Telomere-based crisis: functional differences between telomerase activation and ALT in tumor progression

Abstract

Telomerase activation is a common feature of most advanced human cancers and is postulated to restore genomic stability to a level permissive for cell viability and tumor progression. Here, we used genetically defined transformed mouse embryonic fibroblast (MEF) cultures derived from late generation mTerc −/− Ink4a/Arf −/− mice to explore more directly how telomere-based crisis relates to the evolution of cancer cell genomes and to tumor biology. An exhaustive serial analysis of cytogenetic profiles over extensive passage in culture revealed that the emergence of chromosomal fusions (including dicentrics) coincided with onset of deletions and complex nonreciprocal translocations (NRTs), whereas mTerc-transduced cultures maintained intact chromosomes and stable genomes. Despite a high degree of telomere dysfunction and genomic instability, transformed late passage mTerc −/− Ink4a/Arf −/− cultures retained the capacity to form subcutaneous tumors in immunocompromised mice. However, even moderate levels of telomere dysfunction completely abrogated the capacity of these cells to form lung metastases after tail-vein injection, whereas mTerc reconstitution alone conferred robust metastatic activity in these cells. Finally, serial subcutaneous tumor formation using late passage transformed mTerc −/− Ink4a/Arf −/− cultures revealed clear evidence of telomerase-independent alternative lengthening of telomeres (ALT). Significantly, despite a marked increase in telomere reserve, cells derived from the ALT+ subcutaneous tumors were unable to generate lung metastases, indicating in vivo functional differences in these principal mechanisms of telomere maintenance. Together, these results are consistent with the model that although telomere dysfunction provokes chromosomal aberrations that initiate carcinogenesis, telomerase-mediated telomere maintenance enables such initiated cells to efficiently achieve a fully malignant endpoint, including metastasis.[Keywords: Telomere; telomerase; metastasis; ALT (alternative lengthening of telomeres); chromosomal instability; SKY] Supplemental material is available at http://www.genesdev.org. Tumor formation represents the phenotypic endpoint of a mutational process that endows cells with a critical threshold of cancer-relevant genetic lesions. From a cytogenetic standpoint, epithelial neoplasms or carcinomas fall into one of two major classes-the first possessing microsatellite instability with retention of grossly normal karyotypes, and the second presenting with markedly altered chromosomal counts and structure. These two classes have been operationally defined as "microsatellite instability neoplasias" and "chromosomal instability neoplasias" (MIN and CIN), respectively (for review, see Lengauer et al. 1998). The molecular mechanism underlying MIN is clearly related to defective DNA mismatch repair. The basis for CIN is less well understood, although defects in mitotic checkpoint, nonhomologous end-joining, and DNA replication, as well as increased oxidative stress and telomere dysfu...

Genes and pathways downstream of telomerase in melanoma metastasis

“…These results extend recent studies showing a prometastatic phenotype for oncogene-(including telomerase reverse transcriptase) transformed fibroblasts (29) and melanocytes (30). The fact that the telomerase inhibition reported here reduced the metastatic potential of a spontaneously arising tumor cell line, which has many genetic abnormalities other than up-regulation of telomerase, firmly establishes a role for telomerase in tumor metastasis, highlighting its importance to the lethal stages of tumor progression.…”

Caenorhabditis elegans POT-2 telomere protein represses a mode of alternative lengthening of telomeres with normal telomere lengths

“…Telomerase-independent telomere maintenance that results in telomeres of normal length has been reported for primary and lymphoma cells from mouse telomerase RNA mutants (33), although other studies have revealed long and heterogeneous telomeres in mouse tumors derived in the context of telomerase deficiency (34,35). One immortalized human cell line, C3-cl6, maintained short and homogeneous telomeres in the absence of telomerase (10).…”

Are aneuploidy and chromosome breakage caused by a CINgle mechanism?

“…Telomere shortening in mice neither reproduces the CIN phenotype nor increases carcinogenesis if not aided by additional tumor-inducing treatment. 26,27 Instead, telomere shortening causes cell senescense, effectively blocking uncontrolled proliferation. Over the last few years, it has become clear that telomere shortening is probably a minor factor in the initiation of genomic instability, although telomerase reactivation probably plays an important role in tumor progression.…”