Clones and Subclones in the Lung Cancer Field

Based on a variety of clinical (1,2), histologic (3,4), cytoge-netic (5–7), and molecular (8–10) data, lung cancer, like many other epithelial cancers, is suggested to develop as a multistep tumorigenesis process in a field of cancerization (11,12). Car-cinogen exposure (e.g., tobacco) is believed to induce genetic damage in target cells that accumulates over years of exposure. Moreover, chronic tissue injury leads to chronic wound healing, whereby cells carrying genetic alterations that are important for survival and preferential regrowth expand at the expense of their neighbors. These surviving colonies continue to accumulate new tissue-relevant genetic alterations and eventually evolve into an invasive lesion. Since the whole aerodigestive tract is similarly exposed to carcinogen, one might predict that such clonal out-growths develop throughout the exposed tissue. Until recently, characterization of the fine structure of such a multifocal and multistep tumorigenesis process in lung tissue was technically hampered by the need for tissue dissociation either for cell culture or for bulk molecular genetic or cytoge-netic analyses. The advent of molecular techniques that could be used on smaller amounts of tissue, including polymerase chain reaction on microdissected epithelium (13,14) and in situ hy-bridization techniques (15–17), increased the spatial resolution of the analyses. Moreover, the identification of molecular changes that occurred at relatively high frequencies in lung tu-mors provided informative markers that could be used on small tissue samples to query the nature of the lung cancerization field (18,19). As a result, several groups have now demonstrated the presence of clonal outgrowths in the epithelial field of lung tumors, with higher frequencies of genetic changes detectable in dysplastic lesions than in normal-appearing and metaplastic le