Genomic imprinting disorders: lessons on how genome, epigenome and environment interact

Genomic imprinting, the monoallelic and parent-of-origin-dependent expression of a subset of genes, is required for normal development, and its disruption leads to human disease. Imprinting defects can involve isolated or multilocus epigenetic changes that may have no evident genetic cause, or imprinting disruption can be traced back to alterations of cis-acting elements or trans-acting factors that control the establishment, maintenance and erasure of germline epigenetic imprints. Recent insights into the dynamics of the epigenome, including the effect of environmental factors, suggest that the developmental outcomes and heritability of imprinting disorders are influenced by interactions between the genome, the epigenome and the environment in germ cells and early embryos. Disruption of genomic imprinting can lead to disease. Recent studies suggest that interactions between the genome, the epigenome and the environment in germ cells and early embryos have an impact on developmental outcomes and on the heritability of imprinting disorders.

evidences from humans and animal models that environmental factors may influence genomic imprinting. Finally, we highlight areas requiring additional research that could complete our understanding of imprinting disorders, as well as new technological advances that might correct imprinting errors.

Introduction
In therian mammals, a subset of autosomal genes is preferentially expressed from only one of the two parental chromosomes, some from the maternally inherited allele, others from the paternal allele 1 . This parental origin-dependent expression results from differential epigenetic marking, primarily from methylated cytosine at CpG dinucleotides of genes during gametogenesis in the male and female germline.
These genomic imprints endure for one generation, from their establishment in mature germ cells of an individual to their erasure in the gamete precursors of their progeny. Genomic imprinting thus represents a type of intergenerational epigenetic inheritance. Of note, parent-of-origin-dependent methylation differs from sequencedependent allelic methylation, in which stochastic fluctuation between epialleles [G] is influenced by genetic variants 2 .
In humans, approximately 100 imprinted genes have been identified [3][4][5] . Many imprinted genes have important roles during human development, and alteration of their expression and function can lead to imprinting disorders (Table 1), congenital conditions with a lifelong impact on health and in some cases increased cancer risk 6  This Review focuses on imprints that effect essentially permanent and ubiquitous (rather than tissue-specific or transient (Box 2)) changes on gene expression potential at affected loci. We begin with a brief overview of the genomic basis of imprinting and its control, before reviewing the lifecycle of genomic imprinting and how disruption of the individual factors involved in the establishment, maintenance and erasure of imprints can result in disease. Finally, we discuss the heritability of imprinting defects and the role of environmental insults in imprinting disorders. For details on the evolutionary significance of genomic imprinting 1,9 , the methods for imprinting analysis 10 , the physiological role of imprinted genes 6 or the chromatin mechanisms in imprinting 11 , the reader is referred to previous authoritative reviews.

[H1] The genomic basis of imprinting
The majority of imprinted genes are found in clusters, called imprinted domains, which enables coordination via shared regulatory elements such as long non-coding RNAs (lncRNAs) and differentially methylated regions (DMRs), where DNA methylation differs between the maternally derived and paternally derived alleles.
Each imprinted domain is controlled by an independent 'imprinting centre', which is generally characterized by a germline differentially methylated region (gDMR), also known as primary DMR (Fig. 2). About 35 gDMRs associated with imprinted loci have been identified in the human genome (Table 2) 12 . gDMRs are also characterized by different chromatin configurations on parental chromosomes, with histone marks characteristic of closed chromatin (for example, histone 3 lysine 9 dimethylation (H3K9me2), trimethylation (H3K9me3) and histone 4 lysine 20 trimethylation (H4K20me3)) on the methylated allele, and histone marks characteristic of open chromatin (for example, H3K4me2 and H3K4me3) on the unmethylated allele ( Fig. 2) 4,11,13 . The methylated and unmethylated gDMR alleles are recognized by different transcription factors whose function is to direct differential epigenetic modification and imprinted expression of the locus (Fig. 2) 14 . Whereas maternally methylated gDMRs are more numerous, intragenic and generally correspond to promoters, often of lncRNAs, gDMRs methylated on the paternal chromosomes are intergenic and may function as insulators or enhancers ( Table   2) 1,15 . Of note, in multigenic imprinted domains, the imprinting centre often directs the expression of genes from both the chromosome on which is methylated and the opposite parental chromosome; this situation arises from the regulatory interactions between imprinting centres and the gene products, both coding and noncoding, under their control (Fig. 2).

