Duchenne muscular dystrophy — a novel genetic treatment

Could the innumerable genetic defects underlying Duchenne muscular dystrophy all be treated the same way?  Possibly.  Paradoxically, the treatment involves actually making the gene  even worse.

Understanding how and why this might work involves a very deep dive into molecular biology.  You might start by looking at the series of five background articles I wrote — start at https://luysii.wordpress.com/2010/07/07/molecular-biology-survival-guide-for-chemists-i-dna-and-protein-coding-gene-structure/ and follow the links.

I have a personal interest in Duchenne muscular dystrophy because I ran such a clinic from ’72 to ’87 watching young boys and adolescents die from it.  The major advance during that time, was NOT medical or anything I did, but lighter braces, so the boys could stay ambulatory longer.  Things have improved as survival has improved by a decade so they die in their late 20s.

So lets start.  Duchenne muscular dystrophy is caused by a mutation in the gene coding for dystrophin, a large (3,685 amino acids) protein which ties the contractile apparatus of the muscle cell (actin and myosin) to the cell membrane. Although it isn’t the largest protein we have — titin, another muscle protein with 34,350 amino acids is, the gene for dystrophin is the largest we have, weighing in at 2,220,233 nucleotides.  This is why Duchenne is one of the most common diseases due to a defect in a single gene, the gene is so large that lots of things can (and do) go wrong with it.

The gene comes in 79 pieces (exons) which account for under 1/200 of the nucleotides of the gene.  The rest must be spliced out and discarded.  Have a look at http://www.dmd.nl.  to see what can go wrong — the commonest is deletion of parts of the gene (60 – 70% of cases), followed by duplication of other parts (10% of cases) with the rest being mutations that change one amino acid to another.

Duchenne isn’t like cystic fibrosis where some 600 different mutations in the causative CFTR gene were known by 2003 but with 90% of cases due to just one.  So any genetic treatment for that young boy sitting in front of you had better be personalized to his particular mutation.

Or should it?

Possibly not.  We’ll need to discuss 3 things first

l. Nonsense Mediated Decay (NMD)

2. Nonsense Induced Transcriptional Compensation (NITC).

3. The MDX mouse model of Duchenne muscular dystrophy

Nonsense mediated decay.  Nonsense is a poor term, because the 3 nonSense codons (out of 64 possible) tell the ribosome to stop translating mRNA into protein and drop off the mRNA.  That isn’t nonsense.  I prefer stop codon, or termination codon

An an incredibly clever piece of business tells the ribosome (which is after all an inanimate object) when a stop codon occurs too early in the mRNA when there are a bunch of codons afterwards needed to make up the whole protein.

Lets go back to dystrophin and its 79 exons, and the fact that 99.5% of the gene is made of introns which are spliced out.   Remember the mRNA starts at the 5′ end and ends at the 3′ end.  The ribosome reads and translates it from 5′ to 3′. When an intron is spliced out, a protein complex of several proteins is placed on the mRNA some 20 – 24 basepairs 5′ to the splice site (this happens in the nucleus way before the mRNA gets near a ribosome in the cytoplasm).  The complex is called the Exon Junction Complex (EJC). The ribosome then happily munches along the mRNA from 5′ to 3′ knocking off the EJCs as it moves, until it hits a termination codon and drops off.

Over 95% of  genes do not have introns after the termination codon.  What happens if it does? Well then it is called a premature termination codon (PTC) and there is usually an EJC 3′ (downstream) to it.  If a termination codon is present 50 -55 nucleotides 5′ (upstream) to an EJC then NMD occurs.

Whenever any termination codon is reached, release protein factors (eRF1, eRF3, SMG1) bind to the mRNA.  It there is an EJC around (which there shouldn’t be) the interaction between the two complexes triggers phosphorylation of one of EJC proteins, triggering NMD.

So that’s how NMD happens, when there is a PTC.  Clever no?

Nonsense Induced Transcriptional Compensation (NITC).  I realize that this is a lot to throw at you, but a treatment for Duchenne is worth the effort (not to mention other genetic diseases in which the mechanism to be described also applies).

NITC is something I never heard about until two papers appearing in the 13 April Nature (vol. 568 pp. 179 – 180 (editorial), 193 – 197, 259 – 263).  Ever since we could knock out by placing a PTC early (near the 5′ end) of the gene we’ve been surprised by some of the results –e.g. knocking out some genes thought to be crucial had little or no effect.  Other technologies which didn’t affect the gene, but which decreased the expression of the mRNA (such as RNA interference, aka Post-Transcriptional gene silencing — PTGS) did have big phenotypic effects.

