Earlier this year, Ellen Heber-Katz and her colleagues at the Wistar Institute in Philadelphia, Pennsylvania, announced that they had found a strain of mouse with remarkable healing powers. These animals healed in a way that was quite unusual for mammals - and much more like the tissue regeneration seen in amphibians and other more ‘primitive’ vertebrates. The research team now describes some of the genes that control tissue regeneration, in Proceedings of the National Academy of Sciences.

When an amphibian loses a limb, it can grow a new one. But if a mammal suffers the same injury, the limb doesn’t regrow. Healing in mammals is quite a different process. Evolution has given us an immune system that controls tumour growth - vital for such long-lived creatures as us - but as a consequence we cannot tolerate large-scale regrowth of damaged body parts.

The healer mouse is unusual in that it has a defective immune system. The strain (lovingly referred to as MRL/MpJ) was bred originally to help researchers understand autoimmune diseases. But after punching holes in the earlobes of mice - a common way of marking individuals - Heber-Katz and her colleagues noticed that the ears healed, leaving no trace of the holes.

The tissue did not scar, or even heal around the edges of the hole. It formed new, fully differentiated cartilage, skin and even fur. Later tests showed that the tail tip would also grow back successfully, with new bone, skin and fur, although they have not tested the mice’s ability to regrow limbs.

The findings led to suggestions that, if we knew what was happening in these mice, and how the immune system was different from normal mice, we might start to understand how we could induce regenerative wound healing - perhaps even one day in humans. Understanding the genetic differences in the MRL/MpJ mice is the first step.

The wound-healing ability is inherited, so when Heber-Katz and her colleagues allowed MRL/MpJ mice to breed with normal laboratory mice, the offspring inherited some healer ability, but the extent varied from individual to individual.

The fewer genes involved, the fewer distinct groups of healer abilities should be seen in the offspring. In fact, the amount of variety in the offspring’s healing powers suggested to the researchers that there were at least four genes in control. By measuring the extent to which the offspring inherited the healing trait and by finding ‘marker’ genes that are inherited at the same time (suggesting the genes are close together), the researchers also managed to suggest where on the various chromosomes the healer genes were.

They believe they have found gene locations on chromosomes 8, 12 and 15, with two regions on chromosome 13. There may even be another site on chromosome 7 - so there seem to be a possible six genes, rather than the predicted four. The genes are all inherited from the MRL parent except for one, which comes from the normal lab mice, but is presumable masked in some way by its other genes.

What these ‘healer’ genes are, what they do, and how they are regulated, remains to be discovered, but it is clear that regenerative healing is no simple process. The genes may encode receptors, growth factors, and signalling chemicals - one likely candidate is a receptor for retinoic acid, a chemical which is important during development and skin growth.

But one extremely interesting finding is that the candidate healer genes do not include any of the genes that cause the autoimmune profile of MRL/MpJ. Healing powers are not simply due to diminishing immune system activity.