Identification of a tumor suppressor gene of malignant mesothelioma and functional analysis of the NF2 and p16 genes

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To isolate a new tumor suppressor gene of human malignant mesothelioma, we searched for the abnormalities of chromosome 3p showing frequent loss of heterozygosity and identified that the β-catenin gene (CTNNB1), located at 3p21.33, is homozygously deleted in a malignant mesothelioma cell line, NCI-H28. We constructed a genomic DNA library from NCI-H28, and identified that the breakpoint of the homozygous region was located in intron 1 of β-catenin, which was rearranged to another region located at 3p21, rbc. We further cloned a new gene (MTSK1) of 4153 bp from this region and found that this putative protein product contains a kinase motif at the amino terminal. Northern blot analysis demonstrated that its message sizes are 4.7 and 2.1 kb and that MTSK1 is expressed in the testis at a high level but at low levels in other tissues. We studied for the mutation in 9 malignant mesotheliomas and 68 lung cancers by PCR-SSCP analysis and identified several aberrant mobility shifts, which are now being characterized by sequencing. A protein analysis is also scheduled in the future with antibodies that are under synthesis against MTSK1. Meanwhile, we established new malignant mesothelioma cell lines, Y-Meso8A and Y-Meso8B, from a patient and tested tumorigenecity of these cell lines by transplantation in nude mice. Mutational analysis for tumor suppressor genes (NF2, p16, p53, E-cadherin) and oncogene (KRAS, NRAS, and MDM2) showed that these cell lines do not harbor either mutations of these genes.