Forgotten but not gone

Life is said to have originated in the RNA world.  We all know about the big 3 important RNAs for the cell, mRNA, ribosomal RNA and transfer RNA.  But just like the water, sewer, power and subway systems under Manhattan, there is another world down there in the cell which doesn’t much get talked about.  These are RNAs, whose primary (and possibly only) function is to interact with other RNAs.

Start with microRNAs (of which we have at least 1,500 as of 12/12).  Their function is to bind to messenger RNA (mRNA) and inhibit translation of the mRNA into protein.  The effects aren’t huge, but they are a more subtle control of protein expression, than the degree of transcription of the gene.

Then there are ceRNAs (competitive endogenous RNAs) which have a large number of binding sites for microRNAs — humans have a variety of them all with horrible acronyms — HULC, PTCSC3 etc. etc. They act as sponges for microRNAs keeping them bound and quiet.

Then there are circular RNAs.  They’d been missed until recently, because typical RNA sequencing methods isolate only RNAs with characteristic tails, and a circular RNA doesn’t have any.  One such is called CiRS7/CDR1) which contain 70 binding sites for one particular microRNA (miR-7).  They are unlike to be trivial.  They are derived from 15% of actively transcribed genes.  They ‘can be’ 10 times as numerous as linear RNAs (like mRNA and everything else) — probably because they are hard to degrade < Science vol. 340 pp. 440 – 441 ’17 >. So some of them are certainly RNA sponges — but all of them?

The latest, and most interesting class are the nonCoding RNAs found in viruses. Some of them function to attack cellular microRNAs and help the virus survive. Herpesvirus saimiri a gamma-herpes virus establishes latency in the T lymphocytes of New World primates, by expressing 7 small nuclear uracil-rich nonCoding RNAs (called HSURs).  They associate with some microRNAs, and rather than blocking their function act as chaperones < Nature vol. 550 pp. 275 – 279 ’17 >.  They HSURs also bind to some mRNAs inhibiting their function — they do this by helping miR-16 bind to their targets — so they are chaperones.  So viral Sm-class RNAs may function as microRNA adaptors.

Do you think for one minute, that the cell isn’t doing something like this.

I have a tendency to think of RNAs as always binding to other RNAs by classic Watson Crick base pairing — this is wrong as a look at any transfer RNA structure will show. https://en.wikipedia.org/wiki/Transfer_RNA.  Far more complicated structures may be involved, but we’ve barely started to look.

Then there are the pseudogenes, which may also have a function, which is to be transcribed and sop up microRNAs and other things — I’ve already written about this — https://luysii.wordpress.com/2010/07/14/junk-dna-that-isnt-and-why-chemistry-isnt-enough/.  Breast cancer cells think one (PTEN1) is important enough to stop it from being transcribed, even though it can’t be translated into protein.

Les fleurs du PTEN

Les fleurs du Mal is a volume of poetry by Baudelaire about the beauty of evil and depravity. I have the same esthetic appreciation for the horrible things a mutant of PTEN does. It’s awful, but incredibly elegant chemically.

Back in the day med students used to be told ‘know syphylis and you’ll know medicine’ because of its varied clinical manifestations. PTEN is like that for cellular and molecular biology.

PTEN (Phosphatase and TENsin homolog) is a gene mutated in many forms of cancer. So it was regarded as a tumor suppressor, keeping our cells on the straight and narrow. Naturally cancer cells ‘try’ (note the anthropomorphism) to neutralize it. PI3K is a universal tumor driver, integrating growth factor signaling with downstream circuitries of cell proliferation, metabolism and survival.

Inositol is a 6 membered ring (all carbons) with one OH group attached to each carbon, which are numbered 1 through 6. PI3K puts phosphate on the 3 position, PTEN takes it off. Since this is how PI3K signaling begins, cells lacking PTEN grow faster and migrate aberrantly (e.g. spread).