This turns out to be due NITC, which turns out to be due to increased transcription of genes which are ancestrally related to the mutant. Gene.  Hard to believe.

Time to go back to NMD.  It doesn’t break mRNA down nucleotide by nucleotide, but fragments it.  These fragments get into the nucleus, and bind to complementary genomic sequences of the gene containing the PTC, and also to genes ancestrally related to the mutant gene (so they’ll have similar nucleotide sequences). Then epigenetics takes over because the fragments recruit the COMPASS complex which catalyzes the formation of H3K4Me3 which is part of the histone code which helps turn on transcription of the gene.  The sequence similarity of ancestrally related genes, allows them and only them to be turned on by NITC.  Even cleverer than finding a PTC by the ribosome.

Something so incredible needs evidence.  Well heterozygotic zebrafish can bemade to have one normal gene and one with a PTC. What do you think happens?  The normal gene is upregulated (e.g. more is made).  Pretty good.

Finally the Mdx mouse.  I’ve been reading about it for years.  It has a PTC in exon 23 of the dystrophin gene, resulting in a protein only 27% as long as it should be.  All sorts of therapeutic maneuvers have been tried on it.  Now any drug development chemist will tell you that animal models are lousy, but they’re all we’ve got.

The remarkable thing about the mdx mouse, is that they don’t get weak.  They do have muscle pathology.  All the verbiage above probably explains why.

So to treat ALL forms of Duchenne put in a premature termination codon (PTC) in exon #23 of the human gene. It should work as there are  4 dystrophin related proteins scattered around the genome — their names are — utrophin, dystrophin related protein 2 (DRP2), alpha dystrobrevin, and beta dystrobrevin

There is an even better way to look for a place to put a PTC in the dystrophin gene.  Our genomes are filled with errors — for details see — https://luysii.wordpress.com/2018/05/01/how-badly-are-thy-genomes-oh-humanity-take-ii/.

There are lots of very normal people around with supposedly lethal mutations (including PTCs) in their genomes.  Probably scattered about various labs are at least 1,000,000 exome sequences in presumably normal people.  I’m not sure how much clinical information about them is available (other than that they are normal).  Hopeful their sex is.  Look at the dystrophin gene of normal males (females can be perfectly healthy carrying a mutant dystrophin gene as it is found on the X chromosome and they have 2) and see if PTCs are to be found.  You can’t have a better animal model than that.

At over 1,000 words this is the longest post I’ve written, and hopefully the most useful.

The twists and turns of topoisomerase (pun intended)

It is very sad that my late friend Nick Cozzarelli isn’t around to enjoy the latest exploits of the enzyme class he did so much great work on — the topoisomerases. For a social note about him see the end of the post.

We tend to be quite glib about just what goes on inside a nucleus when DNA is opened up and transcribed into mRNA by RNA polymerase II (Pol II). We think of DNA has a linear sequence of 4 different elements (which it is) and stop there. But DNA is a double helix, and the two strands of the helix wind around each other every 10 elements (nucleotides), meaning that within the confines of our nuclei this happens 320,000,000 times.

I’ve written a series of six posts on what we would see if our nuclei were enlarged  by a factor of 100,000 (which is the amount of compaction our DNA must undergo to fit inside the 10 micron (10 millionths of a meter) in diameter nucleus (since if fully extended our DNA would be 1 meter long. So if you compacted the distance from New York to Seattle (2840 miles or 14,995,200 feet) down by this factor you’d get a sphere 150 feet in diameter or half the length of a football (US) field. Now imagine blowing up the diameter and length of the DNA helix by 100,000 and you’d get something looking like a 2,840 mil long strand of linguini which twists on itself  320,000,000 times. The two strands are 3/8th of an inch thick. They twist around each other every 9/16ths of an inch.

For the gory details start at https://luysii.wordpress.com/2010/03/22/the-cell-nucleus-and-its-dna-on-a-human-scale-i/ and follow the links.

Well, we know that for DNA to be copied into mRNA it must be untwisted, the strands separated so RNA polymerase II (Pol II) can get to it.  Pol II is enormous — a mass of 500 kiloDaltons and 7 times thicker at 140 Angstroms than the DNA helix of 20 Angstrom thickness.

