Telomere elongation in immortal human cells without detectable telomerase activity Telomeres are specialized structures at the ends of eukaryotic chromosomes that protect against degradation and fusion. They consist of repeated TTAGGG sequences and shorten with each cell division, leading to cellular senescence. Telomerase, a ribonucleoprotein enzyme, can elongate telomeres and is typically active in germ cells but not somatic cells. However, this study challenges the notion that telomerase activation is necessary for immortalization. The research examined 35 immortalized human cell lines and found that 15 were telomerase-negative, yet they had very long and heterogeneous telomeres, up to 50 kb. These cells did not require telomerase activity for their immortalization, suggesting that telomere lengthening can occur through alternative mechanisms. Hybrid cells formed between telomerase-negative and -positive cells often senesced, but some later regained proliferation, indicating that telomerase activation is not sufficient for immortalization. The study also showed that telomerase-negative immortal cells could maintain or lengthen telomeres without telomerase activity, possibly through a novel mechanism. Telomere length was not affected by the TRAP assay, and telomerase-negative cells did not contain inhibitors of the assay. Furthermore, telomere length remained stable in some cell lines over many passages, indicating that telomere maintenance can occur without telomerase. The findings suggest that telomere lengthening is necessary for immortalization, which can be achieved either by telomerase activation or by an unknown mechanism. These results challenge the traditional view that telomerase is essential for cellular immortality and highlight the complexity of telomere dynamics in immortal cells. The study also indicates that telomere maintenance in some cells may be achieved through mechanisms not yet understood, emphasizing the need for further research into telomere biology and cellular immortality.Telomere elongation in immortal human cells without detectable telomerase activity Telomeres are specialized structures at the ends of eukaryotic chromosomes that protect against degradation and fusion. They consist of repeated TTAGGG sequences and shorten with each cell division, leading to cellular senescence. Telomerase, a ribonucleoprotein enzyme, can elongate telomeres and is typically active in germ cells but not somatic cells. However, this study challenges the notion that telomerase activation is necessary for immortalization. The research examined 35 immortalized human cell lines and found that 15 were telomerase-negative, yet they had very long and heterogeneous telomeres, up to 50 kb. These cells did not require telomerase activity for their immortalization, suggesting that telomere lengthening can occur through alternative mechanisms. Hybrid cells formed between telomerase-negative and -positive cells often senesced, but some later regained proliferation, indicating that telomerase activation is not sufficient for immortalization. The study also showed that telomerase-negative immortal cells could maintain or lengthen telomeres without telomerase activity, possibly through a novel mechanism. Telomere length was not affected by the TRAP assay, and telomerase-negative cells did not contain inhibitors of the assay. Furthermore, telomere length remained stable in some cell lines over many passages, indicating that telomere maintenance can occur without telomerase. The findings suggest that telomere lengthening is necessary for immortalization, which can be achieved either by telomerase activation or by an unknown mechanism. These results challenge the traditional view that telomerase is essential for cellular immortality and highlight the complexity of telomere dynamics in immortal cells. The study also indicates that telomere maintenance in some cells may be achieved through mechanisms not yet understood, emphasizing the need for further research into telomere biology and cellular immortality.