Residency · Residency · Medical Genetics Genomics

Epigenetics and Imprinting Disorders

Epigenetic Mechanisms

DNA Methylation

DNA methylation involves the addition of a methyl group to the 5-carbon of cytosine in CpG dinucleotides, a reaction catalyzed by DNA methyltransferases. DNMT1 is the "maintenance" methyltransferase that copies existing methylation patterns to the newly synthesized strand during DNA replication, while DNMT3A and DNMT3B are "de novo" methyltransferases that establish new methylation marks. CpG islands, defined as regions greater than 200 base pairs with over 50% GC content and an observed-to-expected CpG ratio above 0.6, are found at approximately 70% of gene promoters. Hypermethylation of a promoter generally silences gene expression by blocking transcription factor binding and by recruiting methyl-CpG binding domain (MBD) proteins. Paradoxically, methylation within gene bodies is associated with active transcription. The TET enzymes (TET1, TET2, and TET3) catalyze oxidation of 5-methylcytosine to 5-hydroxymethylcytosine, initiating the process of active demethylation.

Histone Modifications

Acetylation of histone lysine residues, such as H3K27ac and H3K9ac, is deposited by histone acetyltransferases and opens chromatin to activate transcription. Histone methylation can serve either activating or repressive functions depending on the specific residue modified: H3K4me3 at promoters and H3K36me3 in gene bodies are activating marks, while H3K9me3 and H3K27me3 are repressive. Polycomb Repressive Complex 2 (PRC2) catalyzes the H3K27me3 mark, which is a key feature of facultative heterochromatin. In embryonic stem cells, bivalent domains carrying both H3K4me3 and H3K27me3 mark poised developmental genes, holding them in a state ready for rapid activation or permanent silencing upon differentiation. Several human genetic disorders result from disruption of histone modification pathways, including Kabuki syndrome (caused by mutations in KMT2D or KDM6A), Rubinstein-Taybi syndrome (CREBBP or EP300), and Sotos syndrome (NSD1).

Chromatin Remodeling Complexes

The SWI/SNF (BAF) complex uses ATP hydrolysis to remodel nucleosomes. Mutations in its components, including SMARCB1, SMARCA4, ARID1A, and others, cause Coffin-Siris syndrome and predispose to rhabdoid tumors. The CHD family of chromatin remodelers includes CHD7, whose mutations cause CHARGE syndrome, and CHD8, whose mutations are associated with autism spectrum disorder. The ISWI and INO80 complexes contribute to nucleosome spacing and DNA repair.

Noncoding RNA-Mediated Epigenetic Regulation

The XIST long noncoding RNA coats the inactive X chromosome and recruits PRC2 to establish silencing. Imprinted long noncoding RNAs, such as KCNQ1OT1 and AIRN, silence genes in cis by recruiting chromatin-modifying complexes. MicroRNAs regulate post-transcriptional gene expression and can themselves be subject to epigenetic silencing, creating layered regulatory networks.

Genomic Imprinting

Principles of Imprinting

Genomic imprinting is a form of parent-of-origin-specific gene expression in which only the maternal or paternal allele is expressed while the other is silenced by epigenetic marks. Approximately 100 to 200 imprinted genes have been identified in humans, and they are often clustered in imprinted domains. Imprinting marks, known as differentially methylated regions (DMRs), are established in the germline and maintained after fertilization. Germline (primary) DMRs are set during gametogenesis, while somatic (secondary) DMRs are established post-fertilization under the control of the germline DMRs. Imprinting control regions (ICRs) serve as master regulators for each imprinted gene cluster.

Mechanisms of Imprinting Disruption

Uniparental disomy (UPD) occurs when both copies of a chromosome or chromosomal segment are inherited from a single parent. Isodisomy, in which both copies derive from a single parental homolog, carries the additional risk of unmasking recessive variants. Heterodisomy provides one copy from each of a single parent's two homologs. UPD can arise through trisomy rescue, monosomy rescue, gamete complementation, or post-fertilization mitotic errors. Deletions or mutations of the ICR can disrupt regulation of an entire imprinted domain. Epimutations represent aberrant gain or loss of methylation at DMRs without an underlying DNA sequence change; primary epimutations are stochastic, usually mosaic, and carry low recurrence risk, while secondary epimutations are caused by cis-acting DNA mutations such as ICR deletions and carry high recurrence risk. Chromosomal rearrangements, including translocations or inversions, can also disrupt imprinted loci.

