Spotlight on Research 2012

May 2012

RNA Interference Silences Cell-Signaling that Leads to Excessive Bone Formation in FOP

Scientists supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases have described in a recent issue of Gene Therapy how a gene-silencing process called RNA interference (RNAi) restored normal cell-signaling in the cells of people with the bone disorder fibrodysplasia ossificans progressiva (FOP). These findings, from cell culture experiments, are the first proof of principle that RNAi has potential as a therapy for FOP.

Currently, there are no effective treatments for FOP, a rare disease that causes connective tissues, such as muscles, ligaments and tendons, to harden into bone. The disease is progressive and profoundly disabling, restricting or preventing the ability to bend or move affected areas. Surgery to remove the extra bone is not an option. Explosive bone growth can occur at the site of any trauma to the bone, including the induced trauma of surgery.

In previous research on FOP, the University of Pennsylvania team, led by Eileen M. Shore, Ph.D., and Frederick Kaplan, M.D., discovered a mutation in a gene called ACVR1 that codes for one of the many bone morphogenetic protein (BMP) receptors that control skeletal growth. Specifically, this mutation enhances the signals that cause bone progenitor cells to form the unwanted bone in FOP. The team also found that all cases of classic FOP are caused by the same mutation — with many inherited disorders, this is not the case.

Based on these discoveries, Dr. Shore and her colleagues hypothesized that FOP would be a prime candidate for targeting by allele-specific RNAi for two reasons: First, FOP is an autosomal dominant disorder, which means only one copy, or allele, of the gene is defective; a normal allele remains to coordinate function after the mutant allele is silenced. Second, the FOP mutation involves just a single nucleotide substitution, or "misspelling" of the genetic code.

In RNAi, a short strand of artificial RNA is introduced into cells in order to “interfere” with, or silence, the activity of the messenger RNA that usually translates a gene sequence into protein. Josef Kaplan, Ph.D., on Dr. Shore’s team, generated short RNA molecules that recognize and bind to the defective region of the mutated ACVR1 allele. This would silence only the mutant allele; the normal gene would still make its protein. To test the effect of this targeted RNAi molecule, the team then introduced it into cultured cells recovered from the baby teeth of children with FOP. These cells are ideal for this type of experiment because they are known to become bone cells, and they do not require surgery or a biopsy to extract since baby teeth fall out naturally.

The results were a technical success because the RNAi process specifically bound the mutant messenger RNA in the FOP cells, effectively silencing it. Functionally, the scientists were able to demonstrate, through laboratory analyses, that RNAi brought the signaling in FOP cells down to levels approaching normal. Their measurements also showed that RNAi restored more balanced rates of bone cell differentiation by bone precursor cells in the FOP cell cultures.

The University of Pennsylvania team’s next step is to develop a mouse model of FOP in which they hope to confirm the therapeutic value of RNAi.

The work was also supported by the International FOP Association (IFOPA), the Ian Cali Endowment, the Weldon Family Endowment, the Isaac and Rose Nassau Professorship of Orthopaedic Molecular Medicine, and the Rita Allen Foundation.

The mission of the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), a part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH), is to support research into the causes, treatment, and prevention of arthritis and musculoskeletal and skin diseases; the training of basic and clinical scientists to carry out this research; and the dissemination of information on research progress in these diseases. For more information about the NIAMS, call the information clearinghouse at (301) 495-4484 or (877) 22-NIAMS (free call) or visit the NIAMS website at http://www.niams.nih.gov.

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Kaplan J, Kaplan FS, Shore EM. Restoration of normal BMP signaling levels and osteogenic differentiation in FOP mesenchymal progenitor cells by mutant allele-specific targeting. Gene Ther. 2011 Oct 20. doi: 10.1038/gt.2011.152. [Epub ahead of print]