Mol. Hum. Reprod. Advance Access originally published online on January 18, 2008
Molecular Human Reproduction 2008 14(3):157-168; doi:10.1093/molehr/gam088
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Differential gene expression in cumulus cells as a prognostic indicator of embryo viability: a microarray analysis
1Department of Obstetrics and Gynaecology, Research Institute Growth and Development (GROW), Academic Hospital Maastricht, Maastricht, The Netherlands 2Department of Clinical Genetics, Research Institute Growth and Development (GROW), Academic Hospital Maastricht, Maastricht, The Netherlands 3Department of Population Genetics, Genomics and Bioinformatics, Maastricht University, and Research Institute Growth and Development (GROW), Maastricht, The Netherlands
4Correspondence address. E-mail: avmn{at}sgyn.azm.nl
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Abstract |
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Besides the established selection criteria based on embryo morphology and blastomere number, new parameters for embryo viability are needed to improve the clinical outcome of IVF and more particular of elective single-embryo transfer. Genome-wide gene expression in cumulus cells was studied, since these cells surround the oocyte inside the follicle and therefore possibly reflect oocyte developmental potential. Early cleavage (EC) was chosen as a parameter for embryo viability. Gene expression in cumulus cells from eight oocytes resulting in an EC embryo (EC-CC; n = 8) and from eight oocytes resulting in a non-EC (NEC) embryo (NEC-CC; n = 8) was analysed using microarrays (n = 16). A total of 611 genes were differentially expressed (P < 0.01), mainly involved in cell cycle, angiogenesis, apoptosis, epidermal growth factor, fibroblast growth factor and platelet-derived growth factor signalling, general vesicle transport and chemokine and cytokine signalling. Of the 25 selected differentially expressed genes analysed by quantitative real-time PCR 15 (60%) genes could be validated in the original samples. Of these 8 (53%) could also be validated in 24 (12-EC-CC and 12 NEC-CC) extra independent samples. The most differentially expressed genes among these were CCND2, CXCR4, GPX3, CTNND1 DHCR7, DVL3, HSPB1 and TRIM28, which probably point to hypoxic conditions or a delayed oocyte maturation in NEC-CC samples. This opens up perspectives for new molecular embryo or oocyte selection parameters which might also be useful in countries where the selection has to be made at the oocyte stage before fertilization instead of at the embryonic stage.
Key words: assisted reproductive technology/early cleavage/gene expression/cumulus cells/microarray
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Introduction |
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The only way to prevent a dizygotic twin pregnancy in IVF, which is regarded as one of the most serious complications, is single-embryo transfer (SET). As most patients have more than one embryo available for transfer, selecting the most viable one is of pivotal importance. Most clinics rely for embryo selection on the non-invasive examination of developmental and morphological aspects of the embryos. In every stage of oocyte and embryonic development, characteristics have been defined which appear to be prognostic indicators of successful pregnancy. Among these are zona pellucida thickness and cytoplasmic granularity of the oocyte, size of pronuclei and alignment of nuclear polar bodies in the zygote, early cleavage (EC) in the cleavage stage embryo and number and size of blastomeres, fragmentation and multinucleation in the 4–8-cell stage embryo [see Borini et al. (2005) for a review and Gerris (2005) for a more extensive list of references]. Especially, EC appears to be a good parameter for embryo viability as it is highly correlated with the blastocyst formation rate (Fenwick et al., 2002) and the implantation and pregnancy rate (Shoukir et al., 1997; Sakkas et al., 1998; Lundin et al., 2001), not only in double but also in SETs (Salumets et al., 2003; Van Montfoort et al., 2004). For embryos in the intermediate syngamy state, the pregnancy rate was in between that of EC and non-early cleavage (NEC) embryos (Wharf et al., 2004).
