function of microRNA, tiny strands of genetic material that help
regulate at least 25 percent of a cell's genes.
The new technique could shed light on microRNA's hypothesized role in
tumor development. Malfunctions in microRNA have been linked with
cancer, but very few direct relationships have been established
between specific microRNAs and the genes they regulate.
That could change, however, now that MIT Institute Professor Phillip
Sharp and his colleagues have found a way to inhibit the activity of
microRNA by genetically altering cells.
The technique, described in the August 12 online issue of Nature
Methods, could "provide a tool to identify specific genes that are
being regulated by microRNAs," said Sharp.
MicroRNA consists of short strings of about 22 nucleotides, the
building blocks that make up RNA and DNA. MicroRNA binds to messenger
RNA (mRNA), preventing it from delivering protein assembly
instructions, thereby inhibiting gene expression.
Sharp, who is affiliated with MIT's Biology Department and Center for
Cancer Research, said microRNA exists in every cell and controls a
wide range of cell regulatory activities.
The MIT team has found a way to block microRNA activity by tricking
cells into producing a microRNA "sponge," which soaks up microRNA and
renders it ineffective. By de-activating microRNA, researchers can
observe the resulting effects and determine which genes the microRNA
is targeting.
The new technique could shed more light on microRNA's role in tumor
development: Earlier studies have shown that a type of microRNA known
as let-7 inhibits a cancer-inducing gene called RAS. Abnormally low
levels of let-7 have been found in some types of tumor, said Sharp.
Sharp and MIT biology graduate student Margaret Ebert, lead author of
the paper, decided to block microRNA activity by creating a gene that
produces microRNA sponges and inserting it into their target cells.
Each sponge can bind up to six microRNA molecules, but they could be
engineered to bind more.
The sponge gene also includes a "reporter" gene that causes the cell
to become fluorescent if it has taken up the gene, so the researchers
can know for sure whether the microRNA sponge is being produced in a
particular cell.
Ebert said the new sponge technique is an improvement over an older
method that involves blocking microRNA activity with artificially
synthesized strands of RNA, known as oligos. One advantage is the
inclusion of the reporter gene; another is that the sponge genes can
be expressed continuously, while oligos do not remain in the cell
forever.
More importantly, the sponge technique could be used to create
transgenic animals that express the sponge in all of their cells,
allowing researchers to study microRNA function at the organismal
level. With such animals, sponge genes could be designed so that the
researchers can control when and where they are expressed.
Joel Neilson, a postdoctoral associate in the Center for Cancer
Research, is also an author on the paper. The research was funded by
the National Cancer Institute, the National Institutes of Health, a
Howard Hughes Medical Institute Predoctoral Fellowship, a Paul and
Cleo Schimmel Scholarship, and the Cancer Research Institute.
The MIT Center for Cancer Research (CCR) was founded in 1974, and is
one of eight National Cancer Institute-designated basic research
centers. Its mission is to apply the tools of basic science and
technology to determine how cancer is caused, progresses and responds
to treatment.
mit.edu
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