[H2] Allele-specific expression in somatic cells
Imprinted genes can display monoallelic expression in most or all cell types, but for some genes imprinted expression is restricted to specific tissues (for example, UBE3A 16,17 ) or developmental windows (for example, KCNQ1 18 ), or monoallelic expression and/or methylation can differ between individuals 19-21 . To control the allele-specific expression of imprinted genes in somatic cells, gDMRs direct the establishment of further allele-specific epigenetic features within the imprinted domain during development. These include secondary DMRs (also known as somatic DMRs), which correspond mostly to gene promoters and transcription factor binding sites (Table 2)  Imprinted gene products intensify their exquisite regulation by co-operation in a network (Imprinted gene network, IGN) 32,33 . For example, the transcription factor PLAGL1 32 and the H19 lncRNA 33 have been shown to regulate the mRNA level of several members of the IGN in a DNA methylation-independent manner, in mouse tissues. The human lncRNA IPW, which resides within the Prader-Willi syndrome (PWS) locus on chromosome 15, is able to regulate the expression of MEG3 on chromosome 14 by targeting the EHMT2 H3K9 histone methyltransferase (also known as G9a) to its imprinting centre 34 . Furthermore, many imprinted gene clusters encode microRNAs (miRNAs) and small nucleolar RNAs (snoRNAs), which may be involved in the post-transcriptional control of imprinted genes 35 . These interactions may explain some of the overlaps observed in the phenotypes of imprinting disorders (Table 1).

[H2] Multilocus imprinting disturbances
A subset of patients with imprinting defects exhibits multilocus imprinting disturbances (MLID), that is, imprinting disruption at multiple loci across the genome.
MLID is confined to epimutation subgroups of imprinting disorders (Table 1) and involves loci associated with known imprinting disorders as well as those not currently linked with specific phenotypes 36,37 . To date, most patients with MLID have shown clinical features characteristic of one imprinting disorder, notably BWS, SRS or transient neonatal diabetes mellitus (TNDM), which is probably due to the high frequency of epimutations in these imprinting disorders. However, epigenotypephenotype correlations are not always obvious, possibly because of the spectrum of epimutations involved or their mosaic nature (Box 1) 37-39 .

[H1] The imprinting life-cycle and disease
Throughout their generational lifespan (Fig. 3), genomic imprints must be maintained and preserved from epigenetic reprogramming in somatic cells. Many factors involved in these complex processes and their DNA binding sites can be targets of mutations that cause human imprinting disorders (Table 3).

