p53 (22 page)

The scientists have worked out a method for retrieving that DNA and screening it for aflatoxin-mutated p53. They are also able to monitor the components of the viral genome and to look at
what’s happening with HBx. Unfortunately, however, no such simple test exists for skin cancer, where the carcinogen, sunlight, leaves an equally clear fingerprint on p53. People just have to
be on the lookout themselves for the signs and symptoms of disease.

THE SUN’S FINGERPRINT

I was seven years old when my father was posted to Borneo to run a TB clinic and a general hospital serving
the indigenous Dyak tribespeople. The family
set sail from Liverpool, and I vividly remember the days on deck, far out in the ocean under a blank blue sky and scorching sun. We all got sunburnt, coming out in big blisters on our shoulders
that were too sore to touch and meant we had to sleep, spread-eagled, on our stomachs. Eventually the skin peeled off in long strips like wallpaper, but my younger sister’s nose never seemed
to heal completely and family photos of our Borneo days show her with a patch of sticking plaster across it most of the time. In those days, the 1950s, and indeed for several more decades to come,
we had no idea of the risks we were taking in not protecting ourselves from the sun.

We know now that ultraviolet light (UV) is the main cause of skin cancer, and in the early 1990s Douglas Brash and colleagues at Yale University discovered that it too damages p53 and leaves a
characteristic fingerprint mutation. When Brash first started investigating the effect of UV radiation, a known carcinogen, on skin cells in the late 1980s there were three main theories about how
it causes cancer. One was that it disrupts the immune system so that it fails to remove damaged cells from the surface of the skin as normal; another that sunlight directly stimulates cell growth;
and the third that it damages DNA, knocking out a vital gene or genes.

Several groups in Europe and North America working independently on skin cancer had already discovered that UVB rays are absorbed most readily by the squamous cells, flat disc-shaped cells just
below the surface, and slightly less readily by the basal cells deeper in the skin that nevertheless account for the majority of skin-cancer cases (at that time, the effect of the sun’s rays
on the melanocytes, the cells involved in melanoma – the least common but most deadly form of skin cancer – was still unclear). The researchers had discovered also that UVB radiation
damages DNA – and that it does so in a very specific way: it hits always at the point where the two bases, cytosine (C) and thymine (T),
are adjacent to each other on
a strand of DNA, swivelling them round so that a C is replaced by T, and sometimes two Cs by two Ts. This causes slight but crucial changes in the recipe of the protein the strand produces. This
exact mutation isn’t seen in tumours anywhere else in the body, where sunlight cannot reach, and is thus considered to be a fingerprint of UVB.

In us, as in all living things, DNA damage and mutation happen all the time as we pass through the mill of life. Mutation is, of course, what drives evolution and adaptation to the environment,
so it can be a force for good as well as bad. Brash knew therefore that the fact that UVB causes mutation did not automatically point to this as being the culprit in skin cancer. However, he was
persuaded by the general pattern of the disease – and particularly by some evidence from Australia, which together with New Zealand has by far the highest rates of skin cancer in the world
– that his best bet was to explore the idea of damaged genes.

Typically, skin cancer develops in middle age and beyond. The Australian researchers had noticed that rates among pale-skinned immigrants – generally from the UK and other parts of
northern Europe – who had arrived in the country as adults were lower than among those who had arrived as children. Among the more recent immigrants, skin-cancer rates tended to reflect the
rates in their home countries, while those who had been in Australia since childhood had rates similar to other white Australians. This suggested that the insult to a person’s skin from UV
radiation that had set him or her off on the path to cancer had occurred years earlier, with those who had been in the country longest obviously at greater risk from the powerful Australian sun
than those who had spent their childhoods in the clouds and rain of northern Europe.

