4. Biology Division

Inactivation of tumor suppressor genes as well as activation of oncogenes are critical genetic events in the development of human cancer. As it has been hypothesized that there are still a number of tumor suppressor genes and oncogenes remaining to be isolated, the main focus of the Biology Division is the identification of novel tumor suppressor genes and oncogenes involved in the genesis and progression of human cancer. Functional analyses of these genes are also in progress. Recently, two other research projects have been initiated in the Division. The aim of one is to elucidate the involvement of genomic instability in human carcinogenesis, and that of the other is to clarify molecular mechanisms of invasion and metastasis.

Genetic Alterations in Human Cancers

Tumor suppressor genes are located at loci showing loss of heterozygosity (LOH) or homozygous deletion in tumors. Comparative allelotype analysis of early and advanced stage non-small cell lung carcinomas revealed that the incidences of LOH on chromosomes 2q, 9p, 18q and 22q in brain metastases were significantly higher than those in stage I lung cancer.⁽⁴⁴⁾ Previously, a homozygous deletion was found at chromosome 2q33 in small cell lung carcinoma (SCLC) and a novel phospholipase C family gene, PLC-L/e, was isolated as a candidate tumor suppressor gene from the region of the homozygous deletion. Molecular analysis of the deletion at 2q33 in lung cancers revealed that this region was interstitially deleted in the relevant cell line and that a common region of LOH includes this region. These results further support the presence of a novel tumor suppressor gene in this chromosomal region.^(45,46) Mutational analysis of the PLC-L/e gene as well as a search for other genes in the region of homozygous deletion are now in progress in our laboratory. To identify novel oncogenes involved in human carcinogenesis, molecular analyses have been performed on several amplified or translocated chromosomal regions in human cancers. Arbitrarily primed-PCR genomic fingerprinting analysis revealed that restricted chromosomal regions including the loci of myc-family genes are selectively amplified in SCLC.⁽⁴⁷⁾ A novel gene, designated AF-5a, was cloned and mapped to chromosome 5q12 and recognized as being the 11th partner gene fused with the MLL gene on 11q23 by a complex translocation involving chromosomes 5, 6, 8 and 11.⁽⁴⁸⁾ Two human leukemia cell lines have been established in which the EVI-1 gene is activated either by translocation or inversion involving chromosome 3q26.^(49,50)

Functional Analysis of Tumor Suppressor Genes

There are several projects in the Division aimed at elucidating the pathogenetic significance of tumor suppressor gene inactivation in human cancer. Phosphorylation mechanisms of cyclin-cdk complexes were investigated in association with inactivation of the RB gene. The results indicate that the consensus motif for phosphorylation by cyclin D1-cdk4 is different from that for phosphorylation by cyclin A/E-cdk2.⁽⁵¹⁾ Furthermore, it was shown that cdk2 forms an inactive complex with cyclin D1.⁽⁵²⁾ Tetracycline-dependent inducible systems of p53 gene expression were developed in a SCLC cell line. By using this system, it was shown that apoptosis but not G1 arrest was induced by expression of the wild-type p53 gene in SCLC cells.⁽⁵³⁾

Genomic Instability and Carcinogenesis

Since it has been shown that genomic instability is present in a variety of cancer cells and in cancer prone families,⁽⁵⁴⁾ the prevalence of micro-satellite instability was investigated in gastric cancer prone families and sporadic cervical carcinomas. Genomic instability represented by the replication-error phenotype was present in 2 of 3 gastric cancer patients who had family histories of gastric cancer aggregation, while it was rare in sporadic cervical carcinomas,^(55-57) suggesting that inherited disorders in mismatch repair systems contribute to high susceptibility to gastric cancers in a subset of gastric cancer prone families.

Identification of Genes Involved in Metastasis

To identify genes differentially expressed in association with the metastatic potential of K-1735 murine melanoma cells, the mRNA differential display method was applied to compare mRNAs from high- and low-metastatic cells. Eight genes were identified as being expressed in either high- or low-metastatic cells. Integrin a6 and two unknown genes were expressed in high-metastatic cells, whereas b-tropomyosin, macrophage colony-stimulating factor, inhibin/activin bB subunit, and two unknown genes were expressed in low-metastatic cells.⁽⁵⁸⁾ These results indicate that the acquisition of metastatic potential in tumor cells is regulated by activation and/or inactivation of several specific genes, such as those for cell adhesion molecules, cytoskeletal proteins, and growth factors.

List of papers from this division

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