Cytogenetic testing examines the number and structure of a person’s chromosomes. Chromosomes are tightly bundled packages of DNA present in almost every cell in our bodies. Most people have 46 chromosomes in nearly every cell of their bodies. These chromosomes exist in two sets: we inherit one set of 23 chromosomes from our mother and one set of 23 chromosomes from our father. In this way, DNA is passed from parent to child and contributes to development and health. Cytogenetic testing can detect gains or losses of chromosomes or segments of chromosomes. People with changes in their DNA or in the structure of their chromosomes may have birth defects, behavior issues, delay in development, or learning problems.
What is array CGH?
Microarray-based comparative genomic hybridization (array CGH) is a type of cytogenetic testing. This technology evaluates important areas of our chromosomes to see if there are extra or missing DNA segments that could cause birth defects, mental retardation, or other medical or learning problems.
How is array CGH different from traditional cytogenetic testing?
Array CGH is a technology that evaluates chromosome structure for extra or missing pieces even closer than can be achieved by traditional cytogenetic testing, called karyotyping. Array CGH detects nearly all chromosome deletions and duplications that karyotyping detects but can also detect smaller extra or missing chromosome pieces that will be missed by karyotyping. These smaller alterations, often called “submicroscopic” alterations because they cannot be seen through the microscope, can still cause major birth defects, mental retardation, and genetic syndromes.
Who should consider having array CGH testing?
Array CGH testing should be considered for any pregnancy for which traditional karotyping would be offered. Common reasons for performing karyotyping and array CGH include advanced maternal age, abnormal maternal blood test indicating an increased risk for a chromosome abnormality, previous child or pregnancy with a known chromosome abnormality, abnormal ultrasound findings, and previous miscarriages or stillbirth.
Under what circumstances is array CGH performed prenatally?
Although array CGH should be considered for any pregnancy for which routine karyotyping is offered, many doctors and genetic counselors routinely consider using array CGH technology for pregnancies in which an abnormality is seen by ultrasound evaluation, or in pregnancies in which an abnormality in chromosome number or structure is detected by traditional karyotyping that could benefit from further studies using array CGH.
Is there a risk of miscarriage with array CGH testing?
Array CGH testing itself does not pose a health risk to the fetus or the mother. However, karyotyping and array CGH testing require a sample from the pregnancy, and the procedures performed to obtain the sample, such as amniocentesis or chorionic villus sampling (CVS), have a risk of miscarriage associated with them. Your doctor or genetic counselor can talk to you about these risks and help you determine whether chromosome testing and array CGH are appropriate for your pregnancy.
What are the benefits of array CGH testing?
Array CGH testing, when combined with karyotyping, provides the most comprehensive analysis of chromosome structure available in medicine. A prenatal diagnosis of a specific condition can provide additional information that may enable your doctors to manage your pregnancy better and enable you and your doctors to know what to expect after delivery. Some parents choose to have array CGH testing because problems have arisen in their pregnancy, such as a non-reassuring maternal serum screen, abnormal ultrasound findings, or abnormal karyotype, and they want as much information as possible to make informed decisions. Other parents choose to have array CGH so they can plan and prepare for a child with anticipated medical or health issues. Still other parents choose array CGH testing to provide further reassurance that a chromosome problem does NOT exist in their pregnancy.
If karyotyping is done and the chromosomes are normal, why consider array CGH?
Even if karyotyping is normal, array CGH has the potential to find a deletion or duplication that was either missed or cannot be detected by traditional chromosome analysis. Thus, array CGH offers an enhanced evaluation of chromosomes, and combined with karyotyping, provides a more complete diagnosis in pregnancy.
Can array CGH find all chromosome differences?
No, there are some chromosome differences that array CGH cannot detect, such as balanced rearrangements, changes in structure in areas of the chromosomes not evaluated by the array, and some numerical chromosome differences. For this reason, karyotyping is still an essential cytogenetic test, and we recommend that karyotyping be performed prior to, concurrent to, or following array CGH testing in pregnancy.
What type of sample is needed?
Array CGH is most commonly performed on cells that are obtained from a prenatal diagnosis procedure such as amniocentesis or CVS. It can also be performed on other types of samples, such as stored DNA, blood cells, or cells from a skin sample. Your doctor can help to determine which type of sample would be most appropriate to submit for analysis. Also, we require blood samples from both parents be submitted at the same time as the fetal sample for complete interpretation of results.
Why do you need blood samples from both parents?
Occasionally, a difference in chromosome structure is detected in the fetal sample that has unclear clinical consequences. Not all changes in chromosome structure cause problems in a pregnancy or a person. Some changes run in families and do not cause any differences in health or learning. Other changes in chromosome structure cause birth defects or learning issues, the extent of which cannot be determined until after a child is born and begins to develop. When a change in chromosome structure is detected in a fetal sample, samples from parents are immediately evaluated to get a more complete answer.
What does a positive or abnormal array CGH result mean?
A positive or abnormal result means that a deletion or duplication in the chromosome structure has been detected.
What does a negative or normal array CGH result mean?
A negative result means that array CGH did not detect a deletion or duplication in the chromosome structure of the fetus. Many genetic conditions cannot be diagnosed by array CGH, especially those that are caused by differences in DNA that are too small for even array CGH to detect. Therefore a normal result cannot rule out all genetic differences.
Source: Signature Genomics
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