Background
The sequential genetic changes that drive a cell towards malignancy occur over several years. In view of this, the American Association for Cancer Research Task Force on the treatment and prevention of intraepithelial neoplasia (IEN) has recognized the importance of targeting the treatment of early cancerous lesions to prevent or regress carcinogenesis [
1]. Considerable research has identified molecular markers of IEN that serve as useful targets or endpoints of chemoprevention [
1‐
3]. This laudable effort by the Task Force can be complemented by identification of biomarkers in normal tissues adjacent to tumors (peri-tumoral cancer fields). Validated biomarkers from cancer fields should be useful for primary chemoprevention studies as well.
The idea of field cancerization was conceived by Slaughter almost a decade prior to introducing the term in 1953. In an earlier publication he stated that; "
cancer does not arise as an isolated cellular phenomenon, but rather as an anaplastic tendency involving many cells at once" [
4]. The term "lateral cancerization" was subsequently used to indicate that the lateral spread of tumors was due to progressive transformation of cells adjacent to a tumor, rather than the spread and destruction of the adjacent epithelium by pre-existing cancer cells [
5]. In a more extensive histopathologic review of 783 oral cancer patients, Slaughter and colleagues then used the term field cancerization to describe the existence of generalized carcinogen induced early genetic changes in the epithelium from which multiple independent lesions occur, leading to the development of multifocal tumors [
6]. In some cases, multiple contiguous tumor foci coalesce that partly explain the lateral spread of squamous cell cancers. It was also observed that normal looking cells in close proximity to malignant cells were histologically abnormal and therefore were part of the transformed cells in a particular tumor field, and consequently were responsible for the occurrence of local tumor recurrences. These observations were made at about the era the DNA double helix was discovered by Watson and Crick, hence in the absence of modern molecular techniques. More recent studies using various genetic analyses have provided unequivocal evidence in support of the work of Slaughter and colleagues [
7].
An important unanswered question of field cancerization is exactly how these cancer fields develop. Could a particular carcinogen induce multiple distinct genetic alterations in different cells resulting in polyclonal pre-neoplastic lesions in a particular tissue from which multiple tumors develop? Does a single genetic event occur in a cell that clonally expands and laterally spreads to replace the normal epithelium, and therefore create a large area of pre-neoplastic field from which multiple tumors develop? Alternatively, does an early genetic event occur simultaneously in a group of cells such that subsequent genetic lesions drive some cells towards malignancy? Is it also plausible that some field changes are created during organogenesis when some altered cells proliferate to generate a vast area of preconditioned epithelial surface, such that subsequent genetic lesions in some cells result in multiple cancers? Whereas all these modes of field cancerization are possible, molecular data accrued from studies of head and neck, esophagus, and bladder cancers support the clonal expansion model [
8‐
10]. This model could explain how multiple tumors develop in organs such as skin, colon, esophagus, stomach, bladder, cervix, and vulva that have contiguous epithelium. Though possible, it is less likely that the spread of monoclonal cells is responsible for the development of multiple tumors in glandular tissues like the lung, breast, ovary, pancreas, and prostate that have discontinuous glandular epithelial organization, even when exposed to the same carcinogen. Multiple tumors in these organs are likely to originate from polyclonal tumor stem cells, although these clones may have identical genetic alterations. For example, loss of heterozygosity (LOH) in primary ductal carcinoma in situ (DCIS) were distinct from those in their corresponding cancers, suggesting that some DCIS and subsequent adenocarcinoma in the same breast developed from genetically divergent clones [
11]. Furthermore, a study of microdissected normal glands and their co-existing malignant epithelia from breast cancer specimens demonstrated that only one had identical LOH to the cancer, with the majority harboring LOH at different genetic loci from the tumor [
12].
Notwithstanding the mechanism of precancerous field development, the multistep model of tumorigenesis is probably better understood by field cancerization studies. Importantly, genetic information gained at various stages of cancer development continues to advance our knowledge and understanding of cancer biology, and how such molecular markers can be used clinically. Recent molecular techniques have enabled detection of alterations at all genomic levels. Epigenetic gene silencing, chromosomal anomalies, LOH, DNA sequence analysis for SNP and mutation detection, altered gene expression (transcripts and proteins), and mitochondrial genome changes have all been demonstrated in both precancerous lesions and normal appearing cells close to tumors [
10,
13‐
19]. One can even infer a metabolomic field cancerization profile in various tissues. It could be surmised that the early genetic changes will remain with the tumor cells as well as pre-neoplastic cells from the same organ. If well characterized, such early molecular alterations could have great value in risk assessment, early cancer detection, monitoring of disease progression, and chemoprevention.
A highly informative but almost forgotten genome is that contained in the mitochondrion. This organelle produces almost all the energy required for cellular physiology, and an inevitable byproduct of this reaction is the generation of reactive oxygen species (ROS), which can damage nucleic acids, proteins and lipids. The mitochondrial genome is less protected and has an inefficient repair mechanism compared to the nuclear genome, and yet is bathed by ROS. Not surprisingly therefore, the mitochondrial genome has a high mutation rate, and this may signal the genesis of cancer [
20]. In addition, leakage of ROS into the nucleus can cause mutations in nuclear genes that could initiate the malignant process [
21]. It is thus likely that the modest mtDNA molecule might sustain early genetic damage, and thus an even earlier biomarker indicative of field cancerization compared to nuclear genomic alterations that have been extensively studied.
Since the focus of this review is the identification and utility of genetic alterations in normal adjacent cells or precancerous cells that differ from normal donor control samples, the term field cancerization is used to broadly mean "the process whereby cells in a particular tissue or organ are transformed, such that genetically altered but histologically normal appearing cells predate the development of neoplasia or coexist with malignant cells, irrespective of clonality".
The need for appropriate control tissue in cancer studies
An important message from the above review of the literature is that using histologically normal appearing samples as the sole control tissue in cancer research is probably inappropriate [
88]. At its minimum, the use of donor tissues (ideally obtained under similar conditions as tumor) will serve as better controls for tumor-specific biomarker discovery. Donor control, in addition to normal adjacent to tumors, precancerous lesions, and tumor samples will provide the best sample set for resolution of genetic alterations that are relevant to the disease process. If normal adjacent to tumor tissue must be used as the only normal control, it should first be examined for the absence of genetic abnormalities [
88]. In skin cancer studies for example, additional analysis of UV-induced damage in sun protected skin from the same individual would provide a complementary isogenic control. With regards to mtDNA analysis, it is regrettable that a number of investigators are still using normal appearing cells in the presence of tumors as normal maternal germline control tissue from which somatic mtDNA mutations could be unraveled [
89‐
97]. Obviously this will lead to erroneous mutations being associated with the disease processes, since the adjacent normal tissue in a cancerization field may sustain somatic mtDNA mutations as well. Comparison to the mtDNA profile of the individual's blood, which we have determined to be the authentic germline mitochondrial genome, is necessary.
Competing interests
The author(s) declare that they have no competing interests.