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Updated: Apr 2 2020


  • Overview
    • Changes in gene expression caused by mechanisms other than changes in actual DNA sequence
    • Examples
      • X-inactivation
      • imprinting
      • histone modification
        • see Chromatin structure topic
  • X-inactivation
    • Overview
      • normalizes the genetic amount of males and females (lyonization)
      • inactivates # of X chromosomes - 1 in a Barr body
        • triploid X will have 2 Barr bodies
    • Mechanism
      • mediated by XIST gene
      • inactivation through methylation
      • occurs at blastocyst stage in female embryos
      • X copy chosen for inactivation is random
        • after choosing every subsequent cell will have the same X copy inactivated
    • Clinical relevance
      • mosaicism
        • non-homogenous X inactivation
        • some cells express paternal X and some cells express maternal X
  • Imprinting
    • Overview
      • describes differences in transcriptional activity based on whether the chromosome is of maternal or paternal origin
      • at a single locus
        • 1 allele is active
        • 1 allele is inactive
      • creates a hemizygous state
    • Mechanism
      • inactive ("imprinted") allele is methylated during gametogenesis
        • creates transcriptional inactivity
      • is maternal/paternal specific
        • gene at one locus always methylated on a specific copy
      • all cells of an individual have same imprinting level
      • during gametogenesis of the individual the methylation state is erased
        • reset to be either maternal or paternal depending on the sex
    • Clinical importance
      • Prader-Willi syndrome
        • cause
          • deletion or mutation of normally active paternal allele on 15q
          • remember: Prader = paternal deletion
        • presentation
          • mental retardation
          • hyperphagia → obesity
          • hypogonadism
          • neonatal hypotonia
          • behavior problems
        • affects both male and females
      • Angelman's syndrome
        • cause
          • disruption of the maternally expressed and paternally imprinted gene UBE3A, which encodes an E3 ubiquitin ligase
          • typically results from deletion of the normally active maternal allele on chromosome 15q
            • same region of the genome as Prader-Willi syndrome deletion but opposite chromosome (not the same gene)
        • presentation
          • severe cognitive disability
          • frequent seizures
          • ataxia
          • speech impairment
          • hyperactivity
          • inappropriate laughter
            • "happy puppet"
        • affects both male and females
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