Blinded by Science blog 11: What, if anything, does the father contribute to the epigenome?

Posting the last entry, I realized that I’ve had this blog for over a year.  That’s kinda crazy to think about.  Now I’m not one for introspection . . . okay, that’s a ridiculous lie.  I partake of introspection quite a bit, though it is debatable whether I am able to glean any insights into myself from it.  Wait.  Does that count?  You’ll allow it?  Sweet!

Anyway, it is rather remarkable the differences a year can make.  New city.  New state.  New job.  And yet, some things haven’t changed.  I still really enjoy doing this, communicating science to others.  Hopefully, I am able to continue this with the same level of enjoyment.  And as always, for those of you that read the blog and/or contribute questions, thank you!

Now enough sappiness.  On to the question!

Okay, just a little more sap.

Chris H asks, “Epigenetics is quite the buzz-word of late. What, if anything, does the father contribute to the epigenome? Is it a percentage? Or certain things?”

Yay!  Another biology question!  I really enjoy these.

Before I begin, let’s do a little recap.  Remember how I earlier gave a brief explanation of genetics?  Basically, DNA holds a person’s genetic information; the information is converted to RNA and then translated into proteins which do things for the cell, lots of things.  This whole process, from DNA to RNA to protein to beyond, is very tightly controlled and regulated.  In fact, for most genes, the most important form of control is transcriptional control, which deals with the conversion of the information from DNA to RNA.  This makes sense.  Controlling at this step ensures that the cell does not create unnecessary, and therefore wasteful, intermediates.

One of the methods of transcriptional control is at the level of chromatin structure.  Chromatin is the combination of DNA with proteins and other macromolecules.  It not only protects your DNA but also condenses it so that it can fit inside the nucleus of each of your cells.*  But condensing the DNA enough to fit in the nucleus makes it difficult for the proteins that transcribe the RNA from DNA to read the code and do their job.  So the cell has to loosen the chromatin around genes when they need to be “turned on” and transcribed.  In addition, not all cells need every gene product the genome codes for.  Muscle cells don’t need to make proteins necessary for bone cells, and red blood cells would have problems if they produced the adhesion proteins that keep intestinal cells held to each other without leaking.  So body cells need a way to keep some genes “turned off”.  So the cell will add chemical modifications to the chromatin proteins which cause the structure to loosen (allowing for more gene transcription) or cause the structure to tighten (limiting gene transcription).

Another type of transcriptional control occurs when the chemical structure of the DNA itself is modified.  One of the four DNA bases, cytosine, has a methyl group (a carbon atom that is bonded to three hydrogen atoms) added to it to become 5-Methylcytosine.  It still pairs to guanine, but regions that contain high amounts of this modified base are less active when it comes to transcription.

It is important to understand that neither of these types of control change the underlying DNA code, they merely change the behavior of genes, allowing some to be transcribed more while others are transcribed less.

But what do these transcriptional control mechanisms have to do with the question at hand?  Epigenetics is the study of external or environmental factors that turn genes 'on' and 'off' and affect how cells 'read' genes.  And an epigenetic trait is a “stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence”.  So these two transcriptional control mechanisms are examples of epigenetic modifications that occur in a chromosome.

Additionally, due to their nature and purpose, these changes tend to be inherited by the daughter cells after a cell divides.  Amongst other things, this allows a cell to keep certain genes turned off throughout its descendants, which can function as another layer of protection so muscle cells do not produce proteins that are only necessary for bone cells or brain cells produce proteins that are only necessary for liver cells.

But with regards to the question, do these epigenetic markers survive into the next generation?  And the answer is . . . drum roll please . . .

Eh.  Close enough.

Nope.  They do not.  Except when they do.

This is biology we’re talking about.  Exceptions abound.

Normally, after a sperm and an egg meet and fertilization occurs, the epigenetic markers are wiped clean.  Remember that epigenetic markers help silence genes in one cell type that are only used by a different cell type.  So the markers must be removed so that all the different cell types can develop properly (the markers will be replaced as cellular differentiation occurs).  In mammals, it is thought that this process of removal occurs very early in life, just after fertilization.

But some genes keep their epigenetic markers after fertilization.  Imprinted genes are able to keep their epigenetic markers past fertilization (though they are not the only type of genes that can).  Imprinted genes can be inherited from either the father or the mother.  It is currently estimated that about 1% of mammalian genes are imprinted.

As for how much the father contributes to the epigenetic state of the offspring, it is not currently known.  Epigenetics is still a relatively young field of study.  However, it is known that the father does contribute at least some epigenetic markers so it isn’t zero.  My guess (and it is completely a guess since this is not my field of study and I couldn’t find anything about it while researching for this post) is that the father and the mother each contribute roughly the same amount to what little epigenetic markers survive being wiped clean after fertilization.

*It is estimated that each cell in the human body has about 2 meters of DNA in the nucleus.  It is also estimated that there are about 50 trillion cells in the human body.  So there is about 100 trillion meters of DNA in your body, enough (as this source mentions) to make it to the Sun and back more than three hundred time!