What is Tetrasomy 18p?

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Who am I and why am I doing this? My name is John McGing and I have a son, Sean, who has a rare genetic disorder called tetrasomy 18p. When I went looking for information about this syndrome, I found that there wasn't much available, and the Chromosome 18 Registry didn't really know much, so I set this up to pull things together. So let's start off with a layman's explanation of what this syndrome is.

If youlooked this up in a medical encyclopedia, you'd find this: Tetrasomy 18p is a very rare chromosomal disorder in which the short arm of chromosome 18 (18p) appears four times [this is called a tetrasomy] rather than twice in cells of the body. This is also called isochromosome 18p, [the third chromosome is called an isochromosome but I find tetrasomy easier to say and more descriptive.]

People with a normal chromosomal make-up have two chromosomes, both of which consist of a short arm (called "p"), a long arm (called "q"),and a narrowed region at which the two arms are joined (called a centromere).

People with tetrasomy 18p have this normal pair plus an extra chromosome that consists of two identical short arms (18p) and a centromere (this type of construction is called an isochromosome).

This means that people with tetrasomy 18p have three chromosomes instead of two. And unlike most us us who have two long arms (18q) and two short arms (18p), they have two long arms (18q) and a total of four short arms (18p) present in all (or sometimes only in some) of the cells of their bodies, resulting in the symptoms that characterize this disorder.

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If you've been paying attention, you are asking yourself "Why isn't this called trisomy 18, aren't there 3 number 18 chromosomes present?" That's a good question. The distinction between trisomy 18 and tetrasomy 18p is that a person with tetrasomy 18p has two 18q arms and four 18p arms, while a person with trisomy 18 has three 18q arms and three 18p arms, which makes it a different syndrome.

Confused?  Here is an excellent site that will help you understand the genetics involved and which I highly recommend. This stuff isn't as hard as it looks, at least at the edges of it. And in trying to explain what your child has to doctors, friends etc., it really helps to feel comfortable with this. Please take the time to learn these basic facts; your geneticist or genetics counselor can help you.

The medical literature was very sparse (as the syndrome wasn't forally identified till the mid-1980s) but in recent years things are getting better.  In most documented cases, tetrasomy 18p has been the result of a spontaneous (called "de novo") genetic change (also called a "mutation") early in embryonic development that occurred for unknown reasons. In such cases, the disorder is not inherited from the parents, and siblings are not affected. And more importantly, this means that it is almost a certainty that neither parent did anything during the pregnancy to cause this to happen. Sometimes, things simply happen, and this is one of them.

It appears to affect males and females equally.  Tetrasomy 18p occurs once per every 140,000 live births. Approximately 100 cases have been reported in the medical literature, while the Chromsome 18 Registry has, over 15 years, encountered over 300 affected individuals. And although the science of analysis improves every year, tetrasomy 18p is often initially misdiagnosed as trisomy 18, which it definitely is not.
 

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