Abstract

Human tumors that lack telomerase maintain telomeres by alternative lengthening mechanisms. Tumors can also form in telomerase-deficient mice; however, the genetic mechanism responsible for tumor growth without telomerase is unknown. In yeast, several different recombination pathways maintain telomeres in the absence of telomerase—some result in telomere maintenance with minimal effects on telomere length. To examine non-telomerase mechanisms for telomere maintenance in mammalian cells, we used primary cells and lymphomas from telomerase-deficient mice (mTR−/− and Eμmyc+mTR−/−) and CAST/EiJ mouse embryonic fibroblast cells. These cells were analyzed using pq-ratio analysis, telomere length distribution outliers, CO-FISH, Q-FISH, and multicolor FISH to detect subtelomeric recombination. Telomere length was maintained during long-term growth in vivo and in vitro. Long telomeres, characteristic of human ALT cells, were not observed in either late passage or mTR−/− tumor cells; instead, we observed only minimal changes in telomere length. Telomere length variation and subtelomeric recombination were frequent in cells with short telomeres, indicating that length maintenance is due to telomeric recombination. We also detected telomere length changes in primary mTR−/− cells that had short telomeres. Using mouse mTR+/− and human hTERT+/− primary cells with short telomeres, we found frequent length changes indicative of recombination. We conclude that telomere maintenance by non-telomerase mechanisms, including recombination, occurs in primary cells and is initiated by short telomeres, even in the presence of telomerase. Most intriguing, our data indicate that some non-telomerase telomere maintenance mechanisms occur without a significant increase in telomere length.

Highlights

  • Telomere length is maintained by the ribonucleoprotein complex telomerase [1]

  • To examine whether recombination contributes to nontelomerase mechanisms for telomere maintenance in mouse and human cells, we utilized mouse primary and tumor cells, which were genetically deleted for telomerase

  • In addition to tumor cells, primary cells can utilize nontelomerase mechanisms for telomere maintenance

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Summary

Introduction

Telomere length is maintained by the ribonucleoprotein complex telomerase [1]. telomerase expression in humans occurs primarily in early development, germ cells, and in stem cells and is not detected in primary cells [2,3]. Most human tumor cells have detectable telomerase activity, some proliferating tumors lack telomerase and maintain telomeres by alternative mechanisms that are collectively termed ALT for alternative lengthening of telomeres [4,5]. Late generation mTR2/2 G4–G6 mice have short telomeres and show loss of fertility due to germ cell apoptosis. Wild-derived mouse strains such as CAST/EiJ have significantly shorter telomere length distributions, similar to humans [9]. CAST/EiJ mTR+/2 mice bred for increasing generations show progressive telomere shortening and loss of tissue renewal capacity [10]. Telomere shortening and consequent loss of tissue renewal capacity occurs in CAST/EiJ mice even in the presence of telomerase, and provides the opportunity to examine the effects of short telomeres in the presence of telomerase

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