POT1 (protection of telomeres 1) is the first member of the telomeric shelterin complex found to be mutated in human cancer. There is a Nature Genetics paper on this (Nat Genet. 2013 May;45(5):526-30.) POT1 is mutated in cases of chronic lymphocytic leukemia (CLL), which is the most frequent leukemia in adults. However telomerase dysregulation including expression changes have been seen in other cancers.
Mutations in shelterin complex leads to formation of tumor cells. TRF1 and TANK1 are the negative regulators of telomere length. So telomerase enzyme inhibits TRF1 activity as well as TANKI also.