As the developmental potential of an embryo cannot be fully determined by characteristics visible by microscopy alone, several other markers are studied (Pearson, 2006). For instance, several investigators focused on the influence of the follicular micro-environment on subsequent embryonic development. The follicular fluid LH and growth hormone levels at the time of oocyte retrieval were higher in embryos with good morphology (Mendoza et al., 2002). Furthermore, the concentrations of hormones (17β-estradiol, LH, growth hormone, prolactin, leptin), growth factors (insulin-like growth factor-I), cytokines (interleukin-1) and proteinases (matrix-metalloproteinase-9) in follicular fluid differ according to the probability of pregnancy (Mendoza et al., 2002; Anifandis et al., 2005; Hammadeh et al., 2005; Lee et al., 2005). Also vascularization of the follicles has been examined as a potential marker for the developmental potential of an embryo. The peri-follicular blood flow characteristics are related to oocyte oxygenation (Van Blerkom et al., 1997) and can differ between the follicles in one ovary. Nargund et al. (1996) found, by Doppler imaging of the follicular blood flow, that oocytes from poorly vascularized follicles developed in morphologically inferior embryos as compared to those from well-vascularized follicles. Several other studies confirmed a positive relationship between perifollicular vascularization and pregnancy (Chui et al., 1997; Van Blerkom et al., 1997; Borini et al., 2004). Pregnancies were only achieved with embryos from oocytes which had vascularity detected in >50% of their follicular circumference and live births only from oocytes with >75% follicular vascularity (Chui et al., 1997). Gaulden et al. (1992) suggested that hypoxic intracellular conditions might result in a diminished level of oxidative metabolism in the oocyte and a lower intracellular pH. The latter in turn could lead to meiotic spindle instabilities and chromosomal abnormalities. Indeed,Chui et al. (1997) and Van Blerkom et al. (1997) reported a significantly higher incidence of aneuploidy and spindle defects in oocytes derived from follicles with poor vascularization as compared to those from well-vascularized follicles. In addition, ATP content of the oocyte and dissolved oxygen content of the follicle fluid are related to oocyte/embryo development (Van Blerkom et al., 1995,1997).
As the oocyte is in dialogue with the surrounding cumulus cells via paracrine and gap-junctional signalling (Sutton et al., 2003), we hypothesized that differences in intra-follicular processes which are responsible for oocyte and embryonic development and subsequently implantation are reflected in the gene expression pattern of cumulus cells. The bi-directional communication between the oocyte and the cumulus cells is necessary for oocyte development as oocytes fail to grow in the absence of (a connection with) cumulus cells (Ackert et al., 2001; Matzuk et al., 2002). Zhang et al. (2005) reported that the expression of several genes in cumulus cells, particularly pentraxin 3 (PTX3), was indicative of oocyte and embryo quality. In addition, the expression of cyclooxygenase 2 (COX2), gremlin (GREM) and hyaluronic acid synthase 2 (HAS2) is also positively correlated with embryo quality (McKenzie et al., 2004). In turn, oocyte factors like growth and differentiation factor-9 (GDF-9) are necessary for cumulus expansion (Sutton et al., 2003).
The aim of this study was to analyse the genome-wide expression of genes in cumulus cells as indicators of embryo viability. By analysing gene expression in cumulus cells, the understanding of the regulation of oogenesis and embryonic development might be improved. This information might lead to new molecular non-invasive embryo selection parameters reflected in cumulus cells that can be used in addition to the existing morphological parameters or might result in an oocyte selection tool for those who are obliged to select a limited number of oocytes for fertilization (Ludwig et al., 2000).
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Patients and human cumulus cell collection
Patients visiting the IVF clinic of the academic hospital Maastricht underwent an IVF or ICSI treatment as described previously (Van Montfoort et al., 2006). For the study, which was approved by the local Ethics Committee, in consenting patients, immediately following ultrasound-guided cumulus–oocyte complex (COC) retrieval, a proportion of the cumulus cells surrounding a single oocyte were removed using a sharp needle, lysed in 100 µl Trizol reagent (Invitrogen, Carlsbad, USA) supplemented with 1% (v/v) 2-mercapto-ethanol (Merck, Darmstadt, Germany), snap-frozen in liquid nitrogen and stored at –80°C (cumulus cells from one oocyte per vial). The oocytes were cultured and fertilized individually in 5 µl droplets covered by mineral oil. Between 23–26 h post-injection or 25–28 h post-insemination EC status of embryos was assessed. A 2 h time difference is necessary to compensate for the time difference in early development between IVF- and ICSI-derived embryos (Van Montfoort et al., 2004). Subsequently, on Day 2 of development, the embryos were examined for morphology, number of blastomeres and the presence or absence of multinucleated blastomeres (MNBs) (Van Montfoort et al., 2005).
