You’re probably familiar with DNA, or deoxyribonucleic acid, from watching gory TV crime shows like CSI and Law and Order or reading books and newspaper articles about genetics discoveries. If you paid close attention or really liked science, you may even recall a few things from your high school biology class. If this is the case, you already have the framework to learn about DNA-based genetic tests that doctors use to make a diagnosis. This article will answer questions about the DNA tests that are specially designed for Coffin-Lowry syndrome (CLS).
Genetic tests for CLS involve reading the genetic
code from cells in the patient’s blood or other type of sample. The genetic code
is made up of four chemical structures, which are represented by the letters A,
T, C and G. Each gene is made up of a unique sequence using these four letters
of the DNA alphabet. Just as the words on this page have meaning because of the
order of the letters, so do the letters in a gene. Each gene serves a purpose,
which is to provide instructions for a unique protein to be made by the body.
It’s as if the DNA in genes is a biological recipe. Once the DNA recipe makes a
protein, the protein has a specific function. Proteins must work properly and be
in the right amount to prevent genetic disease.
A specific gene, known as RSK2, was identified by a group of
researchers studying patients with CLS in France, Philadelphia and England
(Trivier, et al. 1996). When they compared the DNA from patients who have CLS to
people who do not have CLS, they found differences in the DNA code. These
differences are called genetic mutations. The researchers wanted to see if the
mutations they found in the RSK2 gene cause CLS, so they studied how the
mutations affect the way the RSK2 protein, called a kinase, works. What they
found was that patients with CLS did not have any normally functioning RSK2
kinase. This led them to conclude that mutations in the RSK2 gene cause the RSK2
kinase to be made improperly, and the creation of an abnormal kinase leads to
all of the symptoms associated with CLS. When a DNA laboratory does the test for
CLS on a patient, they are looking for mutations in the RSK2 gene. Amazingly,
the RSK2 gene is made up of thousands of DNA letters and yet it is not uncommon
for just one letter to be changed in a patient with CLS. The genetic test
requires reading the DNA code from a patient’s sample, letter-by-letter, to find
these small changes.
Genetic testing may indeed be helpful, especially
for younger children. Many young children do not have all of the symptoms of CLS
and it is more difficult for the doctors to make the diagnosis in them. There
may be other genetic conditions that the doctors are considering, many of which
have some of the same symptoms. If the RSK2 gene test shows a mutation, then the
result is “positive,” and the diagnosis of CLS can definitively be made. If it
is negative, or no mutation is found, there may be other things to consider, but
it doesn’t necessarily mean that the diagnosis is not CLS.
There are some symptoms that become apparent later on in CLS
patients, and these can be anticipated if the diagnosis is known early. In
particular, scoliosis (curvature of the spine) is common in adolescents,
particularly boys, with CLS. Because it may not be present when the child is
first evaluated, the doctors will know that a child with a positive DNA test for
CLS has a higher chance than other children to develop scoliosis. The fact that
they can anticipate this potential medical complication allows them to treat it
earlier. The learning problems associated with CLS can also be approached with
early intervention if the diagnosis is known. Lastly, there may be a social and
emotional benefit to having the diagnosis early, because it allows newly
diagnosed children and their families to connect with other parents. Other
parents and families can be invaluable and it is often helpful to talk to others
who understand about the problems, and the joys, unique to CLS.
In addition to confirming the diagnosis and determining the inheritance pattern (discussed later), genetic testing can also provide important information to family members. For example, the mother, sisters, aunts or cousins of a boy or man with CLS may want to have children now or some day in the future. When a child with any genetic condition is born into a family, other family members may want to know if they need to consider the possibility that they may have a child with that condition as well. The most helpful approach to determining the chances of having a baby with CLS is to test the person who has CLS first. If that person shows an RSK2 gene mutation, then other family members can also be tested. This may be difficult to bring up with family members, but it is important to discuss and is something your relatives might be wondering about. Sometimes getting it out in the open is a relief to everyone. If they can find out their chances of bearing a child with CLS, it helps them to prepare for the possibility and consider all of their options for having a family. Anyone with a family history of CLS who wants more information about risks, testing and their reproductive options can meet with a genetic counselor. Genetic counselors can be found by going to http://www.nsgc.org or by requesting a referral from a gynecologist or family doctor.
We’ve already established that CLS is caused by a
mutation in the RSK2 gene. Genes, and the DNA that makes them up, are packaged
into structures called chromosomes. Each of us has 23 pairs of chromosomes, and
the important chromosome in CLS is the X chromosome. Females have two X
chromosomes and two copies of the RSK2 gene. Males have only one X chromosome
and only one RSK2 gene. If a boy has a mutation in his RSK2 gene, he will not
make a functional RSK2 kinase. However, if a female has a mutation in one of her
two RSK2 genes, she will make normal RSK2 kinase from the other copy of the
gene. The normal copy of the RSK2 gene may compensate, totally or in part, for
the deficiency caused by the mutation. There are many females with some mild
features of CLS, however, and these girls and women don’t have enough normal
RSK2 kinase being produced to completely prevent the CLS symptoms. One thing to
note is that it is rare for a female with an RSK2 mutation to have all of the
same features as a male with the condition.
The inheritance of CLS is X-linked, because it is on the X
chromosome. Females with an RSK2 mutation are called “carriers” and have a 50%
chance, with each pregnancy, of passing on the mutation. If the mutation is
inherited by a female, she will be a carrier. If the mutation is inherited by a
male, he will have CLS. Overall, before a pregnancy is conceived, there is a 25%
chance to have a son with CLS and a 25% chance to have a carrier daughter. As
mentioned, female carriers may have some of the symptoms associated with CLS
although they are usually more mildly affected.
