DNA methylation is a key epigenetic mark which undergoes global reprogramming
during early mammalian embryonic development, resulting in almost complete erasure
of the mark after fertilisation of the zygote. Genome-wide patterns of DNA
methylation are subsequently re-established in the implanting blastocyst by de novo
DNA methyltransferases Dnmt3a and Dnmt3b along with their catalytically inactive
co-factor Dnmt3l, while these DNA methylation patterns are maintained through cell
divisions by maintenance methyltransferase Dnmt1. The exact mechanisms by which
these DNA methyltransferase enzymes are targeted to specific genomic regions remain
unclear, but may involve interaction with modified histones and/or the participation of
co-factors. Lsh (lymphoid specific helicase), a putative chromatin remodelling
helicase, has been implicated in facilitating de novo methylation, as Lsh knockout
embryos and derived somatic cell lines display substantial but specific DNA
methylation losses at repetitive elements and single copy genes.
This study aims to define the requirement for Lsh in establishing de novo DNA
methylation and gene expression patterns during the early stages of mouse embryonic
development. The ‘2i’ culture system using two small molecule kinase inhibitors was
harnessed to convert lsh-/- mouse embryonic stem cells (mESCs) to a hypomethylated
‘ground state’ of pluripotency. Culture conditions were then altered to transition these
ground state mESCs to cells representing later, more methylated stages of development
(‘serum’ mESCs, epiblast stem cells and embryoid bodies). Implementation of this
model system suggests that Lsh does not contribute to DNA methylation establishment
in a pluripotent context, but rather is important for facilitating de novo DNA
methylation during differentiation to culture models representing later developmental
stages. These investigations also reveal that Lsh differentially regulates DNA
methylation at major and minor satellite repeats depending on cellular context, and
that this regulation may involve a role for Lsh in maintenance of DNA methylation
