THE EXPANDING FIELD OF CANCER TREATMENT
In our discussion thus far we have focused almost exclusively on the nature of the molecular changes – mutations – that combine, albeit infrequently, to drive cells from normality into cancer. However, an underlying question is also of great interest, namely, in the genetic upheaval associated with producing a tumour cell, what is the critical core of molecular normality that must be retained for the cell to remain viable? That is, in addition to the several hundred genes that can cause tumours by their aberrant action as oncogenes and tumour suppressors, it is self-evident that there are many normal genes whose expression is essential to support the growth and survival of cancer cells. By definition, therefore, these normal, supporting players form a separate category of potential therapeutic targets. In an extension of the concept of oncogene addiction, Stephen Elledge and his colleagues have termed this dependence of tumour cells on normal genes that do not undergo mutation ‘non-oncogene addiction’.
The notion that, notwithstanding the characteristic genomic instability of most tumour cells, many basic molecular pathways remain functional has been implicit in our discussion of cancer biology and we have already encountered several features of tumour cells that encompass non-oncogene addiction. These include perturbed metabolism, the escape from immune surveillance and adaptation to a hypoxic environment. It also seems probable that, although defective DNA repair systems directly promote genetic instability, there is a pressure on cancer cells to retain a level of genomic maintenance consistent with survival. It may be recalled that some cytokines are not readily classifiable in this way, for example, IL6 and TGFβ that can promote or inhibit tumour progression, depending on the context. This functional ambiguity may be reflected in the fact that some immunosuppressant drugs are associated with increased cancer risk, including tocilizumab, an inhibitor of IL6 signalling with FDA approval for the treatment of rheumatoid arthritis.
This concept of a trade-off between rampant genetic instability and the retention of intrinsic proliferative capacity is consistent with the finding in a number of major cancer types (including breast, ovarian, stomach and non-small cell lung) that extreme chromosomal instability correlates with a better clinical outcome. In other words, extreme genomic disruption compromises cell viability.
The identification of specific genes upon which survival of a cancer cell depends has only recently become feasible with the development of methods for gene knock-down using small interfering RNA (siRNA) that now permit the individual interrogation of the majority of human genes. That is, the construction of short hairpin RNA (shRNA) libraries has made it possible to screen cell lines to determine the phenotypic effect of loss of function of each gene in turn. A specific application of this approach has focused on the RAS signalling pathway, initially by using human cells carrying an activating KRAS mutation (G13D). Targeting over 32,000 unique mRNAs, shRNAs were identified that inhibited the activity of the MAPK pathway and displayed synthetic lethality with mutant but not wild-type RAS. From these screens a functionally diverse group of about 100 genes emerged, encoding proteins involved not only in signal transduction, cell proliferation and apoptosis but also in protein and nucleic acid metabolism and intracellular transport. Surprisingly few of these candidate RAS synthetic lethal genes were in the MAPK or PI3K pathways. Genes that were particularly prominent encoded components of the machinery of mitosis and it appears that RAS mutation confers increased mitotic stress. That is, cells with mutant RAS are much more sensitive to inhibition of mitotic spindle function (e.g. by paclitaxel) that are normal cells and, in general, these cells are hypersensitive to mitotic inhibitors (e.g. of the anaphase-promoting complex/cyclosome (APC/C) complex, the E3 ubiquitin ligase that labels cell cycle proteins for degradation by the 26S proteasome, or of the mitotic regulator Polo-like kinase 1 (PLK1)). The effect of inhibition is to cause prometaphase accumulation leading to cell death. A second serine/threonine-protein kinase, STK33, emerged from these RNA silencing screens as being essential for the survival of KRAS-mutant cells. Although STK33 can regulate ribosomal protein S6 kinase (RPS6KB1), an effector of mTOR that can integrate growth factor and nutrient signals and also modulate apoptosis via phosphorylation of BAD, it has not hitherto been associated with RAS signalling. Even more surprisingly, tumour cells with mutant forms of HRAS or NRAS do not show STK33 dependence.
In common with all experimental methods, interference RNA is not without its problems one of which is the variability between shRNAs in the efficiency with which they reduce protein expression. Even so, the power of the approach is indicated by the detection of novel therapeutic targets in RAS mutant cells. A further striking finding is that the expression pattern of genes encoding subunits of APC/C correlates with prognosis in human KRAS-mutant lung tumours while there is no correlation in tumours with normal RAS.
Several proteasome inhibitors are under development and, bortezomib (Velcade®) was approved by the FDA in May 2003 for the treatment of multiple myeloma.
We have referred to some of the seminal contributions to science by the polymath J. B. S. Haldane, who was a victim of colorectal cancer, at the, relatively, early age of 72. In characteristic fashion, when Haldane knew he was dying, he penned a witty poem on the subject of his nemesis in which he noted that ‘Yet, thanks to modern surgeons’ skills, It can be killed before it kills, Upon a scientific basis, In nineteen out of twenty cases.’ His main point was to encourage people to consult a doctor early at the first sign of cancer.