[H2] Imprinting centre methylation dynamics in germ cells
Of the ubiquitous gDMRs present in somatic tissues, all but two originate from the oocyte ( Table 2) 5,12 . This disparity reflects fundamental differences in the mechanisms of methylation acquisition in the female and male germlines, and in the treatment of parent-of-origin-derived methylation in the zygote (Fig. 3) 21 . In primordial germ cells (PGCs), the precursors of sperm and oocytes, germline specification requires remodelling of the epigenome as a pre-requisite for gametogenesis. Our knowledge of these processes comes chiefly from studies in mice 40,41 , and the characterization of human PGCs has revealed subtle interspecies differences, but overall the global erasure of methylation is comparable 42-44 .
A hallmark of PGC remodelling is imprint erasure. Genome-wide de-methylation of 5methylcytosine (5mC) is a passive process during PGC expansion that results from diminished protein levels of the de novo DNA methyltransferase DNMT3A and UHRF1, the recruitment factor of the maintenance DNA methyltransferase DNMT1.
Reprogramming of imprinted methylation follows slower kinetics. In mice, it is associated with oxidation of 5mC to 5-hydroxymethylcytosine (5hmC) by the teneleven translocation 5mC dioxygenase 1 (TET1) and TET2; this modification is not recognized by the maintenance methylation machinery and therefore promotes passive demethylation 40,41,44,45 .
Errors in the erasure process have been observed in patients with rare, sporadic imprinting disorders. In the case of GOM of the PWS/AS imprinting centre (also known as SNURF:TSS-DMR), grandmaternal methylation is not erased in paternal PGCs, and as a result the paternal allele retains this maternal imprint (Fig. 4a) 46 .
Similarly, the hypermethylation of imprinting centres in sperm from subfertile individuals is consistent with incomplete erasure of imprints 47 .
Re-methylation and imprint acquisition occur asynchronously between the sexes, with de novo methylation in the male germline occurring before birth and maintained through many cycles of mitotic division before entry into meiosis, whilst female germ cells remain hypomethylated until maturation (Fig. 3 On the basis of data derived from mouse models, the majority of methylation is deposited in oocytes by DNMT3A and its obligate, catalytically inert, cofactor DNMT3L 51,53,54 , whereas both DNMT3A and DNMT3B contribute in male germ cells 55 . DNMT1 has an auxiliary role ensuring symmetric methylation of CpG sites in oocytes 56 . Transcription and underlying chromatin signature are important factors determining methylation acquisition 50 . Transcription in oocytes is required for methylation at numerous gDMRs 57 , an act that may render the chromatin more accessible to the de novo methylation machinery and/or be associated with specific chromatin changes. The co-transcriptional histone H3K36me3 mark is deposited at intragenic CpG islands and subsequently recognized by DNMT3A and DNMT3B 58,59 . Successive removal of dimethylation and trimethylation of histone H3K4 by KDM1A or KDM1B (known previously as AOF1 and LSD1, respectively) allows for direct interaction with DNMT3L 60-62 . Despite being a generic methylase in mouse oocytes, DNMT3L is not detectable by expression profiling in human oocytes between germinal vesicle phase and meiosis II 63 , suggesting that it is not required for de novo methylation in the human female germline. In mouse male germ cells, transcriptional read-through is involved in acquisition of imprinting centre methylation, whereas histone H3K4 methylation and promoter activity are present at maternal imprinting centres that are protected from de novo methylation 64 .
Failure to establish imprints during gametogenesis can result in imprinting disorders.
Establishment of gDMRs involves several enzymatic steps, any of which may be prone to stochastic errors. In oocytes, deficient transcription through CpG islands destined to be gDMRs can result in failure to establish maternal imprints 57 ; in such cases, there would be no mosaicism (Fig. 4b) 65 . Genetic mutations affecting transcription through the gDMR have been identified in rare patients with BWS with complete and isolated lack of methylation at the imprinting centre 2 (IC2), the imprinting centre of the centromeric domain of the BWS/SRS locus (also known as However, in both humans and mice, whereas most gDMRs lose DNA methylation in pre-implantation stages 49,51 , imprinting centres evade the embryonic wave of epigenetic reprogramming, and studies in both mouse models and human patients with rare imprinting disorders suggest they do so through interaction with critical factors expressed in the oocyte and early embryo. [H2] Oocyte factors DPPA3 (also known as Stella or PGC7) is required for the maintenance of DNA methylation in the early mouse embryo and protects 5mC from conversion to 5hmC in the maternal pronucleus, by associating with chromatin marked by H3K9me2 73 .
DPPA3 is a maternal-effect gene: concepti of maternal null mice rarely progress beyond the two-cell stage, and their genomes are severely demethylated 74 .
Maternal-effect variants in NLRP proteins and associated factors have been implicated in pregnancy outcomes including hydatidiform mole [G] and infertility, as well as monozygotic twinning, pregnancy loss and MLID (Fig. 4c-d)  of FHM suggests that NLRP7 is involved in oocyte-specific imprint establishment ( Fig. 4c) 78 , but hypomorphic maternal NLRP7 variants have been associated with MLID 79 .
In mouse models, NLRP5 and its associated proteins are referred to as the subcortical maternal complex (SCMC) 80 . They are highly expressed in the oocyte, but their mRNA and protein abundance decline to undetectable levels by blastulation 81 . In mouse models, maternal ablation of SCMC gene function compromises embryo development, with frequent demise between the 2-cell and blastula stage, and disruption of processes including maintenance of genome integrity, euploidy, mitochondrial function, and gene transcription and translation [82][83][84] .
A mouse model of maternal Nlrp2 deficiency shows severe reproductive compromise with embryo demise at all developmental stages and mosaic loss and gain of methylation at imprinted loci, indicating that abnormal subcellular localization of DNMT1 and or SCMC members may cause early embryonic loss and imprinting defects 85 .
The effects of maternal SCMC variants suggest a link between DNA methylation, genome integrity and developmental competence in the early embryo. If the embryo's competence is severely compromised, both ploidy and DNA methylation may be intolerably affected, leading to embryo demise. If errors in ploidy and/or methylation are tolerated, the embryo may survive blastulation and continue development, with ongoing differentiation overwriting early epigenetic errorsexcept for imprints, which are indelible in somatic cells. Evidence for this comes from reports of preimplantation genetic diagnosis of embryos with maternal-effect NLRP7 mutations in which all cleavage-stage embryos arrested and had various maternal aneuploidies 86 . Arguably, if an embryo had presented with a normal chromosome complement it would have likely developed into a molar pregnancy or severe MLID due to disturbed maternal imprints. Hence, MLID may be no more or less than evidence of embryonic crises during the critical window encompassing epigenetic reprogramming and ZGA, with an ascertainment bias for live birth and normal ploidy.
Mothers with maternal-effect variants have children with variable disturbance of both paternally-and maternally-methylated imprinting centres (Fig. 4d), and a spectrum of reproductive outcomes including apparent infertility, fetal loss, hydatidiform mole, liveborn children with MLID who exhibit clinical symptoms, and liveborn children with MLID and no clinical phenotypes 37,39 . The only consistent feature of offspring is MLID itself.

