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Individually, rare diseases are... rare.

In the US, we usually say that a disease is rare

when it affects fewer than 200,000 people in the whole country.

And in the EU, a disease is rare

when it affects fewer than one out of every 2000 people.

That's not very often.

But because there are so many different rare diseases

experts estimate there are about 7000

collectively, it's not so rare to have a rare disease.

In fact, about 30 million Americans have a rare disease,

around the same number who have type 2 diabetes!

So, studying them is important in it's own right.

But these sorts of investigations can also reveal larger insights

into how our bodies work.

And because many rare diseases are caused by relatively simple,

known mechanisms, they can also tell us

about the things that can go wrong in much more common diseases.

Sometimes, this even means researchers can come up with a drug

that works for millions of people.

Here are six times research into the most uncommon maladies on the planet

have turned out the benefit the masses.

First up, a bone mineralization disorder called hypophosphatasia, or HPP.

In severe cases, which affect about one in every 100,000 people,

patients have soft bones that can easily break and deform.

Many patients are in chronic pain and often lose teeth prematurely,

and a quarter experience more than 10 fractures in their lifetime.

The disease is caused by a gene mutation that prevents

the body from making correct versions of the enzyme alkaline phosphatase.

In the mid-1960s, researchers learned that this enzyme

regulates the body's production of a molecule called pyrophosphate.

It's found in blood and urine

and prevents the main mineral in our bones from growing.

Without enough of the enzyme, the body has too much pyrophosphate,

so mineralization doesn't happen as well.

While researchers were working to understand HPP,

they realized that pyrophosphate might actually have another use, too.

Previously, they'd found that this molecule had a perk:

It kept bone minerals from dissolving.

So maybe it could help patients with osteoporosis,

a disease of low bone mass that affects 200 million people around the world.

Scientists then searched for compounds that mimicked pyrophosphate,

and they found that the water softening molecule bisphosphonate did the trick.

Now, it's a common osteoporosis drug

although they later realized this treatment actually works for a different reason:

It prevents cells called osteoclasts from breaking down bone.

Because they don't need extra pyrophosphate, artificial or otherwise,

the drug that HPP patients helped give the world won't help them

and might even make them worse.

But if nothing else, it did change how much we know about bone biology

and led to a whole new class of drugs for millions of people.

Next is Gaucher disease,

which affects somewhere around one in every 50,000 or 100,000 people

and shares some interesting parallels to Parkinson's.

Guacher is what's known as a lysosomal storage disease,

which means there's a defect

in the organelle in cells (lysosomes) that digests garbage.

When that happens, the lysosomes can't get rid of the trash fast enough,

and it builds up.

In Gaucher, this is the result of an enzyme deficiency,

specifically one called glucocerebrosidase.

It specializes in breaking down certain glycolipids,

which are basically fats with a sugar attached to them.

So without enough of the enzyme, they build up,

especially in the liver, spleen, and bone marrow, which produces blood cells.

As a result, people with the disease often don't have enough blood cells,

which can make them tired and more prone to bruising and bleeding.

They can also get enlarged spleens and livers.

On rare occasions, Gaucher patients also develop symptoms

ike tremors and slow movements

similar to Parkinson's,

a neurodegenerative disorder that famously affects people's ability to move.

Initially, scientists didn't make much of this.

Then, they noticed something surprising with the relatives of Gaucher patients.

Those who carried the mutation that causes the enzyme deficiency

were more likely to get Parkinson's, too.

In fact, a huge genetic study in 2009 revealed that

around 7% of participants with Parkinson's had a mutation in that gene

the most for any single gene.

In genetics, a finding like that

for a multi-factorial disease like Parkinson's is huge.

Now, scientists are working to figure out what it means.

One possibility is that not having enough of that enzyme

prevents cells from breaking down alpha-synuclein proteins.

These can get misfolded in the brain

and are thought to be one of the main causes of Parkinson's.

That's unlikely to be the whole story,

but it could be important for a subset of cases.

Scientists are hopeful that studying this enzyme

and lysosomes in general may lead to a new understanding of Parkinson's,

and possibly to new treatments for the disease,

and for those with Gaucher, too.

But the fact that Gaucher has already helped

identify the biggest genetic risk factor

to the second-most common neurodegenerative disease is a pretty big deal.

Speaking of lysosomal storage diseases

as you do

scientists are finding that another one,

called Niemann-Pick, might help us combat Ebola virus.

Technically, and fortunately, Ebola hemorrhagic fever is also a rare disease.

But that could change at any time with an outbreak.

Back in 2011, researchers were studying the virus

to figure out how it was getting into cells.

They knew it used a certain glycoprotein to do it,

but they didn't know what on our cells it was targeting.

So, they set up a screen, testing the Ebola glycoprotein

on a series of different cells, each of which had one mutation.

Weirdly, a bunch of the cells that kept the virus out

had a mutation in a gene called NPC1,

which makes a protein that helps shuttle cholesterol around inside cells.

This is the same gene that's mutated in Niemann-Pick disease type C, or NPC,

which affects around one in 150,000 people.

Patients with it end up with build-ups of cholesterol inside neurons,

which can cause dementia at a shockingly early age.

For that reason, it's sometimes called 'childhood Alzheimer's.'

Thankfully, there are some treatments for it,

but the disease itself could also help treat thousands of others.

Because when scientists tried to infect cells from NPC patients with Ebola…

they couldn't.

The mutation was keeping Ebola out.

The fact that Ebola targets NPC1 explains part of why it's so deadly

it's in all cells, so the virus can target any cell of the body,

not just a few like most viruses.

Now, researchers are using this knowledge to create new Ebola drugs.

If they can make molecules that block the NPC1 protein,

they may be able to prevent people from getting infected.

Sometimes, rare diseases are helpful to scientists

because they can confirm that what they've seen in lab animals

also applies to humans.

That's what happened with an extremely rare condition

called congenital leptin deficiency.

As the name implies, people with the disease don't make enough leptin,

a hormone that fat cells produce to tell the body to stop eating.

As a result, they're constantly hungry and eat way too much food.

These people become obese very early in life,

usually within months of being born.

We know of about 30 cases now, but for a long time,

we didn't know the condition existed.

And that became important because for decades,

scientists have been using a mouse with mutations in its leptin genes

to study type 2 diabetes.

The mice become very obese, and if they have the right genetic background,

they develop diabetes quickly, making it easier to study the disease in the lab.

Years of mice experiments suggested that leptin might be important

for our understanding of obesity.

But no one was really sure how relevant it was to people.

That changed in 1997, when researchers identified two severely obese children

who shared the same mutation in their leptin genes.

They made far less leptin than normal,

showing that this hormone was a key player in how our bodies regulate

the amount of food we eat and how much fat we put on.

Like with Niemman-Pick and Ebola, some rare diseases,

it turns out, come with perks.

In the case of something called Laron syndrome,

those advantages are potentially life-changing for the rest of us

if we can figure out how to mimic them.

People with Laron's are very short

under 1.4 meters tall

because of a mutant growth hormone receptor.

Even though they make plenty of growth hormone,

their bodies can't use it normally,

so they never get very tall and their limbs are short.

It's a unique form of dwarfism,

and fewer than 400 cases have been diagnosed worldwide.

The surprising thing is, even though these people are often obese,

they have normal blood pressure,

and they seem impervious diabetes and cancer.