The Complex Exogenous RNA Spectra in Human Plasma: An Interface with Human Gut Biota?

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Published December 10, 2012 in PLOS ONE

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Molecules from foreign organisms, like bacteria, discovered in normal human blood


Human blood contains a wide range of molecules in the fluid outside the cells that carry oxygen and fight infection. This fluid, called plasma, contains proteins and fats, or lipids, sugars and many other molecules, acting as nutrients, or hormones or with other functions. It has recently become clear from several studies that plasma contains RNA molecules as well. Researchers have previously identified many RNA molecules, including small microRNAs, that are involved in regulation of gene expression in cells.

David Galas' laboratory, and their collaborators in the Luxembourg lab of Paul Wilmes, made the surprising observation that a significant fraction of the circulating RNA molecules in the blood plasma appear to be copies that originate, not from the human DNA, the human genome, but rather originate from other species entirely. 

Using new sequencing technologies, and with careful analysis of possible technical errors, sequence error statistics and many other kinds of controls, they demonstrated that there is a wide range of RNA from many different organisms. These include bacteria and fungi that appear to be the ones that colonize the human body (often called the human microbiome), like the bacteria in our digestive system. In addition to these RNA that come from our microbiome, the researchers found RNA molecules from plants that are common as food items, corn, wheat, rice, etc.   

So, the RNA content of the blood includes a wide range of RNA of unexpected origin. These findings, that foreign RNA is found in normal human plasma and includes molecules from our own bacteria, raises the possibility that some of these RNAs may act as signaling molecules (like hormones) mediating, for example, some kind of human-microbiome interaction and may affect and indicate important things about the overall state of human health. 

The researchers at both places, PNDRI and LCSB in Luxembourg, are continuing their research on this exciting new discovery.