Down Syndrome

Because Down Syndrome (DS) individuals have three copies of chromosome 21, the genetic disorder is also known as Trisomy 21.  The syndrome is named after J. Langdon H. Down, a London physician, who observed individuals with distinct characteristics.  His original paper was published in 1866; most is difficult to read, but we do know a lot more since then.

What can happen when there are three copies of chromosome 21?  What is a chromosome?  A chromosome is like a volume of a book.  The book contains recipes to make various things that a cell needs to its job.  A typical diploid cell will have 46 books, two each of the 23 books.  All together, they contain between 20,000 and 25,000 recipes, each recipe can also make on average 3 different products (or variations of similar dishes... akin to my favorite scone recipe that I can make orange, cinnamon, or chocolate scones).  In the human body, some cells will use a set of recipes a lot more than any other.  For example, a muscle cell will use the muscle cell-specific recipes and keep the recipes that a liver cell might use tucked away.  In the science world, a recipe is a gene.

Sometimes, genes become mutated.  This may occur by a number of reasons.  Just as if a recipe suddenly has a typo (add 1 cup of flower instead of flour), the end-product may be quite different.  Since diploid cells have two copies of each recipe, sometimes it's no huge loss if there is a type on one.  The cell will get by with the second good copy.  Cells get into trouble if both copies are bad.  There are also situations when the bad copy over shadows the good; this would be really bad!

So, what happens in DS where there are three volumes of chromosome 21 and three copies of each recipe?  Wouldn't that be a good thing?  Not necessarily.  If it was a good thing, everyone would want to have Down Syndrome.

Individuals with DS have characteristic facial features - almond shaped eyes and protruding tongues.  They have intellectual disabilities with which it usually takes longer to learn a skill.  Babies with DS are floppy because their muscles lack tone; it takes hours of physical therapy to learn how to roll over, sit, crawl, and walk - activities typical people take so much for granted.  Aside from the obvious characteristics, individuals with DS are often born with congenital heart defects, requiring surgeries during infancy.  They are highly susceptible to developing cancers involving white blood cells - lymphomas and leukemias.  Some develop hyperthyroidism and overproduce certain hormones.  Some are highly susceptible to respiratory infections, perhaps because their nasal passages are much narrower than typical.  The list goes on...  (For more information, please visit the National Down Syndrome Society, the Mayo Clinic, or the Genetics Home Reference from the National Library of Medicine websites.)

The underlying question remains.  How does having three copies of chromosome 21 lead to these characteristics and health concerns?  There is a lot that we understand now, but there is much more we still do not know.  My goal with this blog is to explore this question and study what has been found by hard-working scientists all over the world.  Their findings are extraordinary and beyond exciting!