CELL SENESCENCE AND A MECHANISM OF CLONAL EVOLUTION LEADING TO CONTINUOUS CELL-PROLIFERATION, LOSS OF HETEROZYGOSITY, AND TUMOR HETEROGENEITY - STUDIES ON 2 IMMORTAL COLON-CANCER CELL-LINES

Extensive karyotypic analysis was performed on early and late passages of two continuous human cell lines, SW480 and SW620, that were derive d from the same colon cancer patient. We cultivated these two cell lin es in vitro for a period of 24 months and periodically examined their chromosome constitution. SW480 cells, from passage 138, were injected subcutaneously into 20 nude mice. The tumors that grew in nude mice we re then cultivated in vitro for several passages to compare histopatho logic findings and tumor growth patterns with clonal chromosomal profi les. Despite some karyotypic diversity, the two cell lines exhibited c ommon marker chromosomes and followed similar patterns of evolution. D uring subsequent passages, acquisition of new chromosomal abnormalitie s gave rise to sidelines with a near-diploid genome that frequently un derwent endoreduplication. Genomic instability seemed to play an impor tant role in the emergence, growth, and subsequent elimination of the heterogenous sidelines by selection, clonal expansion, and cell death by senescence. Despite continuous growth, both the cell lines occasion ally showed telomeric associations and random dicentric and multicentr ic formations. These lesions were considered evidence of cell senescen ce and were related to the disappearance of particular sidelines throu gh evolution. Successful evolutionary steps were characterized by elim ination of pre-existing marker chromosomes that were subsequently repl aced in the karyotype by their cytologically intact homologous chromos omes possibly after selective endoreduplication. Frequent loss of hete rozygosity for the chromosomes taking part in this process is postulat ed. We suggest that one of the mechanisms by which cancer cells bypass senescence may be related to their potential for continuous clonal di versification.