This is about naming things in science. You have been warned!
The DNA is made of four nucleobases: adenine (A), thymine (T), cytosine (C) and guanine (G). The “letters” of the code. Each of them has been named from where they were originally obtained by the scientists who first identified and/or isolated them.
Adenine was named thus because it was extracted from the pancreas of an ox, which is aden in Greek (the pancreas, not the ox), by the Nobel laureate Albrecht Kossel in 1885.
Thymine comes from thymic acid, which was extracted from the thymus gland of calves by the same Albrecht Kossel and Albert Neumann in 1893.
A year later, the duo named cytosine, another base obtained from the same thymus tissue. Cyto- pertains to cells in Greek.
Fifty years before that, Julion Bodo Unger, a German chemist, extracted the guanine from the guano of sea birds. Why was he looking at bird poop, curious minds inquire? Because he was studying it for its uses as fertilizer. The year of discovery was 1844 and the year of the naming was 1846.
And now you know…
REFERENCE: Unger, JB (1846). Bemerkungen zu obiger Notiz (Comments on the above notice), Annalen der Chemie und Pharmacie, 58: 18-20. From page 20: “ … desshalb möchte ich den Namen Guanin vorschlagen, welcher an seine Herkunft erinnert.” ( “… therefore I would like to suggest the name guanine, which is reminiscent of its origin”.) (Wikipedia translation). Google Books | Google Book PDF
Even the astronauts themselves said their DNA is different and they are no longer twins:
Alas, dear Scott & Mark Kelly, rest assured that despite these titles and their afferent stories, you two share the same DNA, still & forever. You are still identical twins until one of you changes species. Because that is what 7% alteration in human DNA means: you’re not human anymore.
So what gives?
Here is the root of all this misunderstanding:
“Another interesting finding concerned what some call the “space gene”, which was alluded to in 2017. Researchers now know that 93% of Scott’s genes returned to normal after landing. However, the remaining 7% point to possible longer term changes in genes related to his immune system, DNA repair, bone formation networks, hypoxia, and hypercapnia” (excerpt from NASA’s press release on the Twin Study on Jan 31, 2018, see reference).
If I wouldn’t know any better I too would think that yes, the genes were the ones who have changed, such is NASA’s verbiage. As a matter of actual fact, it is the gene expression which changed. Remember that DNA makes RNA and RNA makes protein? That’s the central dogma of molecular biology. A sequence of DNA that codes for a protein is called a gene. Those sequences do not change. But when to make a protein, how much protein, in what way, where to make this protein, which subtly different kinds of protein to make (alternative splicing), when not to make that protein, etc. is called the expression of that gene. And any of these aspects of gene expression are controlled or influenced by a whole variety of factors, some of these factors being environmental and as drastic as going to space or as insignificant as going to bed.
Now, I’d love, LOVE, I tell you, to jump to the throat of the media on this one so I can smugly show how superior my meager blog is when it comes to accuracy. But, I have to admit, this time is NASA’s fault. Although it is not NASA’s job to teach the central dogma of molecular biology to the media, they are, nonetheless, responsible for their own press releases. In this case, Monica Edwards and Laurie Abadie from NASA Human Research Strategic Communications did a booboo, in the words of the Sit-Com character Sheldon Cooper. Luckily for these two employees, the editor Timothy Gushanas published this little treat yesterday, right at the top of the press release:
“Editor’s note: NASA issued the following statement updating this article on March 15, 2018:
Mark and Scott Kelly are still identical twins; Scott’s DNA did not fundamentally change. What researchers did observe are changes in gene expression, which is how your body reacts to your environment. This likely is within the range for humans under stress, such as mountain climbing or SCUBA diving.
The change related to only 7 percent of the gene expression that changed during spaceflight that had not returned to preflight after six months on Earth. This change of gene expression is very minimal. We are at the beginning of our understanding of how spaceflight affects the molecular level of the human body. NASA and the other researchers collaborating on these studies expect to announce more comprehensive results on the twins studies this summer.”
But, seriously, NASA, what’s up with you guys keep screwing up molecular biology stuff?! Remember the arsenic-loving bacteria debacle? That paper is still not retracted and that press release is still up on your website! Ntz, ntz, for shame… NASA, you need better understanding of basic science and/or better #Scicomm in your press releases. Hiring? I’m offering!
