Scientists May Have to Reconsider Everything They Thought They Knew About Genomes
by Masha Vodyanik, age 16
Since the discovery of its structure in the 1950’s, scientists have been decoding our DNA. Our DNA contains genes, the unique ingredients that help determine what traits we possess and who we become. One complete set of genes, containing all the ingredients for one human, is a genome. Until recently, scientists believed that each human contained only one.
Over the past ten years, there have been more and more documented instances of a human containing more than one genome.
Genomes are sequences encoded in all cells, which scientists use to learn everything about a person's genetic information. Scientists used to believe every cell contained the exact same copy of the genome. They thought looking at the genome of a cheek swab sample could teach us about the genomes of cells anywhere else in the body. Genomes between different cell types were not thought to vary significantly.
However, new research reveals that a single individual can have multiple genomes residing in one body. Some people have groups of cells with mutations not found in the rest of the body; others have genomes that came from other people.
Just three years ago, researchers would have been skeptical about such variations. But a flurry of recent discoveries in the field has shaken some of this doubt. The existence of multiple genomes in a single person has huge implications the practice of medicine.
In the last 20 years, the cost of sequencing an entire human genome fell drastically from an estimated three billion dollars to a few thousand. DNA sequencing by geneticists and doctors can now be done in only 50 hours. Never-before-seen links between mutations and diseases are being discovered.
In Greek mythology, a Chimera refers to a fire-breathing creature with a body and head of a lion, a tail of a serpent, and a second head of a goat. In genetics, a chimera refers to a person with multiple genomes. Chimerism, once considered to be rare, is now thought to be pretty common, according to Dr. Linda Randolph, a pediatrician at the Children’s hospital in Los Angeles.
For example, in 1953, a British woman who donated a pint of blood was discovered to have some type A blood and some type O. Scientists who reviewed her case concluded she had acquired some of her blood from her twin brother in the womb, including the genomes in his cells.
At age 53, another woman was discovered to be a chimera. In need of a kidney transplant, she was tested to find a match. The results seemed to indicate that she wasn't the mother of two of her three biological children. Further research showed that she is composed of two genomes. One genome was carried in her blood and in some of her eggs; the second genome was carried by her other eggs.
After multiple studies, it was found that women are likely to gain genomes from their children. After birth, a baby may leave some fetal cells in the mother's body that can travel to different organs and be absorbed by those tissues. In some instances, those cells are known to seek out damaged tissue and help heal it.
A century ago, scientists studied rare creatures with oddly colored patches of fur. Referred to as 'mosaic animals,' scientists believe those animals acquired mutations during fetal development affecting their skin cells to produce a color patch.
Similar mutations in humans, referred to as mosaicism, were difficult to study before DNA sequencing. Only instances with big mutations or effects could be observed. With the current technology, however, mosaicism is considered another way in which people may acquire new genomes.
Latest studies show that mosaicism is quite common, even in healthy cells. Alexander Urban, a geneticist at Stanford University investigated mutations in cells found in connective tissue. Searching for particular cases where DNA was duplicated or deleted, Urban and his colleagues discovered that as much as 30% of these cells contained such mutations. Large portions of mosaic-related mutations are benign – not harmful in effect. In fact, in some circumstances an extra chromosome may be beneficial.
Scientists know that mutations in specific cells could cause some cancers, like leukemia. Researchers are now investigating whether mosaicism is a factor in additional common diseases, like schizophrenia.
With rapidly developing technology and genetic research, scientists are now able to discover new links between mutations and genes. Learning about these phenomena can provide us a more nuanced view of the processes that underlie our existence.
[Source: Science Times]