Diagnosing Williams syndrome

• Clinical diagnosis based on a variety of characteristics. See the Facts about Williams syndrome article for details.

 
• Medical/genetic test confirmation through a blood test: the FISH test.

The FISH test

You can obtain a blood test to confirm the clinical diagnosis of Williams syndrome. A laboratory can use the technique known as fluorescent in situ hybridization (FISH).

FISH is a type of specialized chromosome analysis utilizing specially prepared elastin probes. If a patient has 2 copies of the elastin gene (one on each of their chromosomes #7), they probably do not have WS. If the individual only has one copy, the diagnosis of WS will be confirmed.

Virtually all (98-99%) persons with typical features of WS will have a deletion of the elastin gene.

In more technical terms:

Williams syndrome is the result of a deletion of the 7q11.23 region of chromosome #7 containing the elastin gene and is believed to be a contiguous gene syndrome. Diagnosis of Williams syndrome is confirmed by dual colour FISH using a specific probe and a control probe.

The laboratory will need 5 ml of blood drawn in a Sodium heparin tube. The sample should arrive in the lab the same day it was drawn or on the following day. Results are usually available in 2-4 weeks.

The FISH test is readily available at major hospitals and Cytogenetics laboratories around the country, but it is not a routine test. Therefore not all labs will do FISH-based diagnosis. Families or their physicians should call the lab in advance to make sure they can perform the test.

If you have any doubts that a child may have Williams syndrome, the FISH test will give you a clear cut answer in most cases.

It is important to stress that WS is a genetic diagnosis and an individual who does not have the gene deletion does not have Williams syndrome (i.e. a person who was clinically diagnosed with WS but was later found not be have a deletion in fact does NOT have WS).

NOTE: It is extremely unlikely that any other family member also has Williams syndrome. On the other hand, if the individual with Williams syndrome plans to become a parent, there is a 50/50 chance that his or her child will have Williams syndrome. If such a situation should arise, consult an obstetrician about using the FISH test for prenatal testing of the embryonic cells.