Genetic Testing for the Ichthyoses (2004)
Sherri Bale, PhD, and Cheryl Scacheri, MS, CGC
GeneDx, Inc., Gaithersburg, MD
Introduction Perhaps you or a family member have a diagnosis of ichthyosis and are looking for answers to questions like, "What type of ichthyosis do I have," or "What are the chances that I will pass ichthyosis on to my child?" Sometimes these answers may be provided by routine tests, skin biopsy, or physical exam. At other times genetic testing provides the information that may not be available through any other test or exam. Specifically, genetic tests may be able to determine precisely which type of the many sub-types of ichthyosis a person has. Or it may be used to pinpoint the way ichthyosis is inherited in a family, providing information about each relative's chance to have a child with ichthyosis. Lastly, test results can open the door to a number of reproductive options, including testing embryos before a woman becomes pregnant and other procedures that can be performed during pregnancy. This article will explain what genetic testing is, the reasons one might choose to have a genetic test, what testing can and cannot tell you, and resources you can use to find out if a test is available and where it can be done.
What is genetic testing? Genetic tests involve an examination and analysis of a person's DNA. DNA is a chemical in each cell of the body, which provides instructions for making the proteins and other molecules we need for normal body function. To create this genetic massive textbook of instructions, 3 billion bits of DNA (referred to as "bases") are strung together. Most of these bases are the same from one person to the next. In fact, 98% of my DNA is identical to yours! It is the other 2% that makes us look different from each other, and some of those differences are responsible for causing genetic diseases. If a change is present in a gene and it causes a molecule to be made incorrectly, that change is called a 'mutation.' Genetic testing is a means of looking at a person's DNA to identify the mutation that causes their condition. However, since there is so much DNA to look through, researchers must already know which of the many genes to examine in a particular disease before genetic testing can be offered. Also, a person's doctor must have a good idea of what the clinical diagnosis is for that patient. Genes have not yet been identified for every condition, but there are many genes for the ichthyoses for which we can test. Laboratories often add tests as researchers identify new genes. Up-to-date information about available testing and the contact information for laboratories can be found at an excellent government-supported website: www.genetests.org.
What a genetic test can tell you, your family, and your doctor. Your physician may have reasons for ordering genetic testing and may discuss it with you, or you may wish to bring it up with your doctor yourself. Sometimes, when a patient has ichthyosis, they have features that could fit with several different diagnoses because there are overlapping symptoms between the subtypes. Doctors frequently use genetic testing to help define which ichthyosis a person actually has. This may help them to treat and manage the patient. Another reason to have a genetic test is if you or a family member wants to have children. Genetic testing, which would ideally be performed first on the person with ichthyosis, is often helpful in determining a person's, and their relative's, chances to have a baby with ichthyosis. In some cases, the doctor can determine this by looking at the family tree. However, genetic testing may be recommended if the inheritance pattern is unclear or if you or a family member is interested in reproductive options such as genetic diagnosis before implantation or prenatal diagnosis.
How is genetic testing performed? All genetic tests are very specialized and require highly trained scientists and technicians. In this article, we discuss only DNA-based molecular genetic testing (and do not address the microscope-based test commonly performed for steroid sulfatase deficiency). Samples must be specially prepared to separate out the DNA from a blood, skin, or cheek brush sample from the patient. Genes are usually tested one at a time and sometimes certain regions of a gene, called "mutation hot spots," are tested first. Mutation hot spots are places in the gene where the majority of mutations are found. For most tests, the gene is first prepared for testing by using a technique to make millions of copies of the different regions of the gene. Then, the laboratory may use other tests to look at hot spots. In most situations, however, much of the entire gene must be analyzed, base by base. This is done by a technique called "sequence analysis." With sequence analysis, another piece of equipment is used which reads each letter of the DNA pattern, or sequence. Remember there are 3 billion of these in total, and a single gene usually contains several thousand bases. The recipes that genes encode are quite complex! The sequence in the patient is compared to the sequence that is usually observed in healthy people. This is done by scientists who go through the thousands of DNA letters, meticulously comparing the patient's DNA pattern to the normal sequence. It is almost like editing the chapter in a book, letter by letter. A change is identified when there is a difference between the normal published sequence and sequence observed in the patient. Changes that haven't been seen before must meet rigorous criteria before they are deemed responsible for causing ichthyosis in a patient. The reports issued by laboratories are written for the referring physicians, and an interpretation of the findings is always provided. Once a mutation is identified in a family, other family members can have carrier testing specifically to look for that mutation, and the reproductive options outlined below can be discussed.