P.S. Sometimes is a pain to be obsessed with accuracy (cue in smallest violins). For example, I cannot stop myself from adding something just to be scrupulously correct. Since the day they were conceived, identical twins’ DNAs are starting to diverge. There are all sorts of things that do change the actual sequence of DNA. DNA can be damaged by radiation (which you can get a lot of in space) or exposure to some chemicals. Other changes are simply due to random mutations. So no twins are exactly identical, but the changes are so minuscule, nowhere near 1%, let alone 7%, that it is safe to say that their DNA is identical.
P.P.S. With all this hullabaloo about the 7% DNA change everybody glossed over and even I forgot to mention the one finding that is truly weird: the elongation of telomeres for Scott, the one that was in space. Telomeres are interesting things, they are repetitive sequences of DNA (TTAGGG/AATCCC) at the end of the chromosomes that are repeated thousands of times. The telomere’s job is to protect the end of the chromosomes. You see, every time a cell divides the DNA copying machinery cannot copy the last bits of the chromosome (blame it on physics or chemistry, one of them things) and so some of it is lost. So evolution came up with a solution: telomeres, bits of unusable DNA that can be safely ignored and left behind. Or so we think at the moment. The length of telomeres has been implicated in some curious things, like cancer and life-span (immortality thoughts, anyone?). The most common finding is the shortening of telomeres associated with stress, but Scott’s were elongated, so that’s the first weird thing. I didn’t even know the telomeres can get elongated in living humans. But wait, there is more: NASA said that “the majority of those telomeres shortened within two days of Scott’s return to Earth”. Now that is the second oddest thing! If I would be NASA that’s where I would put my money on, not on the gene expression patterns.
I’m interrupting the series on cognitive biases (unskilled-and-unaware, superiority illusion, and depressive realism) to tell you that I admit it, I’m old. -Ish. Well, ok, I’m not that old. But this following paper made me feel that old. Because it invalidates some stuff I thought I knew about molecular cell biology. Mind totally blown.
It all started with a paper freshly published two days ago and that I’ll cover tomorrow. It’s about what the title says: mRNA can travel between cells packaged nicely in vesicles and once in a target cell can be made into protein there. I’ll explain – briefly! – why this is such a mind-blowing thing.
We’ll start with the central dogma of molecular biology (specialists, please bear with me): the DNA is transcribed into RNA and the RNA is translated into protein (see Fig. 1). It is an oversimplification of the complexity of information flow in a biological system, but it’ll do for our purposes.
DNA needs to be transcribed into RNA because RNA is a much more flexible molecule and thus can do many things. So RNA is the traveling mule between DNA and the place where its information becomes protein, i.e. ribosome. Hence the name mRNA. Just kidding; m stands for messenger RNA (not that I will ever be able to call that ever again: muleRNA is stuck in my brain now).
There are many kinds of RNA: some don’t even get out of the nucleus, some are chopped and re-glued (alternative splicing), some decide which bits of DNA (genes) are to be expressed, some are busy housekeepers and so on. Once an RNA has finished its business it is degraded in many inventive ways. It cannot leave the cell because it cannot cross the cell membrane. And that was that. Or so I’ve been taught.
Exceptions from the above were viruses whose ways of going from cell to cell are very clever. A virus is a stretch of nucleic acids (DNA and/or RNA) and some proteins encapsulated in a blob (capsid). Not a cell!
In the ’90s several groups were looking at some blobs (yes, most stuff in biology can be defined by the all-encompassing and enlightening term of ‘blob’) that cells spew out every now and then. These were termed extracellular vesicles (EV) for obvious reasons. Turned out that many kinds of cells were doing it and on a much more regular basis than previously thought. The contents of these EVs varied quite a bit, based on the type of cells studied. Proteins, mostly, and maybe some cytoplasmic debris. In the ’80s it was thought that this was one way for a cell to get rid of trash. But in 1982, Stegmayr & Ronquist showed that prostate cells release some EVs that result in sperm cell motility increase (Raposo & Stoorvogel, 2013) so, clearly, the EVs were more than trash. Soon it became evident that EVs were another way of cell-to-cell communication. (Note to self: the first time intercellular communication by EVs was demonstrated was in 1982, Stegmayr & Ronquist. Maybe I’ll dig out the paper to cover it sometime).
So. In 2005, Baj-Krzyworzeka et al. (2006) looked at some human cancer cells to see what they spew out and for what purpose. They saw that the cancer cells were transferring some of the tumor proteins packaged in EVs to monocytes. For devious purposes, probably. And then they made to what it looks to me like a serious leap in reasoning: since the EVs contain tumor proteins, why wouldn’t they also contain the mRNA for those proteins? My first answer to that would have been: “because it would be rapidly degraded”. And I would have been wrong. To my credit, if the experiment wouldn’t take up too many resources I still would have done it, especially if I would have some random primers lying around the lab. Luckily for the world, I was not in charge with this particular experiment and Baj-Krzyworzeka et al. (2005) proceeded with a real-time PCR (polymerase chain reaction) which showed them that the EVs released by the tumor cells also contained mRNA.
