Article #5
• Cells of a multicellular organism are genetically homogeneous, but structurally
and functionally heterogeneous b/c of different gene expression
• Epigenetics or “outside conventional genetics” - describes the study of stable
alterations in gene expression potential that arise during development & cell
proliferation
• Such epigenetic processes are essential for development & differentiation, but
they can also arise in mature humans & mice, either by random change or under
environ influence
• also guard against viral genomes that would otherwise hijack cellular fn
• modification of DNA or proteins are recognized by specific proteins that
recognize modifications and facilitate the appropriate biological effects
Establishing and maintaining patterns of DNA methylation
• DNA methylation
o chief contributor to stability of gene expression states, might be
responsible for stable maintenance of particular gene expression pattern
through mitotic cell division and is involved in maintaining X
chromosome inactivation in females
o DNA methylation est. a silent chromatin state by collaborating with
protein that modify nucleosomes
o Post replication modification
o DNA methylation found in cytosines of the dinucleotide sequence CpG
• Extent of DNA methylation changes during mammalian development:
o Wave of demethylation during cleavage
o Genome wide methylation after implantation
o Demethylation is an active process that strips the male genome of
methylation within hours of fertilization.
o Maternal genome is only passively demethylated during cleavage
divisions
o methylation decreases in specific tissues during differentiation
o methylation occurs rarely in postgastrulation – but is seen in cancer!
• DNA methyltransferases (DNMT):
DNMT1 maintenance methylrtansferase (i.e. maintains but not
est maternal imprint), absence results in global demethylation and embryonic
lethality, DNMT1o responsible for maintaining, but not establishing
maternal imprints
DNMT3a, DNMT3b methylation after implantation,
expressed highly during developing embryo
DNMT3L no DNMT activity on own but joins to DNMT3a/b to
imprint on female germ line DNMT2 no DNMT alone, may be responsible for small amount
of non-Cpg methylation in flied
• DNA meth is essential for vertebrate development
• Loss of methylation causes apoptosis in embryos/fibroblasts, NOT in stem cells,
or cancer cells, depression of extopic gene expression, and transcriptional
activation of transposable elements
• Evolution explanation of methylation mechanism to silence transposable
elements? Mediate transcriptional noise reduction? Yet to be resolved
• Non embryonic cells 80% CpG’s are methylated
• CpG islands are associated with genes and all may contain promoters
• methylation is interrupted by short CpG islands generally unmethylated all the
time (few exceptions), if methylated leads to long term shut down of the gene
• cancer cells methylation of CpG islands can contribute to gene silencing
• histone modifications may also tirigger methylation of DNA
• DNA methylation acts as a system of cellular memory that isn’t directly involved
in intiating gene silencing, but senses and propogates the silent state
• Methyl-CpG-binding domain proteins (MBDs) like MeCP2 recruit chromatin
remodelling things…
• Histone deacetylation (HDAC’s)
SO….methylation doesn`t directly shut off genes, it does so my recruiting other
genes that affect histone modification such as deacetylases!!!!
• DNA methylation is just one component of a wider epigenetic program that
includes other postsynthesis modifications of chromatin make feedback loops
so ensure the polarization of chromatin domains so that self-reinforcing steps are
present to prevent going in other direction (pulls back to “pole”)
Interpreting the DNA methylation signal
• Several ways that methylation can repress transcription:
1. exclusion of proteins that affect transcription by the methylation blocking
their binding sites
o CpG methylation blocks the binding of chromatin boundary
element binding protein (CTCF – blocks interactions between
promoter and enhancer)
2. Methyl-CpG-binding domain proteins (MBDs):
o DNA methlylation is repulsive to some DNA binding proteins
but attractive to others
o mediators of transcriptional repression
o Includes MeCP2 and MBD 1-4
o Transciptionally repress MeCP2 interacts with a corepressor
complexes containing HDAC’s (histone deacetylases)
shows that there is an interaction between DNA methylation
and histone modification o mice without MBD2 are viable and fertile but lack methylating
DNA binding complex MeCP1 defective in the methylation-
mediated of genes and significant gene depression
- MBD proteins then recruit additional proteins to the locus, such as
histon
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