Post by Former NIMH Director Thomas Insel: National DNA Day
I don’t know why or how April 25th, the day after Earth Day and the day before Save the Frogs Day (really), was named National DNA Day, but once again we have a reason to celebrate the basic language of biology. In fact, this has been a good year for DNA—that 3 billion base-pair long sequence of nucleotides which constitute the building blocks for the 23 pairs of chromosomes found in almost every human cell. A few new insights about DNA to celebrate and contemplate on National DNA Day:
We now know that DNA is not just about genes. Genes make up less than 2 percent of human DNA. There are many different DNA elements outside of genes that encode important information. Some of these elements, including some just being discovered, regulate genes. Other elements, such as those that control the three-dimensional coiled structure of DNA, are still mysterious.
While the human genome was sequenced a decade ago, we now know that DNA in each of us contains hundreds of differences from this reference sequence. No two humans are genetically identical. Individual differences can extend for a million or more bases without any impact on function; at the same time, differences in only a single base can be critical. But this sequence variation is only part of what makes each of us unique. Even identical twins who started out with identical DNA sequences are never perfectly identical because of mutations that can occur during development and epigenetic differences—that is, differences in how the DNA sequence is expressed.
One of the new, unsettling mysteries about the human genome is that our DNA may not be the same in every cell. Since all of our cells derive from a single stem cell, how can we have multiple genomes? As cells divide, DNA replicates, and this replication is not perfect. There are various enzymes that repair replication errors but not all the mistakes are fixed. In fact, sequence variation is generated continually in dividing cells. Most changes are innocuous, some might even be useful, but others can lead to runaway cell division, which we know as cancer, and some can lead to cell death.
But probably the most important thing to know about DNA in 2014 is that genetics, or what is now called genomics (the study of function and structure of the genome), is no longer just about the study of heredity. Genomics has become the basic tool for all of biology from studies of human evolution to understanding bacterial diversity. We study DNA in disorders in which we suspect environmental factors to play a role, such as asthma and autism. And we use DNA sequences to define the microbiome, the ecosystem of microbes that reside in our guts and on our skin. As the cost of sequencing DNA has dropped by 1 million-fold, genomics has become the cheaper, better, faster way to address a vast array of biological questions. A new PsychChip, designed specifically for discovering the genetic risk factors for mental disorders, will drop the cost of genotyping to $45 per person. The first state-of-the-art exhibition about genome science, Genome: Unlocking Life’s Code, has been captivating visitors at the Smithsonian’s National Museum of Natural history since its opening in June of 2013, and is scheduled to tour the country beginning this summer.
For mental disorders, DNA is certainly less about simple heritability and more about complex mechanisms of risk. Major mutations associated with autism or schizophrenia are spontaneous, not heritable in the classic sense. Yes, there are many common inherited variants, 128 so far in schizophrenia, that in aggregate confer risk. But these are mostly ancient changes, going back hundreds of generations, not really the genetic contribution of mom or dad.
National DNA Day is also the lead in to the USA Science and Engineering Festival, a massive celebration of science this weekend in Washington, D.C. With over 3,000 exhibits and events, this is the biggest science festival of the year—part science fair, part magic show, part music festival. You can be sure that DNA will be celebrated there as well, along with Save the Frogs and Earth Day.