Experimental design
EC was chosen as a marker for embryo viability. Gene expression in cumulus cells from eight oocytes resulting in an EC embryo (EC-CC; n = 8) and from eight oocytes resulting in a non-EC embryo (NEC-CC; n = 8) derived from six patients were analysed using microarrays (n = 16). To exclude a differential gene expression due to differences in patient characteristics, samples were paired. From four patients both an EC-CC and a NEC-CC sample were used. From two additional patients two EC-CC as well as two NEC-CC samples were used. The microarray results were validated by quantitative RT–PCR (qRT–PCR) on the original samples analysed by microarray as well as on 24 new samples.
The cumulus cell samples (for microarray and RT–PCR) from EC and NEC embryos were derived from normally fertilized (2PN) oocytes, which developed into embryos with comparable characteristics on Day 2, i.e. 4-cell with good morphology and no MNBs present.
RNA isolation
Total RNA was extracted using Trizol reagent (Invitrogen) according to the manufacturer’s instructions with some adaptations for the small quantity of RNA. RNA was precipitated with isopropyl alcohol for 2 h and the RNA pellet was washed three times with 75% ethanol. To be able to track the small RNA pellet, 5 µg glycogen (Ambion, Woodward, USA) was added to the sample before RNA precipitation. Total RNA was resuspended in 20 µl RNase-free water and stored at –80°C. For all RNA samples quantity and purity were determined using the Nanodrop ND-1000 spectrophotometer (Nanodrop Technologies, Wilmington, USA) and RNA integrity was determined using the Bioanalyzer 2100 (Agilent Technologies, Palo Alto, USA).
Two cycle amplification and microarray hybridization
Fifty nanogram total RNA was amplified using the two-cycle cDNA synthesis kit (Affymetrix, Santa Clara, USA) in combination with the MEGAscript T7 in vitro transcription system (Ambion). Biotin labelled target complementary RNA was fractionated and hybridized to Human Genome U133A Plus 2.0 Arrays (Affymetrix). Each array contained >54 000 oligonucleotide probe-sets corresponding to 38 500 characterized human genes.
Microarray analysis
To identify probe sets which were differentially expressed between eight EC-CC and eight NEC-CC samples, a three step process was applied. First, Affymetrix GeneChip Operating Software (GCOS, version 1.4) was used to analyse image data. For each transcript represented on the array by a probe set, the expression algorithm computed the detection call (present, absent or marginal), the detection P-value, and the signal which is an average intensity value for each probe set. This resulted in a table with 54 675 probe sets. Second, for each probe set the 16 detection calls were used to determine whether the probe set was reliably detected or not and should or should not be selected for further analysis (McClintick and Edenberg, 2006). To this end, for every group of eight arrays (the early and the non-early) the number of present calls was counted (a number ranging from 0 to 8). If six or more calls were present, the probe set was denoted present. If the probe set was in at least one of the two groups denoted present, it was selected for further analysis. Finally, the over- or underexpression of the remaining probe sets in one of the two groups was analysed using the class comparison method in BRB ArrayTools software package applying a univariate test composed of a paired t-test with random variance model. This was developed by the Biometric Research Branch of the US National Cancer Institute (http://linus.nci.nih.gov/BRB-ArrayTools.html). Hierarchical clustering of samples was also performed using BRB ArrayTools. Samples were clustered by comparing their expression profiles.
