Analysis of tumor cell evolution in a melanoma: Evidence of mutational andselective pressure for loss of p16(ink4) and for microsatellite instability

Tumorigenesis and tumor progression can be considered an evolutionary proce ss. In order to deduce information on the mutational and selective pressure s during melanoma progression we performed microsatellite analysis at 42 au tosomal and two X-linked loci in a microdissected primary melanoma and its nine metastases. Loss of heterozygosity at locus D9S259 was the only geneti c change observed in all metastases. The pattern of loss of heterozygosity at loci D9S162 and D9S171 within the region of common loss on chromosome 9p 21 which encompasses the tumor suppressor gene p16(ink4) enabled the distin ction of four genetically different tumor cell populations. Three cell line ages showed homozygous loss of the p16(ink4) gene, which evolved independen tly in each tumor cell population within the primary tumor. Additional alle le losses could be demonstrated at markers D14S53 and DXS998. The fourth li neage did not demonstrate loss of heterozygosity at loci D9S162 and D9S171 and contained the wild type p16(ink4) gene but was characterized by abundan t microsatellite instability. The evolutionary approach towards tumorigenes is and tumor progression used in this study thus confirms the role of p16(i nk4) inactivation for melanoma progression but not for melanoma initiation; it suggests the existence of additional putative tumor suppressor genes lo cated on 9p as well as on the long arm of chromosome 14 and shows that micr osatellite instability may represent an alternative pathway of tumor cell e volution in malignant melanoma.