Prader-Willi Syndrome (PWS)

Genetics

Prader-Willi syndrome results from loss of function of paternally expressed genes at 15q11.2-q13. Key genes include the SNORD116 snoRNA cluster, SNRPN, MAGEL2, NDN, and MKRN3, with deletion of SNORD116 alone being sufficient to cause the core PWS phenotype. Approximately 70% of cases are caused by paternal deletion (typically about 5 megabases, with two common breakpoint classes: BP1-BP3 and BP2-BP3). About 25% result from maternal UPD, a risk that increases with advanced maternal age due to the trisomy rescue mechanism. Imprinting center defects, either deletions or epimutations of the PWS-IC, account for 2 to 5% of cases, and rare cases involve balanced translocations disrupting the locus.

Clinical Features

Neonatal hypotonia and feeding difficulties, often requiring gavage feeding, are the hallmark early presentations. A dramatic transition to hyperphagia and obesity typically begins between ages 2 and 6 years. Other features include short stature, hypogonadism (cryptorchidism and micropenis in males; primary amenorrhea in females), mild to moderate intellectual disability, and behavioral issues such as temper tantrums, skin picking, and OCD-like behaviors. The characteristic facies includes a narrow bifrontal diameter, almond-shaped eyes, a thin upper lip, and a downturned mouth. Growth hormone deficiency is common, and GH treatment improves height, body composition, and possibly cognition. Patients are at risk for type 2 diabetes, sleep apnea, scoliosis, and osteoporosis.

Diagnostic Testing Strategy

Methylation analysis of the SNRPN locus serves as the first-tier test because it detects all major molecular mechanisms, including deletion, UPD, and IC defects. If methylation analysis shows a maternal-only pattern (abnormal), the next step is FISH or microarray to determine whether a deletion is present. If no deletion is found, microsatellite or SNP analysis distinguishes UPD from an IC defect. When UPD is confirmed, it is important to determine whether it is heterodisomy or isodisomy, as isodisomy warrants evaluation for unmasking of recessive conditions.

Angelman Syndrome (AS)

Genetics

Angelman syndrome results from loss of function of the maternally expressed UBE3A gene at 15q11.2-q13. UBE3A encodes an E3 ubiquitin ligase, and its expression is imprinted (maternal-only) specifically in neurons. Approximately 70% of cases involve maternal deletion of 15q11.2-q13, the same region as in PWS but on the maternal chromosome. About 11% result from UBE3A point mutations or small indels on the maternal copy. Paternal UPD accounts for 3 to 7% of cases, and imprinting center defects explain another 3%. In approximately 10% of cases, the molecular mechanism remains unknown despite a clinical diagnosis.

Clinical Features

Angelman syndrome is characterized by severe intellectual disability with absent or minimal speech. A distinctive movement disorder includes an ataxic, "puppet-like" gait and tremulous limb movements. The behavioral phenotype features frequent laughter and smiling, a happy demeanor, hand-flapping, and excitability. Seizures typically begin between 1 and 3 years of age, involve multiple seizure types, and are often refractory to treatment. Microcephaly develops postnatally, and sleep disturbance with a decreased need for sleep is common. Unlike Rett syndrome, there is no significant regression.

Genotype-Phenotype Correlations

Deletion cases tend to be the most severe. Among deletions, larger ones (BP1-BP3 versus BP2-BP3) may have worse outcomes due to loss of additional non-imprinted genes. UBE3A mutations produce intermediate severity. UPD and IC defect cases are often milder, with some preserved speech and fewer seizures. Mosaicism for IC defects can produce attenuated phenotypes.

Beckwith-Wiedemann Syndrome (BWS)

Genetics

Beckwith-Wiedemann syndrome is an overgrowth syndrome caused by dysregulation of imprinted genes at 11p15.5. This region contains two imprinted domains. IC1 (the H19/IGF2 domain) has ICR1 methylated on the normal paternal allele, with IGF2 paternally expressed and H19 maternally expressed. IC2 (the KCNQ1OT1/CDKN1C domain) has ICR2 methylated on the normal maternal allele, with CDKN1C maternally expressed. Approximately 50% of cases result from loss of methylation at IC2 on the maternal allele (sporadic with low recurrence). About 20% have paternal UPD of 11p15, which is mosaic and segmental and carries the highest cancer risk. Gain of methylation at IC1 on the maternal allele accounts for about 5% of cases and carries high cancer risk, especially for Wilms tumor. CDKN1C mutations on the maternal allele represent about 5% of cases and may be inherited as autosomal dominant with maternal transmission. Cytogenetic abnormalities involving 11p15 account for about 1%, and approximately 15% of clinically diagnosed cases have no identifiable molecular abnormality.