The genes showing significant differential expression between both groups (P < 0.01) were classified into functional groups using the Panther classification system (http://www.pantherdb.org) (Thomas et al., 2003). The gene expression data analysis tool (Thomas et al., 2006) was used to determine which biological process or pathway was significantly overexpressed in one of the two groups. This program uses binomial statistics with Bonferroni correction to analyse whether the proportion of genes from a certain biological process or pathway present in a gene list (i.e. the list of differentially expressed genes from an array study) is significantly different from the proportion of genes in that process or pathway in the whole human genome (P < 0.05).
Quantitative real-time PCR
For the qRT–PCR, TaqMan low density arrays (TLDAs) (Applied Biosystems, Foster City, USA) were used. Each 2 µl well of the TLDA contains user-defined primers and probes selected from an online catalogue (http://myscience.appliedbiosystems.com) for a single gene. One well contains primers and probes for 18S rRNA, a mandatory endogenous control from the manufacturer.
cDNA was prepared from 100 
g total RNA per sample using the High Capacity cDNA archive kit (Applied Biosystems) according to the manufacturer’s instructions. To each cDNA sample (20 µl), 80 µl nuclease-free water and 100 µl 2x TaqMan Universal PCR Master Mix (Applied Biosystems) was added. This mixture was then equally divided over two sample-loading ports of the TLDA, each connected to one set of all the genes of interest. The arrays were centrifuged twice (1', 331 g) to equally distribute the sample over the wells. Subsequently, the card was sealed to prevent an exchange between wells. qRT–PCR amplification was performed using an Applied Biosystems Prism 7900HT sequence detection system with the following thermal cycler conditions: 2 min at 50°C and 10 min at 94.5°C, followed by 40 cycles of 30 s at 97°C and 1 min at 59.7°C.
qRT–PCR analysis
The RQ manager 1.2 software was used to generate Ct values corrected for variances in fluorescent signal strength by using a passive reference dye. The geNorm program (Vandesompele et al., 2002) was used to determine the most stably expressed housekeeping genes. Briefly, the average pair-wise variation of a housekeeping gene with all other housekeeping genes was calculated. Stepwise exclusion of the gene with the highest variation resulted in a combination of two housekeeping genes that have the most stable expression. The geometrical mean of the Ct values of these two genes was used as a normalization factor which was substracted from the Ct values of the genes of interest to obtain normalized Ct values (
Ct). Subsequently, the mean Ct of the NEC-CC samples was substracted from the mean Ct of the EC-CC samples generating a Ct. This Ct was recalculated into a relative expression quantity (2–Ct) of the gene of interest in EC-CC as compared with NEC-CC samples (Livak and Schmittgen, 2001).
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Microarray analysis
For the gene expression analysis, 8 EC-CC and 8 NEC-CC samples (from 6 patients) have been analysed using 16 microarrays. The raw microarray data have been deposited in NCBIs Gene Expression Omnibus (GEO, http://www.ncbi.nlm.nih.gov/geo/) and are accessible through GEO Series accession number GSE9526 [NCBI GEO] . From the 54 675 probe sets on the array, 18 480 had a present call. Most of these probe sets showed similar expression between the EC-CC and NEC-CC group, except for 737 probe sets that were differentially expressed (P < 0.01). For 59 of these probe sets, the corresponding gene is not yet known. Of the 678 remaining probe sets, which correspond to 611 different genes, 162 (24%) were up-regulated and 516 (76%) were down-regulated in EC-CC compared with NEC-CC. Clustering analysis of the arrays based on the 500 most significantly differentially expressed genes perfectly clustered the EC-CC samples and the NEC-CC samples (Fig. 1). The two EC-CC samples derived from the same patient (sample 1-EC a and b and sample 6-EC a and b) and the NEC-CC samples from the same patient (sample 1-NEC a and b and sample 6-NEC a and b) were highly correlated.
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Of the 611 genes that are differentially expressed between EC and NEC (P < 0.01), 426 could be categorized into one or more of the biological processes listed in Table I. Of these processes, protein modification, nucleic acid, lipid, fatty acid and steroid metabolism, apoptosis, general vesicle transport, cell cycle, cell structure and motility, chromatin packaging and remodelling, transport and other signalling are significantly overrepresented (P < 0.05, as compared to the whole human genome) according to the Panther gene expression tool. In Table II, the significantly differentially expressed genes are shown, categorized into their most prominent role. To economize space, only the genes (n = 95) with P < 0.001 are shown. Information on all of the 426 differentially expressed genes (with P < 0.01) is provided in the Supplementary Data, Table S1.
