The Epigenetics Revolution

Occasionally this process of setting up the new imprints on the progenitors of egg or sperm can go wrong. There are cases of Angelman syndrome and Prader-Willi syndrome where the imprint has not been properly erased during the primordial germ cell stage[85]. For example, a woman may generate eggs where chromosome 15 still has the paternal mark on it that she inherited from her father, rather than the correct maternal mark. When this egg is fertilised by a sperm, both copies of chromosome 15 will function like paternal chromosomes, and create a phenotype just like uniparental disomy.

Research is ongoing into how all these processes are controlled. We don’t fully understand how imprints are protected from reprogramming following fusion of the egg and the sperm, nor how they lose this protection during the primordial germ cell stage. We’re also not entirely sure how imprints get put back on in the right place. The picture is still quite foggy, although details are starting to emerge from the haze.

Part of this may involve the small percentage of histones that are present in the sperm genome. Many of these are located at the imprinting control regions, and may protect these regions from reprogramming when the sperm and the egg fuse[86]. Histone modifications also play a role in establishing ‘new’ imprints during gamete production. It seems to be important that the imprinting control regions lose any histone modifications that are associated with switching genes on. Only then can the permanent DNA methylation be added[87]. It’s this permanent DNA methylation that marks a gene with a repressive imprint.

Dolly and her daughters

The reprogramming events in the zygote and in primordial germ cells impact on a surprising number of epigenetic phenomena. When somatic cells are reprogrammed in the laboratory using the Yamanaka factors, only a tiny percentage of them form iPS cells. Hardly any seem to be exactly the same as ES cells, the genuinely pluripotent cells from the inner cell mass of the blastocyst. A group in Boston, based at Massachusetts General Hospital and Harvard University, assessed genetically identical iPS and ES cells from mice. They looked for genes that varied in expression between the two types of cells. The only major differences in expression were in a chromosomal region known as Dlk1-Dio3[88]. A few iPS cells expressed the genes in this region in a way that was very similar to how the ES cell did this. These were the best iPS cells for forming all the different tissues of the body.

Dlk1-Dio3 is an imprinted region on chromosome 12 of the mouse. It’s perhaps not surprising that an imprinted region turned out to be so important. The Yamanaka technique triggers the reprogramming process that normally occurs when a sperm fuses with an egg. Imprinted regions of the genome are resistant to reprogramming in normal development. It is likely that they present too high a barrier to reprogramming in the very artificial environment of the Yamanaka method.

The Dlk1-Dio3 region has been of interest to researchers for quite some time. In humans, uniparental disomy in this region is associated with growth and developmental defects, amongst other symptoms[89]. This region has also been shown to be critical for the prevention of parthenogenesis, at least in mice. Researchers from Japan and South Korea genetically manipulated just this region of the genome in mice. They reconstructed a fertilised egg with two female pronuclei. The Dlk1-Dio3 region in one of the pronuclei had been altered so that it carried the equivalent of a paternal rather than maternal imprint. The live mice that were born were the first example of a placental mammal with two maternal genomes[90].

The reprogramming that occurs in the primordial germ cells isn’t completely comprehensive. It leaves the methylation on some IAP retrotransposons more or less intact. The DNA methylation level of the AxinFu retrotransposon in sperm is the same as it is in the body cells of this strain of mice. This shows that the DNA methylation was not removed when the PGCs were reprogrammed, even though most other areas of the genome did lose this modification. This resistance of the AxinFu retrotransposon to both rounds of epigenetic reprogramming (in the zygote and in the primordial germ cells) provides a mechanism for the transgenerational inheritance of the kinked tail trait that we met in earlier chapters.