A gene therapy lengthens the ends of chromosomes in an old animal, with no rise in cancer
Each time a cell divides, the protective caps on the ends of its chromosomes (telomeres) get a little shorter, and a cell whose caps run out stops dividing. An enzyme called telomerase rebuilds the caps, and most adult cells keep it switched off, partly because cancers switch it on. In 2012 a Madrid group carried the telomerase gene into adult and old mice with a harmless virus, once, and the mice lived longer (by about a quarter when treated at one year old, about an eighth at two) without more cancer. The leaf asks for that result and for its safety, because a longer life with more cancer would not count.
What would count
A living animal given a treatment that lengthens its telomeres (the caps on chromosome ends) afterwards lives longer, or stays healthier for longer, than untreated controls, with no rise in cancer, in a peer-reviewed report. The first-human rung needs measurably longer telomeres in a person after such a gene therapy, with the cancer rate reported, in a peer-reviewed report. For the clinic rung: a regulator approves a telomere-lengthening gene therapy for any condition.
Who settles it: Peer-reviewed reports; the trial registry and report when a first person is treated; the regulator's approval record for the clinic rung.
Where it stands
Shown in animals. Reached May 2012, on the record of Jesus and 6 others, 15 May 2012 (doi.org). Next rung more than 20 years away, if ever.
When it reaches the clinic: the project's estimate
This is the one thing on this page that is a judgment rather than a record. The project's estimate of the year it reaches the clinic is 2044, no earlier than 2036 and no later than not this century; the model draws from that range every run, with the likely year the most probable draw. Set 16 October 2026, a first pass awaiting an outside review.
| Rung | No earlier than | Most likely | No later than | |
|---|---|---|---|---|
| First human | 2029 | 2033 | 2055 | |
| In clinical use | 2036 | 2044 | not this century | feeds the year at the top |
Why. No registered trial of a telomerase gene therapy in people existed when this was written. The first person would come through a rare telomere disease (dyskeratosis congenita, pulmonary fibrosis from short telomeres), where the risk is worth taking; the likely year for that is the early 2030s, and an approval for such a disease the early 2040s. The earliest assumes a sponsor files within three years on the strength of the mouse data; the latest allows for the cancer risk closing the route for good and the rung being reached, if at all, by a very different kind of treatment late in the century. [Note 2026-10-17, from the weekly reading of that date: the first sentence above is wrong as written. A registered gene-therapy trial in people with a telomere disease did exist, for a different gene, ZSCAN4 (registry entry NCT04211714), and it published two patients in 2025 (myers-2025); the checks listed on this entry also name a 2019 trial of a telomerase gene therapy registered in Colombia. The ZSCAN4 report does not meet this entry's first-human test on a plain reading: the gene is ZSCAN4 and not telomerase, the cells were treated outside the body, there are two patients, and no cancer rate is given. The years are unchanged. Whether the ZSCAN4 report should count for the first-human rung is John's ruling; if it does, the first-human estimate falls away with the move.]
Changes to this record
- 17 October 2026, estimate.why: (nothing; a dated note added at the end of the reasoning; no year changed) became [Note 2026-10-17, from the weekly reading of that date: the first sentence above is wrong as written. A registered gene-therapy trial in people with a telomere disease did exist, for a different gene, ZSCAN4 (registry entry NCT04211714), and it published two patients in 2025 (myers-2025); the checks listed on this entry also name a 2019 trial of a telomerase gene therapy registered in Colombia. The ZSCAN4 report does not meet this entry's first-human test on a plain reading: the gene is ZSCAN4 and not telomerase, the cells were treated outside the body, there are two patients, and no cancer rate is given. The years are unchanged. Whether the ZSCAN4 report should count for the first-human rung is John's ruling; if it does, the first-human estimate falls away with the move.]. [reading 2026-10-17] A dated note that the estimate's first sentence ('no registered trial of a telomerase gene therapy in people existed') is wrong as written, and why the ZSCAN4 report does not move the stage; no year changed. Source: Myers and 7 others, 25 February 2025.
