Results of UPD studies in spontaneous abortions
The high frequency of reproductive losses is specific to the human race. The chances of conceiving and giving birth to a healthy child for women between 20 and 30 years old are estimated to be as low as 21-28% per cycle . About 60% of zygotes are eliminated during the pre-implantation or early post-implantation developmental stages and 15-20% of clinically recognized pregnancies are lost during the first trimester . Approximately 50-60% of spontaneously aborted embryos have chromosomal abnormalities which are not compatible with prenatal development . At the same time, the death of another considerable number of embryos with normal karyotypes cannot be explained by existing cytogenetic theories. Considering ontogenesis as a result of the unrolling of the strict developmental program, the epigenetic basis of this process may be of outstanding significance.
According to the classical definition, coined by Conrad Waddington in 1942, epigenetics is a branch of biology which studies “causal mechanisms” by which “the genes of the genotype bring about phenotypic effects” . In its beginning, epigenetics was a synonym for developmental genetics. However, in contrast to classical genetic theories, the subject of epigenetics has a wider diversity of phenomena, which may be unrelated to changes in gene nucleotide sequences . A strong surge of interest in studying the epigenetic basis of human hereditary pathology has been noted over the last several years [6, 7]. New classes of epigenetic diseases, namely chromatin diseases  and imprinting disorders  have been identified. However, little is known about the features and nature of epigenetic abnormalities, i.e., epimutations,  during human prenatal development. In this chapter information on the impact of genomic imprinting abnormalities on embryo development is summarized and discussed.
2. Genomic imprinting and its role in embryogenesis
Genomic imprinting is an epigenetic phenomenon, which is related to differential parent-of-origin gene expression. The term “imprinting” was taken from physiology. It was Konrad Lorenz, an Austrian zoologist, ethologist and ornithologist, who, when working with geese, rediscovered the principle of imprinting (originally described by Douglas Spalding in the 19th century) in the behaviour of nidifugous birds when a young bird acquired several of its behavioural characteristics from one parent.
The term “chromosomal imprinting” was coined in 1960 by Helen Crouse, one of only three PhD students trained by Nobel Laureate Barbara McClintock. Crouse described the selective elimination of paternal chromosomes in the male meiosis in
The first evidence of the parental genome’s memory in mammals came from experiments conducted by the Surani, McGrath and Solter groups with pronuclei transplantation in mouse zygotes in 1984 [13, 14]. These studies were aimed at answering the question about the absence of parthenogenesis in mammalian reproduction. It was discovered that diploid androgenic mouse embryos derived from zygotes, which contained two paternal pronuclei and none of the maternal pronuclei, demonstrated an extensive proliferation of extraembryonic tissues but poor development of the embryo
It is interesting to note that a similar effect of the increasing number of paternal genomes in the zygote is also observed in humans due to an abnormality of fertilization . Fertilization of a diploid oocyte by normal haploid sperm or double fertilization of a normal oocyte by two haploid sperms leads to triploidy in the zygote. In this case, the partial hydatidiform mole (PHM) arises. PHM is characterized by the cystic degeneration of chorionic villi and the presence of a visible embryo in the foetal sac. In the case of the extrusion of the maternal pronucleus from such a triploid zygote, a diploid karyotype is restored through the postzygotic triploid diploidization mechanism , but both haploid genomes are paternal in their nature. This bipaternal karyotype is not compatible with the development of an embryo body, but leads to a hyper proliferation of trophoblasts cells and a complete hydatidiform mole (CHM) with an increased risk of chorioepithelioma. The observed effect may be explained by the double increase in the dose of imprinted genes expressed from paternal chromosomes, which promotes proliferative and invasive activity of the trophoblasts cells as well as an absence of activity of the maternal imprinted genes, which, in turn, must suppress trophoblast proliferation.
