An attack from the body's own immune system may be behind a heart condition which can prove fatal in many cases.

Scientists in the US have discovered that mice, which had been genetically-altered to leave their immune system unregulated, quickly developed a condition almost identical to cardiomyopathy in humans.

Cardiomyopathy is a progressive weakening of the heart muscle cells.

You can deal with bacteria, and to some extent viruses, and cancer, but these are the Cinderella illnesses.

Professor Marlene Rose, Imperial College London

Not only do they lose the ability to pump blood around the body as effectively, but the very structure of the muscle is weakened, allowing the entire organ to be pushed out of shape by the pressure of the blood inside.

Often the heart ends up being twice the size of a healthy heart.

The condition hit the headlines in the UK when six-year-old Sally Slater was found a donor heart with only hours to spare before her own heart failed.

Sally's cardiomyopathy was thought to be caused by a viral infection, but in the majority of cases, doctors have little idea of what has caused it.

Auto-immune diseases are caused when the immune system, for some reason, decides that a certain type of the body's own cells are foreign and starts attacking them.

The best known examples of this are multiple sclerosis and rheumatoid arthritis.

The US researchers, reported Science magazine, used genetic modification techniques to knock out a mouse gene which is responsible for keeping the immune system in check so that it does not launch attacks on itself.

Classic signs

The altered mouse displayed the classic anatomical characteristics associated with cardiomyopathy - their hearts quickly became grossly distended, worked far less well, and died as early as five weeks of age.

Sally Slater suffered from cardiomyopathy

This is strong evidence for there being an immune system component to some human cardiomyopathy cases.

Professor Marlene Rose, an expert in transplant immunology at Imperial College, London, said: "It's confirmation that some of these cases of cardiomyopathy may be caused by auto-immunity.

"Auto-immune diseases are among some of the most difficult to treat - you can deal with bacteria, and to some extent viruses, and cancer, but these are the Cinderella illnesses.

"Patients are being given old-fashioned drugs."