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Last reviewed: September 2019

These resources are designed to give you information that may be useful when your child starts asking questions about why they have SMA, what caused it and how you and the doctors know what equipment and help they need.

We have tried to give you a framework with ways of explaining things that are as accurate as possible without being too complicated, and that can be adapted in ways that work for you and your child.

To understand why some people have SMA, you have to know more about:

Our Bones & Muscles

Inside us we all have a skeleton and its job is to support and protect our body. It’s made of bones that grow as we grow.

The bones that go from our bottom to our head are small and moveable and are called vertebra. They make up our backbone or spine.

As we grow, some of our bones join up together. Where these bones meet up, it’s called a joint – like at our knees or elbows.

Our skeleton can bend at these joints because the bones have muscles attached to them. When our muscles move, so do our bones so that we can do things like wave a hand or nod our head. Our muscles need to stay strong and be able to move when we want them to.

A baby’s body has about 300 bones. As they grow, some bones join up. By the time they’re an adult, they only have 206 bones.

Everyone has more than 640 muscles attached to their bones.

Our brain and nerve cells

One of the jobs our brain has is to tell our body what to do. It has to be able to send instructions to our muscles so that they can move and move our bones. Our brain does this by sending electrical messages through our nerve cells.

Nerve cells are long and thin and different shapes so that they can carry messages from one part of our body to another.

  • The nerve cells you need to know about to understand SMA are our lower motor neurons.

These are inside our spinal cord which is protected by our backbone. Their job is to pick up the messages that come from our brain and down our spinal cord and take them to our muscles and tell them to move.

Everyone’s body is made up of cells – trillions of them. Most cells are so small you’d need a microscope to see them. There are all sorts of cells with different jobs. Some are building blocks that make up our bones, our muscles, our hair and skin. Nerve cells are one sort of cell. Some nerve cells stretch from inside our spinal cord right down to our big toe!

Our genes

Inside every cell there are thousands of genes – these are very important. They control lots of the instructions about us and how our bodies are put together – a bit like a cake recipe. They control things like the colour of our eyes and our hair.

When one of our genes doesn’t work properly, this is sometimes called a ‘faulty’ gene. Having a ‘faulty’ gene is a bit like having a cake recipe with part of the instructions missing.

Lots of people have ‘faulty’ genes of one sort or another – for example, people who are colour blind – but this doesn’t mean this is anybody’s fault.

The important genes you need to know about to understand SMA are:

  • The Survival Motor Neuron 1 or the SMN1 gene

Everyone has two of these SMN1 genes in all the cells in their body; they were passed to them by their parents.

Half our genes come from our mum, half from our dad. The exact set of genes that mums and dads pass on to their children is different each time. This is why brothers and sisters are different from each other.

  • The other gene is called Survival Motor Neuron 2 or the SMN2 gene.

Our proteins

As well as controlling lots of instructions about us, genes also carry information that’s needed to make proteins.

Proteins are even smaller than cells and have different jobs to do, like fighting off infections and building muscles.

Our SMN1 genes make an important protein called: the Survival Motor Neuron Protein or SMN Protein for short.

We need SMN protein to keep our lower motor neurons healthy and working so that they can tell our muscles to move. Lower motor neurons pick up the messages that have come from our brain, all the way down our spinal cord.

Our SMN2 genes are like the SMN1 gene. They can only make a small amount of protein that works but they’re still important. These genes are a bit different – we can have between 0 and 8 copies of them. The more copies we have, the more SMN protein we’re likely to be able to make. These genes are sometimes called ‘back-up’ genes.

Our SMN1 and SMN2 genes work together to make SMN protein.

Your SMN1 and SMN2 genes

If you have SMA, you have two ‘faulty’ SMN1 genes in all the cells of your body. This means these genes can’t make enough SMN protein to keep your lower motor neurons healthy.

The more copies of the SMN2 gene you have to help make up for the ‘faulty’ SMN1 genes, the more SMN protein you should be able to make.

The different amounts of SMN protein that people with SMA can make is one of the reasons why SMA affects people differently.

The chance of inheriting two faulty SMN1 genes

Everyone has two SMN1 genes in all the cells in their body. These are passed to them by their parents – one from their mum, one from their dad.

Dads pass on their genes through their sperm; mums pass on their genes through their eggs. When a sperm and egg join up in the mum’s womb, they form an embryo which grows into a foetus and then a baby.

If the mum and dad are carriers of SMA:

  • A carrier of SMA has one ‘healthy’ SMN1 gene and one ‘faulty’ SMN1 gene.
  • Parents can’t control what genes they pass on, it’s like rolling a dice. For each pregnancy, for a mum and dad who are both carriers, their SMN1 genes can be passed on in one of these four ways:
Mum passes on Dad passes on The baby The baby’s chances
A ‘faulty’ SMN1 gene A ‘faulty’ SMN1 gene Has SMA. Is a carrier of SMA. 1 in 4
A ‘faulty’ SMN1 gene A ‘healthy’ SMN1 gene Doesn’t have SMA. Is a carrier of SMA. 2 in 4
A ‘healthy’ SMN1 gene A ‘faulty’ SMN1 gene Doesn’t have SMA. Is a carrier of SMA. 2 in 4
A ‘healthy’ SMN1 gene A ‘healthy’ SMN1 gene Doesn’t have SMA. Isn’t a carrier of SMA. 1 in 4

Do mums and dads know if they are carries?

Not many people know they’re carriers of SMA and it’s not until they have a child who is diagnosed with SMA that they find out. Because of the way those dice roll, their child born with SMA may be their first child or their second, third or fourth. They don’t find out they are carriers until then.

If the mum or dad has SMA themselves, the dice roll a bit differently.

SMA is a rare condition; this means that not many people have it.

Nobody knows exactly how many people have SMA but, in the whole world, approximately 1 person in every 10,000 are born with SMA each year.

To make this easier to picture, think about the huge crowds of people that go to the Glastonbury music festival or to Wembley Stadium for football matches:

  • 175,000 people go to Glastonbury, so about 28 of them could have been born with SMA.
  • 90,000 people can fit in Wembley Stadium, so that means about 15 of them could have been born with SMA.

The first clues

Working out that someone has got SMA is a bit like solving a mystery or puzzle – doctors know what clues to look for.

One of the first clues was when we started to notice that your muscles weren’t as strong as they usually are for someone the same age as you.

We asked the doctors if they knew why. They did some tests to see what was happening.

One of these tests was to look at a very tiny bit of your blood using a microscope. This is what doctors do a lot of times when they want to work out why someone isn’t feeling or growing well.

What they could see in your blood were the tiniest things called your genes.

They could see your two SMN1 genes weren’t working properly. This was the big clue that told them you had SMA.

They could also see your SMN2 genes and count how many you had. This gave them another clue about your SMA.

The doctors put these clues together and because of them were able to tell us:

  • What Type of SMA you have: