Epigenetics in Obstetrics and Gynecology

1. Definition and Core Concept

2. The Four Mechanisms of Epigenetic Regulation

A. Histone Acetylation

• DNA is coiled around histone octamers (H2A, H2B, H3, H4) in units of ~140 bp called nucleosomes
• HATs (writers) add acetyl groups to histone tails → open, loosely packed chromatin → transcriptionally ACTIVE
• HDACs (erasers) remove acetyl groups → tightly packed chromatin → transcriptionally INACTIVE
• Reader proteins bind acetylated histones via a bromodomain (e.g., SMARCA4/BRG1)
• Phosphorylation of histones also opens chromatin structure and activates transcription

B. Histone Methylation

• Histone methyltransferases (HMTs/writers) add methyl groups to lysine residues on histone tails
• Unlike acetylation, methylation can activate OR repress transcription depending on which residue is modified:
  • H3K9me3 and H3K27me3 (trimethylation of lysine 9 or 27 on Histone 3) → gene repression
  • H3K4me3 (trimethylation of lysine 4 on Histone 3) → gene activation
• Removed by histone demethylases (HDMs/erasers)

C. DNA Methylation

• Cytosine residues are methylated at CpG dinucleotides after embryo implantation by DNA methyltransferases (DNMT1)
• CpG methylation → long-term, stable gene silencing/repression
• During embryonic development, pluripotency genes are repressed by methylation as cells differentiate
• The methylation state is erased in primordial germ cells to enable re-expression of pluripotency genes
• Viral genomes integrated into our DNA are silenced via methylation (repressive marks NOT reset in primordial germ cells - inherited by progeny)
• MECP2 (reader) binds methylated DNA → mutation causes Rett syndrome
• Clinical epigenetic therapies: 5-azacytidine, decitabine (DNA demethylating agents); valproic acid (HDAC inhibitor)

D. MicroRNAs (miRNAs)

• Short (~22 nucleotide), highly conserved, noncoding RNAs that act post-transcriptionally to silence RNA
• Target >50% of all genes expressed during development; each miRNA targets hundreds of genes
• Biogenesis pathway:
  1. RNA polymerase II/III converts DNA → pri-miRNA (nucleus)
  2. Drosha + DGCR8 microprocessor complex cleaves pri-miRNA → pre-miRNA (nucleus)
  3. Exportin-5 exports pre-miRNA to cytoplasm
  4. Dicer (RNase III endoribonuclease) processes hairpin → miRNA duplexes (cytoplasm)
  5. RISC complex converts duplexes → mature miRNA
  6. Mature miRNA → repression of translation OR mRNA degradation → decreased protein function
• Oncomirs: miRNAs dysregulated in cancer
• Germline mutations of DICER1 → familial tumor predisposition syndrome (pleuropulmonary blastoma, cystic nephroma, medulloepithelioma)

3. Epigenetics in Obstetrics

A. Epigenetic Regulation of Placental Development

• IGF-II is an important growth factor in early development
• The maternal allele is hypermethylated (imprinted) = silenced
• IGF-II is therefore expressed only from the paternal allele
• The placenta expresses abundant paternally-inherited/maternally-imprinted IGF-II
• This is a major regulator of placental growth and function
• Insufficient IGF-II expression (e.g., from maternal isodomy) → fetal growth restriction (FGR)
• This has been explained by the "kinship theory": paternal genes promote growth, maternal genes regulate/restrain growth

• Evolutionary concept: paternally expressed genes promote fetal/placental growth
• Maternally expressed genes moderate/limit growth
• Conflict between maternal and paternal genomes in controlling resource allocation to the fetus

B. Genomic Imprinting and Clinical Disorders

• Tumor suppressor gene
• Expressed exclusively from the maternal allele in cytotrophoblast and villous stromal cells
• Absent in complete hydatidiform mole (all paternal DNA)
• Useful immunohistochemical marker to distinguish complete from partial mole

C. Placental DNA Methylation and Pregnancy Complications

• Differential methylation profiles exist in early human development based on parental allele inheritance
• Placental DNA methylation at specific sites is associated with insulin sensitivity in late pregnancy (important in gestational diabetes)
• The placenta can shed fetal cell-free DNA (cffDNA) into the maternal circulation; placenta-specific methylation signatures are used in non-invasive prenatal testing (NIPT) to identify fetal DNA

D. Preeclampsia and Epigenetics

• Circulating miRNAs are altered in preeclampsia and act from the time of implantation
• miRNA dysregulation affects trophoblast invasion and spiral artery remodeling
• DNA methylation changes in placental genes are associated with preeclampsia risk
• Older first-trimester epigenetic age (GrimAge2) is associated with a composite of pregnancy complications (2025 data, PMID 40638919)

E. X-Chromosome Inactivation (Lyon Hypothesis) - Epigenetic in Females

• In females with 2 X chromosomes, one X in each cell is randomly inactivated early in development (lyonization)
• The inactive X condenses to form the Barr body (sex chromatin, visible in interphase nucleus as a dark mass against the nuclear membrane)
• The process involves the X-inactivation center (Xic) and the XIST gene which coats the inactive X
• This is dosage compensation - ensures equivalent X-linked gene expression in males (XY) and females (XX)
• All daughter cells maintain the same inactive X (clonal)
• Clinically relevant in:
  • Carriers of X-linked recessive disorders (variable expression due to skewed X-inactivation)
  • Turner syndrome (45,X): no Barr body
  • Klinefelter syndrome (47,XXY): 1 Barr body

4. Fetal Programming / DOHaD Hypothesis (Barker Hypothesis)

• Environmental stresses during pregnancy cause epigenetic changes that affect the fetus and translate into disease in adult life
• Epigenetic modification of the genome is widespread, occurs primarily during fetal development but extends into adulthood
• Allows the fetus to respond in utero to environmental cues (nutrition, stress, toxins)

• Dutch Hunger Winter (1944-1945): Severe maternal undernutrition → offspring developed hypertension, type 2 diabetes, obesity decades later
• Low birthweight → increased risk for hypertension, diabetes, metabolic syndrome, renal disease in adult life
• Placental insufficiency → most common cause of FGR in the Western world → programs chronic disease in offspring
• Glucocorticoid receptor (GR) promoter methylation in response to early adverse environments → alters HPA axis programming

5. Epigenetics in Gynecology

A. Gynecologic Cancers

• Rett syndrome (MECP2 mutation)
• Rubinstein-Taybi syndrome (EP300/CBP - histone acetyltransferase mutations)
• α-thalassemia/X-linked mental retardation (ATRX mutations)

B. PCOS and Epigenetics

• Adipose tissue dysfunction in PCOS involves epigenetic dysregulation
• Androgen excess may program epigenetic changes affecting metabolic function in future generations
• miRNA profiles are altered in granulosa cells of PCOS patients

C. Endometriosis

• Aberrant DNA methylation of steroidogenic genes (e.g., aromatase/CYP19A1 promoter hypomethylation in ectopic endometrium → excess local estrogen production)
• Progesterone resistance in endometriosis linked to epigenetic silencing of progesterone receptor isoforms
• HDAC inhibitors are being explored therapeutically

6. Epigenetic Clocks and Pregnancy Aging

• Epigenetic clocks (e.g., GrimAge2) measure biological age via DNA methylation patterns
• Pregnancy accelerates within-person epigenetic aging by up to 5.3 years
• Older first-trimester GrimAge2 (biological age older than chronological age) is associated with adverse pregnancy outcomes
• Non-pregnant controls show less epigenetic aging over the same time interval
• These findings suggest gestation influences biological aging and epigenetic age may be a marker of pregnancy health

7. Summary Table for Quick Revision

• The Developing Human: Clinically Oriented Embryology, Chapter 21 (Epigenetics, pp. 1335-1344)
• Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice - Epigenetic Regulation of Placental Development, p. 81
• Thompson & Thompson Genetics and Genomics in Medicine, 9th ed. - X Chromosome Inactivation, Imprinting
• Robbins & Cotran Pathologic Basis of Disease - Cytogenetic Disorders involving Sex Chromosomes
• Panelli et al., "Longitudinal Epigenetic Aging in Pregnancy," Obstet Gynecol 2025 (PMID 40638919)
• Giannubilo et al., "Circulating miRNAs and Preeclampsia: From Implantation to Epigenetics," Int J Mol Sci 2024 (PMID 38338700)

Epigenetics and Assisted Reproductive Technology (ART)

Why ART and Epigenetics Are Inextricably Linked

1. Erasure of parental imprints in primordial germ cells
2. Re-establishment and maintenance of genomic imprints during oocyte maturation and after fertilization

The Two Critical Vulnerable Periods

ART Procedures and Their Specific Epigenetic Risks

1. Ovarian Hyperstimulation (Superovulation)

• Gonadotropins recruit a cohort of follicles, including those that are immature and not yet fully epigenetically reprogrammed
• Oocytes may have incomplete maternal imprint establishment at imprinting control regions (ICRs)
• This is the most implicated single factor in ART-related imprinting errors
• May also affect DNA methylation at non-imprinted loci

2. In Vitro Fertilization (IVF)

• Gametes and embryos are exposed to artificial culture media with suboptimal nutrient composition compared to the fallopian tube environment
• Culture media composition (amino acids, glucose, oxygen tension, growth factors) can alter histone modifications and DNA methylation
• Increased risk of imprinting disorders: BWS risk rises from 1/13,000 (general population) to approximately 1/2,500 after IVF (several-fold increase)
• Associated with low birthweight, preterm birth, and abnormal placentation

