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[nihilistcat]

I should elaborate a bit more. The TERT gene encodes for one of the proteins in the telomerase enzyme complex (telomerase has several subunits, an RNA subunit, TERC, a protein subunit, TERT, and dyskerin or DKC1). In the above mentioned research, a modified mRNA** that encodes for TERT was used to extend telomere. But the important difference is rather than completely restoring telomere, it only adds about 1,000 nucleotides (which increases replicative capacity by up to ~40 additional cell divisions), and then TERT meets the fate of most other proteins, proteolysis (degradation). So while this treatment can (in theory) be readministered, it doesn't create the sort of permanent change in cells that can lead to cancer formation (at least hypothetically). 

**mRNA are "messenger" RNA (ribonucleic acid) involved in the translation of proteins, which work in cooperation with ribosomes.