The McLaren Lab's research focusses on the epigenetic changes that PGCs undergo both during their normal in vivo development, and also when they are cultured with growth factors appropriate for reprogramming into pluripotent EG cells. We are particularly interested in the X chromosome inactivation and reactivation effects shown by female (XX) PGCs and their epiblast precursors, and by the entry of female PGCs into meiotic prophase before birth. Our studies aim to distinguish between cell-autonomous effects programmed into the germ cells themselves, and those induced by signals from the surrounding somatic environment.

Genes subject to genomic imprinting erase their sex-specific DNA methylation during or shortly after their migration to the site of the future gonads. EG cell lines derived from male (XY) or female (XX) PGCs exhibit the appropriate sex-specific methylation pattern on imprinted genes, but studies on EG cell lines made from sex-reversed embryos (XX male, XY female) show that the methylation pattern, at least for H19, depends on the sex-chromosome constitution (XX versus XY) rather than on the sex of the embryo from which they were derived. For the germ cells, the effect of the sex-chromosome constitution is much less marked, perhaps because of the countervailing influence of the tissue environment in which they are developing.

The lab aims to investigate aspects of the tissue environment that might influence sex-specific DNA methylation of imprinted genes. We are also looking at differences and developmental potential among ES cells, EG cells derived shortly after PGC specification, and those derived later, after migration.

Migrating PGCs isolated from the hind gut, cultured in TransWells for 48 hrs, show significantly more reactivation of the silent X chromosome when genital ridge tissue is added to the culture medium than with hind gut tissue alone. The presence of genital ridge tissue appears sufficient to induce reactivation precociously in the 9.5 dpc PGCs. (Chuva de Sousa Lopes S and McLaren A, unpublished)

The sex-specific methylation of differentially methylated sites in imprinted genes is erased in PGCs during and after their migration from the base of the allantois where the germ cell lineage is established, to the site of the future gonads. It is already known that EG cells derived from PGCs during and shortly after migration show significantly less site-specific methylation than their progenitor PGCs, suggesting that the erasure process may continue in the PGCs after they have been placed in culture. We are now looking at the methylation status of EG cells made from earlier PGC stages, before migration has begun.

We have studied sex-reversed and control germ cells, and EG cell lines derived from sex-reversed and control PGCs.

Recent publications:

• Durcova-Hills G, Hajkova P, Sullivan S, Barton SC, Surani MA and McLaren A (2006) Influence of sex chromosome constitution on the genomic imprinting of germ cells. PNAS USA 103, 11184-8

• Durcova-Hills G, Adams IR, Barton SC, Surani MA and McLaren A (2006) The role of exogenous fibroblast growth factor-2 on the reprogramming of primordial germ cells into pluripotent stem cells. Stem Cells 24, 1441-1449

• Moffett A, Loke C and McLaren A (Eds) (2006) Biology & Pathology of Trophoblast, Camb Univ Press

• Ko MS and McLaren A (2006) Epigenetics of germ cells, stem cells, and early embryos. Dev Cell 10, 161-166

• Surani MA and McLaren A (2006) Stem cells: a new route to rejuvenation. Nature [News & Views] 443, 284-5

When PGCs are reprogrammed into pluripotent EG cells in culture, one of the first changes seen (A) is the loss of H3K27me3 (red spot, B), the histone modification indicative of an inactive X chromosome. After 48 hrs, most of the PGCs have lost the spot (C). (Chuva de Sousa Lopes S and McLaren A, unpublished)

Pluripotent stem cell lines can be derived from early peri-implantation embryos (ES cell lines), and also from germ cells soon after PGC specification or later in the germ cell lineage after PGC migration.

Blimp1/GFP—positive PGC precursors, each positive (as are the surrounding epiblast cells) for H3K27me (red spot), indicating an inactive X chromosome.