Targeted Gene Therapy Cures Lysosomal Storage Diseases in Mouse Model

A team of neurologists used enzyme replacement therapy to treat lysosomal storage diseases in mice by targeting the epithelial cells of the blood vessels of the blood brain barrier with a viral vector modified to express specific peptides on the surface while carrying the gene for the missing enzyme in their genome.

Investigators from the University of Iowa (Iowa City, USA) worked with a mouse model system that exhibited central nervous system (CNS) abnormalities due to lysosomal storage disease (LSD). The plan was to use a viral vector to introduce the missing lysosomal gene. A peptide expressed on the surface of the vector would guide the virus to cells of the blood vessels that line the blood brain barrier.

The investigators used a technique known as phage panning to select the most appropriate surface peptides from a library of candidate molecules. Phage panning is a selection technique in which a library of variants of a peptide or protein is expressed on the outside of a phage virion, while the genetic material encoding each variant resides on the inside. This creates a physical linkage between each variant protein sequence and the DNA encoding it, which allows rapid partitioning based on binding affinity to a given target molecule (i.e., antibodies, enzymes, cell-surface receptors). In its simplest form, panning is carried out by incubating a library of phage-displayed peptides with a plate (or bead) coated with the target, washing away the unbound phage, and eluting the specifically bound phage. The eluted phage is then amplified and taken through additional binding/amplification cycles to enrich the pool in favor of binding sequences. In the current study, the panning process was carried out in vivo on actual blood vessel epithelial cells.

Results published in the September 13, 2009, online edition of the journal Nature Medicine revealed a significant difference in peptide (epitope) expression between epithelial cells from normal and diseased animals. Furthermore, epitope expression differed between animals with different types of LSD.

When virus particles expressing the appropriate epitope and carrying the gene for the missing lysosomal enzyme were injected into the animals' peripheral blood stream, the viruses bound specifically to the walls of the blood-brain barrier vessels and then diffused into adjacent brain and nervous tissues. Eventually, reconstituted enzyme activity spread throughout the brain and improved disease phenotypes in two distinct disease models.

"This is the first time an enzyme delivered through the bloodstream has corrected deficiencies in the brain,” said senior author Dr. Beverly Davidson, professor of internal medicine, neurology, molecular physiology, and biophysics at the University of Iowa. "This provides a real opportunity to deliver enzyme therapy without surgically entering the brain to treat lysosomal storage diseases. In addition, we have discovered that these neurological diseases affect not just the brain cells that we often focus on, but also the blood vessels throughout the brain. We have taken advantage of that finding to delivery gene therapy, but we also can use this knowledge to better understand how the diseases impact other cell types such as neurons.”

"Our discovery allowed us to test the idea that the brain cells might be able to make use of the reintroduced enzyme to stop or reverse the damage caused by the accumulated materials,” said Dr. Davidson. "In the treated mice, the affected brain cells go back to looking normal, the brain inflammation goes away and the impaired behaviors that these mice have is corrected.”

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