Now the 1 million dollar, stare-in-your-face question was: is this mRNA functional? Meaning, once delivered to the host cell, would it be translated into protein?
Six months later the group answered it. Ratajcza et al. (2006) used embryonic stem cells as the donor cells and hematopoietic progenitor cells as host cells. First, they found out that if you let the donors spit EVs at the hosts, the hosts are faring much better (better survival, upregulated good genes, phosphorylated MAPK to induce proliferation etc.). Next, they looked at the contents of EVs and found out that they contained proteins and mRNA that promote those good things (Wnt-3 protein, mRNA for transcription factors etc.). Next, to make sure that the host cells don’t show this enrichment all of a sudden out of the goodness of their little pluripotent hearts but is instead due to the mRNA from the donor cells, the authors looked at the expression of one of the transcription factors (Oct-4) in the hosts. They used as host a cell line (SKL) that does not express the pluripotent marker Oct-4. So if the hosts express this protein, it must have come only from outside. Lo and behold, they did. This means that the mRNA carried by the EVs is functional (Fig. 2).
What bugs me is that these papers came out in a period where I was doing some heavy reading. How did I miss this?! Probably because they were published in cancer journals, not my field. But this is big enough you’d think others would mention it. (If you’re a recurrent reader of my blog, by now you should be familiarized with my stream-of-consciousness writing and my admittedly sometimes annoying in-parenthesis-meta-cognitions :D). So how did I miss this? How many more great discoveries have I missed? Am I the only one to discover such fundamental gaps in my knowledge? And thus the imposter syndrome takes root.
Just kidding, I don’t have the imposter syndrome. If anything, I got a superiority illusion complex. And I am absolutely sure that many, many scientists read things they consider fundamental to their way of thinking about the world all the time and wonder what other truly great discoveries are out there already that they missed.
Frankly, I should probably be grateful to this blog – and my friend GT who made me do it – because without nosing outside my field in search of material for it I would have probably remained ignorant of this awesome discovery. So, even if this is a decade old discovery for you, for me is one day old and I am a bit giddy about it.
This is a big deal because of the theoretical implications: a cell’s transcriptome (all the mRNA expressed in a cell) varies not only due to its needs, activity, and experiences, but also due to its neighbors’! A cell is, more or less, its transcriptome. Soooo… if we can change that at will, does that means we can change the type or function of the cell too? There are so many questions that such a discovery raises! And possibilities.
This is also a big deal because it opens up not a new therapy, or a new therapy direction, or a new drug class, but a new DELIVERY METHOD, the Holy Grail of Pharmacopeia. You just put your drug in one of these vesicles and let nature take its course. Of course, there are all sorts of roadblocks to overcome, like specificity, toxicity, etc. Looks like some are already conquered as there are several clinical trials out there that take advantage of this mechanism and I bet there will be more.
Stop by tomorrow for a freshly published paper on this mechanism in neurons.
Nathan Lo is an evolutionary biologist interested in creepy crawlies, i.e. arthropods. Well, he’s Australian, so I guess that comes with the territory (see what I did there?). While postdoc’ing, he and his colleagues published a paper (Sassera et al., 2006) that would seem boring for anybody without an interest in taxonomy, a truly under-appreciated field.
The paper describes a bacterium that is a parasite for the mitochondria of a tick species called Ixodes ricinus, the nasty bugger responsible for Lyme disease. The authors obtained a female tick from Berlin, Germany and let it feed on a hamster until it laid eggs. By using genetic sequencing (you can use kits these days to extract the DNA, do PCR, gels and cloning, pretty much everything), electron microscopy (real powerful microscopes) and phylogenetic analysis (using computer softwares to see how closely related some species are) the authors came to the conclusion that this parasite they were working on is a new species. So they named it. And below is the full account of the naming, from the horse’s mouth, as it were:
“In accordance with the guidelines of the International Committee of Systematic Bacteriology, unculturable bacteria should be classified as Candidatus (Murray & Stackebrandt, 1995). Thus we propose the name ‘Candidatus Midichloria mitochondrii’ for the novel bacterium. The genus name Midichloria (mi.di.chlo′ria. N.L. fem. n.) is derived from the midichlorians, organisms within the fictional Star Wars universe. Midichlorians are microscopic symbionts that reside within the cells of living things and ‘‘communicate with the Force’’. Star Wars creator George Lucas stated that the idea of the midichlorians is based on endosymbiotic theory. The word ‘midichlorian’ appears to be a blend of the words mitochondrion and chloroplast. The specific epithet, mitochondrii (mi.to′chon.drii. N.L. n. mitochondrium -i a mitochondrion; N.L. gen. n. mitochondrii of a mitochondrion), refers to the unique intramitochondrial lifestyle of this bacterium. ‘Candidatus M. mitochondrii’ belongs to the phylum Proteobacteria, to the class Alphaproteobacteria and to the order Rickettsiales. ‘Candidatus M. mitochondrii’ is assigned on the basis of the 16S rRNA (AJ566640) and gyrB gene sequences (AM159536)” (p. 2539).