The parental differences in imprinted genes expression are epigenetic in their nature. They are established during gamete differentiation by sex-dependent epigenetic chromatin modifications, mainly by the differential DNA methylation of promoter regions of imprinted genes or regulatory imprinted centres, which are further stably inherited in the somatic cells of the progeny. These regular and consecutive alterations of chromatin organization are referred to as epigenetic genome reprogramming [17, 18] (Figure 1). This starts in the primordial germ cells when they enter the gonads. Both imprinted and non-imprinted loci become demethylated. This total erasure of epigenetic information is required for the totipontency of future germ cells, imprinting switching and for the prevention of the inheritance of epigenetic defects. The demethylated chromatin’s state remains until the duration of the mitotic arrest in male germ cells and the meiotic arrest in female ones. When mitotic divisions of spermogonia are resumed,
The second wave of epigenetic genome reprogramming, which involves somatic cells, begins immediately after fertilization. The paternal chromosomes became decondensed, protamines in the chromatin are replaced by histones and fast demethylation of paternal genome is triggered. The maternal genome undergoes slow passive demethylation. It is believed that demethylation of parental genomes is required to induce pluripotency in embryonic stem cells. Later, during implantation,
At present, (August 2013), it is reported that there are about 90 imprinted genes in a human genome . Most of them are involved in the regulation of intrauterine foetal development through the control of cell proliferation and the differentiation of placental tissues, regulation of metabolism of some hormones and growth factors . The evolutionary reverse to the haploid expression of a subset of genes in mammalian and flowering plants genomes was a great surprise. Several hypotheses were introduced to explain this intriguing fact, but the “sex conflict” was one the most popular among them . According to this hypothesis, maternal imprinted genes in mammals are responsible for the suppression of foetal growth in order to save maternal resources for subsequent pregnancies. In contrast, paternal imprinted genes are involved in the promotion of foetal growth that provides higher chances of survival for many offspring.
Testing this hypothesis in mouse models, the direct evidence for the significant role of genomic imprinting in mammalian embryo development was obtained. The generation of uniparental disomies (UPD) in progeny of translocation carrier’s mice gives nonviable embryos . This fact leads to the idea of searching for UPD in human spontaneous abortions in order to estimate the impact of genomic imprinting abnormalities on prenatal death.
3. UPD in spontaneous abortions
To date, eight studies have been performed to find UPD in spontaneous abortions [25-32] (Table 1). However, the obtained results were modest. Only seven cases of UPD (2.3%) among a total of 305 spontaneous abortions were found and most of them involved chromosomes which did not contain known imprinted genes. Only in three cases segmental UPD (16p/16q (mat), 14q (pat) and 7q (mat)) embryo death can be connected with a disturbance of the dose of imprinted genes localized on these chromosomal regions. Moreover, spontaneous abortions without previous cytogenetic analysis were included in some studies that could have led to overestimation the obtained rate. As a result, the frequency of UPD for chromosomes, which contain known imprinted genes in spontaneous abortions were estimated to be 1% (3/305) or 1.14 per 1,000 occasions of chromosome inheritance from parents to progeny. The latter figure was obtained from the investigation of 6,156 cases of chromosome inheritance by DNA microsatellite analyses and seven cases of UPD were found in the eight cited studies. This figure does not significantly differ from the expected frequency (1.65:1,000), predicted from data about frequencies of chromosome segregation errors in gametogenesis and early embryogenesis, which can lead to uniparental inheritance in humans .
|Anembryonic pregnancies without cytogenetic analysis||23||UPD(21)mat,|
UPD(21)mat in combination
with trisomies 7 and 9
|- first trimester, without cytogenetic analysis|
|- with normal karyotype||35||0|||
|- 6-22 weeks, with normal karyotype||71||UPD(9)mat|
|- with normal karyotype||24||Maternal segmental heterosomy 16pter-D16S3107|
|- with normal karyotype||81||Segmental UPD(14q)pat,|
|Missed abortions and anembryonic pregnancies:|
- with 46,ХХ karyotype (analysis of X-chromosome inheritance only)
|- with normal karyotype||87*||0||*|
Thus, it seems that UPD is a selectively neutral phenomenon in human reproduction. Moreover, UPD for some chromosomes (6, 7, 11, 14 and 15) are compatible with postnatal life leading to a formation of specific genomic imprinting disorders: transient neonatal diabetes mellitus (TNDM), Silver-Russell syndrome (SRS), Beckwith-Wiedemann syndrome (BWS), Wang and Temple syndromes, Prader-Willi syndrome (PWS) and Angelman syndrome (AS), respectively . It became clear that UPD is a rare cytogenetic phenomenon, which cannot explain the mechanisms of imprinted genes disturbances in human pregnancy loss. The only evidence for the pathogenetic role of genomic imprinting abnormalities in human reproduction remains from studies on a hydatidiform mole which originated from the doubling of paternal genome in conception . However, this conclusion does not offer an answer on the possible mechanisms of imprinting disturbances associated with early pregnancy loss.
