Trisomy 21 – A Deeper Look

Trisomy 21 – A Deeper Look

We know that Trisomy 21 means our child has three chromosomes on the 21st chromosome instead of 2. But what does that actually mean? In order to begin understanding this, lets talk about the chromosomes.

What is a Chromosome?

A good way to get an understanding of chromosomes is to watch these two you tube videos.

A chromosome is a storehouse for our DNA and genes. Having an extra 21st chromosome means our kids have a whole extra set of these genes and DNA. The problem with this is that some of these genes are constantly expressing, meaning there is too much of some proteins being produced in the body and these proteins can cause damage.

What are the proteins?

Proteins are large, complex molecules made up of amino acids. Within the body they have many critical roles. They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs.

Examples of proteins are;

  • Antibodies – Antibodies bind to specific foreign particles, such as viruses and bacteria, to help protect the body. Immunoglobulin E – IgE
  • Enzyme – Enzymes carry out almost all of the thousands of chemical reactions that take place in cells. They also assist with the formation of new molecules by reading the genetic information stored in DNA. E.g. Phenylalanine hydroxylase
  • Messenger – Messenger proteins, such as hormones, transmit signals to coordinate biological processes between different cells, tissues, and organs. E.g. Growth Hormone
  • Structural component – These proteins provide structure and support for cells. On a larger scale, they also allow the body to move. E.g. Actin – Important part of muscle
  • Transport/storage – These proteins bind and carry atoms and small molecules within cells and throughout the body.e.g. Ferritin – carry’s iron

So what Proteins do the 21st Chromosomes Make?

Humans typically have 46 chromosomes in each cell, divided into 23 pairs. Two copies of chromosome 21, one copy inherited from each parent, form one of the pairs. Chromosome 21 is the smallest human chromosome representing 1.5 to 2 percent of the total DNA in cells.

In 2000, researchers working on the Human Genome Project announced that they had determined the sequence of base pairs that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced.

It is estimated that chromosome 21 contains 200 to 300 genes that provide instructions for making proteins.

Fortunately in 71% of these genes are not affected by the trisomy due to regulation within the cell (gene dosage effect). 15% of this 71% are highly variable among individuals – due mainly to their other inherited genetics.

This leaves 29% (87 – 58 genes) of the expressed Chromosome 21 transcripts being overexpressed in cells from people with Trisomy 21. Among these,

  • 22% are increased proportional to the gene-dosage effect (i.e., 1.5-fold) and
  •  7% are amplified (i.e., above the expected 1.5 ratio)

Now it is important to know what these genes are? What affect do they have? And what can we do to counteract or slow these effects?

Which genes are affected by Trisomy 21 and what do they do?