Pig hearts grow uncontrollably inside human recipients

When a pig heart is transplanted into a primate, it continues to grow according to pig physiology, which programs the organ to reach the size needed for a 300+ pound adult pig. Inside a human chest cavity, this means the heart undergoes massive hypertrophy, leading to diastolic heart failure at roughly 1 month post-transplant as the enlarged heart can no longer fill properly. In David Bennett's case, the pig heart grew to nearly twice its transplanted size. Researchers responded by knocking out the growth hormone receptor (GHR) gene in donor pigs, which reduces post-transplant growth and has extended graft survival to 6 months in preclinical models with minimal hypertrophy. But GHR knockout creates a new problem: the donor pigs are smaller and grow more slowly, making it harder to produce organs of the right size for adult human recipients, and the long-term effects of GHR knockout on organ function and durability beyond 6 months are unknown. The field is stuck between two failure modes: leave GHR intact and the heart outgrows the chest, or knock it out and face sizing and unknown longevity issues.