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Furthermore, the significantly (P < 0.05) overrepresented pathways in which the genes differentially expressed between EC and NEC are involved in are Ras, chemokine and cytokine signalling, epidermal, fibroblast and platelet-derived growth factor (EGF, FGF and PDGF) signalling and angiogenesis. In Table III, the genes classified to these pathways are shown.
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qRT–PCR
From six out of eight NEC-CC samples and from all EC-CC samples sufficient RNA remained for a validation of the microarray results by qRT–PCR. Furthermore, an additional 12 EC-CC and 12 NEC-CC samples were analysed to validate the results in independent samples. qRT–PCR was performed on genes selected either because of their highly significant differential expression in both study groups or because of their involvement in a biological relevant pathway or cellular process (n = 25). Furthermore, the most stably expressed housekeeping genes from the microarray (n = 4, EIF4G2, PARK7, SRP14 and RHOA) and 18S (a mandatory control by the manufacturer) were included in the qRT–PCR analysis. SRP14 and RHOA were the two most stably expressed housekeeping genes, i.e. the genes with the lowest variation in expression levels. The expression values of the other 25 genes were normalized against the geometrical mean of these two genes.
In the original samples, which were also analysed by microarray, for 22 out of the 25 selected genes (88%) the differential expression between EC-CC and NEC-CC could be confirmed by qRT–PCR (Fig. 2). When fold changes between 0.9 and 1.1 were excluded 15 of the 25 (60%) genes could be confirmed. Of these, the differential expression of 10 genes (67%) could be confirmed in the independent samples of which 8 remained after exclusion of the fold changes between 0.9 and 1.1 (Fig. 3). These genes were cyclin D2 (CCND2), catenin delta-1 (CTNND1), CXC chemokine receptor 4 (CXCR4), 7-dehydrocholesterol reductase (DHCR7), dishevelled dsh homolog 3 (DVL3), glutathione peroxidase 3 (GPX3), heatshock 27 kDa protein 1 (HSPB1) and tripartite motif-containing 28 (TRIM28).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionSupplementary dataFundingAcknowledgementsReferences |
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To improve the clinical outcome of elective single-embryo transfer (eSET), the embryo selection needs to be optimized. Besides the established selection criteria based on embryo morphology and blastomere number, new selection parameters should be developed. Information about the oocyte and its development might be a valuable contribution to the existing selection criteria. As cumulus cells surround the oocyte inside the follicle, a microarray analysis was performed on these cells. Both cumulus cells from oocytes developing into an EC-CC as well as from oocytes developing into a NEC-CC embryo were compared. Our analysis revealed that 18 480 genes were expressed in cumulus cells, 611 of which showed significant differential expression between EC-CC and NEC-CC. A cluster analysis could perfectly separate the EC-CC and NEC-CC samples, indicating that differences in embryonic implantation potential can already be detected as early as folliculogenesis. These differences were not manifested in blastomere number and morphology of the embryo as these were similar in both groups.
The differences in gene expression could not be due to differences in age, ovarian stimulation or other patient characteristics as from each patient one EC-CC and one NEC-CC (n = 4 patients) sample or two EC-CC and two NEC-CC samples (n = 2 patients) were used. By pairing the samples from each patient, the differential gene expression due to different patient characteristics could be ruled out. Furthermore, while other studies analyzing gene expression in human cumulus cells pooled the cumulus cells from several oocytes (Zhang et al., 2005; Assou et al., 2006), in this study each sample consisted of the cumulus cells from one oocyte. This prevented loss of information.
