"To the best of our knowledge, this is the first direct evidence that overexpression of a microRNA results in the development of a neoplastic disease, highlighting their potential role in human malignancies," says Carlo Croce, director of Ohio State's Human Cancer Genetics Program and professor and chair of the department of molecular virology, immunology and medical genetics.
Over the past several years, scientists have discovered hundreds of microRNAs (miRNAs) and how they regulate gene expression ?basically, by blocking messenger RNA's instructions for protein production. MiRNAs normally help control important biological functions by switching "on" and "off" at different times during cell growth, death, development and differentiation. They can be harmful, though, if they are activated at the wrong time in the wrong place, and that appears to be what happens in some forms of cancer.
The study is published online in the Proceedings of the National Academy of Sciences.
Croce, the senior author of the study and the first to identify a link between miRNAs and cancer, suspected that a particular miRNA, miR155, was a key culprit in some forms of malignant growth. He and his colleagues have been mapping the activity of dozens of miRNAs in various types of normal and malignant tissues for several years. Earlier studies showed that miR155 was unusually active in some types of leukemia and lymphoma, and that its presence indicated a poorer prognosis in patients with breast and lung cancers.
Croce, along with Dr. Stefan Costinean , a r esearch associate, decided to isolate miR155 function by inserting the gene, along with an enhancer, to promote its expression, into fertilized eggs inside a pregnant mouse. The researchers then screened the offspring to find those that had incorporated miR155 into their genomes and followed them to see what effect miR155 might have.
Within three weeks, the transgenic mice developed greatly enlarged spleens, and after six to seven months, they became sick and died. Offspring that did not express miR 155 did not have enlarged spleens and developed normally.
When Costinean examined the spleens of the transgenic mice more closely, he discovered they were full of immature B cells ?and only immature B cells.
"This is significant, because a proliferation of these precursor B cells is one of the hallmarks of some types of leukemia and lymphoma," he said.
B cells, or lymphocytes, are white blood cells that help the body fight infection.
In humans, they are produced from stem cells in the bone marrow and then evolve through several stages before they become mature enough to create antibodies. Antibodies are proteins that sit on the surface of B cells or that are secreted by B cells that can detect the presence of foreign invaders like bacteria, viruses or parasites.
Lab tests revealed that the B cells in enlarged spleens had stopped evolving in the pre-B phase, right at the point where a B cell normally begins to create structures necessary for antibody development.
"We believe that miR 155 initiated the process that blocked further differentiation of these cells," says Costinean.
Croce says the results are consistent with previous study findings. "We know that miRNAs often act just like oncogenes, in that they promote abnormal cell growth that leads to cancer. Others, however, behave more like tumor suppressors, because they block genes that keep abnormal cell division in check or induce prolonged survi val. Our transgenic mouse model clearly shows that miR155 is an oncogene, because it leads to B cell malignancies when it is dysregulated."
Although they now understand that miR 155 overexpression can lead to cancer, the researchers say they still haven't identified the exact mechanism that makes that happen. Still, the study suggests that a newly emerging class of artificially designed molecules (called antagomirs) may be able to block miR 155 expression and be an effective therapeutic strategy in patients with acute lymphoblastic leukemia or high grade lymphomas.