Clinical Features

The core features of BWS include macrosomia (birth weight above the 97th percentile) with postnatal overgrowth, macroglossia that may cause feeding and respiratory difficulties, abdominal wall defects (omphalocele, umbilical hernia, diastasis recti), and neonatal hypoglycemia from hyperinsulinism that may be severe and prolonged. Hemihyperplasia (asymmetric overgrowth) and visceromegaly (particularly nephromegaly and hepatomegaly) are common. Minor features include ear lobe creases and helical pits. The most concerning complication is the risk of embryonal tumors, including Wilms tumor, hepatoblastoma, neuroblastoma, and rhabdomyosarcoma.

Tumor Surveillance

Screening involves abdominal ultrasound every 3 months until age 7 years to detect Wilms tumor and hepatoblastoma, along with alpha-fetoprotein levels every 2 to 3 months until age 4 years for hepatoblastoma. Tumor risk is stratified by molecular subtype: IC1 gain of methylation and paternal UPD carry the highest risk (approximately 28% and 16%, respectively), while IC2 loss of methylation has a much lower risk of about 2.5%. The 2023 consensus guidelines recommend subtype-specific surveillance protocols.

Silver-Russell Syndrome (SRS)

Genetics

Silver-Russell syndrome is a growth restriction syndrome that can be considered the molecular "opposite" of BWS. Approximately 40% of cases result from loss of methylation at IC1 on the paternal allele at 11p15, the reverse of the IC1 gain of methylation seen in BWS. About 10% involve maternal UPD of chromosome 7, and 1 to 2% involve maternal UPD of other chromosomes (14, 16, or 20). Rare causes include 14q32 abnormalities, CDKN1C gain-of-function mutations, HMGA2 mutations, and IGF2 mutations. Approximately 40% of cases have no identifiable molecular cause.

Clinical Features

The hallmark features include severe intrauterine growth restriction (typically birth weight below -2 standard deviations), relative macrocephaly at birth reflecting head sparing, postnatal growth failure with short stature often below -2 SD, body asymmetry with limb length discrepancy, and a distinctive triangular facies with a prominent forehead and small chin. Feeding difficulties in infancy and poor appetite are common, as is fifth finger clinodactyly. Patients face an increased risk of fasting hypoglycemia. Growth hormone therapy is generally recommended.

Netchine-Harbison Clinical Scoring System

A clinical diagnosis of SRS requires 4 of 6 criteria: small for gestational age, postnatal growth failure, relative macrocephaly at birth, body asymmetry, feeding difficulties or low BMI, and protruding forehead. Molecular confirmation is recommended but not required for the clinical diagnosis.

Temple Syndrome (Chromosome 14 Imprinting)

Temple syndrome results from dysregulation of imprinted genes at 14q32 within the DLK1-MEG3 domain. It can be caused by maternal UPD14, paternal deletion of 14q32, or epimutation. Clinical features include prenatal and postnatal growth restriction, hypotonia, early puberty, small hands and feet, and obesity. The syndrome shares features with both PWS (hypotonia and feeding difficulties in infancy) and SRS (growth restriction).

Kagami-Ogata Syndrome

Kagami-Ogata syndrome is the "opposite" of Temple syndrome, resulting from overexpression of paternally expressed genes at 14q32. It can be caused by paternal UPD14, maternal deletion of 14q32, or epimutation. Features include polyhydramnios, placentomegaly, a bell-shaped thorax, a "coat-hanger" appearance of the ribs, abdominal wall defects, and hepatoblastoma risk.

Diagnostic Approaches for Imprinting Disorders

Methylation Analysis

Methylation-specific MLPA (MS-MLPA) is the standard first-tier test because it detects both methylation abnormalities and copy number changes simultaneously. Methylation-specific PCR (MS-PCR) detects aberrant methylation but does not quantify it or detect deletions. Bisulfite sequencing remains the gold standard for locus-specific methylation analysis. Genome-wide methylation arrays, such as the Infinium EPIC array, can identify multilocus imprinting disturbances (MLID) and provide episignatures useful for diagnosis.

