The pieces of DNA that are part of over 90 percent of the genetic
material that are not genes. Researchers now know that this "junk DNA"
contains most of the information that can turn on or off genes. But how
these segments of DNA, called enhancers, find and activate a target gene
in the crowded environment of a cell's nucleus is not well understood.
Now a team led by researchers at Princeton University has captured
how this happens in living cells. The video allows researchers to see
the enhancers as they find and connect to a gene to kick-start its
activity. The study was published in the journal Nature Genetics.
Analyses of how enhancers activate genes can aid in the understanding
of normal development, when even small genetic missteps can result in
birth defects. The timing of gene activation also is important in the
development of many diseases including cancer.
"The key to curing such conditions is our ability to elucidate
underlying mechanisms," said Thomas Gregor, an associate professor of
physics and the Lewis-Sigler Institute for Integrative Genomics. "The
goal is to use these rules to regulate and re-engineer the programs
underlying development and disease processes."
As their name suggests, enhancers switch on the expression of other
genes. In the mammalian genome, there are an estimated 200,000 to 1
million enhancers, and many are located far away on the DNA strand from
the gene they regulate, raising the question of how the regulatory
segments can locate and connect with their target genes.
Many previous studies on enhancers were conducted on non-living cells
because of the difficulty in imaging genetic activity in living
organisms. Such studies give only snapshots in time and can miss
important details.
In the new study, researchers used imaging techniques developed at
Princeton to track the position of an enhancer and its target gene while
simultaneously monitoring the gene's activity in living fly embryos.
"This study provides the unique opportunity to observe in real time
how two regions of DNA interact with each other," said Michal Levo, a
postdoctoral research fellow in the Lewis-Sigler Institute. "We can
monitor in time where the enhancer and the gene are physically located
and simultaneously measure the gene's activity in an attempt to relate
these processes."
The video demonstrates that physical contact between the enhancer and
the gene is necessary to activate transcription, the first step in
reading the genetic instructions. The enhancers stay connected to the
gene the entire time it is active. When the enhancer disconnects, gene
activity stops.
The researchers also found that during transcription, the structure
formed by the enhancer and gene becomes more compact, suggesting a
change in the DNA in that region.
Given that there can be numerous genes between the enhancer and its
target, it is remarkable that enhancers can reach the exact target at
the right time for that gene to become active, the researchers said.
The team believes that the solution may be found in the DNA's unique
wrapping within our cells. The enhancer and gene may be a half-inch
apart when DNA is stretched out in a line, but when packed into the
cell, with specific proteins facilitating physical interactions, they
could be considerably closer.
"Through this study, we can look at the relationship between the
DNA's structural configurations and gene activation," said Hongtao Chen,
a postdoctoral research fellow in the Lewis-Sigler Institute and lead
author on the study.
The video provides evidence against a favored model known as the
"hit-and-run model," where the enhancer does not need to stay attached
to the gene during transcription.
The team also showed that sometimes the enhancer and gene met and
connected but gene activation did not occur, a finding they plan to
explore further.
To capture video of an enhancer contacting a gene, Chen attached
fluorescent tags to the enhancer and its target gene. The enhancers
examined are those of a gene called eve, and they give rise to a pattern
of seven stripes that forms on the surface of the developing embryo
after about three hours.
Additionally, Chen attached a separate fluorescent tagging system to
the target gene that lights up when the gene is activated and undergoes
transcription to produce an intermediary readout of the genetic code, a
molecule called RNA. Gregor's team at Princeton previously developed a
method of adding fluorescent tags to RNA as it is being created to
obtain a real-time readout of gene expression in fly embryos.
The study included work by Lev Barinov, a graduate student in
molecular biology at Princeton, and Miki Fujioka and James Jaynes at
Thomas Jefferson University. Gregor is affiliated with both Princeton
and the Institut Pasteur in Paris.
This study was funded by the National Institutes of Health (grants
U01 EB021239, R01 GM097275, and R01 GM117458) and the National Science
Foundation (grant PHY-1734030).
Story Source:
Materials provided by Princeton University. Original written by Kevin McElwee. Note: Content may be edited for style and length.
Materials provided by Princeton University. Original written by Kevin McElwee. Note: Content may be edited for style and length.
Journal Reference:
- Hongtao Chen, Michal Levo, Lev Barinov, Miki Fujioka, James B. Jaynes, Thomas Gregor. Dynamic interplay between enhancer–promoter topology and gene activity. Nature Genetics, 2018; DOI: 10.1038/s41588-018-0175-z
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