Research Needs for Understanding the Biology of Overdiagnosis in Cancer Screening

Increased Early Cancer Diagnosis: Unveiling Immune-Cancer Biology to Explain Clinical "Overdiagnosis"

Even though clinically small 'early' cancers represent many millions of cells biologically, when removed surgically, these often never recur or regrow, nor reduce the individual's lifespan. However, some early cancers remain quiescent and indolent; while others grow and metastasize, threatening life. Distinguishing between these different clinical behaviours using clinical/pathological criteria is currently problematic. It is reported that many suspicious lesions and early cancers are being removed surgically that would not threaten the patient's life. This has been termed 'overdiagnosis', especially in the sphere of cancer screening. Although a controversial and emotive topic, it poses clinical and public health policy challenges. The diagnostic differentiation between 'non-lethal' and 'lethal' tumor forms is generally impossible. One perspective gathering evidential support is that a dynamic balance exists between the immune response and malignant processes governing 'lethality', where many more cancers are produced than become clinically significant due to the immune system preventing their progression. Higher medical screening "diagnosis" rates may reflect lead-time effects, with more 'non-progressing' cancers detected when an early immune-cancer interaction is occurring. We present a model for this immune-cancer interaction and review 'excess' or 'overdiagnosis' claims that accompany increasingly sensitive diagnostic and screening technologies. We consider that immune tools should be incorporated into future research, with potential for immune system modulation for some early cancers.

Multi-omic cross-sectional cohort study of pre-malignant Barrett's esophagus reveals early structural variation and retrotransposon activity

Barrett's esophagus is a pre-malignant lesion that can progress to esophageal adenocarcinoma. We perform a multi-omic analysis of pre-cancer samples from 146 patients with a range of outcomes, comprising 642 person years of follow-up. Whole genome sequencing reveals complex structural variants and LINE-1 retrotransposons, as well as known copy number changes, occurring even prior to dysplasia. The structural variant burden captures the most variance across the cohort and genomic profiles do not always match consensus clinical pathology dysplasia grades. Increasing structural variant burden is associated with: high levels of chromothripsis and breakage-fusion-bridge events; increased expression of genes related to cell cycle checkpoint, DNA repair and chromosomal instability; and epigenetic silencing of Wnt signalling and cell cycle genes. Timing analysis reveals molecular events triggering genomic instability with more clonal expansion in dysplastic samples. Overall genomic complexity occurs early in the Barrett's natural history and may inform the potential for cancer beyond the clinically discernible phenotype.

Gastric Xanthoma Is Related to the Rapid Growth of Gastric Cancer

Early detection of gastric cancer is important. However, rapid growth of gastric cancers that cannot be resected endoscopically occurs even with periodic check-ups. Accordingly, we assessed factors associated with the speed of gastric cancer growth by examining historical endoscopic images. A total of 1996 gastric cancer cases were screened, and characteristics of lesions with slow and rapid growth were assessed. A total of 114 lesions from 114 patients were included in the assessment. Sixty slow-growing and fifty-four rapidly growing gastric cancers were compared. Female sex and incidence of lesions in the lower part of the stomach were significantly less frequent in the rapid-growth group than in the slow-growth group. History of endoscopic treatment tended to be more frequent in the rapid-growth group. Age, body mass index, histology, Helicobacter pylori status, and medications did not differ significantly between groups. Xanthoma was significantly related to rapid growth of gastric cancer, and map-like redness tended to be more frequent in the rapid-growth group in univariate analysis. Xanthoma was significantly related to rapid growth of gastric cancer on multivariate analysis. Further studies are warranted to clarify the pathophysiological mechanisms involved in the speed of gastric cancer growth.

Oversupply of Limiting Cell Resources and the Evolution of Cancer Cells: A Review

Cancer prevention is superior to cancer treatment—indeed, understanding and controlling cancer risk is a key question in the fields of applied ecology and evolutionary oncology. Ecological cancer risk models offer the dual benefit of being generalizable across cancer types, and unveiling common mechanisms underlying cancer development and spread. Understanding the biological mechanisms of cancer risk may also guide the design of interventions to prevent cancer. Ecological considerations are central to many of these mechanisms; as one example, the ecologically-based hypothesis of metabolic cancer suppression posits that restricted vascular supply of limiting resources to somatic tissues normally suppresses the evolution of somatic cells toward cancer. Here we present a critical review of published evidence relevant to this hypothesis, and we conclude that there is substantial evidence that cancer risk does increase with an abnormal excess of limiting cell resources, including both dietary macronutrients as well as certain micronutrients.

Carcinogenesis: Failure of resolution of inflammation?