Consider the fos gene (which we’ll be talking about later). It contains 380 amino acids (meaning that the gene contains at least 1140 nucleotides ). The actual gene is longer because of introns (3,461 nucleotides), which means that the gene contains 346 complete turns of the double helix, all of which must be unwound to transcribe it into mRNA.

So it’s time for an experiment. Get about 3 feet of cord roughly 3/8 of an inch thick. Tie the ends together, loop one end around a hook in your closet, put a pencil in the other end and rotate it about 100 times (or until you get tired). Keeping everything the same, have a friend put another pencil between the two strands in the middle, separating them. Now pull on the strands to make the separation wider and move the middle pencil toward one end. In the direction of motion the stands will coil even tighter (supercoiling) and behind they’ll unwind.

This should make it harder for Pol II to do its work (or for enzymes which copy DNA to more DNA). This is where the various topoisomerase come in. They cut DNA allowing supercoils to unwind. They remain attached to the DNA they cut so that the DNA can be put back together. There are basically two classes of topoisomerase — Type I topoisomerase cuts one strand, leaving the other intact, type II cuts both.

Who would have thought that type II topoisomerase would be involved in the day to day function of our brain.

Neurons are extended things, with information flowing from dendrites on one side of the cell body to much longer axons on the other. The flow involves depolarization of the cell body as impulses travel toward the axon. We know that certain genes are turned on by this activity (e.g. the DNA coding for the protein is transcribed into mRNA which is translated into protein by the ribosome). They are called activity dependent genes.

This is where [ Cell vol. 1496 – 1498, 1592 – 1605 ’15 ] comes in. Prior to neuronal activity, when activity dependent genes are expressed at low levels, the genes still show the hallmarks of highly expressed genes (e.g. binding by transcription factors and RNA polymerase II, Histone H3 trimethylation of lysine #4 {H3K4Me3 } at promoters).

This work shows that such genes are highly negatively supercoiled (see above) preventing RNA polymerase II (Pol II) from extending into the gene body. On depolarization of the cell body in some way Topoisomerase IIB is activated, leading to double strand breaks (dsbs) within promoters allowing the DNA to unwind and Pol II to productively elongate through gene bodies.

There is evidence that neuronal stimulation leads to dsbs ( Nature NeuroScience vol. 16 pp. 613 – 621 ’13 ) throughout the transcription of immediate early genes (e.g. genes turned on by neural activity). The evidence is that there is phosphorylation of serine #139 on histone variant H2AX (gammaH2AX) which is a chromatin mark deposited on adjacent histones by the DNA damage response pathway immediately after DSBs are found.

Etoposide (a topoisomerase inhibitor) traps the enzyme in a state where it remains bound to the DNA of the dsb. On etoposide Rx, there is an increase in activity dependent genes (Fos, FosB, Npas4). Inhibition of topiosomerase IIB (the most prevalent topoisomerase in neurons) by RNA interference (RNAi) leads to blunted activity dependent induction of these genes. This implies that DNA cutting by topoisomerase IIB is required for gene activation in response to neuronal activity.  Other evidence is that knocking down topoisomerase  using RNA interference (RNAi) stops activity dependent gene transcription.

Further supporting this idea, the authors induced dsbs at promoters of activity dependent genes (Fos, fosB, Npas4) using the CRISPR system. A significant increase in transcription was found when the Fos promoter was targeted.

I frankly find this incredible. Double strand breaks are considered bad things for good reason and the cell mounts huge redundant machines to repair them, yet apparently neurons, the longest lived cells in our bodies are doing this day in and day out. The work is so fantastic that it needs to be replicated.

Social Note: Nick Cozzarelli is one of the reasons Princeton was such a great institution back in the 50s (and hopefully still is). Nick’s father was an immigrant shoemaker living in Jersey City, N. J. Princeton recognized his talent, took him in, allowing him to work his way through on scholarship, waiting tables in commons, etc. etc. He obtained a PhD in biochemistry from Harvard and later became a prof at Berkeley, where he edited the Proceedings of the National Academy of Sciences USA for 10 years. He passed away far too soon of Burkitt’s lymphoma in his late 60s. We were friends as undergraduates and in grad school.

I can only wonder what Nick would say about the latest twists of the topoisomerase story