Doctors Column: Revisiting the human genome

When scientists sequenced the human genome in 2000, it revolutionized biomedical research, much like the invention of the Internet forever changed communications.

The Human Genome Project aimed to identify all the genes in the human genome. At first, scientists estimated that humans had less than 100,000 genes. Then improved methods lowered that to 35,000. Now, a new analysis suggests that humans have no more than 21,000 genes. When considering the complexity of a human being, that number does not seem very high.

However, even the highest of those estimates accounted for less than 20 percent of the DNA sequence in the human genome. The rest of the sequence did not appear to encode genes that led to proteins, and was therefore considered nonfunctional or junk DNA.

Now a recent study by more than 400 researchers at 32 institutions challenges that notion, suggesting that more than 80 percent of the human genome is indeed utilized and therefore important in the overall biology of each person. So much for junk DNA! The Encyclopedia of DNA Elements (ENCODE) project concluded that 20,687 genes produce proteins and an additional 18,400 genes produce RNA involved in coordinating the activity of the genes that produce proteins.

This extensive effort originally focused on the genomes of a small number of human cells but later expanded to include almost 150 different cells, including immune, embryonic, liver tissue, umbilical cord and cancer cells. Specific genes produce proteins for different tissues at different stages of human growth, so using this wide array insured that the analysis included all active genomic regions and gave a broader view of the genome.

The analysis also identified genome regions associated with specific human diseases, creating an opportunity for better understanding these diseases and treating them. In addition, the ENCODE project revealed just how different humans are from other mammals like monkeys, dogs, or dolphins. While previous estimates suggested that just 5 percent of the human genome is unique from other animals, ENCODEs research doubled that estimate to almost 10 percent. Another revelation showed just how complex the control mechanisms of the human genome really are. They signal almost 20,000 genes at the exact time and location to allow a fetus to develop normally and instruct the specific workings of tissues, as in the kidneys, lungs, or brain.

So the action of genes is controlled by layer upon layer of interacting and intricate controls that make each person who they are. Homo sapiens is a species of biological wonder and it will require many years of intense study for us to even begin to understand the mysteries of how genes are regulated to make a human being.

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Doctors Column: Revisiting the human genome