4. Epimutations of imprinted genes in spontaneous abortions
Taking into account the epigenetic nature of genomic imprinting, we proposed a hypothesis that expected the deleterious effect of abnormal imprinted genes expression to be visible at the epigenetic rather than cytogenetic level . Indeed, UPD formation requires a combination of several subsequent errors in chromosomal segregation during parental meiosis, fertilization and embryo development. For example, the most frequent mechanism of UPD formation is trisomy rescue. It arises from chromosomal nondisjunction in meiosis, trisomy formation in the zygote after fertilization and the loss of additional chromosome in some somatic cells during subsequent mitotic divisions. In a third of the cases of such correction, the situation of inheritance of both homologues from one parent may be observed. If involved chromosome contains an imprinted gene, then the double increase or complete loss of expression of imprinted genes may be detected and it is dependent on the parental origin of expressed allele.
On the other hand, the change of the imprinted gene dose may be achieved by epimutations, i.e., abnormal methylation of the expressed allele or demethylation of the silenced allele. From a functional point of view, epimutations and UPD influences on imprinted genes expression should be similar (Figure 2). It is important that epimutation on a single allele is enough to achieve the imprinted gene dysfunction in a dominant manner.
Before the testing of the hypothesis about the influence of methylation defects in imprinted genes on the aetiology of early pregnancy loss, a classification of epimutations was introduced . They were divided into the following several groups depending on their germinal or somatic origin, hyper- or hypomethylation of active or silenced alleles, and affected parental chromosomes (Table 2):
|1. Types of epimutations of imprinted genes by the loci involved:|
|1.1. Epimutations causing a global disturbance of genomic imprinting at the genome level.|
1.2. Epimutations at the imprinting centres causing a disturbance of imprinting of neighbouring genes.
1.3. Epimutations at the imprinted genes.
|2. Types of epimutations of imprinted genes by their origin:|
|2.1. Germinal epimutations|
|2.1.1. Errors of genomic imprinting erasure in primordial germ cells with retention of methyl|
groups (Critical Period 1, CP #1, on Figure 1). These errors may lead to transgenerational inheritance of epigenetic defects.
2.1.2. Errors of imprinting establishment during gametogenesis (CP #2 on Figure 1)
|18.104.22.168. Absence of methylation of imprinted genes alleles that normally should be|
methylated in sperm or oocytes.
22.214.171.124. Aberrant methylation of imprinted genes alleles that normally should be
unmethylated in sperm or oocytes.
|2.2. Somatic epimutations|
|2.2.1. Abnormal hypomethylation of inactive parental alleles of imprinted genes during epigenetic genome reprogramming (CP #3 on Figure 1).|
2.2.2. Abnormal methylation of expressed parental alleles of imprinted genes during de novo DNA methylation upon epigenetic genome reprogramming (CP #4 on Figure 1).
2.2.3. Spontaneous hypomethylation of inactive parental alleles of imprinted genes in somatic cells after epigenetic genome reprogramming (CP #5 on Figure 1).
2.2.4. Spontaneous hypermethylation of expressed parental alleles of imprinted genes in somatic cells after epigenetic genome reprogramming (CP #5 on Figure 1).
|3. Types of epimutations of imprinted genes by their functional consequences and affected parental alleles:|
|3.1. Hypomethylation of the inactive maternal allele of the imprinted gene.|
3.2. Hypomethylation of the inactive paternal allele of the imprinted gene.
3.3. Hypermethylation of the expressed maternal allele of the imprinted gene.
3.4. Hypermethylation of the expressed paternal allele of the imprinted gene.
|Germinal epimutations||Errors of genomic imprinting erasure with retention of methyl groups|
|Not applicable||Not applicable|
|Absence of imprinted genes alleles methylation (CP #2)||BiCHM;|
|Not applicable||Not applicable|
|Aberrant hypermethylation of imprinted genes alleles|
|Not applicable||Not applicable|
|Somatic epimutations||Abnormal hypomethylation of inactive parental alleles of imprinted genes during epigenetic genome reprogramming (CP #3)|
(mosaic forms of AS).
