TSC and genetics
Tuberous Sclerosis Complex is a genetic condition. If you live with the condition, you were born with it
Tuberous Sclerosis Complex (TSC) is a genetic condition, meaning that it is caused by a person’s genes.
If a person is living with TSC it means that they were born with the condition. You cannot ‘catch’ TSC and it is not possible to develop TSC during your life. However, a person living with TSC might not find out that they have the condition until later in life.
Genetics and gene inheritance can be confusing and easily misunderstood. Here, we have provided an overview of genetics in general and also the genetics of TSC.
What are genes?
Genes are made from a person’s DNA and give instructions on how our body should grow and maintain itself.
Each of us has thousands of genes and each gene is responsible for different things.
The genes that make up our bodies are inherited from our parents.
How do genetic disorders happen?
Sometimes, a gene might be have a natural alteration (or ‘mutation’) that would not be usually expected. This mutation means that the gene is unable to instruct the body as it normally would, causing a disruption in normal development and functioning. This is the reason for genetic disorders, such as TSC.
Every genetic disorder is as a result of mutations from different genes.
What genes cause TSC?
TSC is caused by mutations in two genes, called TSC1 and TSC2.
All genes come in pairs, including the genes responsible for TSC. One gene in the pair is inherited by the mother, the other gene in the pair is inherited by the father.
In some genetic conditions, including TSC, a mutation in only one gene (i.e. TSC1 or TS2) will cause TSC.
Although we inherit genes from our parents (see ‘Inherited TSC’ below), it is possible for someone to have the genetic mutations that cause TSC with no family history of the condition (see Non-inherited TSC’ below).
Around one in every three people who live with TSC have inherited the condition from a parent who also has the condition. This is called ‘Dominant inheritance’ and happens because the mutated gene that leads to TSC is passed onto the child.
If one parent is living with TSC, there is a 50% chance in every pregnancy that the child will inherit TSC. This is because genes are handed down to the child in pairs – one from the mother and one from the father. The gender of the parent who lives with TSC and the gender of the child does not change these odds.
In around two in every three people who live with TSC, neither of their parents show any signs or symptoms of also having TSC. This means that the mutation to one of the genes responsible for TSC was sporadic and not inherited.
In around 2% of TSC cases, couples will conceive a child that has non-inherited TSC and then go on to have another child that also has the condition. This is called ‘Mosaicism’ and occurs because the genetic mutation that leads to TSC is found in cells in only the sperm of the man or eggs of the woman. Testing for mosaicism is difficult and possible only if the genetic alteration (i.e. TSC) has been identified in a previous child of the couple.
If a person is living with TSC, they were born with it. This is because TSC happens when a person’s ‘genes’ (the things that make the body grow) don’t act like they are meant to. You are not able to ‘catch’ TSC, like you can with a cold or flu.
A person living with TSC might still not be told that they have TSC until you are older. Possibly even when they are a grown-up. This is because the condition might not have been found until then or was not causing any problems.
When a condition happens because of a person’s genes, it is described as a ‘genetic disorder’. TSC is a genetic disorder.
What are genes?
Genes are what we are made out of. Genes tell the body how it should grow. Everyone has lots of genes. Every gene is responsible for different things, like eye colour or how tall you might be. The genes that make up our bodies come from your parents.
What is a genetic disorder?
Sometimes, a gene does not do what we expect and it change. This happens naturally. A change to a gene might mean that it does not tell the body how to grow as it normally would.
Changes to different genes mean that different things might happen. This is how genetic disorders, like TSC, happen.
TSC happens when two genes, called TSC1 and TSC2, do not do what we might expect.
The genes that make up our bodies come from our parents. If a person’s mum or dad has TSC, it might mean that their son or daughter will also have it. However, a person might also live with TSC even if their mum or dad doesn’t have the condition.
TSC from mum or dad
Around one in every three people who live with TSC has it because their mum or dad gave them one of the TSC genes with unexpected changes to it. This is called ‘Dominant inheritance’.
If a mum or dad is living with TSC, there is a 50% change that a baby they are having will also live with TSC. It does not matter who out of mum or dad lives with TSC, or if the baby is a boy or a girl.
TSC when mum and dad do not have it
Some people who live with TSC have it but their mum or dad don’t. This means that the TSC genes unexpectedly changed but were not given to the child by mum or dad. Usually, if this happens it is only in one of the babies that the mum and dad have.
It does not happen a lot, but a mum and dad who do not have TSC might have more than one baby who lives with the condition. This is because the TSC genes with unexpected changes are not everywhere in the mum or dad’s body, so it does not affect them as much. This is called ‘Mosaicism’ and it is hard to test a mum or dad to see if they have it.
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