Diagnosis of 18p-
There are several different reasons that your family’s physicians might suspect that there is an underlying chromosome change present. Some of the more common reasons include:
- Child missing developmental milestones
- Presence of birth defects
- Health problems
- Minor differences in facial features
- A family history of a chromosome condition
Although 18p- can be detected prenatally by chorionic villus sampling (CVS) or amniocentesis, it is most frequently diagnosed during infancy or early childhood. There are two tests that are commonly performed to identify a deletion on 18p. Both of these tests can be performed on a blood sample.
In this test, white blood cells are grown in the lab. The chromosomes are stained and examined under a microscope by a qualified cytogeneticist. This individual is trained in recognizing extra and missing pieces of chromosomes, as well as other rearrangements, such as translocations and inversions. Results from a chromosome analysis will indicate the number of chromosomes, whether the person is a male or a female, and the general region of the breakpoint of a deletion. The chromosome result of a person with 18p- might look something like this:
46,XX,del(18)(p11)
Note that the location of the chromosome break is indicated by the band number in the parentheses to the right. Because each band can include 50 to 100 genes, this is not a very precise way of determining which genes are included in the deletion. For a more precise determination of the breakpoint, molecular analysis is required.
The most common molecular analysis is chromosome microarray analysis. A microarray analysis is similar to a routine chromosome analysis in that it determines if there is extra or missing pieces of a chromosome, but with much greater resolution. It can detect chromosome deletions and duplications that are too small to be visible under a microscope. Because this is a more expensive test, it is often not the first test ordered during a diagnostic workup. However, it is a much more precise test than a routine chromosome analysis. A microarray result includes the molecular breakpoint of the deletion. It can determine with great specificity which genes are included in the deletion. A microarray result of a person with 18p- might look something like this:
arr 18p11.32p11.21(12,842-15,375,878)x1
In the example above, the “18p11.32p11.21” tells us the chromosome bands involved. The numbers within the parentheses indicate the base pairs that are involved. The “x1” tells us that only one copy of that section of the chromosome is present. Thus, we know that the deletion extends from about the tip of the chromosome to base pair 15,375,878, which is located at the centromere.
Although microarrays are very useful in determining exactly what is missing and what is extra, they cannot determine some important changes that only affects the arrangement (or rearrangement) of the chromosomes. Microarrays cannot detect chromosome changes that do not involve a gain or a loss of chromosome material. For example, they cannot detect balanced rearrangements, such as balanced translocations or inversions or ring chromosomes. Therefore, we typically suggest that individuals have both a chromosome analysis as well as microarray to fully describe the underlying genetic change.