Epigenetic regulation of germline- specific genes
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Hackett2010.doc (1.561Mb)
Date
24/11/2010Author
Hackett, Jamie Alexander
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Abstract
In mammals, epigenetic modifications and trans-acting effectors coordinate gene
expression during development and impose transcriptional memories that define
specific cell lineages and cell-types. Methylation at CpG dinucleotides is an
epigenetic mechanism through which transcriptional silencing is established and
heritably maintained through development. Functionally, DNA methylation regulates
key biological processes such as X-chromosome inactivation, transposon repression
and genomic imprinting. However, the extent to which DNA methylation is the
primary regulator of single-copy gene expression and the precise mechanism of
methylation-dependent silencing remain undetermined. Here, I identify a novel set of
germline-specific candidate genes putatively regulated by DNA methylation.
Analysis of one candidate gene, Tex19, demonstrates that promoter CpG methylation
is the primary and exclusive mechanism for regulating developmental silencing in
somatic lineages. Genetic or pharmacological removal of CpG methylation triggers
robust de-repression of Tex19 and loss of transcriptional memory. Moreover, Tex19
critically relies on de novo methylation, mediated by Dnmt3b, to impose silencing in
differentiating ES cells and somatic cells in vivo from embryonic day (E)7.5.
Reporter gene and ChIP analysis demonstrate that Tex19 is strongly activated by
general transcription factors and is not marked by repressive histone modifications in
somatic lineages, consistent with differential DNA methylation per se being the
primary mechanism of regulating expression. Full transcriptional silencing of Tex19
is critically dependent on the methyl-binding protein (MBP) Kaiso, which is only
recruited to methylated Tex19 promoter. The reliance on DNA methylation and
Kaiso for silencing in somatic cells establishes an epigenetic memory responsible for
maintaining expression in germline and pluripotent cell types through successive
developmental cycles. This thesis represents the first causal report of lineagespecific
promoter DNA methylation directing silencing of an in vivo gene through
recruitment of an MBP.