Genomic Imprinting

Definition And Concept

Genomic imprinting is an epigenetic process where specific genes are expressed in a parent-of-origin-specific manner.
Unlike standard Mendelian inheritance, where both maternal and paternal alleles are expressed, imprinting results in the silencing of one allele via epigenetic markers, leaving only one functional copy.
This silencing occurs during gametogenesis and is primarily mediated by deoxyribonucleic acid (DNA) methylation.

The lifecycle of an imprint involves three distinct stages:

Erasure: Inherited parental imprints are cleared in the primordial germ cells of the developing fetus.
Establishment: New imprints are established during gametogenesis (oogenesis or spermatogenesis) according to the sex of the individual.
Maintenance: Imprints are maintained during DNA replication and mitosis post-fertilization to ensure tissue-specific gene expression throughout the individual's life.

DNA Methylation: The addition of a methyl group to cytosine residues in CpG islands creates Differential Methylated Regions (DMRs), which are transcriptionally silent.
Imprinting Control Regions (ICR): Cis-acting DNA sequences that regulate the imprinting of a gene cluster, potentially controlling several megabases of DNA.
Histone Modification: Processes like acetylation, methylation, and phosphorylation of histone tails alter chromatin structure (euchromatin vs. heterochromatin), influencing gene accessibility to transcriptional machinery.
Long Non-Coding RNA (lncRNA): Antisense transcripts can interfere with sense strand expression or recruit chromatin-modifying complexes to silence target genes.

Clinical syndromes arise when the normally active allele is lost or silenced through four main mechanisms:

Chromosomal Deletion: Physical loss of the active allele, representing the most common cause (e.g., approximately 70% of Prader-Willi and Angelman syndrome cases).
Uniparental Disomy (UPD): Inheritance of both copies of a chromosome from one parent and none from the other.
Imprinting Defects (Epimutations): Failure to erase or reset the imprint during gametogenesis despite having a normal DNA sequence.
Point Mutations: Pathogenic variants within the active gene itself, such as the UBE3A mutation in Angelman syndrome.

Syndrome	Chromosome / Gene	Imprinting Defect	Key Clinical Features
Prader-Willi Syndrome (PWS)	15q11-13	Loss of paternal expression (e.g., SNRPN)	Neonatal hypotonia, childhood hyperphagia, morbid obesity, short stature, hypogonadism, mild to moderate intellectual disability.
Angelman Syndrome (AS)	15q11-13	Loss of maternal expression (e.g., UBE3A)	Severe intellectual disability, postnatal microcephaly, seizures, paroxysms of laughter, ataxia, absent speech.
Beckwith-Wiedemann Syndrome (BWS)	11p15.5	Overexpression of paternal IGF2	Macrosomia, hemihyperplasia, macroglossia, omphalocele, high embryonal tumor risk.
Russell-Silver Syndrome (RSS)	11p15.5	Loss of paternal IGF2	Severe intrauterine growth restriction, relative macrocephaly, clinodactyly, body asymmetry.
Temple Syndrome	14q32	Maternal UPD 14	Early puberty, hypotonia, short stature.
Kagami-Ogata Syndrome	14q32	Paternal UPD 14	Coat-hanger ribs, bell-shaped thorax, abdominal wall defects.
Pseudohypoparathyroidism Type 1a	GNAS	Defective maternal expression	Albright Hereditary Osteodystrophy phenotype with parathyroid hormone resistance.

Management requires a multidisciplinary approach focused on the specific syndrome features:

Nutritional Support: Feeding tubes are required for neonatal PWS; strict caloric restriction is mandatory in older children with PWS.
Endocrinology: Growth hormone therapy is standard in PWS and RSS to improve height and body composition.
Neurology: Aggressive seizure management is necessary in Angelman syndrome.
Oncology: Serial screening for embryonal tumors (e.g., Wilms tumor, hepatoblastoma) with abdominal ultrasonography and alpha-fetoprotein is critical in BWS every three months until age 8.
Genetic Counseling: Recurrence risk varies significantly based on the molecular mechanism. The risk is <1% for UPD or de novo deletions, but up to 50% for inherited imprinting center mutations or specific active gene mutations.