Of the 611 differentialy expressed genes 24% was overexpressed in EC-CC, whereas 76% was overexpressed in NEC-CC. The most abundant functions or pathways these genes were involved in were EGF, FGF and PDGF signalling as well as chemokine and cytokine signalling, lipid, fatty acid and steroid metabolism, cell cycle, apoptosis and angiogenesis. Twenty-five genes were selected for validation by quantitative real-time PCR. The gene expression profile found by microarray analysis could be validated for 15 of the 25 (60%) selected genes. In literature, a 84–88% concordance between microarray and quantitative real-time PCR has been described (Rajeevan et al., 2001a; Dallas et al., 2005). Microarray results can be influenced by labelling and hybridization efficiency, whereas quantitative PCR is dependent on the efficiency of the enzymes and primers. Especially, small fold changes between study groups are sensitive to these variations, which explains the lower concordance in our study (Rajeevan et al., 2001b). The differential expression of 8 of the 15 genes (53%) could be verified in extra independent samples. This indicates that gene expression validation in independent samples is very important to control for genes not consistently over- or underexpressed in the tested conditions and that with microarray analysis alone some genes can show differential expression between the examined conditions by chance. The validated genes are CCND2, CTNND1, CXCR4, DHCR7, DVL3, GPX3, HSPB1 and TRIM28.
CCND2 is an important cell cycle regulator and plays an essential role in cumulus cell proliferation as a null mutation in Ccnd2 in mice impairs cumulus cell proliferation and leads to small follicles unable to ovulate (Sicinski et al., 1996). Cumulus cell proliferation is regulated by factors secreted by fully grown oocytes (Eppig, 2001). At least in bovine oocytes, this cumulus cell promoting activity decreases upon meiotic maturation (Lanuza et al., 1998). The increased proliferation in the NEC-CC group as compared to the EC-CC group therefore suggests that the enclosed oocyte in the first group was less mature at the time of oocyte retrieval. This delayed maturation might also explain the increased CTNND1 expression in NEC-CC. 
-Catenin forms, together with E-cadherin, a major cell–cell adhesion protein complex (Keirsebilck et al., 1998). This might result in an enhanced cumulus cell adhesion as compared to EC-CC, and thus a delayed cumulus expansion, which was not visible by microscopy at the time of cumulus cell collection. The cumulus cell membrane receptor CD44 (Schoenfelder and Einspanier, 2003), which attaches the extracellular matrix to the cumulus cells (Yokoo et al., 2002) was significantly underexpressed in NEC-CC, also confirming a disturbed cumulus cell expansion. Cumulus expansion is necessary for a proper oocyte and embryo development (Zhang et al., 1995) and is induced by the oocyte factors BMP-15 and GDF-9 (Sutton et al., 2003). In our study, no indication was found for underexpression of BMP-15 or GDF-9, as the expression of the target genes PTGS2, HAS2, GREM1 and PTX3 (McKenzie et al., 2004; Zhang et al., 2005) was not different between the two groups. There might be an oocyte factor other than BMP-15 or GDF-9 that induces cumulus expansion.
An increase in cumulus cell apoptosis has also been associated with immaturity of oocytes and with an impaired fertilization (Host et al., 2002) and pregnancy rate (Lee et al., 2001). In our study, of the 10 differentially expressed genes (P < 0.01) whose most prominent function is a role in apoptosis, 8 point to an increase in apoptosis in the NEC-CC samples (6 pro-apoptotic genes were overexpressed and 2 anti-apoptotic genes were underexpressed as compared to EC-CC). It is not exactly known how these apoptotic signals exert their negative effect on oocyte and embryo development. The apoptotic signals can easily be transferred from the cumulus cells to the oocyte through the gap junctions. After the LH peak, at the time the oocyte reaches metaphase I, these junctions close (Sutton et al., 2003). The timing of gap junction closure might be important for oocyte and embryo development. A delayed response to LH also emerges from the increased expression of both CCND2 (Muniz et al., 2006) and CTNND1 (Sasson et al., 2004), whose expression is inhibited by LH and the increased expression of CD44, which is induced by LH (Schoenfelder and Einspanier, 2003).