Brash reasoned that if UV radiation affected the immune system or triggered runaway growth of cells directly, the effect would be much more immediate and transient, and
the age at immigration would make no difference to the risk. The fact that there was a difference pointed to a mutated gene, which has a lasting effect, as the most likely cause. His
task therefore was to find out which gene was affected. Once again it was like searching for a needle in a haystack: the human genome had not yet been sequenced and everyone still believed it
contained at least 100,000 genes, not fewer than 30,000, as we now know to be the case.

Brash and his group followed several fruitless lines of enquiry, looking at known oncogenes, before their luck changed. One day, Arnie Levine appeared at Yale to give a talk in which he
mentioned that p53 was found to be mutated in many cancers. Brash heard the talk and suddenly the pieces fell into place: a tumour-suppressor gene was a much more likely candidate for skin cancer
than an oncogene because, to cause malignancy, a tumour-suppressor gene requires both alleles to be damaged – or both sets of brakes to fail, to return to our car analogy of
Chapter 7
. These
events could be years apart, thus accounting for the typically slow development of skin cancer often many years after the victim first suffered a bad dose of sunburn.

And there was another intriguing clue that p53 might be involved. People suffering from an extremely rare skin disease called Lewandowsky-Lutz dysplasia develop warty growths, particularly on
their hands and feet, that can be profuse and that readily turn malignant when exposed to the sun. Lewandowsky-Lutz disease is caused by infection with certain strains of the human papilloma virus,
HPV, which is known to target and destroy p53 protein and lead to cancer in other organs, notably the cervix.

To start the investigation, Brash’s group pulled from the medical archive blocks of tissue taken from non-melanoma skin tumours of patients in New York City and in Uppsala, Sweden, where
Jan Pontén of the University Hospital had become interested and joined the research effort. All the tumour samples came from sites on the patients’ bodies
exposed to the sun, such as face and hands. The researchers extracted DNA from each and homed in on the p53 gene, looking for mutations. They found them in 90 per cent of the samples
– the great majority bearing the fingerprint of UV radiation and thus capable of producing an active protein with the characteristic modifications to the recipe. Brash and his fellow
researchers wrote up their findings in
PNAS
in 1991. But this was before anyone knew how normal p53 worked, and it took another few years – and research by many other groups as well
as theirs – for a clear picture to emerge of what happens in the normal course of events when we sit out in the sun, and what can go wrong to cause skin cancer. In essence the picture looks
like this: at some point in their lives, most probably during childhood, people who develop skin cancer will have suffered an episode of sunburn which caused a cell or cells to sustain mutation to
the p53. We now know that UV radiation causes extensive damage to DNA, but our bodies have an efficient mechanism for repairing it if it’s not too serious: enzymes in our cells snip out the
damaged stretch of DNA and replace it with a healthy copy. However, the mechanism can fail, and the mutation hot spots are the sites in the genes where, for some reason, the repair process is least
efficient.

Peter Hall and David Lane’s maverick experiment with the sun lamp on Hall’s arm showed that p53 is activated in our skin when we sit out in the sun. Other researchers have since
found that in the normal course of events, this activated p53 protein will trigger apoptosis in cells that cannot repair their sun-damaged DNA. But a cell in which p53 itself is damaged will resist
apoptosis and will sit around, reproducing the fateful mutation from one generation to the next, until further insults to the skin, often decades later, turn it cancerous.

At this point, says Brash, sunlight delivers a double whammy. A rogue cell with mutant p53 is surrounded by normal cells that, when damaged by sunlight, will respond,
as
they should, by committing suicide. This gives more elbow room for the rogue cell to spread. ‘By inducing healthy cells to kill themselves off, sunlight favours the proliferation of
p53-mutated cells,’ he explained in an article for
Scientific American
with fellow skin-cancer specialist David Leffell. ‘In effect, sunlight acts twice to cause cancer: once
to mutate the p53 gene and then afterwards to set up conditions for the unrestrained growth of the altered cell line.’ This is the crux of the matter, for it is the expansion of clones of
that single mutated cell that precipitates cancer.

