Establishing imprint marks in germ cells
A fundamental question in imprinting is how a discrete set of genes is marked in the gametes. A principal component of this marking is DNA methylation at imprinting control regions, but the factors that govern which sequences become methylated in gametes are poorly understood. Our recent work has uncovered an essential role for transcription in establishment of DNA methylation marks (Chotalia et al. 2009).
We are currently investigating the mechanistic links between transcription, histone modifications and DNA methylation at imprinted domains in germ cells, using a combination of genetic and highly sensitive molecular approaches. We are also seeking to explain why some genes are imprinted in the female germline and others in the male germline. The discovery of a transcriptional component in imprint establishment has important implications for how imprinted domains evolve and may help explain how imprinting is disrupted in some imprinted gene disorders.
Figure 1: (Click to enlarge)
Factors participating in establishment of DNA methylation imprint marks in germ cells.
Epigenomic landscape of gametes
We still know very little of the epigenetic landscape in gametes and whether the known imprinted genes are a special case. Recent advances in sequencing technologies, combined with our ability to tailor assays to very small numbers of cells, have enabled us to profile DNA methylation extensively and at single base-pair resolution in male and female gametes (Smallwood et al. 2011).
We are using these new data to gain insights not only into the mechanisms of DNA methylation, but to track gamete-derived methylation in early embryos, to investigate what determines whether methylation is maintained during preimplantation development and to determine what consequences it has for gene activity. Genome-wide methylation maps are vital for establishing the extent of imprinting in our genomes, and should also form an important reference for investigating epigenetic errors as a possible cause in infertility.
Figure 2: (click to enlarge)
CpG island methylation profiles for chromosome 19 from oocytes (MII) and sperm. From Smallwood et al. 2011 Dynamic CpG island methylation landscape in oocytes and preimplantation embryos. Nat. Genet. http://dx.doi.org/10.1038/ng.864
Imprinting and epigenetic influences on optimal adult health
It is thought that epigenetic modifications can be programmed by environmental factors, such as diet and early life experiences, and such changes can be perpetuated long after these exposures and potentially to future generations. Imprinted genes are a paradigm for how epigenetic events in gametes of one generation dictate gene activity in embryos of the next.
Our methylation mapping of the egg and sperm suggests that this epigenetic influence may extend well beyond the small number of known imprinted genes. Using genome-wide approaches, we are tracking the fate of gamete-derived DNA methylation, the stability of these marks throughout the lifetime and how they affect activity of associated genes.
Updated 22 August, 2011