Biomarkers and the genetics of early neoplastic lesions.

Two great challenges in cancer diagnosis and prevention are 1) the detection of specific pre-invasive neoplastic lesions that give origin to malignant tumors and 2) the identification of those prognostic factors of tumors that predict the outcome of individual cancer patients. As personalized medicine becomes more established, the use of biomarkers to predict responses to various therapeutic regimens also will become much more important. Also, the ability to detect neoplastic transformation at its earliest presence is a requisite for improving preventive interventions and reducing cancer incidence. In this chapter we will discuss the molecular aspects of early pre-invasive neoplastic lesions; specifically, alterations in genes regulating proliferation, differentiation, apoptosis and invasiveness as well as chromosomal aberrations and microsatellite instability will be addressed. We will argue that the identification of molecular changes associated with neoplastic transformation will lead to the development of new molecular markers for the early detection of pre-invasive neoplastic and malignant lesions, assessment of cancer risk, assessment of responses to preventive or therapeutic interventions, and separation of less aggressive neoplastic lesions from more aggressive lesions. 1.1. Early Neoplastic Lesions: Definition and General Concepts Neoplastic lesions may derive from epithelial or nonepithelial cellular populations. Often, pre-invasive neoplastic lesions are small, multiple and may display wide ranges of diversity [1]. Morphological alterations characterizing pre-invasive neoplastic lesions include nuclear pleomorphism consisting of increased variation in nuclear size, shape, and hyperchromatism, abnormal mitoses, abnormal nucleolar patterns and altered or absent differentiation. The evolution of these types of lesions may take three directions: spontaneous regression, progression toward a fully malignant phenotype, or stasis, remaining as a pre-invasive neoplastic lesion (2). Although there are discrepancies in terminology used by pathologists to describe these types of early neoplastic lesions, they are most frequently referred to as pre-invasive neoplasia, dysplasia, carcinoma in situ, intraepithelial neoplasia, or as incipient cancer (3,4). Based exclusively on morphological features, lesions which progress to malignant tumors cannot be easily distinguished from lesions which spontaneously regress. It has been shown that normal epithelium and normal-looking tissue in the proximity of a tumor may carry mutated cancer genes, suggesting that the first steps in early pre-invasive neoplasia do not have a distinctive morphologic correlate (5–8). Similarly, molecular changes in lesions known to be at risk for developing morphologic pre-invasive neoplasia such as ulcerative colitis, actinic skin damage, and Barrett’s esophagus, may precede histomorphologic changes by many months (9,10). Development of molecular markers, which clearly reflect various stages of neoplastic transformation, may redefine the diagnostic criteria for precursor lesions of cancer (11). Carcinogenesis is an indivisible continuum of molecular and morphological changes that may culminate in the development of invasive tumors. A growing body of evidence suggests that a tumor may initially start as a stem cell that develops mutations and subsequently produces a clonal population of cells harboring these and additional mutations which uniquely suit the clone to its microenvironment. Because early neoplastic lesions exhibit phenotypic heterogeneity, multicentricity, genetic instability and other chromosomal alterations, it is not always easy to predict their biological behavior or risk of progression [12]. Studies in which the molecular features of pre-invasive neoplastic lesions are correlated with molecular features of matching invasive lesions, for example, a colorectal cancer with an adenomatous polyp forming its edge, should be useful in identifying molecular features associated with greater risks of progression (13). Similarly, studies of prognostic factors in specific cancers may identify molecular features associated with aggressive behavior (14). Good transgenic mouse models of a cancer (e.g., pancreatic cancer) also may be useful in identifying molecular features leading to the development of metastatic disease (15,16). In some cases it is the combination of molecular changes, not one molecular change, that leads to aggressive behavior. For example, in mouse models mutations in K-ras frequently lead only to pancreatic intraepithelial neoplasia (PanIN) and only loss of p16/p19 (Ink4a/Arf) may not cause major changes in transgenic mice; however, activation of K-ras-2 by mutation plus loss of Ink4a/Arf leads to metastatic pancreatic cancer (16) (See Figure 1). Of note is that some molecular features in one tumor may identify an aggressive subset of tumors, but these same markers e.g. p53, may not identify aggressive features in a different tumor (13,17). The study of early neoplastic lesions is further complicated by observations that cells carrying chromosomal and genetic alterations may disappear during the spontaneous regression of intraepithelial neoplasia, as reported for cervical, skin and bronchial lesions (1,2,17). Fig. 1 The development of micro-invasive pancreatic cancer via the development and progression of pancreatic intraepithelial neoplasia. (PanIN). Thus it is apparent that molecular changes precede histological and morphological alterations and these findings may have implications for developing a new classification of early neoplastic lesions, based on molecular parameters (11); however, unless there are grossly observable histopathologic changes (e.g., increased vascularity or inflammation), that mark areas in which molecular changes are likely, molecular changes alone are unlikely to be of practical use in identifying the boundaries of pre-invasive neoplastic lesions because the pattern of sampling would be too complex and the many assays to demark the boundary of the lesion would be too costly.