glycosylation, as well as interaction with transcription factors, and genomic rearrangements of the LHB/CGB cluster due to highly homologous segments. For ectopic pregnancies, expression of CGB genes comparable with normal first trimester pregnancy contrasts with the low concentration of HCG in the serum, indicating the problems with stable hormone assembly and transport rather than pathological CGB variants and low transcriptional activity.
During implantation, trophoblasts exhibit the same behaviour as tumour cells, such as tissue invasiveness, metastasis, loss of contact inhibition, escape from immune surveillance, and massive proliferative ability (Willey, 1999). The breast, ovarium, bladder, lung, renal and some other malignancies have been reported to express the HCG
-subunit-coding genes CGB, CGB5 and CGB8 in contrast to poorly expressed CGB7 (Lazar et al., 1995
; Bellet et al., 1997
; Giovangrandi et al., 2001
; Span et al., 2002
, 2003
). As the prognostic value of CGB gene expression in predicting tumour progression is still controversial (Bieche et al., 1998
; Hotakainen et al., 2002
), detailed studies on the expression patterns of total mRNA levels of CGB genes as well as on the contribution of each individual gene are needed.
In conclusion, as the main results of the study we would like to highlight the detection of high expression of CGB, CGB5 and CGB8 in the placenta throughout pregnancy with a minor decrease during the second trimester; significant reduction of CGB genes expression in cases of recurrent miscarriage; low placental expression activity of CGB7, possibly diverging from its current function; a putative role of CGB1 and CGB2 in implantation and placentation; and the presence of an alternatively spliced product of CGB1.
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Acknowledgements |
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Submitted on May 6, 2005; resubmitted on June 17, 2005; accepted on July 18, 2005.
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