A transplanted heart adopts the biological age of its new host, not the donor

A transplanted heart adopts the biological age of its new host, not the donor

The groups of Vadim Gladyshev and Steve Horvath have shown in mice and patients that the biological age of a transplanted heart rapidly shifts toward the age of the new body rather than retaining the donor's age. The effect is visible in DNA marks, gene activity, and cardiac function, and it is confined to the transplanted heart itself, leaving the rest of the recipient's body unchanged.

For decades, transplant medicine has treated the heart as an organ of fixed age: the graft carries whatever years the donor had. This is the basis for caution toward older donors. Few programs accept hearts from donors over 50, even though hundreds of patients die on the waiting list each year for lack of organs.

A preprint posted on bioRxiv on September 15 tested this assumption directly. In the mouse model, the donor heart was implanted alongside the animal's own heart rather than replacing it. This design provides a built-in control: the graft can be compared with the native heart of the same animal. Heart age was measured by three independent "aging clocks," methods that predict the biological age of a tissue from chemical marks on the DNA (methylation). By all three clocks, young hearts transplanted into old mice read older than the donor age; old hearts placed in young mice read younger. The recipient's native heart, liver, and blood did not change: the effect was locked inside the graft. Verification at individual methylation sites confirmed the direction of the shift independently of the clock algorithm, and gene expression data showed that the genes shifting most strongly and consistently were those of mitochondria and metabolism, the very pathways that typically decline with accumulated cellular damage during aging.

The same group had previously shown, through parabiosis (shared circulation between a young and an old organism), that the bloodstream carries aging signatures across the entire body in both partners simultaneously. The graft effect is different: only the transplanted organ changes, while the recipient's own heart, liver, and blood remain as before.

In humans, the authors obtained archival cardiac biopsies from 11 patients whose donor-recipient age gap ranged from −24 to +50 years: the methylation-based age deviation of the graft correlated with the recipient's age. In a much larger cohort of hundreds of patients at Brigham and Women's Hospital, functional cardiac capacity and peak oxygen consumption under exercise one year after surgery were independently predicted by the recipient's age, not the donor's, precisely the functional verification that Horvath himself had recommended.

"An old heart placed in a young recipient does not simply continue aging along its original trajectory. Its biological age shifts toward the age of the new, younger organism. We call this biological age assimilation," wrote one of the lead authors, Jesse Poganik.

The authors propose a mechanism: the recipient's blood and tissues exchange accumulated molecular damage with the graft. An old body adds damage to the graft and ages the organ; a young body dilutes the damage and rejuvenates it. Intentionally allocating older hearts to younger recipients could expand the organ pool available for transplantation.