Ellen Thomas: Genomics 101 - What is genetic or genomic testing?

In this explainer episode, we’ve asked Ellen Thomas, Interim
Chief Medical Officer at Genomics England, to explain what
genetic and genomic tests are, why someone might do a test, and
how they are performed, in less than 10 minutes.


You can also find a series of short videos explaining some of the
common terms you might encounter about genomics on
our YouTube channel.


If you’ve got any questions, or have any other topics you’d like
us to explain, feel free to contact us
on info@genomicsengland.co.uk.


You can download the transcript or read it below.


Naimah: What is genetic or genomic testing? Today, I’m joined by
Ellen Thomas, interim chief medical officer for Genomics England,
who’s going to explain more. So, first of all Ellen, what is a
genetic test?  


Ellen: Well, genetic tests examine a person’s genes to see if
they have any changes in their DNA which might explain their
symptoms. We all have DNA in most of the cells of our bodies, we
inherit it from our parents and pass it on to our children. DNA
provides the blueprint for our genes, and the proteins which
build and run our bodies. Nearly all of our DNA is exactly the
same across all of us, but around 5 million out of our 3 billion
DNA letters are different, and each of these we call a genetic
variant. The pattern of genetic variants that we all carry helps
to make us who we are, and genetic testing is designed to examine
some of these variants to help inform our healthcare.  


Naimah: So, why are they sometimes called genetic tests and
sometimes called genomic tests?  


Ellen: Well, the words genetic and genomic are often used in
exactly the same way, but broadly, genetic tests are usually used
to look at just one or a small number of a patient’s genes, while
a genomic test will look at hundreds or even thousands of genes
at the same time. In general, it’s fine to use
either.  


Naimah: If you want to hear more about the difference between
genetics and genomics, you can find another explainer episode
with Rich Scott on our website, which goes into more
detail.  


Okay, so coming back to you, Ellen, what are the reasons we might
do a genomic test?  


Ellen: Some rare health conditions are caused by DNA variants in
our genes, conditions such as cystic fibrosis, Huntington’s
disease or sickle cell disease. In these 3 conditions, there is
usually just one gene that is responsible, the same gene for all
patients. That means that you can often find the DNA variant
which has caused a patient’s symptoms by doing a test which looks
just at that gene, or even sometimes just at a part of the gene.
But for other genetic conditions, a variant could be found in any
of dozens or even hundreds of genes, which could cause the same
condition or a group of conditions, and examples of that include
familial forms of epilepsy or developmental disorders in
children.  


For these conditions, to find an answer you often need to do a
broader genomic test, looking at many genes at the same time, and
also sometimes in between the genes. Finding the variant in a
patient’s DNA which has caused the condition is useful, because
it helps understand how the condition is passing down in the
family, and whether it could affect anyone else in the family in
the future. It is also increasingly used to work out which
treatment an individual patient might respond to
best.  


Genomic tests are also used to help diagnose and treat cancer. A
tumour develops and spreads because new variants in the DNA build
up inside the tumour, which are not present in the patient’s
healthy cells. By testing the DNA of the tumour, you can
sometimes understand more about why it happened and what
treatment might be most effective.  


Naimah: So, can you tell me a bit about what sort of questions
you can and can’t address with genomic testing, and how has this
changed over time?  


Ellen: Well, at the most basic level, if a condition is not
caused by DNA variants, then a genomic test will not provide any
useful information. So, doctors use genomic tests when they
suspect that a patient might have an explanation of their
symptoms in their genes, but we don’t always find an answer.
Sometimes patients with a genomic cause and those with a
different cause may have very similar symptoms.  


We do constantly learn more about the ways in which genetic
variants cause disease through research. Patients may have a gene
variant causing their condition, but it’s so rare that it hasn’t
yet been discovered, or so complex that it can’t be seen in the
test analysis, so the test won’t identify the cause. Sometimes
new understanding through research can then find the answer,
which can be many years after the patient first developed
symptoms.  


Naimah: And how are these tests performed? For example, are they
a blood sample?  


Ellen: Yes, for most rare conditions, the tests use a blood
sample. In cancers, a sample of the tumour needs to be tested
after it’s removed by surgery or biopsy. The blood or the tumour
is then processed to extract the DNA, and then there’s a range of
different tests which can be used to read the DNA sequence.
Expert scientists in the NHS then review variants in the DNA to
make sense of the results and provide information to clinicians
and patients about what it means for their diagnosis or
treatment.  


Naimah: And why are there lots of different genomic tests which
can be used in healthcare?  


Ellen: There are different types of genetic variants, and there
are tests available that are specialised for these different
types of genetic variant. Some tests look at a single gene, some
tests look at many genes, often known as a panel of genes. Some
also compare genomic information from a patient and their parents
to understand which DNA variants are likely to be important. The
tests will be selected to match what is understood about the
patient’s symptoms and their likely cause, and to provide the
best chance of finding information which will be useful for their
care.  


Naimah: That was Ellen Thomas explaining genetic and genomic
testing. If you’d like to hear more explainer episodes like this,
you can find them on our website at www.genomicsengland.co.uk.
Thank you for listening.