TRIM28 (also known as KAP-1 or TIF1beta) is a universal co-repressor of gene transcription that acts via interaction with KRAB domains in Kruppel-type zinc-finger proteins. It also has a role in DNA repair. Upon DNA damage, KAP1 is phosphorylated through which the damaged DNA can decondensate (Ziv et al., 2006). The increased expression of TRIM28 in NEC-CC might thus lead to a reduced response to DNA damage or to transcriptional suppression of several genes.
The chemokine receptor CXCR4 was up-regulated in NEC-CC samples. The expression of Cxcr4 in cumulus cells was confirmed by Hernandez et al. (2006) who localized the protein at the cell surface. The role of this receptor and its ligand CXCL12 in oocyte development is however still undefined. CXCR4 in endothelial and cancer cells is expressed via hypoxia inducible factor 1
(HIF1)-mediated transcription, which in turn is activated under hypoxic conditions via the phosphatidylinositol 3-kinase (PI 3-kinase) pathway (Schioppa et al., 2003; Staller et al., 2003; Phillips et al., 2005). EGF can also induce PI 3-kinase signalling and up-regulate CXCR4 transcription via HIF-1 (Phillips et al., 2005). HIF-1 is a regulator of oxygen homeostasis by functioning as a transcription factor for genes involved in angiogenesis, erythropoiesis, glycolysis and cell proliferation and survival (Semenza, 2002). How CXCR4 can relieve hypoxic stress in cumulus cells needs further investigation.
Another gene which might indicate a hypoxic environment in NEC-CC samples is GPX3. Hypoxia is a strong transcriptional regulator of this gene through its HIF-1 binding sites. It is the only glutathione peroxidase in which these binding sites have been detected (Bierl et al., 2004). Hypoxia leads to the formation of reactive oxygen species (ROS) which can cause lipid peroxidation, enzyme inactivation and cell damage, resulting in apoptosis (Buttke and Sandstrom, 1994) not only in cumulus cells, but also in the oocyte (Tatemoto et al., 2000). Both hypoxia (Van Blerkom et al., 1997) and a concentration of ROS above a certain level in follicular fluid have also been negatively associated with embryonic development, pregnancy outcome (Pasqualotto et al., 2004; Das et al., 2006) and a significantly higher incidence of aneuploidy and spindle defects in oocytes (Chui et al., 1997; Van Blerkom et al., 1997). Nowadays some clinics screen the embryos for aneuploidy by fluorescence in-situ hybridization on one or two blastomeres biopsied from the embryo (Munne et al., 2003). Although our data need further examination, it would be very promising if intrafollicular hypoxia and thus the enhanced chance for aneuploidy in the corresponding embryo can be analysed by using the cumulus cells instead of removing one or two blastomeres from the embryo.
A common response to hypoxia is the stimulation of angiogenesis. Recently, it has been established that Wnt signalling, besides in embryogenesis, also plays a role in angiogenesis (Masckauchan et al., 2006). WNT5A, which is up-regulated in NEC-CC (only analysed with microarray, but not included in the qRT–PCR assay) acts via non-canonical Wnt signalling to promote angiogenesis. It exerts its signal through DVL3 phosphorylation (also up-regulated in NEC-CC) (Schulte et al., 2005).
In agreement with the existence of a stressful environment in NEC-CC is the increased expression of the stress-induced apoptosis inhibitor HSPB1 (Arya et al., 2007). HSPB1 can however also be expressed in the presence of estrogen instead of stress factors and act as a co-repressor of estrogen signalling by binding to the estrogen receptor beta (Al-Madhoun et al., 2007).
DHCR7 converts 7-dehydrocholesterol to cholesterol, which is a component necessary for progesterone and estrogen synthesis. A suppression of the cholesterol biosynthesis pathway in COC led to a decrease in progesterone production and subsequently to a decreased rate of germinal vesicle breakdown (Yamashita et al., 2003). It is however unknown how an increased cholesterol biosynthesis is related to a reduced embryonic development (as is the case in NEC samples), but it is consistent with the estrogen-related HSPB1 expression and it fits with the theory of a delayed follicle maturation as estrogen and progesterone levels start to rise during follicle maturation.