Inflammation in the tumor microenvironment is a hallmark of cancer and is recognized as a key characteristic of carcinogens. However, the failure of resolution of inflammation in cancer is only recently being understood. Products of arachidonic acid and related fatty acid metabolism called eicosanoids, including prostaglandins, leukotrienes, lipoxins, and epoxyeicosanoids, critically regulate inflammation, as well as its resolution. The resolution of inflammation is now appreciated to be an active biochemical process regulated by endogenous specialized pro-resolving lipid autacoid mediators which combat infections and stimulate tissue repair/regeneration. Environmental and chemical human carcinogens, including aflatoxins, asbestos, nitrosamines, alcohol, and tobacco, induce tumor-promoting inflammation and can disrupt the resolution of inflammation contributing to a devastating global cancer burden. While mechanisms of carcinogenesis have focused on genotoxic activity to induce mutations, nongenotoxic mechanisms such as inflammation and oxidative stress promote genotoxicity, proliferation, and mutations. Moreover, carcinogens initiate oxidative stress to synergize with inflammation and DNA damage to fuel a vicious feedback loop of cell death, tissue damage, and carcinogenesis. In contrast, stimulation of resolution of inflammation may prevent carcinogenesis by clearance of cellular debris via macrophage phagocytosis and inhibition of an eicosanoid/cytokine storm of pro-inflammatory mediators. Controlling the host inflammatory response and its resolution in carcinogen-induced cancers will be critical to reducing carcinogen-induced morbidity and mortality. Here we review the recent evidence that stimulation of resolution of inflammation, including pro-resolution lipid mediators and soluble epoxide hydrolase inhibitors, may be a new chemopreventive approach to prevent carcinogen-induced cancer that should be evaluated in humans.

Cancer overdiagnosis: a biological challenge and clinical dilemma

For cancer screening to be successful, it should primarily detect cancers with lethal potential or their precursors early, leading to therapy that reduces mortality and morbidity. Screening programmes have been successful for colon and cervical cancers, where subsequent surgical removal of precursor lesions has resulted in a reduction in cancer incidence and mortality. However, many types of cancer exhibit a range of heterogeneous behaviours and variable likelihoods of progression and death. Consequently, screening for some cancers may have minimal impact on mortality and may do more harm than good. Since the implementation of screening tests for certain cancers (for example, breast and prostate cancers), a spike in incidence of in situ and early-stage cancers has been observed, but a link to reduction in cancer-specific mortality has not been as clear. It is difficult to determine how many of these mortality reductions are due to screening and how many are due to improved treatments of tumours. In cancers with lower incidence but high mortality (for example, pancreatic cancer), screening has focused on high-risk populations, but challenges similar to those for general population screening remain, particularly with regard to finding lesions with difficult-to-characterize malignant potential (for example, intraductal papillary mucinous neoplasms). More sensitive screening methods are detecting smaller and smaller lesions, but this has not been accompanied by a comparable reduction in the incidence of invasive cancers. In this Opinion article, we focus on the contribution of screening in general and high-risk populations to overdiagnosis, the effects of overdiagnosis on patients and emerging strategies to reduce overdiagnosis of indolent cancers through an understanding of tumour heterogeneity, the biology of how cancers evolve and progress, the molecular and cellular features of early neoplasia and the dynamics of the interactions of early lesions with their surrounding tissue microenvironment.

Long-term relationships between screening rates, breast cancer characteristics, and overdiagnosis in US counties, 1975-2009

Effects of mammography screening in the general population are disputed. Screening rates differ greatly between US counties, providing a natural opportunity to investigate effects of screening. We compared mammography screening rates with the types and outcomes of breast cancers diagnosed in US counties. The county screening rate was defined as the proportion of women age ≥40 with ≥1 mammogram in the past 2 years (range, 34-91%). Two periods were analyzed: 1975-2009 (612,941 breast cancer cases, 195 counties) and 1996-2009 (645,057 cases, 211-547 counties). Multiple signs of overdiagnosis were observed: First, breast cancer incidence increased as screening became common. Second, incidence stopped increasing once screening rates stabilized. Third, the increases in incidence were limited to age groups receiving screening. Fourth, the increases were larger in counties where screening became more common. Fifth, the increases were limited to small and early-stage breast cancers (which are consistent with overdiagnosis). Sixth, compensatory reductions in large and advanced-stage breast cancers were much smaller than the increases. Difference-in-differences regression analysis suggested 31% (95% CI: 28-34%) of breast cancers diagnosed in 1996-2009 were overdiagnosed. Screening rates correlated with increased incidence for all hormone receptor statuses, HER2 statuses, and grades. Reductions in breast cancer mortality during 1975-2009 were similar in screened and unscreened age groups. Overall, we found repeated signs that breast cancer overdiagnosis is widespread in the US, but the biological nature of overdiagnosed tumors remains unclear. Mortality benefits of screening, though they may be present and substantial, could not be detected at the population level.

