Seeing a New World

May I wish readers a Happy New Year – and indeed extend my felicitations to non-readers with the hope that they too will become followers! What a good idea! Not least because I suspect many are viewing the new year with a mixture of anxiety and despair. But I can promise there’s nothing like the sanity of science to restore you after a few minutes contemplating how we’re doing on the economic and political fronts.

Your starter for 2017

By happy chance a few weeks ago I tried to explain how it’s now possible to ‘re-write the manual of life’ – that is, to engineer our DNA, to fix broken genes if you like. This means that, in theory, it’s possible to correct errors in our genetic code that cause genetic diseases. As there are over 6,000 of these and they include Down syndrome, cystic fibrosis and Alzheimer’s disease, there’s no need to say it’s important. There are several ways of going about this but the one I described is called CRISPR and it’s had a lot of media coverage.

Right on cue

Well done then Keiichiro Suzuki, Juan Carlos Belmonte and friends from the Salk Institute in California together with colleagues from other centres in Spain, Saudi Arabia and China for their December paper describing a new CRISPR twist. They used a rat model of retinitis pigmentosa, a genetic disease that is a major cause of inherited blindness, afflicting about one and a half million people worldwide (one in 4,000 in the UK).

The CRISPR-Cas9 system is great but it works best in dividing cells (e.g., in skin and gut that are renewing all the time) and it’s particularly useful for knocking out genes rather than inserting new DNA. The latest modification allows a new gene to be inserted into a specific site in the DNA of cells that are not dividing (e.g., those of the eye or brain).

The bits of CRISPR-Cas9, which insert DNA at very precise locations within the genome, are delivered to target cells as part of an inert virus. However, the package also includes DNA that encourages the cells to use a repair process that can be turned on even in non-dividing cells. So CRISPR-Cas9 cuts the cell’s DNA at an exact sequence and the cell then repairs the double-strand breaks (by a process called non-homologous end joining (NHEJ) that glues the broken ends directly together). Give the cell a new bit of DNA (e.g., your favorite gene) and that will get patched in – bear in mind that the cell doesn’t ‘know’ what it’s doing: it just tries to fix damaged DNA with whatever’s at hand.

And the target?

Retinitis pigmentosa occurs when a chunk of a gene called Mertk is lost. After quite a lot of experiments to show that their method worked, Suzuki, Belmonte & Co made a viral carrier that included a normal Mertk gene and injected it under the retina of rats with the disease. After about 5 weeks the rats were making Mertk RNA as a result of the gene being correctly ‘knocked-in’ to eye cells. The light-detecting region of the eye, greatly reduced by the disease, was significantly restored, with associated appearance of MERTK protein.

      Diseased    Normal     Treated                         Diseased         Normal         Treated

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Left trio: Sections of the light-detecting layers of the eye in diseased (left), normal (centre) and diseased post-treatment rats (right). Right trio: corresponding fluorescence images showing MERTK expression (red: highlighted by white arrows); Cells labeled blue. (Suzuki et al. Nature 1–6 (2016) doi:10.1038/nature20565)

How did the rats see it?

Well, after treatment they were able to detect light and had significantly recovered their visual functions, albeit not to completely normal levels.

The usual caveats apply: the method isn’t hyper-efficient and a human treatment is still a long way off. Nevertheless, it’s a significant step.

The same group has also shown, using a way of re-programming the expression of just four genes, that it’s possible to arrest the signs of ageing. In other words, in mice this time, tinkering with these genes can increase lifespan – and yes, we have versions of these genes and in us they also control cell renewal.

So the New Year message is clear to see. If we can avoid turning the planet into a desert or blowing ourselves to smithereens the future is really rosy – and maybe even infinite!

References

Suzuki, K. et al. (2016). In vivo genome editing via CRISPR/Cas9 mediated homology-independent targeted integration. Nature 540, 144-149.

Ocampo, A. et al. (2016). In Vivo Amelioration of Age-Associated Hallmarks by Partial Reprogramming. Cell 167, 1719–1733.

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In the beginning … 

You may have noticed that the American actress Angelina Jolie, who is now employed as a  Special Envoy  for the  United Nations High Commissioner for Refugees, has re-surfaced in the pages of the science media. She first hit the nerdy headlines by announcing in The New York Times that she had had a preventive double mastectomy (in 2013) and a preventive oophorectomy (in 2015).

We described the molecular biology that prompted her actions in A Taxing Inheritance. The essential facts were that she had a family history of breast and ovarian cancer: genetic testing revealed that she carried a mutation in the BRCA1 gene giving her a 87% risk of breast cancer and a 50% chance of getting ovarian cancer.

A star returns

BRCA1 and breast cancer are back in the news as a result of a paper by Jane Visvader, Geoffrey Lindeman and colleagues in Melbourne that asked a very simple question: which type of cell is driven to proliferate abnormally and give rise to a tumour by mutant BRCA1 protein? That is, pre-cancerous breast tissue contains a mixture of cell types: does cancer develop from one in particular –  and, if you blocked proliferation of that type of cell, could you prevent tumours forming?

