Investigating spontaneous activation of two telomere maintenance mechanisms in the same cancer cells

Abstract

Telomeres are biological constructs that protect the genomic information from DNA damage repair pathways and chromosomal fusions. Telomeres shorten every cell division and when the telomeres become critically short the cells become senescent. Cancer cells overcome this hindrance and proliferate infinitely via the activation of a telomere maintenance mechanism (TMM). Current literature suggests most cancers maintain their telomeres via two known mechanisms. The first involves telomerase, a ribonucleoprotein, and the other TMM, Alternative Lengthening of Telomeres (ALT), is independent of this ribonucleoprotein. Normal somatic cells do not utilise a TMM, TMMs are ideal targets for the generation of selective anti-cancer therapeutics. There is a widespread assumption in this field of research that ALT and telomerase are mutually exclusive. A prior study in the Reddel laboratory investigated the possibility that spontaneous activation of both TMMs within a single cell can occur. A melanoma cell line, LOX IMVI, was characterised to have telomerase activity in addition to the phenotypic characteristics of ALT. To identify whether both TMMs actually contributed to telomere length maintenance in LOX IMVI cells, telomerase activity was abrogated. My main contribution to this study was to examine telomere lengths in these telomerase-null cells over the course of 200 population doublings. Telomere lengths continued to decrease throughout that entire time. Therefore, no evidence was obtained that ALT contributes to telomere length maintenance in LOX IMVI cells. I then found that another human cell line, 1301, which is derived from a paediatric acute lymphoblastoid leukaemia, has telomerase activity and features of ALT activity in every subclone. To determine whether ALT and telomerase are contributing to telomere length maintenance in these cells, I used CRISPR/Cas9 to knock out the TERC gene, which encodes the RNA subunit of telomerase. Knockout was confirmed in eight subclones and were passaged long-term with seven controls. In seven of the treated clones telomere length declined detectably, but in one clone there was a negligible rate of decline. To my knowledge, this is the first evidence indicating that functional levels of telomerase and ALT activity may be spontaneously activated in the same cancer cells.