Interfering in the Response to Short Telomeres Improves Immune System Function in Old Mice
Telomeres are repeated DNA sequences found at the ends of chromosomes. A little telomere length is lost with each cell division, and short telomeres trigger cell senescence or programmed cell death. It is a part of the system ensuring the Hayflick limit on the replication of somatic cells. The stem cells that create replacement somatic cells can lengthen their own telomeres, but there are very few stem cells in comparison to the number of somatic cells making up the majority of tissue. This is how evolution reduces cancer to an acceptable level, by dramatically restricting the number of cells capable of unfettered replication, and thus reducing the odds of a malfunction leading to runaway replication.
With age stem cell function declines, reducing the pace at which stem cells deliver replacement somatic cells with long telomeres. As a result, average telomere length falls and the proportion of cells with very short telomeres increases in tissues throughout the body. This has a meaningful negative effect on health, a driver of chronic inflammation, increased numbers of senescent cells, and impaired tissue function. In today's open access paper, researchers report on their efforts to specifically sabotage the cascade of mechanisms that emerge in response to short telomeres in a cell, showing that it improves health in aged mice, at least in the short term. The flip side of the coin, not investigated here, is that this could increase cancer risk by promoting damage to DNA via the continued operation of damaged cells, usually avoided because cells with very short telomeres are destroyed on some timescale.
Telomeres progressively shorten and accumulate damage with aging, and this contributes to cellular senescence and hematopoietic dysfunction. When critically short, telomere ends are detected as DNA damage and trigger a telomeric DNA damage response (tDDR), a signaling cascade involving posttranslational protein modifications, such as phosphorylation of histone H2AX at serine 139 (known as γH2AX), which promotes recruitment of DDR factors including phosphorylated KRAB-associated protein 1 (pKAP1) at damaged sites. Persistent tDDR drives cellular senescence and cell death. The inability of senescent cells to proliferate impairs tissue regeneration, and their secretion of proinflammatory factors, collectively known as the senescence-associated secretory phenotype, promotes chronic, low-grade inflammation, disrupting the local microenvironment and eventually causing systemic frailty.
Whether the tDDR causally impairs hematopoiesis remained unclear. Here we show in telomerase-deficient Telomerase RNA component (TERC) knockout mice, which recapitulate telomere-driven hematopoietic dysfunction and aging, that targeting telomeric noncoding RNAs with telomeric antisense oligonucleotides (tASO) suppresses tDDR in hematopoietic organs, reduces senescence and inflammation, alleviates hematopoietic dysfunction, and enhances hematopoietic stem cell fitness and repopulating potential in vivo. Similar observations were recapitulated in aged wild-type mice, and ex vivo treatment with tASO improved the function of human hematopoietic stem cells from aged donors.
Taken together, our results identify tDDR as a pathogenic driver of hematopoietic decline and support tASO-mediated tDDR inhibition as a potential therapeutic strategy for telomere biology disorders and age-associated hematopoietic aging.