Understanding the Role of Lysine-Specific Demethylase 1 in Embryonic Gene Regulation

Histone proteins provide a means of packaging DNA over 10,000-fold in order to allow the accommodation of genetic material as chromatin in the nucleus of the cell. However, the chemical manipulation of histones underpins an array of additional biological functions of chromatin. The unstructured N-terminal tails of histones are covalently modified in a variety of fashions, with many of these modifications implicated in the regulation of gene expression. Lysine specific demethylase 1 (LSD1), which demethylates mono- and di-methylated histone H3 lysine 4 (H3K4) as part of a complex including CoREST and histone deacetylases (HDACs), is essential for embryonic development in the mouse beyond embryonic day (E)6.5. The aim was to determine the role of LSD1 during this early period of embryogenesis through generation and analysis of conditional knockout mouse embryonic stem (ES) cells, which are the in vitro counterpart of the epiblast. Prior analysis of post-implantation loss-of-function genetrap embryos revealed that LSD1 expression, and therefore function, is restricted to the epiblast. Conditional deletion of LSD1 in mouse ES cells revealed a reduction in CoREST protein and associated HDAC activity, resulting in a global increase in histone H3K9 and H3K56 acetylation, but only minor increases in global H3K4 methylation. Despite this biochemical perturbation, LSD1 deleted ES cells proliferate normally and retain stem cell characteristics. However, differentiation of these ES cells is associated with significant cell death. Loss of LSD1 causes the aberrant expression of 588 genes, including transcription factors with roles in anterior/posterior patterning and tissue specification. Brachyury, a key-regulator of mesodermal differentiation, is a direct target gene of LSD1 and is over-expressed in E6.5 Lsd1 genetrap embryos. Thus, LSD1 regulates the expression and appropriate timing of a key developmental regulator, as part of the LSD1/CoREST/HDAC complex, during early embryonic development. Notably, rescue experiments show that the catalytic activity of LSD1 is not required for gene repression and it is proposed that the regulatory role of LSD1/CoREST/HDAC complex is governed by the ability to target HDACs to genomic regions or to prevent promoter access of gene-activating complexes with H3K4 tri-methylation catalytic activity.