Multilocus Imprinting Disturbances (MLID)

In some patients, aberrant methylation affects multiple imprinted loci simultaneously. These multilocus imprinting disturbances are associated with variants in genes encoding components of the subcortical maternal complex, including NLRP2, NLRP5, NLRP7, PADI6, OOEP, and KHDC3L. Patients can present with overlapping features of multiple imprinting disorders. Biallelic variants in NLRP7 cause recurrent hydatidiform mole (biparental complete mole).

<image>A diagram of the chromosome 15q11.2-q13 imprinted region showing the arrangement of genes from centromere to telomere: MKRN3, MAGEL2, NDN, SNRPN-SNURF with the PWS imprinting center (PWS-IC), the SNORD116 snoRNA cluster, UBE3A, and the AS imprinting center (AS-IC). Maternal and paternal chromosomes are shown in parallel. On the paternal chromosome, MKRN3 through SNORD116 are shown as active (green) while UBE3A is silenced (red). On the maternal chromosome, UBE3A is active (green) while the paternal-expression genes are silenced (red). Common deletion breakpoints (BP1, BP2, BP3) are marked with vertical dashed lines. Methylation status of the PWS-IC is indicated (unmethylated on paternal, methylated on maternal).</image>

<image>A schematic of the 11p15.5 imprinted region showing two domains. IC1 domain: IGF2 (paternally expressed) and H19 (maternally expressed) with the ICR1 differentially methylated region shown as methylated on the paternal allele and unmethylated on the maternal allele, with CTCF insulator binding only to the unmethylated maternal allele. IC2 domain: CDKN1C (maternally expressed) and KCNQ1OT1 (paternally expressed) with ICR2 shown as methylated on the maternal allele. Arrows indicate the molecular defects in BWS (gain of methylation at IC1, loss of methylation at IC2, paternal UPD, CDKN1C mutations) and SRS (loss of methylation at IC1 on paternal allele) with color-coded labels.</image>

<image>A flowchart for the diagnostic workup of a patient with suspected imprinting disorder. Starting with clinical suspicion based on phenotype, proceeding to methylation-specific MLPA as the first-tier test. Branches show: if methylation pattern is abnormal for PWS (maternal-only) proceed to FISH/microarray for deletion vs. UPD testing; if abnormal for AS (paternal-only) proceed similarly plus UBE3A sequencing if no deletion/UPD found; if abnormal for BWS/SRS proceed to determine IC1 vs IC2 involvement and UPD analysis. A separate branch shows genome-wide methylation array for multilocus imprinting disturbances when multiple imprinted loci are abnormal.</image>

Clinical Pearls

Methylation analysis of the appropriate locus is the single best first-tier test for imprinting disorders because it detects all major molecular mechanisms, including deletion, UPD, IC defects, and epimutations. Recurrence risk depends critically on the molecular mechanism: most epimutations carry less than 1% recurrence risk, whereas IC deletions and CDKN1C mutations may carry up to 50% recurrence risk. Paternal UPD15 in Angelman syndrome and maternal UPD15 in Prader-Willi syndrome are associated with milder phenotypes compared to deletion cases. In Beckwith-Wiedemann syndrome, the molecular subtype dictates both the cancer risk and the recommended surveillance approach; IC1 gain of methylation and paternal UPD carry the highest Wilms tumor risk. Advanced maternal age increases the risk for UPD-related imprinting disorders due to trisomic rescue of meiotic nondisjunction events. Isodisomy, as opposed to heterodisomy, carries an additional risk of unmasking recessive conditions and should prompt consideration of genomic sequencing for the affected chromosome. Multilocus imprinting disturbances should be considered when clinical features overlap between multiple imprinting syndromes, and genome-wide methylation analysis with maternal-effect gene testing is indicated in these cases.

References

• Buiting K et al. Clinical utility gene card for: Prader-Willi Syndrome. Eur J Hum Genet. 2014;22(9).
• Dagli AI et al. Angelman Syndrome. GeneReviews. 2021.
• Brioude F et al. Expert consensus document: Clinical and molecular diagnosis, screening and management of Beckwith-Wiedemann syndrome. Nat Rev Endocrinol. 2018;14(4):229-249.
• Wakeling EL et al. Diagnosis and management of Silver-Russell syndrome: first international consensus statement. Nat Rev Endocrinol. 2017;13(2):105-124.
• Mackay DJG et al. Multilocus imprinting disturbances and the impact on human health. Hum Mol Genet. 2023;32(R1):R16-R23.
• Monk D et al. Genomic imprinting disorders: lessons on how genome, epigenome and environment interact. Nat Rev Genet. 2019;20(4):235-248.

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