In this lecture,
we will take you from the basic science into the clinic,
and I will give you an example of how the rules of monogenic diseases apply
to a clinical case which I will present as the one on cystic fibrosis.
But let me first rehearse the basic principles of autosomal recessive diseases.
First, autosomal recessive diseases strike like lightning.
So, the parents are carrier but have
no clue that they are a carrier of this rare disease and
then find themselves with a boy or a girl that has this rare recessive disease.
So, it means that the parents are not at risk of developing
the disease but the siblings or the patients are.
And there is a one in four chance that the siblings will be affected,
and both of the sexes are affected equally.
So, boys and girls are at risk of having these diseases.
Now, the example is cystic fibrosis because it's a more frequent rare disease,
recessively inherited in the western population,
and it has a frequency of about one in 1/2500-1/3300.
The gene is called the cystic fibrosis transmembrane conductance
regulator which is a name that was given to the gene when it was identified.
And actually, the cystic fibrosis gene was
cloned by positional cloning about 30 years ago.
So, it's one of the prime examples of referred genetics where
people had no clue what gene they were looking for and then eventually
identified it by linkage analysis.
It's a relatively big gene and it encodes a chloride channel.
So now that we know the gene,
we can offer diagnostic testing but we can also offer carrier testing in the family
and we could even offer carrier testing to the whole population if that were an option.
Now, cystic fibrosis, to give you a little clinical insight,
it's a chronic disease of the lungs, so, eventually,
the lungs will be destroyed by recurrent infections.
It also affects the pancreas.
So, also the pancreas needs a lot of secretion and if that secretion is not normal,
the pancreatic function is not normal either.
Newborns will be born often with what is called
Meconium ileus which is actually an obstruction of the small bowel,
and also the males will be infertile because they are born
with the absence of the Vas Deferens.
There's also a few presentations which are more mild than
this classical cystic fibrosis and they are called cystic fibrosis-related diseases.
Now, cystic fibrosis as I said is the most common lethal genetic diseases among
the western population and it has differences in
the frequency depending on the ethnic and the geographical region of the patient.
The diagnosis is mainly based on clinical science.
So actually, we don't need a DNA test to confirm a diagnosis.
What people normally use is what is called the sweat test because there is
this abnormal chloride concentration even at the level of the skin.
Now for diagnostic testing,
what's important to know is that
the cystic fibrosis gene affects about 99% of the cystic fibrosis cases.
So, it means that this disease is not heterogeneous because it's one gene, one disease.
However, at the level of mutations,
we have a significant heterogeneity.
So, there is more than 1500 different ways to inactivate the cystic fibrosis gene.
And this is shown schematically in this slide where we
have this schematic presentation of the cystic fibrosis gene,
and we can see that there is missense mutations,
nonsense mutations, frameshift mutations, splice site mutations
at different positions induce genes and every patient will have two of these mutations.
Now if we look at the list of mutations,
what is apparent is that one of those mutations which is
the F508del mutation is very frequent.
It accounts for approximately 70% of all the mutant alleles.
And then comes a series,
the small series of mutations that are relatively
frequent like five percent in the western population.
And after that you have a long list of rare mutations.
This means that if we would design a test that covers this handful of mutations,
we can already confirm the diagnosis in most of the patients.
If this is not true,
one can move to more comprehensive testing, and in that case,
we would do Sanger sequencing to sequence the entire coding sequence,
or nowadays, we are using massive parallel sequencing to indeed sequence the entire gene.
Now, recessive diseases, what is more important in
the clinic is actually the implications for the family members.
As we've shown before,
if a patient is affected,
both parents will be carriers and they have recurrence
risk of one in four of having another child with cystic fibrosis.
But what is now the risk for cousins?
Well, the first thing which we can easily calculate
is the risk for the siblings of the parents.
So, for the uncles and the aunts,
their risk is one in two of being a carrier,
because they have either inherited the normal or the abnormal allele from their parents.
What's more intriguing is to know
the frequency of the carrier status in the normal population.
And we have no clue how much that frequency would be.
We have only one figure to calculate this frequency.
And here I come back to the Hardy-Weinberg equilibrium
that was explained to you by Yury in one of the previous courses.
And this is a equilibrium,
this is an equation to calculate the frequency of
disease of alleles in a normal population.
So, the only figure we have is the frequency of the disease which is q²,
and let me use 1/2500 because that's an easy figure.
Well, q² is the frequency of the disease,
meaning that q is the frequency of the abnormal allele.
And if we have 2pq,
we have two times the allele frequency,
which means we have 2√q² which would be 1/50.
So, the carrier frequency of the disease would be 1/25.
So, if we use that figure of 1/25 as the carrier frequency in the normal population,
we can calculate that the risk for these cousins would be 1/200
which is 10 times above the frequency of the disease in the general population.
Apart from cystic fibrosis,
there is hundreds of these different autosomal recessive diseases.
There's one that would like to mention which is
the sickle-cell anaemia which is very frequent
in the African population because of the presence of malaria.
And this gives you the first of one of the interesting features of recessive diseases.
One of the features of
recessive diseases is that there is a founder effect in some populations.
So, mutation occurs somewhere in
the population and then stays in that population for different generations.
If you combine this founder effect which is called a selective advantage,
one of the mutations may become very frequent.
And that's exactly the case for this F508del mutation.
It has occurred about 50,000 years ago.
And through all these generations,
it has been selected,
and people believe that it has been selected because of its resistance to cholera.
Another aspect which is very important in the recessive disorders is the consanguinity.
What we know is that in inbred populations,
the risk for cystic fibrosis and the risk for
other rare diseases is remarkably increased.
So, consanguinity is a risk factor for recessive diseases.
And one last thing as a molecular feature is what is called uniparental disomy.
There's a very few cases where the patient would have
homozygous mutations but one of the parents would not be a carrier.
And rather to invoke a known paternity what has happened in
those families is that there is a maternal uniparental disomy,
meaning that chromosome 7 has come
twice from the mother and the mother happens to be a mutation carrier,
so, it means that the boy or girl will be affected.
In this case, the recurrence risk is of course not 1/4,
but it's about 1 / 1 000 000.
Now, what are the treatment options?
The problem with many rare diseases is that we don't have any treatment at all,
partly because there are inborn errors,
and secondly, because there is no interest because they are so rare.
Now, for cystic fibrosis,
people have been looking for treatment for at least 30 years.
There is the symptomatic treatment.
One can treat the infections,
one can try to supplement the pancreatic enzymes.
So basically, one would love to have a fundamental basic treatment.
And there is one example now of a treatment that
has been approved and it's called Ivacaftor
which is a drug that actually modulates the efficiency of one mutant protein.
So, this is a protein that has the G551D mutation.
This aberrant mutant protein makes it to the plasma membrane.
So basically, it sits where it should sit but it doesn't function well.
And the Ivacaftor comes as a potentiator,
comes as a kind of a chaperone,
helps the protein to fold properly and to let the chloride transfer the protein.
So, we do have a treatment for cystic fibrosis but
the main limitation is that this mutation has to be present.
So, this treatment is limited to those patients who have this mutation.
And as you will recognize,
this is not the most frequent mutation.
So, for cystic fibrosis,
we don't have a treatment yet,
for the most frequent mutation,
we only have a treatment for a mutation that
occurs in about five percent of the patients.
This is the study of cystic fibrosis,
and I hope it has been helpful to tell you
one particular case to show how we take basic science to the clinic.
