Cellular senescence likely evolved as a defense against cancer: Damaged cells stop dividing, preventing cells that have sustained oncogenic mutations from proliferating and forming tumors. One down side is that the proliferative capacity of the organ goes down; another is that persistent senescent cells secrete factors that damage the extracellular matrix and contribute to age-related decline in tissue function.

Because senescent cells accumulate with age, senescence has been considered a biomarker of aging — that is to say, a measurable feature of a tissue that would allow us to calculate its “biological” or “physiological” age without necessarily knowing the chronological age of the donor. This makes abundant sense: tissues with more senescent cells would have lower proliferative capacity and higher levels of deleterious secreted factors; given two samples of the same chronological age, one could argue that the one containing more senescent cells was “older” in some meaningful way.

This idea is central to a recent paper demonstrating that senescence correlates negatively with the efficacy of a transplanted tissue. Grafts from older donors have poorer outcomes than those from younger donors, but some older tissues work just fine — what’s the difference between an old graft that works and an old graft that doesn’t? The answer, in part, is the level of expression of senescence markers: Higher expression of senescence-related genes predicts a poorer outcome in a transplant. In other words, controlling for chronological age, physiological age is a negative correlate of transplant efficacy. From McGlynn et al.:

Cellular senescence in pre-transplant renal biopsies predicts post-operative organ function Older and marginal donors have been used to meet the shortfall in available organs for renal transplantation. Post transplant renal function and outcome from these donors is often poorer than chronologically younger donors. Some organs, however, function adequately for many years. We have hypothesised that such organs are biologically younger than poorer performing counterparts. We have tested this hypothesis in a cohort of pre-implantation human renal allograft biopsies (n=75) that have been assayed by Real Time-PCR for the expression of known markers of cellular damage and biological ageing, including CDKN2A, CDKN1A, SIRT2, and POT1. These have been investigated for any associations with traditional factors affecting transplant outcome (donor age, cold ischaemic time) and organ function post transplant (serum creatinine (SC) levels). Linear regression analyses indicated a strong association for SC with pre transplant CDKN2A levels (p=0.001) and donor age (p=0.004) at six months post transplant. Both these markers correlated significantly with urinary protein to creatinine ratios (p=0.002 and p=0.005 respectively), an informative marker for subsequent graft dysfunction. POT1 expression also showed a significant association with this parameter (p=0.05). Multiple linear regression analyses for CDKN2A and donor age, accounted for 24.6% (p=0.001) observed variability in SC levels at six months and 23.7% (p=0.001) at one year post transplant. These data indicate that allograft biological age is thus an important novel prognostic determinant for renal transplant outcome.

Note that the authors measured the levels of senescence-associated gene expression, rather than the number of cells in the tissue that were senescent. This is a subtle but important point, given what we know about the biology of senescence: Gene expression is an imperfect marker for the extent of senescence within a tissue: A given amount of RNA could mean a small number of cells with intense expression or a larger number of cells with lower expression.

Distinguishing between these alternatives may be clinically important, depending on whether the increased senescence is causing the poorer transplant performance — and if so, why? Regeneration is likely necessary for optimal organ performance after a transplant; since senescent cells are permanently nondividing, the presence of a large number of them would predict poorer regenerative capacity. Alternatively (though not mutually exclusively) the protein factors secreted by senescent cells might have deleterious consequences on the function of the tissue itself, and this might interfere with transplant efficacy.

Figuring out which is more decisive — the number of senescent cells in a tissue, or their aggregate production of secreted factors — will become very important if we’re someday able to selectively destroy senescent cells. In the “increased numbers” case, such treatments wouldn’t help (the cells are equally unable to divide whether they’re alive or not), but in the “increased secretion” case, anti-senescence therapeutics could make the difference between success and failure in organ transplants of many kinds.