Caveat Emptor

 

It must be unprecedented for publication of a scientific research paper to make a big impact on a significant sector of the stock market. But, in these days of ‘spin-off’ companies and the promise of unimaginable riches from the application of molecular biology to every facet of medicine and biology, perhaps it was only a matter of time. Well, the time came with a bang this June when the journal Nature Medicine published two papers from different groups describing essentially the same findings. Result: three companies (CRISPR Therapeutics, Editas Medicine and Intellia) lost about 10% of their stock market value.

I should say that a former student of mine, Anthony Davies, who runs the Californian company Dark Horse Consulting Inc., mentioned these papers to me before I’d spotted them.

What on earth had they found that so scared the punters?

Well, they’d looked in some detail at CRISPR/Cas9, a method for specifically altering genes within organisms (that we described in Re-writing the Manual of Life).

Over the last five years it’s become the most widely used form of gene editing (see, e.g., Seeing a New World and Making Movies in DNA) and, as one of the hottest potatoes in science, the subject of fierce feuding over legal rights, who did what and who’s going to get a Nobel Prize. Yes, scientists do squabbling as well as anyone when the stakes are high.

Nifty though CRISPR/Cas9 is, it has not worked well in stem cells — these are the cells that can keep on making more of themselves and can turn themselves in other types of cell (i.e., differentiate — which is why they’re sometimes called pluripotent stem cells). And that’s a bit of a stumbling block because, if you want to correct a genetic disease by replacing a defective gene with one that’s OK, stem cells are a very attractive target.

Robert Ihry and colleagues at the Novartis Institutes for Biomedical Research got over this problem by modifying the Cas9 DNA construct so that it was incorporated into over 80% of stem cells and, moreover, they could switch it on by the addition of a drug. Turning on the enzyme Cas9 to make double-strand breaks in DNA in such a high proportion of cells revealed very clearly that this killed most of them.

When cells start dying the prime suspect is always P53, a so-called tumour suppressor gene, switched on in response to DNA damage. The p53 protein can activate a programme of cell suicide if the DNA cannot be adequately repaired, thereby preventing the propagation of mutations and the development of cancer. Sure enough, Ihry et al. showed that in stem cells a single cut is enough to turn on P53 — in other words, these cells are extremely sensitive to DNA damage.

Gene editing by Cas9 turns on P53 expression. Left: control cells with no activation of double strand DNA breaks; right: P53 expression (green fluorescence) several days after switching on expression of the Cas9 enzyme. Scale bar = 100 micrometers. From Ihry et al., 2018.

In a corresponding study Emma Haapaniemi and colleagues from the Karolinska Institute and the University of Cambridge, using a different type of cell (a mutated line that keeps on proliferating), showed that blocking P53 (hence preventing the damage response) improves the efficiency of genome editing. Good if you want precision genome editing by risky as it leaves the cell vulnerable to tumour-promoting mutations.

Time to buy?!

As ever, “Let the buyer beware” and this certainly isn’t a suggestion that you get on the line to your stockbroker. These results may have hit share prices but they really aren’t a surprise. What would you expect when you charge uninvited into a cell with a molecular bomb — albeit one as smart as CRISPR/Cas9. The cell responds to the DNA damage as it’s evolved to do — and we’ve known for a long time that P53 activation is exquisitely sensitive: one double-strand break in DNA is enough to turn it on. If the damage can’t be repaired P53’s job is to drive the cell to suicide — a perfect system to prevent mutations accumulating that might lead to cancer. The high sensitivity of stem cells may have evolved because they can develop into every type of cell — thus any fault could be very serious for the organism.

It’s nearly 40 years since P53 was discovered but for all the effort (over 45,000 research papers with P53 in the title) we’re still remarkably ignorant of how this “Guardian of the Genome” really works. By comparison gene editing, and CRISPR/Cas9 in particular, is in its infancy. It’s a wonderful technique and it may yet be possible to get round the problem of the DNA damage response. It may even turn out that DNA can be edited without making double strand breaks.

So maybe don’t rush to buy gene therapy shares — or to sell them. As the Harvard geneticist George Church put it “The stock market isn’t a reflection of the future.” Mind you, as a founder of Editas Medicine he’d certainly hope not.

References

Ihry, R.J. et al. (2018). p53 inhibits CRISPR–Cas9 engineering in human pluripotent stem cells. Nature Medicine, 1–8.

Haapaniemi, E. et al. (2018). CRISPR–Cas9 genome editing induces a p53-mediated DNA damage response. Nature Medicine (2018) 11 June 2018.