[H2] Zygotic factors
The KRAB zinc-finger protein (KRAB ZFP) ZFP57 acts as the focus for a multiprotein complex that protects imprinting centres from both passive and active demethylation [87][88][89] . ZFP57 recognizes a hexameric motif enriched in all maternally and paternally methylated imprinting centres in mouse 90,91 . KRAB ZFPs are a large, expanding family; their rapid evolution seems to keep pace with the endogenous retroviruses (ERVs) whose expression they suppress through DNA hypermethylation 92  In summary, it seems that imprinting centre sequences have characteristics that support allele-specific gene expression, chromatin organization and DNA methylation in the early embryo, enabling these patterns to evade early-embryonic reprogramming and subsequently persist in somatic tissues. Genetic variants associated with imprinting centre epimutations can demonstrate variable clinical presentation and incomplete penetrance 28 or apparent anticipation

[H2] Intergenerational inheritance of imprinting defects
[G] with increased clinical severity over multiple generations 103 . These findings suggest that whereas highly-penetrant variants, such as those disrupting transcription factor binding, exhibit patent and penetrant phenotypes, genomic variants with lower penetrance may need to be identified by comprehensive sequencing efforts. Consistent with this hypothesis, a recent study demonstrated that frequent sequence variants have subtle effects on imprinted methylation, expression and phenotype 20 , suggesting that imprinting is a more quantitative than categorical phenomenon.

[H1] Environmental influences on imprinting
In addition to genetic causes of imprinting centre epimutations, environmental factors may also influence the imprinting process. In humans, evidence for this phenomenon derives from assisted reproductive technologies (ART) 104 . Other environmental influences on imprinting centres may include nutritional status or exposure to chemical pollutants in utero 105 . In many cases, changes in methylation represent increased variability on the methylated allele, likely relating to a failure of maintenance, or an adaptive response to the external stimuli.

