Brainstorming

 

It’s the first day of a New Year and, as is well known, Scottish folk world-wide make a big celebration of yesterday (Hogmanay), New Year’s Day and indeed quite often the next few days for good measure. Even in the far north-west of England as a youngster with more or less black hair (deemed to be important for some reason) I was trundled round the neighbours in one of the rituals — ‘first-footing’, i.e. being the first guest of the new year, despite our family having no Scottish connections that I knew of.

Scots Wha Hae

Most such jollifications seem to require mournful dirges accompanying incomprehensible lyrics by Robert Burns. To be fair I should note that Max Bruch and Hector Berlioz, wonderful composers both, saw fit to include a musical reference to ‘Scots Wha Hae’ in the Scottish Fantasy and in the concert overture Rob Roy. Mind you, Berlioz himself described his overture as “long and diffuse” and it was so badly received that he burned the score the night of its premier.

However, there is something else that Scots make quite a fuss about, given half a chance, and here perhaps we can agree they have a point. It’s the number of notable scientists and physicians their country has produced. Wikipedia’s List of Scottish engineers and scientists runs to over 150 names — remarkable for a population that even today is only about five million. The listed luminaries feature some household names: Alexander Graham Bell, James Watt, James Clerk Maxwell, Lord Kelvin and Joseph Lister just to be going on with.

But there’s a slightly unnerving thing about Wikipedia’s List in that, long though it is, there are some serious omissions. I spotted this the other day when I was searching for a bit of background about one of the heroes of this New Year’s story. The first missing star I noted was John Hunter, generally thought to have carried out the first surgical removal of a malignant melanoma (skin cancer) in 1787. Worse still, I found no mention of William Macewen: it was his first successful removal of a brain tumour (in 1879) that makes him directly relevant to our story. He was a truly remarkable figure. Thought of as the ‘Father of neurosurgery’, he was a pioneer in  surgery of the brain and other organs. But the really outstanding thing about Sir William Macewen CB., FRS., FRCS, to give him his full handle, was his approach to surgery. Thus, for example, in treating brain tumours he applied his profound knowledge of anatomy to work out from the patient’s symptoms the precise location of the abnormal growth so he knew where to take surgical aim. Amazing!

Very slow progress

Nearly 60 years after Macewen’s pioneering surgery the American composer George Gershwin would have appreciated his genius as treatments had made little progress by the 1930s when Gershwin succumbed to a brain tumour (specifically a glioblastoma multiforme). It took until 1958 for the first useful drug treatment for brain tumours to emerge and until the mid-1970s for radiation therapy come into use. Indeed it was only the introduction of CT scans towards the end of the 20th century that permitted tumour localisation without needing Macewen’s extraordinary gifts.

Something very odd

In parallel with these advances has emerged the evidence for an unexpected feature of brain tumours. You might guess that brain tumours would start in the brain but it turns out that most do nothing of the sort. The vast majority (about 90%) are secondary cancers: that is, they arise when tumour cells spread from another part of the body — commonly breast or lung. In other words most brain tumours are metastases — and they are mighty important. About 24,000 people in the United States discover they have these abnormal growths every year and they cause about 18,000 deaths. The rates are much the same in the UK where deaths from brain and related tumours number just over 5,000.

But also familiar …

Those who follow developments on cancer will know that metastasis is one of the hottest potatoes. Until very recently we had no idea of the molecular goings on that turn a cell in a primary tumour into a wanderer that can leave its site of origin, get into the bloodstream, get out at some other location and there establish a new, secondary colony. The mists are beginning to lift as the wonders of modern biology are applied to this pressing problem.

Step forward one of the main movers and shakers in the field who is the modern hero of today’s piece: David Lyden of the Gale and Ira Drukier Institute for Children’s Health, Weill Cornell Medicine, New York.

So topical is this issue of metastasis that I’m relieved to note that the contributions of the Lyden group have featured regularly in these pages (Keeping Cancer CatatonicScattering the Bad Seed and Holiday Reading (4) – Can We Make Resistance Futile). A succinct summary of those contributions would be: (1) cells in primary tumours release ‘messengers’ into the circulation that ‘tag’ metastatic sites before any cells actually leave the tumour, (2) the messengers that do the site-tagging are small sacs — mini cells — called exosomes, and (3) they find specific addresses by carrying protein labels that home in to different organs — we represented that in the form of a tube train map in Lethal ZIP Codes.

