Male: There is a condition which a lot of people have asked me about genetically, Digeorge syndrome, genetically speaking, what is the genetics of Digeorge syndrome for instance?
Female: It is not an inherited condition, so it is not recessive, it is not dominant, it is not an abnormality within an individual gene, it has actually got several causes, but what we have discovered is that most cases of Digeorge syndrome are actually caused by a very tiny piece of the 22nd chromosome being deleted.
Male: No one knows why? Female: Well, certain chromosomes are more prone to this kind of breakage and there is something in the structure of the chromosome. This is one of the commonest chromosome deletions that we know of, actually. It maybe even like one in 3000 or something. Male: If someone was with it, can you diagnose it in pregnancy?
Female: We do not have screening tests for relatively rarer conditions like Digeorge syndrome, we would test for it if there was a suspicion based on ultrasound abnormalities or something like that. Male: Okay, but if you found something suspiscious, how would they know?
Female: You can test for it.
Male: How would you test for it?
Female: You could do the amniocentesis or you could obtain fecal material and you can actually do what is called a FISH test. Now the word FISH stands for, it is an acronym for Fluorescence in situ hybridization. So this wonderful people came up with the test where you can make an artificial copy of part of a chromosome, attach a little signal to it, it is called a probe and you take a patient's DNA or the patient's chromosome slide, actually, you made a chromosome preparation, you take that slide, you make it single strand, your probe is also single strand and you know that DNA has a tendency to stick to each other, so you throw the slide into the solution of the probe and then you take it out and then you look at it under the microscope and if that region is present in the person's chromosomes, a signal will show. So that is a normal result, so that means that area is present and it is picked up by the probe. So if you make a probe to the 22nd chromosome where you have this piece missing, normally, people with Digeorge syndrome and you do this FISH test from them. They will only show one signal, instead of two. Normally, they should show two signals, one for each copy of the chromosome. If they show only one signal, it means that the second chromosome has a deletion on it for that area. So we have this set of tests now, we call FISH test which combine this kind of technology and which you can use for very small deletions or rearrangements within the chromosomes.
Male: If you had one kid with Digeorge, and you have another kid, do you have a risk of having it and a second or third or fourth?
Female: Generally not.
Male: So it could be a one-shot deal. Female: It is usually a one-shot deal, but we offer parents testing for that same deletion, just from the off chance that one of them has a very-- Male: Have you had a feeling they had two Digeorge?
Female: No.
Male: But have you heard--?
Female: We have and people say that about a quarter of the time, which is a fairly high number and in my experience, it is probably lower, but about a quarter of parents, maybe a carrier, but then they will have something. They will have some learning problems or a heart defect.
Male: What heart defect is most commonly seen with that?
Female: This 22nd chromosome, the deletion, the syndrome you mentioned, Digeorge syndrome is a far relatively well-defined group of problems. It is interrupted aortic arch, trunkus and along with it, they have an absent thymus or hypoparathyroidism.
Male: So it is kind of like look, then you would be called in to see if it really is, is that correct? You do the FISH test.
Female: But the same deletion actually can cause another syndrome, it is called velo-cardio-facial syndrome, so the same deletion is very variable kind of expression.
Male: You know why you get two different looks from it?
Female: There is some explanation for that. It is not a simple explanation again, but it is something to do with the genes and the location that are actually deleted versus a present and extra--
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