mosaic forms of TNDM
|Partial hypomethylation of ||Not applicable||Not applicable|
|Abnormal hypermethylation of expressed parental alleles of imprinted genes during epigenetic genome reprogramming (CP #4)||Not applicable||Not applicable|
|Stochastic epimutations (hypo- and hypermethylation) in somatic cells after epigenetic genome reprogramming (CP #5)|
(mosaic forms of PWS);
The first evidence of epimutations in imprinted genes in reproductive losses came from studies of biparental complete hydatidiform mole (BiCHM, MIM #231090). This pathology, as opposed to classical androgenic complete mole, arose in the case of normal biparental karyotype  (Figure 3). It was shown that imprinted genes, which are methylated on maternal chromosomes in normal embryos, became hypomethylated in the case of BiCHM both on the paternal and maternal homologues . Functionally, this epigenetic status is the same as an androgenic complete mole. Subsequent studies revealed that BiCHM arose due to germinal epimutations, namely the absence of
The first study devoted to the analysis of methylation status of
Our further studies on imprinted genes
It was remarkable also that all detected epimutations were confined to one placental tissue (cytotrophoblast or extraembryonic mesoderm) only indicating their somatic origin in post-implantation stages of development after the divergence of embryonic and extraembryonic cell lineages. This observation has an important value for the discussion on the increase of genomic imprinting disorders in children born after the application of assisted reproductive technologies (ART) [20, 41-44]. Indeed, several cases of children with BWS, SRS and AS born after
Evidence for epimutations of imprinted genes in spontaneous abortions was also noted in other subsequent studies (See tables 3 and 4). For example, multiple hypermethylation of imprinted genes was detected in 4% (2 out of 55) of spontaneous abortions and 18% (10 out of 57) of stillbirths . In this study,
|SA||29||CC||10.3% (3)||-||-||-||1↓, 2↑||2↑||-||1↑||-||-||-||-||-||1↑||-|||
|Total||406||12.1% (49)||5.0% (5/100)||46.2% (12/26)||34.6% (9/26)||7.1% (11/154)||3.2% (13/406)||34.6% (9/26)||7.4% (30/406)||53.8% (14/26)||30.8% (8/26)||30.8% (8/26)||30.8% (8/26)||23.1% (6/26)||1.8% (1/55)||38.5% (10/26)|
|↓ - 16.0% (65/406); ↑ - 18.7% (76/406)|
|Total||406||12.1% (49)||23.1% (6/26)||50.0% (13/26)||42.3% (11/26)||46.2% (12/26)||3.7% (15/406)||3.7% (9/241)||3.9% (6/154)|
|↓ - 10.3% (42/406); ↑ - 7.4% (30/406)|
It is important to note that all the above mentioned studies were performed by the “candidate gene” approach, i.e., only several interesting genes were tested based on their supposed functions in embryogenesis. A new era in this area of research began with the application of array technologies. They provide comprehensive and unbiased analysis of the human imprintome. Results of the first studies revealed a new, intriguing phenomenon of multilocus methylation defects (MLMD) on imprinted genes. This effect is presented by multiple epimutations (hypo- and/or hypermethylation) affecting several imprinted genes simultaneously in the genome in different combinations. In fact, the first example of MLMD is a BiCHM, which results from errors of imprinting establishment on maternal chromosomes during epigenetic genome reprogramming in oocytes leading to the hypomethylation of maternal alleles of different imprinted genes . MLMD were detected in human reproductive losses  as well as in patients with genomic imprinting disorders . Importantly, in one recent study 15 (8%) probands among 194 patients with clinical features of an imprinting disorder but no molecular diagnosis had methylation anomalies, including missed and unexpected molecular diagnosis .
Evidence of MLMD in 13 first trimester spontaneous abortions were obtained in our study by DNA methylation array the GoldenGate Methylation Cancer Panel I (Illumina) analysis of 51 imprinted genes . Multiple methylation defects affecting from four to 12 genes in each embryo were found. Epimutations were presented by the hypomethylation of paternal alleles of
Summarizing the published data, it is possible to note that MLMD were observed in 49 of 406 investigated spontaneous abortions (12%) (See tables 3 and 4). It is evident that this value, of a significant magnitude greater than UPD frequency in miscarriages, indicates an appreciable effect of epigenetic defects on imprinted genes in pregnancy losses. The incidence of epimutations for different genes varied from 1.8% (
The results of the studies suggest possible mechanisms of the selective influence of epimutations of imprinted genes on early embryo development. As it was mentioned early, the “sex conflict” hypothesis is the most popular in explaining the imprinted mode of gene expression. From its point of view, the expected suppressive effect of epimutations should emerge from the hypomethylation of paternal alleles, which leads to a loss of imprinting and the biallelic expression of maternal genes responsible for foetal growth suppression, as well as from hypermethylation of paternal alleles, which leads to the absence of products responsible for foetal growth stimulation. The total incidence of such types of epimutations in spontaneous abortions is 29% (10.3 + 18.7). At the same time, the total incidence of epimutations, which can lead to the promotion of foetal growth (hypo- and hypermethylation of maternal alleles), was 23.4% (16 + 17.4). The differences between frequencies of “suppressive” and “promotion” epimutations were not statistically significant (p = 0.07). However, this is a relative estimation for several reasons, including complete acceptance of the “sex conflict” hypothesis for each imprinted gene (maternal genes are suppressors, paternal genes are activators) and the idea that there is a strong reverse correlation between gene methylation and its expression. To obtain more precise and weighted estimations, further studies of imprinted gene functions and molecular mechanisms of their expression regulation are encouraged.
