Tag Archives: PVT1

Chemistry and Biochemistry can’t answer the important questions but without them we are lost

The last two posts — one concerning the histone code and cerebral embryogenesis https://luysii.wordpress.com/2018/06/07/omar-khayyam-and-the-embryology-of-the-cerebral-cortex/ and the other concerning PVT1 enhancers promoters and cancer https://luysii.wordpress.com/2018/06/04/marshall-mcluhan-rides-again/ — would be impossible without chemical and biochemical knowledge and technology, but the results they produce and the answers they seek and lie totally outside both disciplines.

In fact they belong outside the physical realm in the space of logic, ideas, function — e.g. in the other half of the Cartesian dichotomy — the realm of ideas and spirit.  Certainly the biological issues are instantiated physically in molecules, just as computer memory used to be instantiated in magnetic cores, rather than transistors.

Back when I was starting out as a grad student in Chemistry in the early 60s, people were actually discovering the genetic code, poly U coded for phenylalanine etc. etc.  Our view was that all we had to do was determine the structure of things and understanding would follow.  The first xray structures of proteins (myoglobin) and Anfinsen’s result on ribonuclease showing that it could fold into its final compact form all by itself reinforced this. It also led us to think that all proteins had ‘a’ structure.

This led to people thinking that the only difference between us and a chimpanzee were a few amino acid differences in our proteins (remember the slogan that we were 98% chimpanzee).

So without chemistry and biochemistry we’d be lost, but the days of crude reductionism of the 60s and 70s are gone forever.  Here’s another example of chemical and biochemical impotence from an earlier post.

The limits of chemical reductionism

“Everything in chemistry turns blue or explodes”, so sayeth a philosophy major roommate years ago.  Chemists are used to being crapped on, because it starts so early and never lets up.  However, knowing a lot of organic chemistry and molecular biology allows you to see very clearly one answer to a serious philosophical question — when and where does scientific reductionism fail?

Early on, physicists said that quantum mechanics explains all of chemistry.  Well it does explain why atoms have orbitals, and it does give a few hints as to the nature of the chemical bond between simple atoms, but no one can solve the equations exactly for systems of chemical interest.  Approximate the solution, yes, but this his hardly a pure reduction of chemistry to physics.  So we’ve failed to reduce chemistry to physics because the equations of quantum mechanics are so hard to solve, but this is hardly a failure of reductionism.

The last post “The death of the synonymous codon – II” puts you exactly at the nidus of the failure of chemical reductionism to bag the biggest prey of all, an understanding of the living cell and with it of life itself.  We know the chemistry of nucleotides, Watson-Crick base pairing, and enzyme kinetics quite well.  We understand why less transfer RNA for a particular codon would mean slower protein synthesis.  Chemists understand what a protein conformation is, although we can’t predict it 100% of the time from the amino acid sequence.  So we do understand exactly why the same amino acid sequence using different codons would result in slower synthesis of gamma actin than beta actin, and why the slower synthesis would allow a more leisurely exploration of conformational space allowing gamma actin to find a conformation which would be modified by linking it to another protein (ubiquitin) leading to its destruction.  Not bad.  Not bad at all.

Now ask yourself, why the cell would want to have less gamma actin around than beta actin.  There is no conceivable explanation for this in terms of chemistry.  A better understanding of protein structure won’t give it to you.  Certainly, beta and gamma actin differ slightly in amino acid sequence (4/375) so their structure won’t be exactly the same.  Studying this till the cows come home won’t answer the question, as it’s on an entirely different level than chemistry.

Cellular and organismal molecular biology is full of questions like that, but gamma and beta actin are the closest chemists have come to explaining the disparity in the abundance of two closely related proteins on a purely chemical basis.

So there you have it.  Physicality has gone as far as it can go in explaining the mechanism of the effect, but has nothing to say whatsoever about why the effect is present.  It’s the Cartesian dualism between physicality and the realm of ideas, and you’ve just seen the junction between the two live and in color, happening right now in just about every cell inside you.  So the effect is not some trivial toy model someone made up.

Whether philosophers have the intellectual cojones to master all this chemistry and molecular biology is unclear.  Probably no one has tried (please correct me if I’m wrong).  They are certainly capable of mounting intellectual effort — they write book after book about Godel’s proof and the mathematical logic behind it. My guess is that they are attracted to such things because logic and math are so definitive, general and nonparticular.

Chemistry and molecular biology aren’t general this way.  We study a very arbitrary collection of molecules, which must simply be learned and dealt with. Amino acids are of one chirality. The alpha helix turns one way and not the other.  Our bodies use 20 particular amino acids not any of the zillions of possible amino acids chemists can make.  This sort of thing may turn off the philosophical mind which has a taste for the abstract and general (at least my roommates majoring in it were this way).