Simple question but their paper summarises about 10 years of work to come up with a clear answer.

And the villain is …

The mature mammary gland is made up of lots of small sacs (alveoli) lined with cells that produce milk – called luminal cells. Groups of alveoli are known as lobules, linked by ducts that carry milk to the nipple. Most breast cancers start in the lobular or duct cells.

Breast fig copy

Left: Normal breast lobule showing alveoli lined with milk-producing luminal cells connected to duct leading to the nipple. Right: Normal milk sac, non-invasive cancer, invasive cancer.

Things are complicated by there being more than one type of progenitor cell but the Melbourne group were able to show that, in mice carrying mutated BRCA1, one subtype stood out in terms of its cancerous potential. These cells carried a protein on their surface called RANK (which is member of the tumour necrosis factor family). They had gross defects in their DNA repair systems (so they can’t fix genetic damage) and they’re highly proliferative. Luminal progenitors that don’t express RANK behave normally.

Slide1 copy

Scheme representing normal and abnormal cell development. The basic idea is that different types of cells evolve from a common ancestor. The Australian work identified one type of luminal progenitor cell that carries a protein called RANK on its surface (pink cell) as being a prime source of tumours. RANK+ cells have defective DNA repair systems so they accumulate mutations (red cells) more rapidly than normal cells, a feature of tumour cells.

In mice with mutant BRCA1 a monoclonal antibody (denosumab) that blocks RANK signalling markedly slowed tumour development. In a small pilot study blockade of RANK inhibited cell proliferation in breast tissue from human BRCA1-mutation carriers.

Next?

How effective blocking the activation of RANK signalling will be in preventing breast cancer is anyone’s guess but the idea behind the work of the Australian group cannot be faulted. Being able to prevent the ‘starter’ cells from launching themselves on the pathway to cancer driven by mutation in BRCA1 would mean that women in Angelina Jolie’s position would not have to contemplate the drastic course of surgery. The question is: will the preliminary mouse results lead to something that works in humans and, moreover, does so with high efficiency. As ever in cancer, watch this space – but don’t hold your breath!

References

Nolan, E. et al. (2016). RANK ligand as a potential target for breast cancer prevention in BRCA1-mutation carriers.

 

Lethal Lifesaver

Almost exactly three years ago (goodness me, it seems like a couple of months!) I wrote a piece about one of the novel approaches to cancer therapy being tried around the world. This exploits an effect called synthetic lethality that refers to the death of a cell as a result of a combination of mutations in two or more genes whilst mutation in either of these genes alone leaves the cell perfectly functional. The example involved two distinct pathways that repair damaged DNA – recall that our genetic material is being continuously assaulted in a variety of ways and that we’ve evolved very effective repair strategies. One of these involves a pair of familiar ‘cancer genes’, BRCA1 and BRCA2, mutated forms of which can be inherited to give rise to several types of cancer. The other requires an enzyme called PARP (for poly (ADP-ribose) polymerase). So the idea is that if BRCA mutations block that route the cell becomes dependent on PARP. Stop PARP functioning and the cell accumulates genetic damage that it is eventually unable to live with. Result: death of a cancer cell.

Blog fig

Synthetic lethality. If there are two distinct signaling pathways in a cell, each of which can be blocked without harming the cell but when both are inhibited simultaneously the cell dies, the effect is called synthetic lethality. The enzyme PARP (poly (ADP-ribose) polymerase 1) normally repairs single-strand DNA breaks. When this pathway is blocked by PARP inhibitors single-strand DNA breaks accumulate together with double-strand DNA breaks. If cells have normal BRCA, the double-strand breaks are repaired by a second pathway involving BRCA and the cell survives. However, in cancer cells with mutant BRCA this pathway is impaired. The use of PARP inhibitors means that neither pathway can work and the inhibitors, in effect, selectively target and kill cancer cells with BRCA mutations.

‘Three cancers for the price of one’ summarized small-scale clinical trials of several related PARP inhibitors, including one called olaparib, treating breast, ovarian and prostate cancers (BRCA mutations cause about 5% of breast cancers and 10% of ovarian cancers and they can also give rise to prostate cancer). The drugs showed effects on all three tumour types but in a subsequent trial there was no significant survival benefit for breast cancer patients.

Whilst that was a set-back I was sufficiently prescient to comment that ‘the PARP story is far from over’ and indeed further trials have shown significant effects on ovarian cancer, olaparib prolonging progression free survival from 4.3 months to 11.2 months. On this basis  Lynparza (aka Olaparib) was approved in December 2014 in both Europe and the USA for the treatment of advanced ovarian cancer with mutated BRCA.

This is only one more small step along the road to equipping us with a comprehensive anti-cancer drug cabinet but it is, of course, good news for the patient group who should benefit. For my colleague Steve Jackson and his team who developed this approach it must be a wonderful moment and they can look forward to following the success of the drug, now being marketed by Astra-Zeneca.

