Oncogene (2002) 21, 7421 – 7434 ª 2002 Nature Publishing Group All rights reserved 0950 – 9232/02 $25.00 www.nature.com/onc Genetic alterations of multiple tumor suppressors and oncogenes in the carcinogenesis and progression of lung cancer Hirotaka Osada1 and Takashi Takahashi*,1 1Division of Molecular Oncology, Aichi Cancer Center Research Institute, 1-1 Kanokoden, Chikusa-ku, Nagoya 464-8681, Japan Lung cancer has become the leading cause of cancer number of deaths. Multiple morphological steps have death in many economically well-developed countries. been recognized in the carcinogenic process of lung Recent molecular biological studies have revealed that cancer. In the development of squamous carcinoma at overt lung cancers frequently develop through sequential the bronchial epithelium, oncogenic triggering converts morphological steps, with the accumulation of multiple normal bronchial epithelium into hyperplastic, meta- genetic and epigenetic alterations affecting both tumor plastic and dysplastic lesions. After these premalignant suppressor genes and dominant oncogenes. Cell cycle stages, lung cancer develops as a carcinoma in situ progression needs to be properly regulated, while cells (CIS), and then emerges as an overt squamous cell have built-in complex and minute mechanisms such as carcinoma. Adenocarcinoma is also considered to cell cycle checkpoints to maintain genomic integrity. develop at least in part from premalignant precursor Genes in the p16INK4A-RB and p14ARF-p53 pathways lesions such as atypical adenomatous hyperplasia appear to be a major target for genetic alterations (AAH). Molecular biological studies have demon- involved in the pathogenesis of lung cancer. Several strated that overt cancers carry multiple genetic and oncogenes are also known to be altered in lung cancer, epigenetic alterations, indicating inactivation of tumor leading to the stimulation of autocrine/paracrine loops suppressor genes and activation of dominant onco- and activation of multiple signaling pathways. It is genes during the processes of carcinogenesis and widely acknowledged that carcinogens in cigarette subsequent progression of lung cancer. In fact, smoke are deeply involved in these multiple genetic accumulating evidence points to the multi-step accu- alterations, mainly through the formation of DNA mulation of genetic and epigenetic changes, which adducts. A current understanding of the molecular often accompany the sequential morphological changes mechanisms of lung cancer pathogenesis and progres- (Figure 1). sion is presented in relation to cigarette smoking, an It is widely acknowledged that cigarette smoking is absolute major risk factor for lung cancer development, a dominant risk factor for lung cancer, and that by reviewing genetic alterations of various tumor carcinogens in the smoke are main initiators of lung suppressor genes and oncogenes thus far identified in cancer by inducing multiple genetic alterations, lung cancer, with brief summaries of their functions and mainly through the formation of DNA adducts. regulation. Fingerprints of such genetic insults can be seen, for Oncogene (2002) 21, 7421 – 7434. doi:10.1038/sj.onc. example, in the mutational spectra of p53 and KRAS. 1205802 Proper cell cycle regulation and checkpoints are crucial for maintaining genomic integrity, and their Keywords: tumor suppressor; oncogene; lung cancer abrogations are thought to contribute to genomic instability (Dhar et al., 2000), thereby playing an important role even in the early steps of cancer Introduction development. Many of the tumor suppressor genes and oncogenes altered in lung cancer are known to Lung cancer has become the leading cause of cancer play a role in the regulation of cell cycle progression death in many industrialized countries claiming more in either a direct or an indirect manner, and a than 160 000 and 50 000 lives annually in the United considerable proportion of the lung cancer-related States and Japan, respectively (Minna et al., 1997; genes are a component of the checkpoint mechan- Statistics and Information Department, Minister’s isms. In addition to the genetic and epigenetic Secretariat, 1999). A better understanding of the changes to genomic DNA, the presence of genetic molecular pathogenesis of this fatal disease is thus instability in lung cancer is also reflected by frequent urgently needed in order to achieve a preventive or structural and numerical changes of chromosomes therapeutic breakthrough aiming at a reduction in the (Testa, 1996; Haruki et al., 2001). In this study, we will mainly focus on the genetic changes, in relation to cigarette smoking, and their roles in the pathogen- *Correspondence: T Takahashi; E-mail: [email protected] esis and progression of lung cancer. Genetic alterations of lung cancer H Osada and T Takahashi 7422 Figure 1 Accumulation of alterations in the multi-step progression of lung carcinogenesis. Oncogenic triggering events, most sig- nificantly due to exposure to carcinogens in cigarette smoke, result in the accumulation of genetic and epigenetic alterations, which convert normal bronchial epithelium into hyperplastic, metaplastic and dysplastic lesions, and then carcinoma in situ (CIS) and overt squamous cell carcinomas. Adenocarcinoma is also thought to develop similarly in a multi-step fashion at least in part from pre-malignant lesions in the peripheral airways such as atypical adenomatous hyperplasia (AAH). Supposed biological consequences resulting from the genetic and epigenetic alterations are indicated Tumor suppressors and growth inhibitory signals components of checkpoints and growth inhibitory pathways as discussed below (Figure 2). During the processes of lung carcinogenesis and progression, multiple tumor-suppressor genes are p53/MDM2/p14ARF pathway (p53, 17p13.1; MDM2, inactivated. Alterations inactivating tumor suppressor 12q14.3-q15; p14ARF, 9p21) genes usually involve two events: deletion of a large chromosomal DNA segment of one allele and a smaller The p53 tumor suppressor gene is now regarded as a mutational or epigenetic event in the other allele. guardian of the genome, playing a central role in cell Previous studies on loss of heterozygosity (LOH) have cycle checkpoints. Upon damage of DNA in the provided clues to the localization of tumor suppressor genome, the ATM and ATR kinases are activated, genes involved in the pathogenesis. Deletions of 3p, resulting in the phosphorylation and consequential 9p21, 13q14, and 17p13 are frequently observed even in activation of the p53 protein directly at Ser 15 and the early lesions preceding the conversion into overt indirectly via CHK2 or CHK1 at Ser 20 (Figure 3). lung cancer. Allelic loss of 3p is among the most The activated p53 transactivates its target genes frequent and earliest events in lung cancer develop- including p21CIP/WAF1, 14-3-3s, BAX, and GADD45 ment, suggesting that an important tumor suppressor (Yu et al., 1999). Up-regulation of p21CIP1/WAF1 gene(s) may reside in this region. The 9p21, 13q14 and (Harper et al., 1993; el-Deiry et al., 1993) and 14-3-3s 17p13 regions harbor the p16INK4A/p14ARF/ (Hermeking et al., 1997) by p53 imposes G1 and G2 p15INK4B, Retinoblastoma (RB) and p53 genes, arrest, respectively, while the activated p53 is also inactivation of which has been shown to play a believed to participate in DNA repair through the fundamentally important role in the pathogenesis of induction of p53R2 expression (Yamaguchi et al., lung cancer. It is of note that these tumor suppressor 2001). Alternatively, unrepairable DNA damage may proteins compose functional linkages, i.e., p14ARF- lead to apoptotic cell death via p53-dependent p53 and p16INK4A-RB pathways, and that they are induction of the expression of various downstream Oncogene Genetic alterations of lung cancer H Osada and T Takahashi 7423 Figure 2 Alterations of tumor suppressor pathways in lung cancer. Two major pathways (i.e., p14ARF-p53 and p16INK4A-RB) involved in the pathogenesis of lung cancer are functionally linked and play roles as a component of checkpoints and growth in- hibitory pathways. In addition, signals from various oncogenes and growth factors are also linked to these pathways. Note that many of the components are inactivated in lung cancer. Molecules with frequent activating alterations in lung cancers are marked in red. Molecules showing frequent and infrequent inactivations are indicated in dark and light blue, respectively genes including the pro-apoptotic Bcl-2 family (BAX, resultant bulky DNA adducts, which are mainly NOXA,andPUMA) (Miyashita and Reed, 1995; Oda formed by carcinogens in cigarette smoke, preferen- et al., 2000a; Yu et al., 2001), APAF-1 (Robles et al., tially cause a guanine to thymine transversion. Highly 2001), PIG3 (Venot et al., 1998) and p53AIP (Oda et sensitive densitometric examination has revealed that al., 2000b). Phosphorylation at Ser 46 of the p53 smoking induces p53 mutations even in bronchial protein by HIPK2 has been suggested to play a key epithelial cells of normal morphological appearance role in the induction of apoptosis by p53 in concert (Hussain et al., 2001). In addition to a role for p53 with the phosphorylation of Ser15 and Ser20, described inactivation at the early stages of lung cancer above (D’Orazi et al., 2002; Hofmann et al., 2002). The development, accumulating evidence suggests a possi- regulation of centrosomal duplication has also been ble relation to a poor prognosis after surgical removal linked to p53 and its effecter p21CIP1 (Tarapore