George Lucas gave his blessing to the Christening (of course he did).
Acknowledgements: Thanks go to Ms. BBD who prevented me from making a fool of myself (this time!) on the social media by pointing out to me that midichloria are real and that they are a mitochondrial parasite.
Polymerases are enzymes that synthesize nucleic acids. The main types of polymerases are DNA polymerases and RNA polymerases. Everything alive has them. Saying that you cannot have cellular life on Earth without them is like saying you cannot have a skeleton without bones.
The first polymerase was discovered by Arthur Kornberg in 1956. Of note, his (and his two postdocs and lab technician) discovery was rejected for publication by The Journal of Biological Chemistry basically on the grounds that they don’t know what they’re talking about or they’re not qualified to talk about it. It took a new Editor-in-Chief to push the publication which finally appeared in the July 1958 issue. Talk about politicking in academia…
Anyway, less than a year since publication, in 1959, Kornberg (but not his co-authors) received the Nobel Prize for the discovery of the polymerase. Which he isolated from a bug called E. Coli, the same bacterium that can be found in your intestines and poop or can give you food poisoning (same species, but not necessarily the same strain).
Reference: Lehman IR, Bessman MJ, Simms ES, & Kornberg A (July 1958). Enzymatic Synthesis of Deoxyribonucleic Acid. I. Preparation of Substrates and Partial Purification of an enzyme from Escherichia Coli.The Journal of Biological Chemistry, 233:163-170. FREE FULLTEXT PDF|2005 JBC Centennial Cover
Transgenerational epigenetic inheritance (TGI) refers to the inheritance of a trait from one generation to another without altering the DNA code (normally, evolution is driven by changes in the DNA itself). Instead, it happens by modifying the proteins that wrap around the DNA, the histones; these histones, in turn, control what genes will be expressed and when. Until a decade ago, TGI was considered impossible, nay, a scientific heresy since it had too close of a resemblance to Lamarckian evolution. But, true to its guiding principles, the scientific endeavor had to bite the bullet in front of amassing evidence and accept the fact that it may have been a kernel of truth to the so called ‘soft inheritance’.
Anway et al.’s paper was one of the first to promote the concept, ten years ago. They exposed pregnant rats to the pesticides vinclozolin or methoxychlor (only vinclozolin is still used widely in U.S.A. and several EU countries, particularly in agriculture, wine production, and turf maintenance; methoxychlor was banned in the early noughts). The authors found out that more than 90% of the male offspring had “increased incidence of male infertility”. These effects were transferred through the male germ line to nearly all males of all subsequent generations examined” up to great great grandsons, inclusively (Anway et al., 2005). (I don’t want to speculate how they managed to breed the low fertility males…). That doesn’t mean that the F5 generation was OK (the great great great gransons); it means that they stopped investigating after the F4 generation (or they couldn’t breed the F4s). Moreover, the mechanism of inheritance seems to be altered methylation of the DNA histones of the male germline, and not alteration of the DNA itself. Females were affected too, but they didn’t have enough data on that experiment (the Ph.D. student that did the work had to graduate sometime…).
Although the authors used higher amounts of pesticides than they suspected back then, in 2005, to be found in the environment, the study still gives pause for thought. After all, it has been 10 years since this paper plus the previous 20 years of use of the stuff. And no, you cannot get rid of it by washing your grapes and vegetables really thoroughly.
Reference: Anway, M. D., Cupp, A. S., Uzumcu, M., & Skinner, M. K. (3 June 2005). Epigenetic transgenerational actions of endocrine disruptors and male fertility. Science, 308(5727): 1466-1469. DOI:10.1126/science.1108190. Article + Science Cover + FREE PDF