If you’re interested in how far reductionism can take us  have a look at http://wavefunction.fieldofscience.com/2011/04/dirac-bernstein-weinberg-and.html

Were my two philosopher roommates still alive, they might come up with something like “That’s how it works in practice, but how does it work in theory? 

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Marshall McLuhan rides again

Marshall McLuhan famously said “the medium is the message”. Who knew he was talking about molecular biology?  But he was, if you think of the process of transcription of DNA into various forms of RNA as the medium and the products of transcription as the message.  That’s exactly what this paper [ Cell vol. 171 pp. 103 – 119 ’17 ] says.

T cells are a type of immune cell formed in the thymus.  One of the important transcription factors which turns on expression of the genes which make a T cell a Tell is called Bcl11b.  Early in T cell development it is sequestered away near the nuclear membrane in highly compacted DNA. Remember that you must compress your 1 meter of DNA down by 100,000fold to have it fit in the nucleus which is 1/100,000th of a meter (10 microns).

What turns it on?  Transcription of nonCoding (for protein) RNA calledThymoD.  From my reading of the paper, ThymoD doesn’t do anything, but just the act of opening up compacted DNA near the nuclear membrane produced by transcribing ThymoD is enough to cause this part of the genome to move into the center of the nucleus where the gene for Bcl11b can be transcribed into RNA.

There’s a lot more to the paper,  but that’s the message if you will.  It’s the act of transcription rather than what is being transcribed which is important.

Here’s more about McLuhan — https://en.wikipedia.org/wiki/Marshall_McLuhan

If some of the terms used here are unfamiliar — look at the following post and follow the links as far as you need to.  https://luysii.wordpress.com/2010/07/07/molecular-biology-survival-guide-for-chemists-i-dna-and-protein-coding-gene-structure/

Well that was an old post.  Here’s another example [ Cell vol. 173 pp. 1318 – 1319, 1398 – 1412 ’18 ] It concerns a gene called PVT1 (Plasmacytoma Variant Translocation 1) found 25 years ago.  It was the first gene coding for a long nonCoding (for proteins RNA (lncRNA) found as a recurrent breakpoint in Burkitt’s lymphoma, which sadly took a friend (Nick Cozzarelli) far too young as (he edited PNAS for 10 years).

So PVT1 is involved in cancer.  The translocation turns on expression of the myc oncogene, something that has been studied out the gazoo and we’re still not sure of how it causes cancer. I’ve got 60,000 characters of notes on the damn thing, but as someone said 6 years ago “Whatever the latest trend in cancer biology — cell cycle, cell growth, apoptosis, metabolism, cancer stem cells, microRNAs, angiogenesis, inflammation — Myc is there regulating most of the key genes”

We do know that the lncRNA coded by PVT1 in some way stabilizes the myc protein [ Nature vol. 512 pp. 82 – 87 ’14 ].  However the cell experiments knocked out the lncRNA of PVT1 and myc expression was still turned on.

PVT1 resides 53 kiloBases away from myc on chromosome #8.  That’s about 17% of the diameter of the average nucleus (10 microns) if the DNA is stretched out into the B-DNA form seen in all the textbooks.  Since each base is 3.3 Angstroms thick that’s 175,000 Angstroms 17,500 nanoMeters 1.7 microns.  You can get an idea of how compacted DNA is in the nucleus when you realize that there are 3,200,000,000/53,000 = 60,000 such segments in the genome all packed into a sphere 10 microns in diameter.

To cut to the chase, within the PVT1 gene there are at least 4 enhancers (use the link above to find what all the terms to be used actually mean).  Briefly enhancers are what promoters bind to to help turn on the transcription of the genes in DNA into RNA (messenger and otherwise).  This means that the promoter of PVT1 binds one or more of the enhancers, preventing the promoter of the myc oncogene from binding.

Just how they know that there are 4 enhancers in PVT1 is a story in itself.  They cut various parts of the PVT1 gene (which itself has 306,721 basepairs) out, and place it in front of a reporter gene and see if transcription increases.

The actual repressor of myc is the promoter of PVT1 according to the paper (it binds to the enhancers present in the gene body preventing the myc promoter from doing so).  Things may be a bit more complicated as the PVT1 gene also codes for a cluster of 7 microRNAs and what they do isn’t explained in the paper.

So it’s as if the sardonic sense of humor of ‘nature’, ‘evolution’, ‘God’, (call it what you will) has set molecular biologists off on a wild goose chase, looking at the structure of the gene product (the lncRNA) to determine the function of the gene, when actually it’s the promoter in front of the gene and the enhancers within which are performing the function.

The mechanism may be more widespread, as 4/36 lncRNA promoters silenced by CRISPR techniques subsequently activated genes in a 1 megaBase window (possibly by the same mechanism as PVT1 and myc).

Where does McLuhan come in?  The cell paper also notes that lncRNA gene promoters are more evolutionarily conserved than their gene bodies.  So it’s the medium (promoter, enhancer) is the message once again (rather than what we thought the message was).