Genomic imprinting breaks the rules of typical gene expression. It's like a genetic game of favorites, where some genes only work when they come from mom or dad, not both. This quirk can lead to unique inheritance patterns and even certain disorders.
The molecular mechanisms behind imprinting are like genetic on/off switches. DNA methylation and histone modifications act as tags, telling genes when to be quiet or speak up. These tags are set early on and stick around, influencing how genes behave throughout life.
Genomic Imprinting
Genomic imprinting and inheritance
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Epigenetic phenomenon where gene expression depends on parent-of-origin of allele
Certain genes expressed differently based on inheritance from mother or father (IGF2 , H19 )
Imprinted genes typically expressed from only one allele, either maternal or paternal, while other allele silenced
Deviates from classical Mendelian inheritance patterns, where both alleles expressed equally
Leads to parent-of-origin effects, where offspring phenotype depends on which parent allele inherited from (Prader-Willi syndrome , Angelman syndrome )
Influences inheritance of certain genetic disorders and traits
Molecular mechanisms of imprinting
DNA methylation key mechanism
Addition of methyl groups to cytosine residues in CpG dinucleotides of DNA
Methylation patterns established during gametogenesis and maintained throughout development
Methylation of promoter regions typically leads to gene silencing
Histone modifications also play a role
Acetylation and deacetylation can influence gene expression
Specific modifications, such as H3K9 methylation, associated with repressed chromatin states and gene silencing
Imprinting control regions (ICRs) regulate expression of imprinted genes
ICRs are differentially methylated regions (DMRs) that control expression of nearby genes
Methylation status of ICRs established during gametogenesis and maintained in offspring
Parent-of-origin effects in genes
Phenotype of offspring depends on which parent allele inherited from
Expression of allele determined by parent who passed it on, rather than allele itself
Examples of imprinted genes:
IGF2 (Insulin-like growth factor 2): Expressed from paternal allele; involved in fetal growth and development
H19: Expressed from maternal allele; long non-coding RNA that regulates IGF2 expression
SNRPN (Small nuclear ribonucleoprotein polypeptide N): Expressed from paternal allele; involved in Prader-Willi syndrome
UBE3A (Ubiquitin-protein ligase E3A): Expressed from maternal allele in brain; involved in Angelman syndrome
Imprinting in human disorders
Prader-Willi syndrome (PWS):
Caused by loss of function of paternally expressed genes on chromosome 15q11-q13
Characteristics include hypotonia, intellectual disability, behavioral problems, and hyperphagia leading to obesity
Angelman syndrome (AS):
Caused by loss of function of maternally expressed UBE3A gene on chromosome 15q11-q13
Characteristics include intellectual disability, speech impairment, seizures, and happy demeanor
Imprinting disorders arise from disruptions in normal imprinting patterns
Uniparental disomy (UPD): Inheritance of both copies of chromosome from one parent
Deletions or mutations in imprinted regions
Epigenetic alterations, such as abnormal DNA methylation patterns (Beckwith-Wiedemann syndrome, Silver-Russell syndrome)