Enter Proc. Natl. Acad. Sci. vol. 112 pp. 13976 – 13981 ’15 which carefully studied a PTEN mutant found in an unfortunate man with aggressive prostate cancer. It just changed one of the 403 amino acids (#126) from alanine to glycine. Not a big deal you say,it’s just a change of CH3 (alanine) to H (glycine). #126 is near the active site of the enzyme. One might expect that the mutation inhibits PTEN’s phosphatase activity (e.g. its enzymatic activity). Not so — the mutations shifts the activity so the enzyme. Instead of removing phosphate from the 3 position of inositol, the phosphate at the 5 position is removed (leaving the 3 position alone). This shifts inositol phosphate levels in the cell with hyperactivation of PI3K signaling (which requires inositol phospholipids containing phosphate at the 3 position).

What happens is that inositol phosphates fit into the mutant active site with the 5 position near the catalytic amino acid (cysteine). Essentially the 6 membered ring rotates the 3 position away from cysteine and puts the 5 position there instead. This changes PTEN from a tumor suppressor (anti-oncogene) to an oncogene.

To a chemist this is elegant and beautiful (apologies Baudelaire).

PTEN has taught us a huge amount about the control of protein levels, pseudogenes, competitive endogenous RNA (ceRNA). You can read all about this in https://luysii.wordpress.com/2014/01/20/why-drug-discovery-is-so-hard-reason-24-is-the-3-untranslated-region-of-every-protein-a-cerna/

That’s fairly grim, so here’s a link to one of the great comedians of years past — Jonathan Winters

http://biggeekdad.com/2013/04/jonathan-winters-stick/

It’s politically incorrect and sure to offend the humorless pompous prigs. Enjoy ! ! !

None dare call it junk

There has been a huge amount of controversy about whether all the DNA we carry about has some purpose to carry out — or not. Could some of it be ‘junk’?.

At most 2% of our DNA actually codes for the amino acids comprising our proteins. Some (particularly the ENCODE consortium) have used the criterion of transcription of the DNA into RNA (a process which takes energy) as a sign that well over 50% of our genome is NOT junk. Others regard this transcription as the unused turnings from a lathe.

All agree however, that bacteria use a good deal of their small genomes to code for protein. The following paper http://www.pnas.org/content/112/14/4251.full quotes a figure of 84 – 89%.

Consider the humble leprosy organism.It’s a mycobacterium (like the organism causing TB), but because it essentially is confined to man, and lives inside humans for most of its existence, it has jettisoned large parts of its genome, first by throwing about 1/3 of it out (the genome is 1/3 smaller than TB from which it is thought to have diverged 66 million years ago), and second by mutation of many of its genes so protein can no longer be made from them. Why throw out all that DNA? The short answer is that it is metabolically expensive to produce and maintain DNA that you’re not using

If you want a few numbers here they are:
Genome of M. TB 4,441,529 nucleotides
Genome of M. Leprae 3,268,203 nucleotides
1,604 genes coding for protein
1,116 pseudoGenes (e.g. genes that look like they could code for proteins, but no longer can because of premature termination codons.

This brings us to the organism described in the paper — Trichodesmium erythraeum — a photosynthetic bacterium living in the ocean. When conditions are right it multiplies rapidly causing a red algal bloom (even though it isn’t an algae which are cellular). It’s probably how the Red Sea got its name.

The organism only uses 64% of its genome to code for its protein. The most interesting point is that 86% of the nonCoding (for protein anyway) DNA is transcribed into RNA.

The authors wrestle with the question of what the nonCoding DNA is doing.

“Because it is thought that many bacteria are deletion-biased (47, 77), stable maintenance of these elements from laboratory isolates to the natural samples suggest that they may be required in some fashion for growth both in culture and in situ.”

Translation: The nonCoding DNA probably isn’t junk.

They give it another shot.

“Others have hypothesized that the conserved repeat structures observed in some bacteria could function as recombination-dependent “promoter banks” for adaptation to new conditions, thereby allowing relatively quick “rewiring” of metabolism in subpopulations”

Plausible, but why waste the energy transcribing the DNA into RNA if it isn’t doing anything for the organism doing the transcribing?

Never assume that what you can’t measure or don’t understand is unimportant.