[H2] Assisted reproductive technologies
ART is usually performed for male and/or female infertility and includes procedures such as ovarian hyperstimulation to obtain multiple oocytes for retrieval, in-vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI) and embryo culture and transfer, all of which coincide with critical events in epigenome reprogramming.
Reports of ART-conceived children with rare imprinting disorders (for example, AS and BWS) first suggested a potential link with the occurrence of epimutations at imprinting centres (Fig. 6)  In addition to ART-related procedures, infertility per se has been linked to the pathogenesis of imprinting disorders (Fig. 6). The frequency of AS with epimutations was shown to be increased in subfertile couples, independent of IVF, ICSI or embryo culture 115 . More recently, impaired methylation of imprinting centres was reported in sperm of subfertile men 47 . Furthermore, unrecognized ART-associated epigenetic alterations may play a role in the increased risk of low birthweight and congenital anomalies that have been reported in ART-conceived children 116 and animal models 117 . Maternal age and delayed ovulation or fertilization are associated with depletion of oocyte proteins and RNA stores and altered developmental fitness of embryos 118-120 , suggesting that maternal effect genes may be critically vulnerable to these or other challenges that occur during ART (Fig. 6).
Disentangling the effects of infertility and ART in the aetiology of ART-associated imprinting disorders in humans is difficult and not all studies have shown an association between ART and altered methylation, with some reports suggesting that there is no increase in mosaicism or methylation aberrations at imprinted gDMRs 121,122 . Other groups have reported perturbed imprinting in pre-implantation embryos suitable for transfer 123 , suggesting that -similar to aneuploidyepigenetic mosaicism in early embryos may be a normal occurrence. The rarity of ART-associated imprinting disorders suggests that they may result from a combination of multiple interacting factors, including specific aspects of ART protocols, infertility, genetic susceptibility and stochastic effects (Fig. 6). Moreover, epidemiological surveys often have ascertainment bias for liveborn offspring with clinically blatant phenotypes associated with imprinting disorders, while the frequency of clinical pregnancy, though well-known to be limited in ART, is not considered. Potentially, individuals with imprinting disorders represent the subset of IVF outcomes with the least pervasive disturbance and the most recognizable clinical features, and a more definitive study will require consideration of nonviable products of conception at all stages, including both epigenome and genome integrity.

[H2] Nutrition and metabolic disorders
Certain developmental windows are especially vulnerable to abnormal nutritional Nutritional status may also affect epigenetic profiles at imprinted loci in a variety of ways. It is possible that the availability of free methyl donors, such as Sadenosylmethionine, a substrate for DNA and protein methylation, is limited, with evidence that methyl-deficient diets, folate levels and genetic variants in proteins involved in one-carbon metabolism all affect imprinted methylation patterns, at the 11p15.5 imprinted gene cluster 97,126,127 . In these studies, the presence of missense amino acid substitutions in genes regulating the S-adenosyl methionine (SAM) or the inhibitory S-adenosylhomocyteine (SAH) abundance correlate with aberrant imprinted methylation 126,127 , which also revealed a link between low vitamin B12 levels and H19 methylation maintenance 126 Functional genetic variants of DNMT1 in BWS patients were also observed in combination with SNV of folate metabolism pathway genes, suggesting that decreased DNMT1 enzymatic activity could be exaggerated by extreme SAM/SAH ratios 97 . Furthermore, the ZFP57 locus is a folate-sensitive region, and its genomic binding regions are metastable epialleles responsive to periconceptional conditions 128, 129 . In mouse, withdrawal of maternal dietary protein permanently altered imprinted expression of Cdkn1c in offspring, which was maintained into adulthood and occurred through a folate-dependent mechanism of DNA methylation loss 130 . However, not all studies on isocaloric protein restriction during pregnancy have resulted in altered imprinted methylation in the newborn 131 , suggesting that any deregulation is likely a consequence of a general effect on global methylation. Recent evidence suggests that cells have important energy status sensors that protect the cells against metabolic stress by directly regulating epigenetic processes. The nicotinamide adenine dinucleotide (NAD)dependent deacetylase, SIRT1 has been shown to protect methylation at imprinted loci by directly regulating acetylation of DNMT3L, at both the promoter and protein level in mouse embryonic stem cells 132 .