Does MLMD arise spontaneously in different loci or is it driven by mutations in a candidate’s genes responsible for genomic imprinting establishment and maintenance? To answer this question the evidence of MLMD in patients with imprinting disorders should be discussed.
5. Multiple methylation defects in patients with imprinting disorders
The first evidence of multiple epimutations of imprinted genes was obtained in 2005 in the comparative analysis of methylation status of two genes –
In 2006, Mackay and colleagues reported on a study of two patients with TNDM and IUGR who had hypomethylation of
Later in 2006, Mackay and colleagues described another 12 patients with TNDM and hypomethylation of
Another specific finding of this study which is also very intriguing, is that the level of mosaicism for the DNA methylation index varied in different investigated tissues (blood, mouthbrushes, fibroblasts) and affected different genes. The presence of mosaicism indicates postzygotyic errors of imprinting maintenance in somatic cells. However, it is more significant that such errors affected only maternally, but not paternally inherited alleles. This type of epigenetic mosaicism may explain another interesting fact that all patients with MHS had a major clinical manifestation of TNDM but no other genomic imprinting disorders that can be expected from the involvement of different imprinted genes. It is reasonable to assume that germinal epimutations at
Later, MLMD were also reported in other imprinting disorders. However, both maternal and paternal alleles were affected in comparison with MHS. Hypermethylation and hypomethylation of imprinted genes were reported also. For example, in some cases of BWS, loss of methylation at
It was mentioned earlier that microarray technologies allow us to obtain comprehensive data sets about the methylation status of imprinted genes over the whole genome. In a recent study, 65 patients with different genomic imprinting disorders (BWS, SRS, PWS, AS, TNDM and pseudohypoparathyroidism (PHP-1B)) were investigated by using “GoldenGate Cancer Panel I” (Illumina) DNA methylation microarray . MLMD were detected in all the diseases except PWS and AS, which demonstrated methylation defects at
Thus, examinations of patients with imprinting disorders indicate that the epigenetic basis of these diseases may, in some cases, be supplemented by multiple methylation defects in several other imprinting genes in addition to epimutation at the disease-specific gene, which is responsible for the pathogenesis of major clinical features of a given syndrome. The incidence of MLMD in different imprinting disorders varies from 8.9% (SRS) to 56.3% (TNDM) (Table 5). The incidence of methylation defects at different genes varies also. The
Is the combination of imprinted genes affected by MLMD in different syndromes and pregnancy losses non-random? Are there any specific features of nucleotide sequences and mechanisms of expression regulation of imprinted genes with different incidence of methylation defects? Does an interaction between imprinted genes that may lead to formation of specific epigenotype and phenotype exist? Further studies are necessary to obtain answers to these questions. However, there is some data which indicates the existence of coordination mechanisms in the regulation of epigenetic status of human imprintome. For example, it was shown that epigenetic changes in the cases of BiCHM and TNDM affected imprinted genes on maternal chromosomes only, whereas other non-imprinted genes were not a subject for epimutations [68, 69]. On the other hand, multiple methylation defects were observed by DNA methylation array analysis both in imprinted and non-imprinted genes in spontaneous abortions . It is also possible that MLMD may have a different molecular nature in comparison with a methylation defect at a single locus. For example, multiple methylation defects in patients with BWS were observed only in the cases of
|SRS||23||8.2% (2)||0||0||1↓||0||n.a.||1↓, 2↑||1↓||23↓||2↓|||
|↓ – 75.6% (360/476); ↑ – 0.6% (3/476)||↓ – 52.9% (252/476); ↑ – 4.8% (23/476)|
6. Genetic control of epigenetic status of imprinted genes
The story begins again with BiCHM. Recurrent cases of this pathology or classical CHM and PHM in anamnesis, the occurrence of several cases within one pedigree, and the appearance in consanguineous couples provide evidence for an autosomal recessive mode of inheritance of BiCHM by maternal lineage. The first candidate’s genes were DNA methyltransferases, however, the sequencing of it in women with a history of BiCHM did not reveal the presence of mutations. Subsequent genome-wide association studies and homozigosity mapping indicated the linkage of BiCHM with chromosomal segment 19q13.42, in which the