The fact that EGF as well as PDGF and FGF signalling is overrepresented is mainly because the pathways share common components like AKT1, ARAF, MAPK2K, RAF1 and SOS2. These factors were all more abundant in NEC-CC as compared to EC-CC. The EGF-like ligands epiregulin, amphiregulin and betacellulin, which have recently been discovered to be induced by LH (Conti et al., 2006) were not differentially expressed in this study. As these are the ligands that are mainly involved in oocyte maturation, this suggests that a disturbed EGF signalling might not be the cause of the decreased implantation potential in the NEC-CC group. The FGF signalling specific FGFR2 was up-regulated in EC-CC as compared to NEC-CC and in the PDGF signalling PDGFA was down-regulated and PDGFC up-regulated, all known to be involved in angiogenesis. Chen et al. (2006) found in mice ovaries that FSH, partly via estradiol, modulated PDGF members, including e.g. Pdgfa and Pdgfc, in an opposite way, suggesting different functions for these two proteins. Angiogenesis is important for folliculogenesis as the concentration of VEGF in follicular fluid has been negatively correlated to IVF outcome and embryo development (Friedman et al., 1998; Barroso et al., 1999).
Although the ultimate goal was to find a new parameter predicting a pregnancy, in this study, EC, which has previously been shown to be a good marker for pregnancy, was used as an end-point. To correlate a differential gene expression directly to pregnancy, only cycles with SET should be included, as with double-embryo transfer it is not known which embryo implanted. SET, however, would make it impossible to perfectly match samples with a positive and samples with a negative pregnancy outcome for several patient characteristics which probably influence gene expression in cumulus cells. Furthermore, as pregnancy or implantation not only depends on embryonic factors, but also on e.g. endometrial receptivity, a considerable number of extra microarrays would have been needed in order to find significant intrafollicular differences in gene expression. As EC is a good parameter for pregnancy, independent of blastomere number and morphology (Salumets et al., 2003; Van Montfoort et al., 2004), this was chosen as a marker. Besides, there are indications that whether or not an embryo cleaves early is determined during oogenesis as the human embryonic genome is only activated between the four- and eight-cell stage (Braude et al., 1988; Eichenlaub-Ritter and Peschke, 2002). This means that the mature oocyte at ovulation must contain the proteins and mRNA necessary for fertilization and the early stages of embryonic development, including the first cleavage division. Several intrafollicular processes might influence the accumulation of these transcripts. Most investigators of EC concluded indeed after eliminating several explanations for EC like differences in oocyte maturity, that EC might be the result of an as yet unknown intrinsic oocyte factor (Shoukir et al., 1997; Sakkas et al., 1998; Lundin et al., 2001; Fenwick et al., 2002).
In conclusion, this study provides evidence that embryo viability is reflected in differential gene expression in the cumulus cells. The molecular discrimination of cumulus cells from different oocytes might lead to an improved embryo selection with improved eSET results or might serve as a tool for oocyte selection necessary in countries where not all oocytes are allowed to be fertilized. Although some genes point to hypoxic conditions as a negative regulator or to a delayed oocyte maturation, other processes or conditions might be disturbed as well. Probably, among different IVF patients, different processes can be disturbed leading to an impaired embryo development. To generate a group of genes predictive of embryo quality or even pregnancy, genes belonging to several processes that might be disturbed should be included.
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Supplementary data |
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Supplementary data are available at http://molehr.oxfordjournals.org
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Funding |
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This study was supported by a research grant (945-12-014) from the Dutch Organisation for Health Research and Development (ZonMW) and the Dutch Health Insurance Board (CvZ) in a joined research program on health technology assessment of infertility.
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Acknowledgements |
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The authors want to thank Bieke Vanherle for her technical assistance and John Baeten and Ronald Bergkamp from Applied Biosystems for their equipment support. Microarray analyses were performed using BRB ArrayTools developed by Dr Richard Simon and Amy Peng Lam.
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References |
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Submitted on September 19, 2007; resubmitted on November 6, 2007; accepted on November 27, 2007.
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