Today lets talk about what happens to the DNA/RNA that enters our bodies from things that we eat!

This post will predominantly focus on work published in the journal RNA on February 25, 2017 titled "Survey of 800+ datasets from human tissue and body fluid reveals XenomiRs are likely artifacts."

As I am certain we are all aware, DNA is transcribed into RNA which is translated into a Protein. Proteins are the molecular machines of life and are responsible for allowing the necessary chemical reactions happen in our cells that are required for the cells to live.

When ever you eat many foods, you are eating a part of another living organism (whether it be a plant, or an animal). These organisms all have genetic material, so they all have these sets of instructions for creating proteins that do a multitude of functions.

Why doesn't that genetic material start producing proteins in our bodies?

It's a good question, because there isn't anything particularly special about another organisms gene sequences, from our own (I mean the sequences are different, but our DNA replication and protein production machinery doesn't care where the DNA comes from, it just sees it and performs its function). It is this aspect of our bodies, and life in general, that allows scientists to produce proteins from various organisms in bacteria in order to study the proteins function. The bacterial machinery don't care that the gene sequence didn't come from a bacteria, they treat the DNA the same.

There are some epigenetic means by which self vs. non self DNA can be discriminated for a cell, which involves methylation patterns on the DNA but this only discriminates against DNA that is wildly different, like bacterial vs eukaryotic. This is a subject for a different post where I talk more about epigenetics (part 2 of a series I have started, but keep getting side tracked into writing other things), I will at some point publish this piece. :D

There were some articles published studying a type of RNA called miRNA

What is miRNA

miRNA or microRNA is a short usually 22 nucleotide long single stranded RNA piece that functions as a part of an RNA silencing complex.^[2]

The miRNA complex goes over to RNA transcripts and does this...well not really

What this means is there are proteins in the body whos job is to use these miRNAs to bind to other RNA transcripts and chew them up so the transcripts can not form proteins. That pathway looks like this:

Source

MicroRNAs are chopped out of naturally made RNA transcripts, and are bound to proteins which cells can then use to chop up what ever RNAs the microRNA binds to. Our bodies and cells naturally use this pathway to control how much of our own proteins are expressed (as the cell is a very fine tuned machine and needs specific amounts of things, so these miRNAs serve as a regulatory mechanism). However every other organism that becomes our food also uses these miRNAs. Do these miRNAs get absorbed into our bodies? If they do, can they result in a silencing function for proteins in our cells, that we do not want?

This is the result that a few research groups had reported

Exogenous miRNAs (ones from outside the body) or as they were terming it XenomiRNAs (alien miRNAs) were observed to be present in blood samples. One study reported that a miRNA called MIR168a, which is prominently found in rice, could be absorbed in our guts and enter our blood stream (in people with diets high in rice content). ^[3] Upon entering the bloodstream the researchers observed that this particular miRNA would cause the RNA transcript of a human protein involved in cholesterol shuttling to get chopped up. This is a pretty huge deal (and potentially very troubling), as it implied that these miRNAs from foreign sources could have an impact on the expression levels of proteins in our bodies.

Some other studies attempted to replicate what these researchers reported, and artificially induce it in animal models. However they were not successful! ^[4] This lead to people questioning... okay, are these miRNAs really able to be absorbed, or were those first researchers just reporting based on contaminated samples?

Researchers decided to look more into this, and to take advantage of recent advances in DNA sequencing

What Did They Look At?

There are databases where sequencing data has been deposited for anyone to look at and re-analyze. One such type of data-set deposited in these databases is a complete description of all of the various RNA transcripts present in a sample. Researchers used this data from 824 different samples and tried to explore for these XenomiRNAs in the samples. AKA they were looking for whether foreign RNA was present in human RNA sequencing data when it shouldn't be. As if this were a pervasive phenomenon they should be all over the place!

What Did They Find?

XenomiRNAs are absent in brain, liver and blood cells

The authors reported that xenomiRNAs were absent from most human tissue samples. In the samples where they are present, they are very lowly abundant.

XenomiRNAs ARE present in bodily fluids

The "foreign RNAs" were present in 69% of samples, and the most common sources for the foreign RNAs were from rodents, dicots (flowering plants) and insects. The researchers noted that these foreign RNAs were present in most of the studied bodily fluids, although at very low levels.

Most XenomiRNAs originate from sources unlikely to be food

They saw a disproportionately high amount of these foreign RNA's from insect sources, and a disproportionately low amount from plants, or common human meats (birds, fish). They concluded that the composition they saw does not reflect what humans eat (and if it were coming from food sources it should).

Controlled feeding of rats does not change the XenomiRNA composition found present in them

The researchers suspected that if the diets of rats were controlled to favor certain foods, that if miRNA's were being absorbed into the rats bodies then they should see disproportionately high amounts from the foods that they are eating. However no such result was observed, serving as evidence against the reported results discussed earlier.

Authors Conclusions

The authors concluded by stating that it is unlikely that transfer of dietary miRNAs from foods into human bodies occurs. They instead suggest that the detection of these miRNAs in previous studies (and in their own analysis here) is due to the sampling techniques employed when originally obtaining the data sets. AKA the miRNAs detected by previous papers are likely to be the result of contaminating their samples.

Answering The Original Question

Why doesn't foreign DNA start producing proteins and such in our bodies after we eat it? Because it just isn't absorbed in the first place. At least thats what this article argues in favor of.

I am certain, considering the potential ramifications of the other published works, that studies on this will continue, and a lot more than 800ish data sets will be looked through to further solidify whether or not foreign DNA can effect our own bodily functioning. At this point however, the evidence seems fairly strong that the answer is in fact NOPE.

Sources

1. http://rnajournal.cshlp.org/content/early/2017/01/06/rna.059725.116.full.pdf
2. https://en.wikipedia.org/wiki/MicroRNA
3. http://www.nature.com/cr/journal/v22/n1/full/cr2011158a.html
4. https://www.ncbi.nlm.nih.gov/pubmed/23669076

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