Improving cancer diagnostics complements efforts to develop novel therapeutics
Cancer is a difficult disease to cure for a variety of reasons (as discussed in an earlier post). It is a highly dynamic disease involving rapidly evolving tumor cell populations. These tumor cells often possess a diversity of mutations that each require a different targeted treatment. Furthermore, tumors frequently originate from a single cell that divides uncontrollably. In the case of certain cancers (like that of the pancreas), detection often comes too late for effective treatment because symptoms only occur once a tumor reaches a certain size.
In addition to developing hundreds of targeted cancer therapeutics, the field of oncology has also focused efforts on improving early diagnostic tools. The hope is that better prognostic outcomes will result from detecting cancer in earlier stages, especially before metastasis occurs. Here, I review the principle cancer detection assays currently in clinical use and discuss their advantages and disadvantages.
Some diagnostic approaches depend on the phenotype—or the gross morphology—of a tumor or cancer cells. These methods include 1.) diagnostic imaging and 2.) histology.
A variety of biological imaging techniques have been developed to identify tumor masses once they have formed (Figure 1). These include Magnetic Resonance Imaging (MRI), Positron Emission Tomography/Computed Tomography (PET-CT), ultrasound, and X-ray imaging.
The main advantage of these techniques is that physicians typically do not need to have any foreknowledge of the specific location or type of tumor being screened. Some of these methods can be used to scan the whole body, such as MRIs and PET-CT scan, allowing physicians to effectively assay for metastasis. However, these tests have varying levels of accuracy. The PET-CT scan is arguably one of the most accurate imaging methods because it involves the use of a radioactive tracer molecule that preferentially accumulates in tumors.
Unfortunately, not all of these imaging methods are as accurate as the PET-CT scan, and they display varying frequencies of false-negatives and false-positives. Moreover, some of these imaging tests are disadvantageous because they are mutagenic, increasing a patient’s lifetime risk of developing cancer in the future. This includes the radioactive tracer used in PET-CT scans and X-ray-based techniques (such as mammography). Some of these tests are quite expensive, requiring highly trained staff to administer and interpret the results (Figure 2). They also cannot be performed on all patients. MRIs, for example, often cannot be used effectively on children or people suffering from claustrophobia because movement in the machine blurs the images. MRIs have decreased effectiveness in patients with metal implants, such as a hip replacement or a pacemaker, because the metal may cause image distortions or heat up/shift in the tissue because of attraction to the magnet in the apparatus. Some patients can even have allergic reactions to contrast agents used for some of the imaging methods like PET-CTs and MRIs.
Another phenotype-based method for diagnosing cancer is to obtain a piece of the tumor (a biopsy) and examine the cells under light microscopy (histology; Figure 3). The primary advantage of this approach is that it is typically considered the “gold standard” method for diagnosing cancer.
It enables the physician to stage the diseases by examining, for example, how far into a tissue the cancerous cells have invaded.
The most obvious disadvantage of biopsy/histology-based cancer diagnostics is that it requires surgical intervention. This can be expensive, especially if a patient does not have medical insurance. Biopsies increase the risk of surgical complications and of spreading the cancer to other tissues by breaking apart the tumor. Biopsies also require fore-knowledge of the location of the tumor, which does not permit high-throughput screens for early-stage tumors that are suspected to be hiding somewhere in the body. Furthermore, biopsies only give a binary diagnostic answer (yes or no). They do not provide information about the mutational heterogeneity of a tumor and, thus, which therapeutic(s) should be prescribed.
To overcome many of the disadvantages associated with phenotype-based cancer diagnostics, researchers have been developing methods for identifying cancer using “molecular signatures.” These signatures are composed of biomarkers associated with different types of cancer. Biomarkers include any of the three main biological molecules (DNA, RNA, and protein), in addition to small molecule metabolites (such as volatile organic compounds [VOCs]). These biomarkers are either released by cancer cells themselves or, in the case of metabolites, they are released as a result of altered metabolism in healthy cells. Biomarker assays work by detecting mutations, modifications, or (in the case of metabolites) the simple presence or absence of these biological molecules. Using this information, improved diagnoses, prognoses, and personalized treatments can be delivered to patients.
To perform a biomarker diagnostic assay, biological samples must be obtained from a patient. Samples extracted as a biopsy during surgery or during an outpatient procedure are often sent off for biomarker analysis. Samples can also be obtained through less invasive means. For example, “liquid biopsies” are now feasible, and they can be performed on various biological fluids, such as blood (e.g., Guardant, Freenome, Grail), urine (e.g., Trovagene, UroSEEK), saliva, cerebrospinal fluid, and pleural secretions. Companies like Owlstone Medical are even developing technologies for “breath biopsies.”
Most of biomarker diagnostics work in one of three ways. Some involve sequencing the DNA or RNA obtained from a biopsy using various next-generation sequencing or genotyping technologies (Figure 4).
Others involve chip-based biosensors. Of these chips, there are three basic designs that allow the detection of minuscule amounts of nucleic acids or proteins. They all involve a “biorecognition” molecule bound to the surface of a material known as the “transducer.” When the cancer biomarker in the patient’s sample binds the biorecognition molecule, the transducer converts this interaction into a measurable readout: typically, an electrochemical, optical, or mass-based signal (Figure 5). The third type of assay is used for breath biopsies. It deploys similar technology as alcohol breathalyzers, which measure the mobility of exhaled volatile metabolites using a technique called Ion Mobility Spectrometry (Figure 6).
Ultimately, biomarker diagnostics have a number of key advantages over phenotype-based assays. Many biomarker methods are highly sensitive, allowing the detection of cancers in much earlier stages of the disease. The information they provide gives physicians the opportunity to tailor treatments to each patient: an approach called precision medicine. Given the improvements in next-generating sequencing technologies in recent years, many of the sequencing-based assays in particular are less expensive than others involving surgical procedures or full-body imaging. Likewise, the ease of obtaining samples for liquid and breath biopsies allows the close monitoring of cancers for mutations, recurrence, or metastasis over time. Many of these biomarker assays can also be readily adapted to test for new cancer biomarkers as we learn more about these diseases.
Unfortunately, biomarker diagnostics do have some downsides. The first disadvantage is that these methods rely upon our existing knowledge of which biomarkers are associated with different types of cancer. Some biomarkers may only be associated with a given cancer under certain circumstances, which might be influenced by the patient’s genetics. Furthermore, some biomarkers may not necessarily be cancer-specific if the gene or protein being assayed is also expressed in other tissues where it plays roles in normal physiological processes. Thus, we are limited by our current understanding of cancer biology and an imperfect set of biomarkers currently on hand for assay development. The second key disadvantage of biomarker-based diagnostics is that these tests do not necessarily yield precise spatial information like imaging methods can. In a blood biopsy, for example, the physician may only know that mutated DNA or proteins are present. The assay does not necessarily reveal where the cancer is located in the body.
The future of cancer diagnostics
Going forward, the field of cancer diagnostics should continue to utilize both phenotype-based and biomarker-based assays, since each have their own clinical advantages and limitations. New innovations in these diagnostic assays, along with the continued development of targeted therapeutics, afford patients the hope that cancer will one day become a chronic, but entirely treatable, condition.
Featured image credit: Prostate cancer cells, 2017 Nikon Small World Photomicrography Winner, James E. Hayden, FBCA, RBP