A Radiant Visitor

In an historic first, Cancer For All welcomes a guest, Stacey McGowan, who is a physicist just starting a Ph.D. on something called Proton Therapy. She is a member of the Department of Oncology in Cambridge and you can find out more about her in her blog www.planningforprotons.com but today she is going to take us into her world with a simple guide to radiotherapy in the treatment of cancer.

As undergraduate there was a lot of pressure to know what you wanted to do after graduation. I knew I wanted to stay in physics as it was what I loved; I also knew I wanted a job that meant something to me. I did not want to work in finance or for a defence company. At the time I also didn’t think I wanted to go into research! This seemed to have left me with two options, to work in the energy industry, or in medicine.

A lot of people, including my undergrad self, are unaware of medical physicists and their role in the hospital and in treating patients. After an inspiring talk at a careers event from a medical physicist working in the NHS I knew that this was what I wanted to do after graduation: I wanted to be a medical physicist.

There are three main methods for treating cancer; surgery, chemotherapy and radiotherapy. A patient will usually receive one or more of these methods as part of their treatment. Of the cures achieved about 49% of them involve surgery, 11% involve chemotherapy and 40% involve radiotherapy. However of the NHS’s cancer budget surgery costs around 22%, chemotherapy 18% and radiotherapy just 5%. This makes radiotherapy both a successful treatment option, sometimes on its own but usually in combination with surgery or chemotherapy, and it is extremely cost effective. Despite this many people don’t really know what radiotherapy is and the prospect of it as a treatment often makes patients apprehensive. As much as radiation sounds scary, we are exposed to it all the time in nature from the sun and soil and nowadays in our homes from electrical devices including Wi-Fi and mobile phones. In addition, we use it in many diagnostic applications including X-rays, CT scanners and nuclear medicine.

The difference between the radiation used for cancer treatment and that received from other sources is in the amount of radiation, or dose, delivered. When I talk about dose, think of it in the same way you would any other type of medicine. An oncology doctor will prescribe a course of radiotherapy with a specific dose to be delivered to the patient every weekday for between 4 and 6 weeks. The radiation is delivered in the form of X-rays – highly energetic particles of light – delivered at higher energies and doses than those used to image a broken bone (Editor’s enlightenment: physicists tend to use the word ‘light’ to mean electromagnetic radiation of any wavelength – not just what the eye sees). To create such highly energetic light we need a powerful machine that can also precisely deliver the X-rays to the part of the patient where the cancer lies. This machine is known to the medical community as a linac, and to the scientific community as a linear accelerator!

The linacs used in the hospital differ from those used in physics research as medical linacs have a very different role and it is the medical physicists’ job to ensure they work as intended. The X-rays delivered to the patient will harm cells in their body, both cancerous and healthy, by damaging their DNA. It is extremely important that the cancer cells receive the dose necessary to kill them so that they cannot continue to grow, resulting in a cure. It is also a priority that healthy tissue receives the smallest possible radiation dose to ensure a low chance of long term side effects. To accomplish these goals linacs are designed to rotate about the patient so that the tumour can be targeted from more than one direction. Treatment is usually delivered in daily doses (known as fractions) over a period of a few weeks because healthy cells are better at repairing damage to their DNA than cancer cells, so they can recover from each dose, whereas damage will accumulate in the tumour cells. Cumulative DNA damage leads to cell death, stopping the cancer in its tracks.

Linacs can also shape the beam so that it will match the shape of the tumour, shielding the adjacent healthy tissue from the highest radiation doses. To produce such patient-specific and intricate treatments powerful computer programs are used to design the treatment based on images of the patient (usually CT scans). Oncologists and physicists will work together, distinguishing cancer tissue from healthy, choosing beam directions and designing beam shapes to ensure that the patient receives the optimal treatment.

Many types of cancers respond to radiotherapy including those of the lung, breast, prostate, brain and spine and the method can be used to treat both adults and children. The short term side effects from radiotherapy vary depending on the region being treated. For example, radiation of the abdominal area may cause digestive and bowel discomfort or if the head and neck is the target, the patient may experience difficulty swallowing and develop a dry mouth. Generally radiotherapy can lead to tiredness, nausea and skin irritation in the targeted areas. Long term side effects can include secondary cancer, more probably in young patients, and growth problems in children.

The future of radiotherapy in the NHS is to use of protons and not X-rays to deliver radiation for specific types of cancer. The nature of protons makes the aim of cure without complication more achievable and is the topic of my PhD research.

Unlike X-rays, protons have a finite range (we can choose where they stop) which reduces the amount of radiation exposure to the patient, making this form of therapy especially beneficial for spine and brain tumours in adults and for most cancers in children. Proton therapy is particularly attractive for treating childhood cancers because it is less likely than conventional radiotherapy to cause growth defects and other health complications, including the development of cancers in later life.

Despite the UK lacking the facilities necessary to treat cancer using proton radiotherapy, a limited number of NHS patients are currently offered this option abroad as part of the NHS Proton Overseas Programme. The Government also announced in April 2012 that two proton centres will be established in England, in Manchester and in London. It is hoped that these will start to treat patients by early 2017.

Stacey McGowan

Department of Oncology, University of Cambridge

http://www.planningforprotons.com