Advances in organ transplant could pave the way for the use of animal tissues in human beings
The English appreciate the wonder that is the cow far better than those Indians who see her as their mother. We remain at a very preliminary stage of research, no better than the poor schoolchildren who, generation after generation, write the legendary school essay on the properties of the cow—an enumeration of its horns, hooves and tail. Our savants are futilely funded to assay the products of the cow, such as the radioactive shield capabilities of its dung, but in the irreligious West, they profitably study the properties of the cow itself. And so, in London recently, the life of a child born with the autosomal-dominant Holt-Oram syndrome was saved by using bovine tissues. It was used as a transplant to repair cardiac defects characteristic of the condition.
This was not the first instance, either. In 2009, an American child born without heart valves between the atria and the ventricles had these crucial structures rebuilt using bovine pericardium. A decade earlier, a valve from a vessel in a cow’s neck was used to replace the heart valve of a boy. The technique has found use in India, too. In July, a Hyderabad woman’s heart valve was reconstructed using bovine cardiac tissue.
Implanting animal tissues is an old surgical strategy for dealing with extreme conditions. The pioneer was pig skin, which has been used as a life-saving dressing for severe and extensive burns for decades. It found use as a sanitation barrier, protecting damaged underlying tissues from infection and dessication until they could repair themselves. This strategy has since come to be classed as a xenograft—an implant from an unrelated animal—and the idea of xenotransplantation has come into vogue.
The first real xenotransplant recipient was an American child known as Baby Fae in the literature. Born with the life-critical left side of the heart underdeveloped, she received a baboon heart transplant. She did not survive because of immunological reasons—she was blood group A, the donor baboon was AB, and rejection followed. The universal donor group O is extremely rare among baboons. Yet xenotransplantation offers hope to the lakhs of people who die for want of an organ donor every year. It could represent a huge business opportunity in healthcare, competing with stem cell therapy. Failures like Baby Fae’s actually suggest that with the support of genetic intervention, it can develop into a sector within the future industry for the prolongation of life.
The choice of donor is conditioned by the possibility of pathogens jumping species, breaking the barrier between humans and animals. Having weathered the AIDS pandemic, the medical community is especially wary about this. Hence the choice of a baboon donor for Baby Fae—the chimpanzee, the first cousin of humans, is not considered because while its tissues are less likely to be rejected, its viruses and bacteria are more likely to be transmitted. However, the lack of baboons of blood group O (less than 1% of population) dramatically reduces options.
Barnyard animals are likely to remain popular donors. Pigs are suitably distant on the evolutionary tree but have organs roughly the same size as those of humans. Besides, the chance of diseases jumping species is remote. Humans and farm animals have been in contact for millennia, and any possible cross-infections must have happened already. But the need for genetic intervention is obvious. A team led by David H Sachs at Massachusetts General Hospital has suggested breeding pigs while editing out the genes which code for an enzyme synthesising a galactose sugar at the cell surface. This sugar is what human systems reject, resulting in transplant failures.
Minus this enzyme, pig organs should be widely implantable in humans. While bovines have larger organs which cannot be transplanted into humans, their tissues may be used for repairing structures like heart valves. The only limiting condition would be ageing. Pigs live for about 15 years, and humans at least four times longer. It is not known how xenografted tissues would perform beyond the lifespan of the donor, but that should not deter science from exploring possibilities for the prolongation of healthy life.
As the quality of medical care improves, organ failure will become the only or main cause of death and, in the absence of fresh incentives which are hard to imagine, donors will remain scarce. Scarcity would only benefit illegal speculators, like India’s infamous kidney bandits. This scarcity picture would be dramatically altered if other mammals could be regarded as an inexhaustible pool of spare parts, to be retooled by genetic engineering. The implications for the prolongation of life and our understanding of the life well lived would be utterly changed.