Endocrine disruptors
In addition to micronutrient availability, prenatal exposure to estrogenic endocrinedisrupting compounds (EDC), such as bisphenol A (BPA), results in deregulation of genomic methylation and hydroxymethylation 133,134 , with imprinting and methylation anomalies being reported in both mouse placenta 135 and developing gametes 136,137 .
Endorsing the vulnerability of imprinted loci to EDCs, prenatal BPA exposure in humans has been associated with changes in methylation at the MEST locus and is linked with early childhood obesity 138 . Dnmt1 expression was found to be decreased in BPA-treated mouse spermatogonia 137 , and BPA exposure during oocyte maturation altered other epigenetic marks, specifically the abundance of histone modifications, which was linked to induced oxidative stress 139 . Exposure-induced oxidative stress was shown to alter both TET enzyme expression and function, leading to altered 5-hmC levels at numerous imprinted loci 134 , which indicates that environmental toxicants also alter long-term imprinted gene regulation (Fig. 6).
Indirect effects of the toxic compounds on DNA methylation could also be exerted as consequence of developmental and metabolic alterations 140 .
In summary, combined genetic and environmental predispositions may erode the gametic and zygotic competence to reprogramme the epigenome, with consequences on imprint maintenance, and insights into these effects in humans may be gained by delineating the aetiology of apparently sporadic primary epimutations in individuals with imprinting disorders.

[H1] Conclusions and perspectives
The maintenance of differential DNA methylation of imprinting centres is fundamental for the survival of imprinting marks in the early embryo. Some of the key factors and genomic sequences involved in this process have been identified, but the causation and timing of their interactions require further clarification. This is particularly true for the SCMC proteins and possibly further oocyte-specific factors that affect DNA methylation maintenance in the early embryo, whose mechanisms of action and relationship with ZGA are still ill-defined. Importantly, further human-based studies are required, firstly to resolve key differences from mouse in the timing and mechanisms of epigenetic remodelling, and secondly to learn from rare cases of imprinting disorders by identifying genetic variants that predispose to imprinting

Box 1 | Epigenetic mosaicism in imprinting disorders.
Numerous patients with imprinting disorders, with or without MLID, have somatic mosaicism, in which tissues contains cells with imprinting aberrations as well as those with appropriate allelic methylation. Mosaicism is observed with all types of primary and secondary epimutations, with the exception of erasure and establishment errors (Fig. 4), indicating a more common post-zygotic aetiology 36  Patients with SRS or BWS often present with body asymmetry, a feature accredited to mosaicism, with recent mouse models for these two imprinting disorders identifying mosaicism in bilateral organs with asymmetric growth 100 . Mosaic H19 hypomethylation is common in SRS, for which severity differs markedly between patients 147 . Detailed studies in another imprinting disorder, Angelman Sydrome, explored the timing of such an event. In a female patient with mosaic SNURF hypomethylation, X-chromosome analysis showed that cells with the imprinting defect had either the paternally-derived or maternally-derived X chromosome inactivated, suggesting that the insult occurred before X-inactivation and implantation 148 . In principle, somatic imprinting errors may occur at any time in dividing cells. Immediately following replication, the methylation pattern on the template strand is recognized by the UHRF1-DNMT1 maintenance methyltransferase complex and copied onto the daughter strand. A failure to recognize or copy this pattern will result in a sustained hemimethylated profile that will segregate in subsequent cell divisions in a tissue-restricted manner.           99,103,205,206 PGCs, primordial germ cells.

Genomic imprinting
The epigenetic marking of a gene on the basis of parental origin, which results in monoallelic expression.

Anticipation
A phenomenon whereby the symptoms of a genetic disorder become apparent at an earlier age or with greater severity in succeeding generations.

Assisted reproductive technologies
(ART). Techniques used to achieve pregnancy during the treatment of infertility. ART covers a wide spectrum of treatments including the use of fertility drugs, intrauterine insemination and in vitro fertilization/intracytoplasmic sperm injection.

