A rare disease is one that affects fewer than five in 10,000 people. You might wonder why anyone would dedicate their life to studying a disease when only a handful of people would benefit from a cure. Why not study one of the big killers, such as cancer, tuberculosis or malaria?

The significance of rare diseases is often not appreciated by people outside of the field. Rare diseases can provide valuable insights into the causes and progression of far more common diseases. The study of these diseases can also lead to the development of life-saving drugs. In fact, that’s how statins were developed. The cholesterol-lowering drug is one of the most frequently prescribed medicines in the developed world and it has saved countless lives.

The journey to develop drugs that reduce cholesterol began in the late 1960s when a young Japanese biochemist named Akira Endo scoured thousands of strains of fungi looking for something that inhibited cholesterol production. He assumed that there are enzymes involved in producing cholesterol and something must inhibit one or more of these enzymes.

On discovering a product in fungi that did exactly this, tests moved to experiments in petri dishes. This was followed by clinical trials with participants who had dangerously high levels of cholesterol and were susceptible to heart attacks in their early years. The trial participants suffered from familial hypercholesterolemia, a rare but life threatening condition, the most severe form of which affects one in 300,000 people.

Although Endo’s drug, Mevastatin, didn’t make it to the market, his study prompted further research into similar drugs, eventually culminating in the statins that are available on the market today and are used to treat more than 30m people globally.

Insights into osteoporosis

Similarly, a trio of rare diseases – two of which have fewer than 50 sufferers each worldwide – have advanced our understanding of “bone turnover and remodelling” in the human body. Bone turnover and remodelling ensure we have just the right amount of bone for the things we do in life, whether working in an office, competing as a professional athlete or going into space. Whatever we do on a daily basis, the cells that make and break down bone need to ensure that the right amount is there – enough to keep us strong enough to function but not so much that it becomes too heavy and slows us down.

Understanding these rare diseases has also aided the development of therapies to help treat bone diseases including osteoporosis. The first of the three diseases – hypophosphotasia – is an inherited disorder where sufferers struggle to make hard bones. It affects one in 100,000 people. Van Buchem disease and sclerosteosis have about 30 and 40 reported cases respectively and sufferers of these two conditions make so much bone that it traps nerves needed to control breathing, hearing and other essential functions.

Osteoporosis is often treated using a group of drugs called bisphosphonates. These drugs mimic a molecule in the body which inhibits cells taking away bone; discovered from studies into understanding hypophosphotastia. The two rarer conditions have opened up our understanding of pathways that are involved in helping cells make new bone and this has potential for development of further treatment for osteoporosis and other conditions.

Black bone disease

More recently, another rare disease, alkaptonuria (“black bone disease”), has yielded clues about a very common condition: osteoarthritis.

People with alkaptonuria do not have enough of a particular enzyme, leading to a build-up of a chemical in the body called homogentisic acid. The build-up of this acid results in black and brittle bones. About one in every 250,000 people has this genetic disorder.

In my own research, my colleagues and I examined bone samples from people with black bone disease and people with osteoarthritis. We discovered new bone formations that were common to both conditions. This finding helps us to understand a bit more about osteoarthritis and potentially offers new options to treat it. The new bone formations appear microscopically inside bones. Although we don’t know exactly how they are formed, when we find out, we may be able to use this understanding to treat osteoporosis or help repair broken bones more quickly.

A new mineralised structure in cartilage that forms and contributes to joint damage has also been found in people with alkaptonuria. It is unusual to see hard mineralised structures in cartilage, which is very soft. The presence of these new mineralised structures in cartilage may help us to understand how cartilage is mechanically damage in osteoarthritis. If we can understand how they are formed, we may be able to stop them damaging cartilage.

These recent discoveries offer new opportunities for understanding common diseases and highlight the significance of rare diseases to everyone.

Two British physicians understood the value of studying rare and unusual disorders many years ago. William Harvey, discoverer of the circulatory system in the 17th century, said: “Nature is nowhere accustomed more openly to display her secret mysteries than in cases where she shows tracings of her workings apart from the beaten paths; nor is there any better way to advance the proper practice of medicine than to give our minds to the discovery of the usual law of nature, by careful investigation of cases of rarer forms of disease.”

William Bateson, an English biologist and coiner of the term “genetics”, put it more succinctly: “Treasure your exceptions!”

Adam Taylor is director of the Clinical Anatomy Learning Centre & Senior Lecturer in Anatomy at Lancaster University.