Testing on a pregnancy or before a pregnancy is achieved. Genetic testing during a pregnancy, called "prenatal diagnosis," has been around for decades and usually involves one of two procedures to obtain a sample from the pregnancy. Before becoming pregnant, it is critical to get started by testing the person in the family who has ichthyosis. Once the mutation in the family is known, genetic tests are performed on the DNA from the developing baby and can tell you if the baby has inherited the mutation. Both of the procedures have a small chance of complications, which should be discussed with your doctor or genetic counselor. Amniocentesis is the more common procedure. It is performed around 16 to 18 weeks of pregnancy and the doctor uses a needle to obtain amniotic fluid, which has some of the baby's cells floating in it. Chorionic villus sampling can be performed earlier in pregnancy, at around 11 to 13 weeks. The doctor obtains a piece of the placenta, which has the same genetic material as the baby, for testing. Genetic testing on a pregnancy may be done to obtain information about the baby that might be helpful for delivery or decision-making about continuing the pregnancy. "Pre-implantation diagnosis" is a relatively new technique in which embryos are created outside of the body through in vitro fertilization. Genetic testing is performed on the embryos. Only the embryos that did not inherit the mutation are transferred into the woman's womb to create a pregnancy. This method is performed at only a few specialized centers in the world. Because each of these procedures is invasive and have both benefits and limitations, it is best to have a thorough discussion with your physician and the specialists who perform them if you are considering prenatal diagnosis or pre-implantation diagnosis.
What genetic testing cannot tell you. Results of genetic tests, even when they identify a specific mutation, can rarely tell you how mild or how severe a condition will be in any particular individual. There may be a general presentation in a family or consistent findings for a particular diagnosis, but it's important to know that every individual is different. The result of a genetic test may be "negative," meaning no mutation was identified. This may help the doctor exclude certain diagnoses, although sometimes it can be unsatisfying to the patient. "Inconclusive" results occur occasionally, and this reflects the limitation in our knowledge and techniques for doing the test. But we can be optimistic about understanding more in the future, as science moves quickly and new discoveries are being made all the time. For more information about genetic tests performed at GeneDx, visit our website at www.genedx.com.
Tests currently available for the ichthyoses include: (disease name - gene(s)involved, inheritance pattern)
- CHILD Syndrome - CDPX2, NSDHL X-linked Dominant
- Conradi-Hunermann syndrome - EBP X-linked dominant, PEX7 Autosomal recessive
- Autosomal recessive congenital ichthyosis (lamellar or CIE) - TGM1, Autosomal recessive
- Ichthyosis Bullosa of Siemens - KRT2e Autosomal dominant
- Epidermolytic ichthyosis (formerly epidermolytic hyperkeratosis) - KRT1, KRT10, Autosomal dominant
- Sjogren-Larsson syndrome - FALDH, Autosomal recessive
- Vohwinkel Syndrome - GJB2 Dominant and recessive
- Erythrokeratoderma variabilis - GJB3, GJB4 Autosomal dominant
- Darier disease - ATP2A2, Autosomal dominant
- Hailey-Hailey disease - ATP2C1, Autosomal dominant
- KID syndrome - GJB2, Autosomal dominant
- Multiple Sulfatase Deficiency - SUMF1, Autosomal recessive
- X-linked ichthyosis - ARSC1, X-linked (steroid sulfatase deficiency)
- Epidermolytic Palmar-Plantar Keratoderma - KRT9, Autosomal dominant
- Pachyonychia Congenita - KRT16, KRT17, KRT6a, KRT6b Autosomal dominant
- Netherton syndrome - SPINK5, Autosomal recessive
If you have any questions about this article or genetic tests, please feel free to contact GeneDx, Inc. by email or by calling 301.519.2100.