3. ICSI (Intracytoplasmic Sperm Injection)

• Bypasses natural sperm selection processes
• Men needing ICSI often have oligospermia/azoospermia with underlying genetic/epigenetic sperm abnormalities (genome-wide alterations in histone modifications and DNA methylation)
• Sperm from infertile men show abnormal genome packaging and DNA methylation compared to fertile men
• ICSI has been more consistently linked to multilocus imprinting disturbances (MLID) than conventional IVF
• Placentas from ICSI children show global H3K4me3 (histone methylation) differences compared to naturally conceived children
• Concern about overuse of ICSI beyond strict male-factor infertility

4. Embryo Culture Conditions

• Culture medium composition directly influences the epigenome of the pre-implantation embryo
• High glucose, suboptimal amino acids, non-physiological O₂ tension → aberrant de novo methylation
• Different culture media brands have been associated with different birthweight outcomes in animal and human studies

5. Cryopreservation (Embryo/Oocyte Freezing)

• The freeze-thaw process may induce epigenetic changes
• Paradoxically, frozen embryo transfer (FET) is associated with higher birthweights than fresh transfer (possibly due to absence of supraphysiologic hormone levels from stimulation)
• Risk of epigenetic disturbance from cryoprotectants and osmotic stress during freezing is under investigation

ART-Associated Imprinting Disorders

• The 11p15 region has two imprinting domains:
  • IC1 (telomeric): Contains IGF2 (paternal, growth-promoting) and H19 (maternal). IC1 normally methylated on paternal allele → IGF2 expressed, H19 silenced.
  • IC2 (centromeric): Contains CDKN1C (p57KIP2), KCNQ1, KCNQ1OT1. IC2 normally methylated on maternal allele → CDKN1C expressed (growth suppressor), KCNQ1OT1 silenced.
• In ART-associated BWS: predominantly IC2 hypomethylation (loss of maternal methylation) → KCNQ1OT1 expressed, CDKN1C silenced → overgrowth
• BWS and RSS are clinical and molecular mirror images of each other at 11p15

Multilocus Imprinting Disorder (MLID)

• Normal imprinting disorders affect one locus (e.g., BWS affects 11p15)
• In ART conceptions, DNA methylation alterations occur not only at specific loci but at multiple imprinted loci simultaneously
• This is called MLID (Multilocus Imprinting Disorder)
• Children with MLID may present with features of a single known imprinting disorder, or overlapping features of multiple imprinting disorders
• MLID can result from:
  • ART processes themselves (hormone therapy, culture medium)
  • Primary subfertility of the couple (poor oocyte/sperm quality)
  • Pathogenic variants in maternal effect genes (NLRP5, PADI6, ZFP57) that impact imprinted loci in trans

Epigenetic Sperm Abnormalities and Paternal ART Risk

• Sperm from infertile men (required for ICSI) exhibit genome-wide alterations in:
  • Histone modifications (genome packaging)
  • DNA methylation patterns
• Compared to fertile men's sperm on chip microarray analysis
• Epigenetic changes in sperm may be acquired transgenerationally or from conditions such as obesity
• Episona test: commercial test for sperm epigenetic assessment (though limited clinical utility currently)
• Weight loss in obese men may reverse/ameliorate epigenetic sperm changes through chromatin remodeling
• The underlying male factor infertility (not just ICSI procedure itself) contributes to epigenetic risk

Transgenerational Epigenetic Inheritance

• Concern that epigenetic errors introduced by ART may not just affect the child conceived, but potentially future generations (grandchildren)
• ART bypasses natural epigenetic checkpoints of reproduction
• Sperm or oocyte epigenetic abnormalities can be transmitted to offspring and may persist

Clinical Outcomes in ART Pregnancies (Epigenetic Basis)

Summary: Key Exam Points

• Thompson & Thompson Genetics and Genomics in Medicine, 9th ed. - "The Environment Interacts with the Epigenome" + "BWS and ART" (pp. 157-161)
• Berek & Novak's Gynecology - "Epigenetic Sperm DNA Assessment" + "Risks of ICSI"
• Emery's Elements of Medical Genetics and Genomics - In Vitro Fertilization
• Kagami et al., "Association between imprinting disorders and ART," Epigenomics 2025 (PMID 40033833)
• Zhang et al., "ART and imprinting errors: analyzing underlying mechanisms," Hum Fertil 2023 (PMID 37929309)
• Sciorio & El Hajj, "Epigenetic Risks of Medically Assisted Reproduction," J Clin Med 2022 (PMID 35456243)