TI - mTERT-/- and mTR-/- cells do not show a difference in DNA damage response induction compared to wild-type . AB - Changes in gene expression levels might be expected if telomerase components play a direct role in transcription ; however , some phenotypes might be due to functional changes that do not involve transcriptional changes . To further examine a potential telomere length maintenance -independent role of mTR and mTERT in DNA damage , we used a sensitive functional assay using mTR-/- G1 , mTERT-/- G1 and wild-type MEFs . Three independent early-passage MEF cultures of each genotype were treated with two different DNA-damaging agents , camptothecin and gamma rays , to induce a DNA damage response . Camptothecin is a topoisomerase I inhibitor that generates double-strand breaks ( DSBs ) during DNA replication ( 44 ) . Gamma rays act directly on DNA , inducing DSBs in the sugar-phosphate backbone . In each cell line , we monitored p53 and H2AX ( gamma-H2AX ) PHOSphorylation in response to DNA damage as examples of well-characterized damage responses ( 45,46 ) . The phosphorylaTION levels were monitored with PHOSphorylated specific antibodies on western blots , using beta-actin as an internal loading control . We also measured the accumulation of gamma-H2AX and PHOSphorylated 53BP1 ( 47 ) in DNA damage-induced foci using immunofluorescence . Finally , we monitored the rate of cell proliferation in response to both camptothecin and gamma irradiation .