Seminar
Circulating free tumor DNA and colorectal cancerADN libre circulant d’origine tumorale et cancer colorectal

https://doi.org/10.1016/j.gcb.2009.04.015Get rights and content

Summary

Cancer is characterized by multiple somatic genetic and epigenetic alterations that could be useful as molecular markers for detecting tumor DNA in different bodily fluids. In patients with various diseases as well as in healthy subjects, circulating plasma and serum carry small amounts of non-cell-bound DNA. In this free circulating DNA, tumor-associated molecular alterations can be detected in patients who have cancer. In many instances, the alterations identified are the same as those found in the primary tumor tissue, thereby suggesting tumor origin from a fraction of the circulating free DNA. In fact, various types of DNA alterations described in colorectal cancer have been detected in the circulating free DNA of patients with colorectal cancer. These alterations include KRAS2, APC and TP53 mutations, DNA hypermethylation, microsatellite instability (MSI) and loss of heterozygosity (LOH). Also, advances in polymerase chain reaction (PCR)-based technology now allow the detection and quantification of extremely small amounts of tumor-derived circulating free DNA in colorectal cancer patients. The present report summarizes the literature available so far on the mechanisms of circulating free DNA, and on the studies aimed at assessing the clinical and biological significance of tumor-derived circulating free DNA in colorectal cancer patients. Thus, tumor-derived circulating free DNA could serve as a marker for the diagnosis, prognosis and early detection of recurrence, thereby significantly improving the monitoring of colorectal cancer patients.

Résumé

Les altérations génétiques ou épigénétiques somatiques des cellules tumorales constituent un moyen d’identification moléculaire de la présence d’ADN tumoral dans un prélèvement biologique. De l’ADN libre est présent dans le sang et, chez les patients atteints de cancer, une fraction de cet ADN est d’origine tumorale. Chez des patients atteints de cancer, l’origine tumorale d’une fraction de l’ADN libre circulant a été démontrée au moyen de la détection d’altérations génétiques de l’ADN libre circulant plasmatique et/ou sérique identiques à celles mises en évidence au niveau de l’ADN extrait de la tumeur. Les altérations génétiques et épigénétiques les plus communes décrites dans le cancer colorectal ont été détectées au niveau de l’ADN libre circulant plasmatique et/ou sérique de patients atteints de cancer colorectal. Il s’agit principalement de mutations de l’oncogène KRAS2, de mutations des gènes suppresseurs de tumeurs APC et TP53, d’altérations de marqueurs microsatellites et d’anomalies de la méthylation de l’ADN tumoral. Grâce aux développements technologiques considérables réalisés dans le domaine de l’analyse de l’ADN, la recherche de nouveaux biomarqueurs du cancer colorectal basée sur la détection de l’ADN libre d’origine tumorale sérique ou plasmatique ou extrait d’autres prélèvements biologiques ouvre de nouvelles voies pour la mise au point de tests relativement simples et peu onéreux pour le dépistage, le diagnostic précoce de récidive, l’évaluation du pronostic et la prédiction de la réponse aux traitements.

Introduction

A number of alterations occurring in cells and biological molecules during the course of neoplastic processes can be considered markers of cancer, and advances in genomics and proteomics now make it possible to identify such markers in plasma and serum, as well as in other bodily fluids such as urine, pancreatic juice, fecal matter and saliva. This makes these biological media highly useful for screening and making the diagnosis, and for determining the patient's prognosis and therapeutic assessment. In patients with solid tumors, management decisions are made on the basis of clinical, histological and, more rarely, molecular factors, yet the overall picture—given the heterogeneous nature of tumorigenesis—remains imperfect, making it difficult to determine the most appropriate diagnostic or therapeutic strategy. However, one of the characteristic features of cancer cells is the presence of genetic anomalies (amplifications, deletions, point mutations, chromosomal translocations, microsatellite instability [MSI]) and/or epigenetic alterations (promoter gene hypermethylation, acetylation) that perturb the expression of the genes controlling critical cell processes, such as proliferation, differentiation, the cell cycle, apoptosis and angiogenesis [1].

Thus, an improved knowledge of the molecular biology of cancer can be expected to open the way for new perspectives not only in fundamental research (better understanding of carcinogenesis), but also in drug development (for example, the currently rapidly developing ‘target-therapy’ approach) and clinical management (use of biomarkers to establish the diagnosis or prognosis, or to predict response to treatment). Previous research has demonstrated that either plasma or serum can contain a small quantity of free (non-cell-bound) circulating DNA. Concentrations of this free circulating DNA, in the order of a few nanograms per milliliter (ng/mL), increase significantly in patients with cancer compared with healthy subjects [2]. Also, there is clear evidence that, in cancer patients, a fraction of this DNA is tumor-derived, as the genetic and/or epigenetic molecular alterations characteristic of the tumor are found in the free DNA circulating in the plasma or serum. In fact, molecular alterations characteristic of the majority of solid tumors have been detected in the serum or plasma of cancer patients [3]. However, most of the research into the detection of free tumor-derived DNA in blood samples, and in other biological samples such as fecal matter, urine or pancreatic juice, is still in the preliminary stage. Nevertheless, this molecular approach to the detection of free circulating tumor-associated DNA in non-tumor biological samples introduces exciting new perspectives for the diagnosis and follow-up of patients with cancer.

Section snippets

Free cirulating DNA in plasma and serum

Free circulating DNA was first reported in serum from patients with systemic lupus erythematosus, rheumathoid arthitis and glomerulonephritis [4], [5], [6], [7], [8]. Leon et al. [2] were the first to report that cancer patients presented with higher levels of such non-cell-bound circulating DNA in their blood than did patients with non-cancerous diseases. These findings were later confirmed in a series of patients with gastrointestinal cancer [9]. The mean concentration of free DNA circulating

Molecular alterations of plasma or serum free circulating DNA in colorectal cancer

When precisely characterized, genetic or epigenetic molecular alterations of cancer-cell DNA should offer a means of identifying the presence of cancer-cell DNA in biological samples. PCR has enabled the development of sensitive, specific and reproducible molecular-biology techniques that can demonstrate the presence of one altered gene copy in 10,000 to 100,000 normal copies. Such diagnostic techniques have been used for years to detect residual disease in bone-marrow samples from patients

Free tumor-derived circulating DNA in plasma or serum: a biomarker with prognostic value and/or predictive of treatment effect in colorectal cancer

In light of the growing complexity of management strategies for colorectal cancer, it is essential to validate any new prognostic factors that could be used to identify subgroups of patients who may be expected to benefit from the various therapeutic strategies. For a given type of cancer at a given stage, the expected beneficial effect of a particular therapeutic strategy is never observed in all patients. The reason is undoubtedly related to the biological heterogeneity of most solid tumors.

Conclusion

The identification of new markers of colorectal cancer responds to the need for new sensitive and specific biological tests to enable earlier diagnosis, more accurate evaluation of the prognosis and earlier detection of recurrent disease, while also offering new directions towards the development of other diagnostic and/or therapeutic strategies to improve the prognosis. Cell-free circulating tumor-associated DNA, easily detectable in blood and possibly urine, is a potentially attractive

Conflict of interest statement

None.

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