- 17 October 2026, notes: (nothing; text added at the end of the notes) became Added 2026-10-17, from the weekly reading of that date: the one human result of lengthened telomeres from a gene therapy is a 2025 report of two people with a rare telomere disease, whose own blood stem cells were treated outside the body with the gene ZSCAN4 (myers-2025); in one the average length in a class of white blood cells rose from 3.6 to 6.7 thousand base pairs. It was not entered as the first-human rung, for the four reasons given in the estimate's dated note. FOR JOHN: whether that report counts is a reading of the test.. [reading 2026-10-17] Records the ZSCAN4 report and leaves the first-human question for John. Source: Myers and 7 others, 25 February 2025.
The last readings
- 17 October 2026 Left for the author
Nothing in the window 2026-10-10 to 2026-10-17, but the search turned up an older published result that the leaf does not know and that bears on its first-human rung: a trial at Cincinnati Children's of a person's own blood stem cells treated outside the body with a gene-carrying vector for the gene ZSCAN4, reported in the journal NEJM Evidence in 2025, in which two treated patients showed lengthened telomeres, one patient's lymphocyte telomere length rising from 3.6 to 6.7 thousand base pairs. On a literal reading this is not the leaf's first-human rung: the gene carried is ZSCAN4 and not telomerase, the cells were treated outside the body rather than the animal or person being dosed, there were two patients with five and twenty-four months of follow-up, and no cancer rate is reported, which the test asks for. It does, though, contradict one sentence of the leaf as written, so it is flagged. On the leaf's own verify list, the Libella telomerase gene therapy registered in Colombia in 2019 (registry number NCT04133649) still shows no posted result, and its sister trial record was last changed on 2019-10-28, so that item is unchanged. Left for the author: The change I would make is to the estimate's reasoning, which says that no registered trial of a telomerase gene therapy in people existed when the leaf was written: a registered gene-therapy trial in people with telomere disease did exist (registry number NCT04211714) and had published telomere lengthening in two patients in 2025, so that sentence is wrong as written, and I would replace it with a line saying that the one published human result used the gene ZSCAN4 outside the body and so does not meet this leaf's test. Two things make it the author's call. First, whether the ZSCAN4 result counts for the first-human rung is a reading of the test, which only the author settles, and if it counts the leaf moves a rung and the first-human estimate of 2033 goes with it. Second, correcting an entry the record missed is the author's by the reading's own rule. I would also not change the estimate's years myself. To log in the publications log once its page has been read, as the reading's rule asks: Clinical Use of ZSCAN4 for Telomere Elongation in Hematopoietic Stem Cells (NEJM Evidence volume 4, issue 3, article EVIDoa2400252 (2025), 2025; https://doi.org/10.1056/EVIDoa2400252).source 1, source 2, source 3, source 4
What the story so far says about it
On the record
- 15 May 2012Telomerase gene therapy in adult and old mice delays aging and increases longevity without increasing cancer
- 25 February 2025Clinical Use of ZSCAN4 for Telomere Elongation in Hematopoietic Stem Cells
Notes
The same Madrid group later reported telomerase gene therapy repairing lung scarring and anaemia in mice with short telomeres. A separate leaf on this branch holds the one human result of lengthened telomeres, which came from an ordinary hormone drug and not a gene therapy. The reason this leaf's horizon is far is the cancer question: a treatment that lengthens telomeres in every cell lengthens them in any cancer cell too, and a regulator will want decades of data. Added 2026-10-17, from the weekly reading of that date: the one human result of lengthened telomeres from a gene therapy is a 2025 report of two people with a rare telomere disease, whose own blood stem cells were treated outside the body with the gene ZSCAN4 (myers-2025); in one the average length in a class of white blood cells rose from 3.6 to 6.7 thousand base pairs. It was not entered as the first-human rung, for the four reasons given in the estimate's dated note. FOR JOHN: whether that report counts is a reading of the test.
Not yet verified
These facts in the record were written from memory or from a secondary source and have not yet been checked against the paper, the regulator's record or the registry. Nothing here is presented as settled until it is cleared, by checking it and either deleting the line or correcting the record.
- That a company chief executive, Elizabeth Parrish of BioViva, said in 2015 that she had taken a telomerase gene therapy herself is confirmed (MIT Technology Review, 2015-10-14); a 2022 single-patient case report on the same therapy (journal and subject unconfirmed) and a 2019 registered AAV-hTERT trial in Colombia (NCT04133649, no result found) still need to be read.