CLS is not always inherited, however. Based on a relatively small
number of cases in which there was no family history of CLS, the medical
literature suggests that over half of the mothers of most boys with RSK2
mutations are not carriers (Jacquot, et al. 1998). If the mother of a child with
CLS has no symptoms and no family members with CLS, genetic testing may be the
only way to determine if she is a carrier or not. Mothers who are not carriers
still have a small chance of having a daughter who is a carrier or more than one
affected son; however, this risk is likely to be low (on the order of 1%). If a
woman who has a child with CLS is not a CLS carrier, her mother, sisters,
cousins and nieces would also not be carriers and they do not have an increased
chance of having a child with CLS. The family members of an unaffected father of
a child with CLS also need not be concerned about an increased chance for having
a baby with CLS, because of the X-linked inheritance pattern.
Genetic testing for CLS currently only looks at
the DNA sequence of the RSK2 gene. If a mutation is present, the diagnosis of
CLS is confirmed. If no mutation is found, there are a few possible
explanations. For one, there is some evidence in mice that another gene exists
which causes the same symptoms as CLS (Yang, et al. 2004). A negative result of
RSK2 testing may be due to the fact that there is a DNA mutation, but it lies in
another gene. At this point, the role of another gene or genes in CLS has not
been confirmed in patients with CLS.
A second possibility is that a mutation is present in the RSK2
gene, but was not picked up by the methods used in the lab. This is a limitation
of the technology that is available to us at this time. Because CLS is a rare
condition, it is difficult to know how many mutations are not being detected,
but the estimate is that it is likely to be low. Certain types of
rarely-occurring mutations cannot be detected in females, and testing an
affected boy in the family (if one is available) may be recommended.
Lastly, some doctors order RSK2 testing for patients that don’t
completely fit the CLS phenotype, meaning that they may have only some of the
symptoms associated with CLS and may even have other symptoms that are not
typically seen in CLS patients. This is not uncommon, and many tests may be
ordered to get to the bottom of the diagnosis. Sometimes one of the tests comes
back positive, and then the diagnosis is known. Other times, we are limited by
our current understanding of genetics and the available technology, and the
tests are all negative. Because of our rapidly advancing knowledge in genetics,
checking in with your geneticist or genetic counselor on an annual basis is a
good idea, as they can discuss new discoveries with you as they are made.
The testing is currently being offered at two
laboratories in the US: GeneDx, Inc. (http://www.genedx.com)
and Greenwood Genetic Center (http://www.ggc.org).
RSK2 gene testing must be ordered by a physician or genetic counselor, who will
discuss the results with the patient and family. The testing is slightly
different between these two labs, and your doctor or genetic counselor can help
you decide which lab is best for you. Testing can be done on a blood sample that
can be shipped to the lab, so there is no need to travel if you want the testing
done. The Greenwood Genetic Center (GGC) requires a special tube that the blood
must be shipped in, which they will send to your doctor’s office free of charge.
The special kit is only necessary for the first person tested in the family.
After that, blood samples for carrier testing can be sent in the tubes available
at any drawing station. If you’d prefer not to have blood drawn at all, GeneDx
can send a brush kit to your doctor or your home to collect cheek cells for DNA
testing. The accuracy, turnaround time and cost are the same for blood and cheek
brush samples. Unfortunately, unlike the quick results the investigators seem to
get on TV programs, the results of DNA testing for CLS usually take several
weeks, and many steps go into the analysis. The time is well-spent, however, and
the laboratory works hard to insure that the results they report to your doctor
are accurate.
Because of the methods they use, GGC prefers doing the initial
testing on males with CLS while GeneDx has experience testing both affected
males and females. Both labs require testing the person with CLS first, and can
then offer carrier testing to other family members. Testing a sample from a
pregnancy is also available when a mutation is known in a family. It is
important to be aware of the billing policy of the lab before sending a sample,
as the testing is expensive ($1,300 - $2,600, for the first person in the
family; $250 - $1,050 for carrier testing and testing on a pregnancy). Insurance
frequently covers RSK2 testing, but you will probably want to check with your
insurance company to see if you will be responsible for all or part of the cost
of testing.
This article covers some commonly-asked questions about genetics
and DNA testing. The next time you’re watching a crime show on TV or reading an
article that describes DNA, you’ll have a better understanding of genetics and
the methods used to examine genetic material. Of course, it’s especially nice to
know that not only is this technology helpful in solving crimes, but it can also
be used to help families like yours.
If there are any questions about this article or genetic testing
for CLS, please feel free to contact the author at
cheryl@genedx.com.
Cheryl Scacheri, MS, CGC
Director of Genetic Counseling Services
GeneDx, Inc.
Gaithersburg, MD
http://www.genedx.com/services/dis_cls.php
References:
Trivier, et al (1996) Mutations in the kinase Rsk-2 associated with Coffin-Lowry
syndrome. Nature. 384:567-570.
Jacquot, et al (1998) Germline mosaicism in Coffin-Lowry syndrome. European
Journal of Human Genetics. 6:578-582.
Yang, et al (2004) ATF4 is a substrate of RSK2 and an essential regulator of
osteoblast biology: Implication for Coffin-Lowry syndrome. Cell. 117:387-398.