Homozygous c.295G>T (p.Glu99X) and heterozygous c.1970A>T (p.Asp657Val) mutations were observed in a woman with four hydatidiform moles . Her sister with two moles and brother were compound heterozygotes for these mutations. Her father had a homozygous p.Asp657Val mutation. Her mother had a homozygous p.Glu99X mutation, three successful pregnancies and a stillbirth in anamnesis. In another studied family, a brother and his three sisters with recurrent hydatidiform moles (two, three, and five cases in anamnesis, respectively) were homozygous for p.Arg693Pro mutations, but the brother did not have any reproductive problems. The authors of this study made a very important conclusion that mutations of
Several studies combined methylation analysis of imprinted genes and the search for
A search for
There are current reports on mutations at almost all exons and introns of
As mentioned previously,
NLRP7 protein has no DNA-binding motifs in its sequence, which is why it is unclear how it may be involved in imprinting recording during oogenesis. In this situation, an alternative hypothesis is attractive. According to this hypothesis, the involvement of NLRP7 in BiCHM pathogenesis may be related to its participation in inflammation and autoimmune response. It was found that patients with
Mutations at the
It is probable that some patients with imprinting disorders and MLMD also have mutations in two other genes –
In 2008, mutations at the
Two years later, Mackay and Temple reported about
Twenty-seven BWS patients with
The protein encoded by
Taking the discussed results together, it is possible to make an unexpected conclusion that some part of imprinting diseases and reproductive disorders associated with abnormal imprinting are related to defects in gene (or genes) involved in the establishment and maintenance of epigenetic organization of imprinted loci. In other words, imprinting diseases, or at least some part of them, that were usually considered epigenetic in nature, have, in fact, a single gene basis sometimes modified by parental-of-origin effects. A similar situation is specific for chromatin diseases (ICF, Rett, Rubinshtein-Taybi, Coffin-Lowry, ATR-X syndromes), which arise due to mutations in genes involved in the control of chromatin organization . From this point of view, the presence of one form of TNDM, a classical imprinting disorder, in the OMIM catalogue (MIM # 601410) as a result of
Considering the high incidence of reproductive losses in humans and the elevated level of methylation defects at different imprinted loci in spontaneous abortions, the search for mutations in genes involved in the control of genomic imprinting is a challenge for modern reproductive epigenetics and medicine. In our preliminary study, 11 first trimester missed abortions with MLMD of imprinted genes were tested for the presence of
The reviewed data clearly indicates that epigenetic abnormalities are the leading cause of imprinted gene dysfunction in pregnancy complications and losses. This is not surprising because of the fact that the rate of epimutations is estimated to be one or two orders higher than the incidence of classical gene mutations. It is import to also note that epimutation in one allele is enough to cause the loss of imprinting or the silencing of an imprinted gene due to one of its main inherent features, namely monoallelic expression.
The application of genome wide technologies of DNA methylation analysis revealed the phenomenon of multiple methylation defects at imprinting genes both in spontaneous abortions and in some patients with imprinting disorders. Today we are witnesses of data accumulation about spectrum and the incidence of this type of methylation abnormalities in different diseases. However, cautious estimations should be provided because of a lack of current data about possible benign epipolymorphisms of imprinted genes. However, obtained results change and supplement existent concepts about pathogenesis of imprinting disorders. One of the most intriguing findings is that some part of epigenetic imprinting defects has, in fact, a genetic nature due to mutations in genes, which are responsible for imprinting regulation. This remark may have obvious significance for the likes of molecular genetic diagnosis in the light of the application of high-throughput genomic and post-genomic technologies and for medical genetic counselling. Carriers of mutations in imprinting control genes may have incorrect or instable epigenomes in their gametes or progeny which will be not compatible with fertilization, implantation, normal prenatal development or the delivery of a healthy child. Preimplantation genetic diagnosis for the excluding of embryo transfer with mutations in such genes may be a successful reproductive choice for such couples.
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