In the 50 years since Haldane’s death, the field of cancer has been transformed. Nevertheless, his advice is as valid as ever because early detection and surgery remains the first and most effective treatment for many cancers. This in turn suggests that mass screening programmes for major cancers can only be beneficial. It has transpired though that screening programmes have inherent deficiencies that present problems when it comes to making an overall cost versus benefit assessment. These have become particularly prominent in the context of breast cancer. The plus side of the balance sheet is that screening saves lives when early detection permits appropriate treatment. On the negative side, four factors make a contribution. (1) Any screening system will miss some tumours that should be treated, that is, it will give false negatives. For breast screening about 10% of tumours are missed. (2) Conversely, there will be a false positive rate, typically 7%, most of which (>80%) have a benign cause. (3) Tumours are identified and treated, even though they would not have developed to become life-threatening during the normal lifespan of the patient – an occurrence referred to as over-diagnosis. (4) Finally, although the radiation exposure required for an X-ray or CT scan is low it is nevertheless weakly carcinogenic. Collectively these factors cause patient distress, increase the cost and lead to unnecessary treatment.
The problem is one of risk assessment and judgement – the probability of saving a life compared with the overall risk of doing harm. The US Preventive Services Task Force has reviewed eight large trials and concluded that the benefits of breast cancer screening were so marginal that it recommended reducing the US screening programme from annually for women over the age of 40 to biannually commencing at age 50. In other words, concluding that benefit is negligible for women under 50. A corresponding analysis concluded that the Danish screening programme had conferred no significant benefit. The suggestion of the US Task Force has not been implemented but it brings into sharp focus the question of how best to use resources that will inevitably be limited.
To illustrate this problem, consider the Cochrane Collaboration report of 2009 concluding brought about an overall reduction in breast cancer deaths of 15%. This translates to one life prolonged from every 2,000 women screened over a ten-year period but in that time there will be ten healthy women who have undergone unnecessary treatment as a result of over-diagnosis. These findings have been predictably controversial because they rely on the estimation of risk ratios that are small and vary between different studies. Overall, however, it seems reasonable to conclude that, in the UK, breast cancer screening has played a similar role to the various chemotherapy advances summarised earlier. Each has made small but significant contributions to the national trend of survival rate. Similar concerns have also been raised over other screening programmes notably that for prostate cancer, have already mentioned the shortcomings of the PSA test for this disease. Much less controversial has been the highly effective screening method for cervical carcinoma. Because this detects pre-malignant lesions that can be treated and led to a progressive decline in the UK deaths from this disease over the last 40 years to the current level of about 1,000.
Despite the statistical variation, the evidence indicates that, although some individuals will undoubtedly benefit from screening programmes, there are substantial drawbacks and that these might be made clearer to the general public. In the end, the question revolves around the best use of financial resources in circumstances that are in continuous flux as technologies develop. For example, over the last ten years, the number of metabolites identifiable by the metabolomics methods has risen from 30 to over 1,500. While very few of these are likely to emerge as useful in cancer diagnosis, due to the broad range of natural variation, it nevertheless seems reasonable to believe that an extended repertoire of tumour biomarkers, particularly those provided by genomic analysis, will progressively improve screening quality, so that we might anticipate that the shortcomings in such programmes will diminish as the technology is refined.
In this review of cancer biology, we have encountered remarkable technical developments that cover improvements in screening methods, a progressively increasing range of biomarkers with enhanced sensitivity of detection, the complete identification of the mutational profile of individual tumours and the identification of the pattern critical non-oncogenes that characterise specific types of tumours. To these advances is coupled the promise of being able to monitor tumour response to treatment from the earliest stages by the essentially non-invasive techniques of imaging and/or genomics. Together with developments in drug design and testing that are already accelerating the rate at which treatments progress to clinical application, these prospects represent the spectacular scale upon which cancer can now be tackled at the molecular level – a breadth of strategies of the immense potential that would have been almost inconceivable just a few years ago. As we have noted, the success of these approaches depends critically on the capacity to develop drugs and therapeutic methods to exploit the targets. The fact that resources are limited means that the cost of screening will have to be balanced against the expansion of drug development and testing as one of the anticipated benefits from WGS. This implies that diagnosis will become a more precise science and offers the hope that specific chemotherapy could become the first line of defence, assuming an adequate supply of new drugs of appropriate specificity and with acceptable side effects. That desirable goal will not, however, remove the need for continued public debate on how to apportion cancer funding, a discussion in which clear and informed input from the scientific community will be an essential contribution.
The completion of the sequence of the human genome in 2003 was one of the great milestones in scientific history. It made an almost immediate impact on cancer with the discovery of a major mutation in melanoma that rapidly led to a new drug treatment. Equally dramatic has been the advances in technology that now mean complete genomes can be sequenced at low cost in a day or so. This is generating an avalanche of data from which has already emerged not only new cancer genes but the capacity to sub-classify cancers using molecular signatures. We are still at a very early stage in the genomic revolution but it has already had direct effects on patient treatment. As the application of genomics and associated high-throughput screening methods gathers pace breathtaking horizons are opening before us. The possibility of using chips to isolate single tumour cells from the circulation that can then be completely sequenced offers the possibility of biomarker detection at the earliest stages of cancer progression. If the massive screening programmes being applied to drug identification can fulfil their promise, the dream that chemotherapy might supplant surgery as the most effective first-line treatment for cancer may yet be realised.