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Over the years, I have often talked and written about the fact that chromosome abnormalities are a common occurrence among all conceptions, but most do not result in live births. The latest frequency estimates were shared during last December’s World Chromosome 18 Day presentation and can be viewed on the Chromosome 18 Registry website (https://www.youtube.com/watch?v=alMkEiIc9Oc&t=11s).
In the July 2021 issue of this newsletter, we shared data indicating that the children of people exposed to radiation after the 1986 Ukrainian Chernobyl Nuclear Power Plant meltdown did not have an excess number of DNA mutations compared to the general population. Additionally, there are no data indicating that any other type of environmental exposure can cause a chromosome abnormality. This supports the idea that chromosome abnormalities occur from random mutations during cell division in egg and sperm cells and are not the result of environmental exposures.
It is important to differentiate, however, that chromosome abnormalities can cause birth defects. Even though environmental exposures have not been found to cause chromosome abnormalities, there are environmental exposures that are known to cause birth defects. The classic example is that of Thalidomide prescribed for morning sickness in the late 1950s and early 1960s. Many women who took this drug had babies born with multiple birth defects (principally shortened or absent limbs). Today, we know about numerous environmental exposures including some drugs and alcohol can cause birth defects. These compounds are referred to as “teratogens.”
It appears that there are two different mechanisms that can cause birth defects: First, birth defects can be due to a chromosome abnormality. Second, birth defects can be caused by a teratogen exposure. The mechanism underlying the development of a birth defect as a consequence of either of these is very different.
As an analogy, imagine you are building a house. The chromosomes are like the blueprints directing how that house will be built. If a page of the blueprints is lost or duplicated, the resulting house may have some issues and not quite be what you were expecting – a house defect. On the other hand, teratogen exposure is more like a sledgehammer. If you take a sledgehammer to your house while under construction and knock out a wall or use it to make holes in the floor, your house may also have some problems and not look like the original plan – a house defect. Both scenarios may result in a less than perfect house, but the mechanism is entirely different. One is a change in the plans and the other is a change in the construction. The change in the plan is like a chromosome abnormality. The change in construction is like the effect from a teratogen. Either one can result in birth defects.
Of course, because there are two different mechanisms an individual could theoretically have a birth defect that is the result of the combination of these two mechanisms. Somone with a chromosome 18 abnormality could have issues unrelated to the chromosome abnormality and are caused by environmental exposure. Such birth defects would most likely be the ones not found in others with the same chromosome condition.
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