Pitt Hopkins Syndrome (PTHS) is caused by changes in the TCF4 gene on chromosome 18.
Genes are like instruction booklets that tell the cells in our bodies how to make proteins. We have two copies of every gene – one from our mother, one from our father. Mutations change the DNA code of genes so that they cannot make a proper protein.
Children with PTHS have a mutation in one copy their tcf4 gene so that it no longer codes for a TCF4 protein. The brain cannot develop properly without both working copies of tcf4
In most cases, there is no family history of PTHS and the child is the first person in the family to have the tcf4 mutation. Doctors call this a ‘de novo’ mutation. De novo mutations usually arise in the mother’s egg, the father’s sperm or very early in the child’s embryonic development. This means that the chance of having another child with PTHS is low.
So far, there have only been two families reported in literature that have had more than one child with PTHS. This probably happened because one parent had the mutation in several egg or sperm cells. This is called mosaicism and the chances of this happening are thought to be less than 1%.
If you have a child with PTHS and are having another baby, you can ask for genetic testing while you are pregnant to see if your new baby has PTHS. The lab will check your new baby’s DNA for the same mutation that your child with PTHS has.
TCF4 mutations cannot be found in a small proportion of children who have PTHS symptoms. There are several possible reasons for this:
- The child has an unusual type of mutation that we are unable to see with existing gene testing techniques
- There is a second, unknown, gene that causes PTHS
Even if no tcf4 mutation is found, if the child has symptoms of PTHS then a diagnosis can be made. If a diagnosis is made based on symptoms, it is called a clinical diagnosis .Healthy siblings of people with PTHS have no risk of being mosaic , so are at no increased risk of having a child with PTHS.
There are two syndromes that are very similar to PTHS but are even rarer and caused by different genes. They are called Pitt-Hopkins-like syndrome 1 (PTHSL1) and Pitt-Hopkins-like syndrome 2 (PTHSL2).
PTHSL1 is caused by mutations in the CNTNAP2 gene on chromosome 7.
PTHSL2 is caused by mutations in the NRXN1 gene on chromosome 2.
PTHSL1 and 2 are both recessively inherited, meaning that the child has mutations in both copies of CNTNAP2 or NRXN1 and both parents are carriers, each having one mutation. In these families, there is a 1 in 4 chance of having another child with PTHSL1 or 2.
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