21.TUMOR SUPPRESSION AND FUNCTIONAL GENOMICS PROJECT
Inactivation of a Tumor Suppressor Gene TSLC1 in Human Cancers
Non-small cell lung cancer (NSCLC) is one of the most common human malignancies in the world. Elucidation of the molecular mechanism of lung tumor suppression is therefore crucial to overcome this refractory cancer. TSLC1 is a tumor suppressor gene in human NSCLC that was identified by this project through functional complementation based on the suppression of tumorigenicity in nude mice. Two-hit inactivation by promoter methylation associated with loss of heterozygosity (LOH) on chromosomal loci on 11q23 was observed in 21 of 48 (44%) primary NSCLC tumors as well as 6 of 12 NSCLC cell lines. Inactivation of the TSLC1 gene was further observed in 7 of 22 (32%) primary prostate cancers, 4 of 14 (29%) hepatocellular carcinomas, and 3 of 12 (25%) pancreatic cancers (256,257). Promoter methylation of the TSLC1 was also detected in 15 of 97 (16%) primary gastric cancers by collaboration with others (258). These findings suggest that the alterations of TSLC1 would be involved in many human cancers. Furthermore, restoration of TSLC1 expression strongly suppressed subcutaneous tumor formation of a human prostate cancer cell line, PPC-1, or metastasis of a human NSCLC cell line, A549, from the spleen to the liver in nude mice, suggesting that TSLC1 could provide a novel therapeutic target for these tumors (259).
Characterization of TSLC1 Protein and Its Cascade of Tumor Suppression
TSLC1 contains 3 immunoglobulin (Ig)-like C2 type loops in its extracellular domain and shows significant homology with NCAM. Biochemical and cell biological studies have revealed that TSLC1 is a glycosylated membrane protein that forms cis homo-dimers and is expressed at the cell-cell attachment site. In polarized cells, TSLC1 is preferentially expressed in the lateral membrane. Over-expression of TSLC1 induces cell aggregation in a Ca2+/Mg2+_independent manner, indicating that TSLC1 is a member of the Ig super-family cell adhesion molecules (260). It is also demonstrated that TSLC1 directly associates with DAL-1, a gene product of another lung tumor suppressor belonging to the protein 4.1 family. TSLC1 further interacts with the actin filament through DAL-1 at the cell-cell attached site, where the complex formation of TSLC1 and DAL-1 is dependent on the integrity of the actin cytoskeleton. Re-distribution of both TSLC1 and DAL-1 to the newly generated membrane-ruffling areas suggests that these proteins are also involved in cell motility accompanying the actin rearrangement (259). These findings, together with frequent loss of their expression in lung cancers, suggest that TSLC1 and DAL-1 play a critical role in the same pathway involved in the suppression of lung tumor formation and metastasis. To further elucidate the physiological and pathological roles of TSLC1, a murine orthologue of the Tslc1 was isolated (261), and a mouse model that is genetically deficient in the TSLC1 is being constructed.
Analysis of Genetic Alterations in Human Cancers
Genetic alterations in the chromosomal regions including 11q were extensively examined in human cancers, and the homozygous deletion was detected on the chromosomal region 5q12 in 2 cell lines from human small cell lung cancer (262).
Investigation of the Genetic Basis of Cancer Susceptibility
To elucidate the molecular basis of the cancer susceptibility of individuals, numbers of cancer-related genes were examined for single-nucleotide polymorphisms (SNP), and several SNPs were newly identified. Allele-specific expression of these genes is being analyzed.