Blastocyst
Developmental stage of mammalian embryo just before implantation consisting of an inner cell mass which will form the embryo, and a cavity with an outer layer called trophoblast, which will give rise to the placenta.

cis-acting element
DNA sequence regulating the expression of a gene that is present on the same chromosome.

Copy number variation
(CNV). Type of structural variation of a chromosome consisting in duplication or deletion of DNA sequence.

Endogenous retrovirus
Repetitive genetic element present in the genome that, similarly to retroviruses, uses the activity of reverse transcriptase to move from one locus to another (also known as retrotransposons).

Epiallele
Epigenetic profile which is maintained in somatic tissues resulting in interindividual variation.

Epigenome
Chromatin modifications influencing genome function and not involving the underlying DNA sequence that can be propagated through cell division.

Epigenetic reprogramming
The erasure of pre-existing epigenetic marks that allow for subsequent remodelling of chromatin.

Epimutation
When referred to imprinting disorders, epigenetic change that affects the regulation of imprinted loci. The epimutation is primary if there is no detectable genetic cause, secondary if it is associated with a genetic cause.

Imprinting disorders
Diseases associated with disruption of imprinted gene expression that can be caused by genetic or epigenetic defects.

Incomplete penetrance
A situation in which not all individuals carrying a dominant deleterious genetic variant express the associated clinical phenotype.

Gain of methylation
(GOM). When referred to imprinting disorders, gain of methylation on the unmethylated allele of imprinting centre. It is detected in patients and causes deregulation of the imprinted genes in the domain controlled by the imprinting centre.

Genome activation
The initiation of gene expression in the developing embryo. The initial burst of expression is termed zygotic genome activation (ZGA) and is regulated by pioneer transcription factors during the oocyte-to-embryo transition. Initiation of expression in cleavage embryos is referred to as embryonic genome activation.

Germline differentially methylated region
(gDMR): Regions of differential DNA methylation between parental alleles in somatic cells that originate from the gametes. gDMRs that survive embryonic reprogramming are generally associated with imprinted genes.

Haploinsufficiency
A situation in which half of the normal level of a gene product, usually consequence of a loss-of-function mutation, is not sufficient for the normal function.

Hydatidiform mole
Benign gestational trophoblastic disease developing during pregnancy and resulting from an abnormal fertilization. It is characterized by trophoblastic proliferation and little or no embryonic tissue. It is commonly sporadic and contains only sperm DNA.
Occasionally, it can be biparental, recurrent and familial following an autosomal recessive mode of inheritance.

Imprinting centre
A function definition for gDMRs that have been shown to regulate imprinted genes expression through either genetic targeting in mouse or through mutations in patients. Also known as imprinting control region (ICR). Not all gDMRs have been shown to be imprinting centre regions.

Loss of methylation
(LOM). When referred to imprinting disorders, loss of differential imprinting centre methylation detected in patients and causing deregulation of the imprinted genes in the domain controlled by the imprinting centre.

Multi-locus imprinting disturbance
(MLID). Methylation anomalies at imprinted DMRs in patients with imprinting disorders in addition to those that are normally associated with the disease.
Maternal effect gene development of the embryo.

Penetrance
Proportion of individuals in a population with a specific genotype who show an associated phenotypic trait.

Primordial germ cells
(PGCs). Stem cell-like cells found in the gonadal ridge of developing embryos that develop into gametes following sex-specific epigenetic reprogramming and meiosis.

Pronucleus
The haploid nucleus from a male or female gamete before the genetic material fuse at syngamy.

Protamines
Basic proteins that largely replace histones in the nucleus of mature sperm for more condensed DNA packaging.

Secondary differentially methylated region
A region of differential DNA methylation between parental alleles that does not originate from the germline. They are often referred to as somatic DMRs and are regulated in a hierarchical fashion by a nearby imprinting centre region.

Subcortical maternal complex
(SCMC). A large multi-protein complex comprising of NLRP5, OOEP, TLE6, PADI6 and KHDC3L that localises to the outermost regions of the cytoplasm in oocytes and excluded from regions of cell-to-cell contact in cleavage embryos.

trans-acting factor
Protein regulating the expression of a gene.