‘What most people don’t realise,’ says Brash, ‘is that clonal expansion is numerically more important to cancer than making the initial mutation. Exposing yourself to the
sun five times will make five times as many mutations. But favouring cell division of the p53 mutant five times will make many more mutant cells. Like compound interest at the bank, this
exponential increase soon leads to very large numbers.’ The pre-malignant lesion thus formed provides an increasingly easy target for the mutagen – in this case UVB radiation – to
inflict the crucial ‘second hit’ on a p53-mutant cell that will turn it cancerous.

***

What we’ve been discussing here are somatic mutations – ones that occur by chance in individual cells of the body at some point in a person’s life. Sometimes
the cell that receives the hit is a sperm or an egg, and the mutant gene can then be passed on to future generations. This is called a germline mutation and it can be very bad news for those who
inherit it, because every cell in their bodies will carry the mutant gene.

***

When he was a small boy growing up on the outskirts of Boston, Massachusetts, John Berkeley took a tumble one day while playing in the garden. The fall raised a bump on the
back of his head and when, some days later, it had failed to go down, his parents took him along to the doctor, who diagnosed rhabdomyosarcoma, a rare form of cancer growing in the muscle attached
to the bone at the base of his skull. John was four years old, and over the next two years and more he was in and out of Boston’s Dana-Farber Cancer Institute (then known as the Sidney Farber
Cancer Center). He was treated in the specialist Jimmy Fund Clinic, named after the boy with lymphoma of the guts who had been the poster child for the campaign to establish a research centre into
children’s cancers back in the late 1940s.

John had surgery to remove a tumour the size of a golf ball from his skull, followed by nearly a year of radiotherapy.
‘I recall lying on a table and they would
literally tape your body and your head down to the table with what seemed like masking tape, so you wouldn’t move,’ he told me when I spoke to him over the phone. ‘I’m sure
it was very difficult to keep a child still in order to perform the radiation treatment, but I just remember it was very unpleasant to be taped down to a table at that age.’

His treatment regime included chemotherapy, which lasted two years and meant daily trips to the outpatient clinic where he was hooked up to an intravenous drip delivering a debilitating cocktail
of drugs for eight hours at a time. ‘Towards the end of my treatments was the time I started going to kindergarten and I had lost my hair, I was a bald child,’ he said. ‘Kids can
be cruel, as they say, and that was certainly the case for me – I vividly recall going to school and being picked on by the other kids.’

The effort to keep John alive despite a poor prognosis from his cancer team was an emotional rollercoaster for his mother and father too. ‘I learnt later on that it nearly tore their
marriage apart,’ he remembered. ‘Back in the 1970s, cancer services weren’t even close to being established like they are today. For the families to which this sort of thing
happened, it was very . . . I would use the word “barbaric”. You were brought in; you got your treatments and you’d go home. There was no friendly atmosphere, no family advocacy
support, no sugar coating on anything.’

But the return to robust good health of their plucky small son was not the end to the Berkeleys’ troubles. John had a brother, born around the time that his treatment began. In an uncanny
rerun of John’s story, the boys, then aged 10 and six, were playing with friends in the garden when his brother was hit on the leg by a baseball. The lump from the knock didn’t go away,
and when his anxious parents took him to the hospital to have it checked out, they were told he had osteosarcoma, a tumour on the bone, and his leg would have to be amputated.

‘The next thing that raised the red flag for our family was that my father developed some soft-tissue sarcomas in his early forties,’ said Berkeley. The
family’s medical history raised flags too for the specialists at the Dana-Farber Institute. This was the early 1990s, and the genetic basis of cancer was by now firmly established.
What’s more, a young Chinese-American oncologist and epidemiologist, Frederick Li, who was part of the team taking care of the Berkeleys, had a particular interest in cancer that ran in
families. His name had recently become associated with a cancer-predisposition syndrome that he and Joseph Fraumeni, an epidemiologist working at the NIH in Bethesda, had identified.