Reprogramming of DNA observed in human germ cells for first time

June 4, 2015
This stylistic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). The chromosome is X-shaped because it is dividing. Introns are regions often found in eukaryote genes that are removed in the splicing process (after the DNA is transcribed into RNA): Only the exons encode the protein. The diagram labels a region of only 55 or so bases as a gene. In reality, most genes are hundreds of times longer. Credit: Thomas Splettstoesser/Wikipedia/CC BY-SA 4.0

A team of researchers led by the University of Cambridge has described for the first time in humans how the epigenome - the suite of molecules attached to our DNA that switch our genes on and off - is comprehensively erased in early primordial germ cells prior to the generation of egg and sperm. However, the study, published today in the journal Cell, shows some regions of our DNA - including those associated with conditions such as obesity and schizophrenia - resist complete reprogramming.

Although our genetic information - the 'code of life' - is written in our DNA, our genes are turned on and off by epigenetic 'switches'. For example, small molecules attach to our DNA in a process known as methylation and contribute to the regulation of gene activity, which is important for normal development. Methylation may also occur spontaneously or through our interaction with the environment - for example, periods of famine can lead to methylation of certain genes - and some methylation patterns can be potentially damaging to our health. Almost all of this epigenetic information is, however, erased in prior to transmission to the next generation

Professor Azim Surani from the Wellcome Trust/Cancer Research UK Gurdon Institute at the University of Cambridge, explains: "Epigenetic information is important for regulating our genes, but any abnormal methylation, if passed down from generation to generation, may accumulate and be detrimental to offspring. For this reason, the information needs to be reset in every generation before further information is added to regulate development of a newly fertilised egg. It's like erasing a computer disk before you add new data."

When an egg cell is fertilised by a sperm, it begins to divide into a cluster of cells known as a blastocyst, the early stage of the embryo. Within the blastocyst, some cells are reset to their master state, becoming , which have the potential to develop into any type of cell within the body. A small number of these cells become primordial germ cells with the potential to become sperm or .

In a study funded primarily by the Wellcome Trust, Professor Surani and colleagues showed that a process of reprogramming the epigenetic information contained in these is initiated around two weeks into the embryo's development and continues through to around week nine. During this period, a genetic network acts to inhibit the enzymes that maintain or programme the epigenome until the DNA is almost clear of its methylation patterns.

Crucially, however, the researchers found that this process does not clear the entire epigenome: around 5% of our DNA appears resistant to reprogramming. These 'escapee' regions of the genome contain some genes that are particularly active in neuronal cells, which may serve important functions during development. However, data analysis of human diseases suggests that such genes are associated with conditions such as schizophrenia, metabolic disorders and obesity.

Walfred Tang, a PhD student who is the first author on the study, adds: "Our study has given us a good resource of potential candidates of regions of the genome where epigenetic information is passed down not just to the next generation but potentially to future generations, too. We know that some of these regions are the same in mice, too, which may provide us with the opportunity to study their function in greater detail."

Epigenetic reprogramming also has potential consequences for the so-called 'dark matter' within our genome. As much as half of human DNA is estimated to be comprised of 'retroelements', regions of DNA that have entered our genome from foreign invaders including bacteria and plant DNA. Some of these regions can be beneficial and even drive evolution - for example, some of the genes important to the development of the human placenta started life as invaders. However, others can have a potentially detrimental effect - particularly if they jump about within our DNA, potentially interfering with our genes. For this reason, our bodies employ methylation as a defence mechanism to suppress the activity of these retroelements.

"Methlyation is effective at controlling potentially harmful retroelements that might harm us, but if, as we've seen, methylation patterns are erased in our germ cells, we could potentially lose the first line of our defence," says Professor Surani.

In fact, the researchers found that a notable fraction of the retroelements in our genome are 'escapees' and retain their - particularly those retroelements that have entered our genome in our more recent evolutionary history. This suggests that our body's defence mechanism may be keeping some epigenetic information intact to protect us from potentially detrimental effects.