MicroRNA-433 targets AKT3 and inhibits cell proliferation and viability in breast cancer

Breast cancer is the most frequently diagnosed malignancy in women. However, the molecular mechanisms underlying breast cancer pathogenesis are not fully understood. The present study examined the role of miR-433 in breast cancer and investigated its underlying molecular mechanisms of action. Reverse transcription-quantitative polymerase chain reaction and western blot analysis were performed to analyze the level of microRNA (miRNA/miR)/mRNA and protein expression, respectively. Additionally, MTT assay was used to determined cell proliferation and viability. Cell apoptosis was measured by flow cytometry. A dual-luciferase reporter assay was used to confirm the identity of the downstream target of miR-433. The results revealed that miR-433 was downregulated in breast cancer tissues and cell lines. Overexpression of miR-433 inhibited cell proliferation and cell viability in BT-549 cells, whereas downregulation of miR-433 increased cell proliferation and cell viability in MDA-MB-231 cells. Further flow cytometry analysis revealed that miR-433 was able to induce apoptosis and also alter the levels of proteins expression of B-cell lymphoma-2 and Bcl-associated X. Bioinformatics analysis showed that RAC-γ serine/threonine-protein kinase (AKT3) was one of the downstream targets of miR-433, and luciferase reporter assay further confirmed that AKT3 is a direct target of miR-433. The knockdown of AKT3 was able to inhibit proliferation and viability in BT-549 cells. Overexpression of AKT3 prevented the inhibitory effects of miR-433 on proliferation and viability in BT-549 cells. The level of AKT3 mRNA expression was upregulated in breast cancer tissues compared with normal tissues and was inversely correlated with miR-433 expression levels. In summary, the results of the present study results indicate that the tumor-suppressive role of miR-433 may be mediated by regulating AKT3. miR-433 may therefore serve as a potential therapeutic target for breast cancer.

Genomics, Endoscopy, and Control of Gastroesophageal Cancers: A Perspective

In The Cancer Genome Atlas the goals were to define how to treat advanced cancers with targeted therapy. However, the challenges facing cancer interception for early detection and prevention include length bias in which current screening and surveillance approaches frequently miss rapidly progressing cancers that then present at advanced stages in the clinic with symptoms (underdiagnosis). In contrast, many early detection strategies detect benign conditions that may never progress to cancer during a lifetime, and the patient dies of unrelated causes (overdiagnosis). This challenge to cancer interception is believed to be due to the speed at which the neoplasm evolves, called length bias sampling; rapidly progressing cancers are missed by current early detection strategies. In contrast, slowly or non-progressing cancers or their precursors are selectively detected. This has led to the concept of cancer interception, which can be defined as active interception of a biological process that drives cancer development before the patient presents in the clinic with an advanced, symptomatic cancer. The solutions needed to advance strategies for cancer interception require assessing the rate at which the cancer evolves over time and space. This is an essential challenge that needs to be addressed by robust study designs including normal and non-progressing controls when known to be appropriate.

Low-Dose Mixture Hypothesis of Carcinogenesis Workshop: Scientific Underpinnings and Research Recommendations

BACKGROUND

The current single-chemical-as-carcinogen risk assessment paradigm might underestimate or miss the cumulative effects of exposure to chemical mixtures, as highlighted in recent work from the Halifax Project. This is particularly important for chemical exposures in the low-dose range that may be affecting crucial cancer hallmark mechanisms that serve to enable carcinogenesis.

OBJECTIVE

Could ongoing low-dose exposures to a mixture of commonly encountered environmental chemicals produce effects in concert that lead to carcinogenesis? A workshop held at the NIEHS in August 2015 evaluated the scientific support for the low-dose mixture hypothesis of carcinogenesis and developed a research agenda. Here we describe the science that supports this novel theory, identify knowledge gaps, recommend future methodologies, and explore preventative risk assessment and policy decision-making that incorporates cancer biology, environmental health science, translational toxicology, and clinical epidemiology.

DISCUSSION AND CONCLUSIONS

The theoretical merits of the low-dose carcinogenesis hypothesis are well founded with clear biological relevance, and therefore, the premise warrants further investigation. Expert recommendations include the need for better insights into the ways in which noncarcinogenic constituents might combine to uniquely affect the process of cellular transformation (in vitro) and environmental carcinogenesis (in vivo), including investigations of the role of key defense mechanisms in maintaining transformed cells in a dormant state. The scientific community will need to acknowledge limitations of animal-based models in predicting human responses; evaluate biological events leading to carcinogenesis both spatially and temporally; examine the overlap between measurable cancer hallmarks and characteristics of carcinogens; incorporate epigenetic biomarkers, in silico modelling, high-performance computing and high-resolution imaging, microbiome, metabolomics, and transcriptomics into future research efforts; and build molecular annotations of network perturbations. The restructuring of many existing regulatory frameworks will require adequate testing of relevant environmental mixtures to build a critical mass of evidence on which to base policy decisions. Citation: Miller MF, Goodson WH III, Manjili MH, Kleinstreuer N, Bisson WH, Lowe L. 2017. Low-Dose Mixture Hypothesis of Carcinogenesis Workshop: scientific underpinnings and research recommendations. Environ Health Perspect 125:163-169; http://dx.doi.org/10.1289/EHP411.

Synthesis and deposition of a Tröger’s base polymer on the electrode surface for potentiometric detection of a neuroblastoma tumor marker metabolite

We propose an innovative approach to detect a low molecular weight metabolite of neuroblastoma.