In One More Small Step the same team looked closely at exosomes and found that a wide variety of tumour cell types secrete two sizes of exosomes (big and small! — see blog for details!!). Amazingly these sacs carry about 1000 different types of protein — suggesting that they might have powerful effects.

Breaking the barrier

With that in mind Lyden’s group have now turned their attention to how tumour cells find their way to the brain. How do they achieve the feat of crossing the ‘blood-brain barrier’ — the layer of (endothelial) cells that encloses the brain and controls the types of molecules that can move to and from circulating blood — and are exosomes involved? In other words, are they little bags of trouble that play a role in helping most brain tumours to grow?

Answer ‘yes’ of course, or we wouldn’t have spent so long getting up to speed on the subject. Gonçalo Rodrigues, Lyden & Co. set up a brain slice culture system and pre-treated the slices with exosomes from human breast cancer metastatic cells that were known to spread preferentially to different tissues (brain, lung or bone).

Photos of brain slices showing how exosomes help to provide a niche for human breast cancer metastatic cells to invade, attach and grow. These are fluorescence microscopy images: brain blood vessels (vasculature) are red; cancer cells are green (GFP). Left: no pre-treatment; Right: pretreatment with exosomes. White arrowheads show vasculature-associated cancer cells. White bar = 100 micronsFrom Rodrigues et al. 2019.

The photos show a typical experiment using brain-seeking exosomes. There is a huge increase in the number of green cancer cells attaching to the brain slice as a result of exosome pre-treatment (right) by comparison with no exosome addition (left). Corresponding experiments with exosomes that direct migration to lung or bone show no effect: cancer cell attachment remains low (as in the left hand photo).

How do they do it?

The group took their studies a stage further by looking at the 1000 or so proteins in the exosomes for any that seemed to specify migration to the brain — in other words, to act as addresses of the kind we described in Lethal ZIP Codes. They came up with one in particular: a protein called CEMIP  (if you’re interested that stands for ‘cell migration inducing hyaluronidase 1’. It’s an enzyme that chops up long chains of sugars (called hyaluronic acid). These chains form scaffolds to support proteins in various tissues including the brain — and their disruption may play a role in cancer cell movement).

The levels of CEMIP are higher in exosomes that promote brain metastasis but not in those associated with lung or bone metastatic cells. Thus pre-conditioning the brain microenvironment with CEMIP+ exosomes drives invasion. When they are depleted invasion and tumour cell association with the brain vasculature is disrupted. This remarkable new work has revealed how exosomes help wandering tumour cells to storm the blood-brain barrier. Immediately this opens the possibility of isolating exosomes from small samples of blood and screening them for proteins — i.e. using them as a ‘biomarker’ for metastatic cancer targets. But of course the great goal is to be able to interfere with their actions, an intervention that could dramatically cut the incidence of brain tumours. What a triumph that would be!!

We began with a Scottish tradition. Let’s end with another by raising a mental glass to scientists all over the world who, step by perspiring step are inching towards the goal of controlling cancer. Keep it up guys — and back to your benches!!

Reference

Rodrigues et al. (2019). Tumour exosomal CEMIP protein promotes cancer cell colonization in brain metastasis. Nature Cell Biology 21, 1403–1412.

 

 

Holiday Reading (4) – Can We Make Resistance Futile?

For those with a fondness for happy endings we should note that, despite the shortcomings of available drugs, the prospects for patients with a range of cancers have increased significantly over the latter part of the twentieth century. The overall 5-year survival rate for white Americans diagnosed between 1996 and 2004 with breast cancer was 91%; for prostate cancer and non-Hodgkin’s lymphoma the figures were 99% and 66%, respectively. These figures are part of a long-term trend of increasingly effective cancer treatment and there is no doubt that the advances in chemotherapy summarised in the earlier Holiday Readings are contributory factor. Nonetheless, the precise contribution of drug treatments remains controversial and impossible to disentangle quantitatively from other significant factors, notably earlier detection and improved surgical and radiotherapeutic methods.