Taking the MYC out of cancer

It has been famously said, though no one quite knows who gave first utterance, that England and America are two nations divided by a common language. In deference to readers from the US of A, therefore, we need a word about English before we embark on the current topic. You can get quite a long way in the States on an English accent, partly because the inhabitants are, by and large, very tolerant and cosmopolitan souls and also because they perceive Brits as being rather weird but harmlessly entertaining – at least since they gave up the idea of owning America, signed the Treaty of Paris and slung their hook. But that last phrase is an example of how things can get sticky when you talk to an American audience and it slips your mind that 1783 was quite a long time ago – long enough in fact for a good deal of divergent language evolution. Put another way, use idioms unthinkingly and you can die the death – leaving your listeners wishing that you would indeed sling your hook (American translation: beat it). One of the odder things about this linguistic separation is that American doesn’t have a good phrase that means gently making fun of someone. You can pull a Yankee leg it is true and you may mess them about – but that one’s really fraught as it carries a different innuendo in English. So it’s a pity that over there you can’t extract the Mick, take the Mickey or remove the Michael. It’s deeply regrettable that this phrase probably owes its origins to Cockney rhyming slang referring to the act of urination but strawberries grow in manure and all that. Similar pratfalls work in the other direction, of course, and British audiences are likely to look blank if you mention your keister: start talking about booty and mussing with someone and they’ll really be baffled.

To the current topic. We saw in Mission Impossible that many different pathways pass signals saying ‘grow’ from the outside world to the nucleus at the centre of a cell. Many of these relays use RAS proteins – they’re a major junction in the cellular network so they’re a very tempting target for disruption of signaling. But if all roads lead to Rome, so to speak, is there not an even better target – a main gate, the critical portal through which everything that drives cell proliferation must pass? There is and it’s called MYC (pronounced ‘mick’ – Ah! Now all is clear!!), the gene encoding a protein of the same sound that is a unique master regulator. MYC coordinates the expression of a large panel of genes involved in cell growth and division – it’s essential for cell proliferation.

 MYC pic

Cell signaling. Many messengers turn on lots of relays that focus on the nucleus telling cells to grow and divide. The MYC protein is a master coordinator.

But there’s an obvious problem: to survive we need to make new cells all the time – about one million every second, just to maintain the status quo. It seems hardly worth pointing out that if you gave someone a drug that blocked MYC it would be fatal: the body simply couldn’t survive for very long with a blocked cell production line. Indeed it’s been known for some time that knocking out the MYC gene in mice is fatal: they fail to develop beyond an early embryonic stage. And yet there’s a huge temptation to ask ‘What would inhibiting MYC do to tumors: might it actually kill them?’ – a curiosity fuelled by the knowledge that MYC is deregulated in most – perhaps all – cancers. That is, an almost invariable upshot of the mutation patterns found in tumors is that excessive amounts of MYC protein are made – and, as it drives cells round the cell cycle and into division, more MYC equals abnormal cell growth, aka cancer.

So, in an experiment that all logic said would not work, Gerard Evan and his colleagues in San Francisco and Cambridge devised a way to switch on an inhibitor of MYC in transgenic mice to ask the unaskable: ‘Is blocking MYC fatal and, if is isn’t, what does it do to tumors?’ The method is to use a trick of genetic engineering to give mice a novel gene that is switched on by dosing them with a drug added to their drinking water. Omitting the drug switches it off. The gene encodes a protein that sticks to the MYC protein and prevents it from interacting with its normal partner so that the dynamic duo that drives cell growth can’t be formed.

The results were startlingly dramatic. First of all, MYC blockade didn’t kill the mice although it certainly had some effects. A shaved patch of fur doesn’t re-grow quickly as it normally would and males become infertile because they can’t make new sperm. But the mice aren’t aware of these little problems and in general are as full of beans as their chums with normal levels of MYC activity – and in any case these mild effects are reversible. Switch off the MYC block and they return rapidly to normal.

That was surprising enough but the really staggering result came from introducing the MYC blockade into mice that develop lung tumors (driven by the expression of a mutant RAS gene). Inhibition of MYC has an almost immediate effect on tumor size: tumors regress and the side effects remain mild and reversible. A single burst of MYC blockade results in a significant extension of life span for tumor-bearing mice. Even more remarkable, successive episodes of MYC inhibition (a ‘metronomic’ regime) leads to the gradual eradication of the tumors – and the elimination of lung cancer means that the mice now have a normal life span.

It should be emphasized that so far this has only happened in mice and the Appian Way of cancer therapy is littered with the corpses of brilliant ideas that worked a treat in those wonderful little models but were utterly useless when it came to humans. However, science is the practice of eternal optimism and there are sound grounds for hope here. Switching a gene on and off by genetic engineering is fine in mice. It won’t do for us but already some small molecule inhibitors have been made that appear to work in mice. The hope is that both the anti-cancer effect and the mild side-effects will be recapitulated in humans – and that, because all pathways do indeed lead to it, taking the MYC out of cancer will kill tumours. Then the really optimistic bit – that even crafty cancer cells will be unable to find a way round such a block. Because all proliferation signals lead to MYC there will be no adaptive mechanism to which cells can turn to ensure their survival.

In short, the tumour will be stuffed – which rather brings us back to where we started except that, having taken the MYC, this needs no translation.

Reference

Soucek, L., Whitfield, J.R., Sodir, N.M., Massó-Vallés, D., Serrano, E., Karnezis, A.N., Swigart, L.B. and Evan, G.I. (2013). Inhibition of Myc family proteins eradicates KRas-driven lung cancer in mice. Genes Dev., 27, 504-513.