Explore further: Egg and sperm race: Scientists create precursors to human egg and sperm in the lab

More information: Tang, WWC et al. A unique gene regulatory network resets the human germline epigenome for development. Cell; 4 June 2015

Related Stories

Egg and sperm race: Scientists create precursors to human egg and sperm in the lab

December 24, 2014
Scientists at the University of Cambridge working with the Weizmann Institute have created primordial germ cells - cells that will go on to become egg and sperm - using human embryonic stem cells. Although this had already ...

Epigenetic reprogramming: Research discovers how epigenetic information could be inherited

January 24, 2013
New research reveals a potential way for how parents' experiences could be passed to their offspring's genes. The research was published today, 25 January, in the journal Science.

Scientists reveal epigenome maps of the human body's major organs

June 1, 2015
For more than a decade, scientists have had a working map of the human genome, a complete picture of the DNA sequence that encodes human life. But new pages are still being added to that atlas: maps of chemical markers called ...

Homing in on developmental epigenetics

August 23, 2013
Germ cells have unique molecular features that enable them to perform the important task of transmitting genetic information to the next generation. During development from their embryonic primordial state, germ cells are ...

Inherited 'memory' of environmental impact on health may be limited

July 10, 2014
When a pregnant mother is undernourished, her child is at a greater than average risk of developing obesity and type 2 diabetes, in part due to so-called 'epigenetic' effects. A new study in mice demonstrates that this 'memory' ...

Genetic guides to epigenetics

February 10, 2015
Dirk Schübeler and his group at the Friedrich Miescher Institute for Biomedical Research (FMI) identify determinants that set epigenetic marks along the genome. The new study, published in Nature, shows that genetic activity ...

Recommended for you

Genome editing reveals role of gene important for human embryo development

September 20, 2017
Researchers have used genome editing technology to reveal the role of a key gene in human embryos in the first few days of development. This is the first time that genome editing has been used to study gene function in human ...

A piece of the puzzle: Eight autism-related mutations in one gene

September 19, 2017
Scientists have identified a hotspot for autism-related mutations in a single gene.

Scientists identify key regulator of male fertility

September 19, 2017
When it comes to male reproductive fertility, timing is everything. Now scientists are finding new details on how disruption of this timing may contribute to male infertility or congenital illness.

New assay leads to step toward gene therapy for deaf patients

September 18, 2017
Scientists at Oregon State University have taken an important step toward gene therapy for deaf patients by developing a way to better study a large protein essential for hearing and finding a truncated version of it.

Biologists identify gene involved in kidney-related birth defects

September 18, 2017
A team led by University of Iowa researchers has identified a gene linked to rare, often fatal kidney-related birth defects.

Genomic recycling: Ancestral genes take on new roles

September 18, 2017
One often hears about the multitude of genes we have in common with chimps, birds or other living creatures, but such comparisons are sometimes misleading. The shared percentage usually refers only to genes that encode instructions ...


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (1) Jun 04, 2015
Journal article excerpt:
Methylation at alternative promoters or splice sites can affect transcript variants expression, whereas enhancer methylation may modulate gene expression (Jones, 2012). These are potential mechanisms for translating DNA methylation information to phenotypes.

The conserved molecular mechanisms are RNA-mediated and they link DNA methylation to RNA-mediated amino acid substitutions that differentiate all cell types in all individuals of all genera.

See: From Fertilization to Adult Sexual Behavior http://www.hawaii...ion.html
Small intranuclear proteins also participate in generating alternative splicing techniques of pre-mRNA and, by this mechanism, contribute to sexual differentiation....

See also: Nutrient-dependent/pheromone-controlled adaptive evolution: a model.

Cell type differentiation is typically biophysically constrained.
not rated yet Jun 04, 2015
See also: Nutrient-dependent/pheromone-controlled adaptive evolution: a model.

See in response:

"Cell type differentiation is typically biophysically constrained."

The real science of cell differntiation

"That's not what cellular differentiation is. It's the developmental process by which stem cells in a multicellular organism specialize."


Read more at:

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.