Peering into the future there is no question that the gradual introduction of new anti-cancer drugs will continue and that those coming into use will be more specific and therefore less unpleasant to use. By developing combinations of drugs that can simultaneously poke the blancmange at several points it may be possible to confront tumor cells with a multiple challenge that even their nimbleness can’t evade, thereby eliminating the problem of drug resistance. Perhaps, therefore, in 20 years time we will have a drug cabinet sufficiently well stocked with cocktails that the major cancers can be tackled at key stages in their evolution, as defined by their genetic signature.

However, on the cautionary side we should note that in the limited number of studies thus far the effect of drug combinations on remission times has not been startling, being measured in months rather than years or decades. Having noted the durability of cancer cells we should not be surprised by this and the concern, of course, is that, however ingenious our efforts to develop drug cocktails, we may always come second to the adaptability of nature.

Equally perturbing is the fact that over 90% of cancer deaths arise from primary tumors spreading to other sites around the body. For this phenomenon, called metastasis, there are currently very few treatment options available and the magnitude of this problem is reflected in the fact that for metastatic breast cancer there has been little change in the survival rates over the past forty years.

Metastasis therefore remains one of the key cancer challenges. It’s over 125 years since the London physician Stepen Paget asked the critical question: ‘What is it that allows tumour cells to spread around the body?’ and it’s a daunting fact that only very recently have we made much progress towards an answer – and thus perhaps a way of controlling it. To the fore in this pursuit has been David Lyden and his colleagues at Weill Cornell Medical College in New York. Their most astonishing finding is that cells in the primary tumour release messengers into the circulation and these, in effect, tag what will become landing points for wandering cells. Astonishing because it means that these sites are determined before any tumour cells actually set foot outside the confines of the primary tumour. Lyden has christened this ‘bookmarking’ cancer. That is a quite remarkable finding – but, as ever in science, it merely shifts the question to ‘OK but what’s the messenger?’

A ray of sunshine

It might appear somewhat churlish, especially after all that funding, to end on a note of defeatist gloom so let’s finish with my ray of sunshine that represents a radical approach to the problem. It relies on the fact that small numbers of cells break away from tumors and pass into the circulation. In addition, tumours can release both DNA and small sacs – like little cells – that contain DNA, proteins and RNAs (nucleic acids closely related to DNA). These small, secreted vesicles are called exosomes – a special form of messengers, communicating with other cells by fusing to them. By transferring molecules between cells, exosomes may play a role in mediating the immune response and they are now recognized as key regulators of tumour growth and metastasis.

Step forward Lyden and friends once more who have just shown in a mouse model of pancreatic cancer that exosomes found their way to the liver during the tumour’s earliest stages. Exosomes are taken up by some of the liver cells and this sets off a chain of cell-to-cell signals that eventually cause the accumulation of a kind of molecular glue (fibronectin). This is the critical ingredient in a microenvironment that attracts tumour cells and promotes their growth as a metastasis (secondary growth). So you can think of exosomes as a kind of environmental educator.

Exosome Fig

Exosomes released from primary tumours can mark a niche for metastasis.

The small sacs of goodies called exosomes are carried to the liver where they fuse with some cells, setting off a chain reaction that produces a sticky protein – fibronectin – a kind of glue for immune cells and tumour cells. (from Costa-Silva, B., Lyden, D. et al., Nature Cell Biology 17, 816–826, 2015).

The recent, remarkable technical advances that permit the isolation of exosomes also make it possible to fish out circulating tumour cells and tumour DNA from a mere teaspoonful of blood.

Circulating tumour cells have already been used to monitor patient responses to chemotherapy – when a treatment works the numbers drop: a gradual rise is the earliest indicator of the treatment failing. Even more exciting, this approach offers the possibility of detecting the presence of cancers years, perhaps decades, earlier than can presently be achieved. Coupling this to the capacity to sequence the DNA of the isolated cells to yield a genetic signature of the individual tumor can provide the basis for drug treatment. There are still considerable reservations attached to this approach but if it does drastically shift the stage at which we can detect tumors it may also transpire that their more naïve forms, in which fewer mutations have accumulated, are more susceptible to inhibitory drugs. If that were to be the case then even our currently rather bare, though slowly expanding, drug cabinet may turn out to be quite powerful.