Developing RNA-Based Gene Therapies
Non-coding RNA (ncRNA) molecules have achieved superstar status in genetics and molecular biology.
These are small RNA molecules that do not encode proteins or stable RNAs (rRNA or tRNA)
but appear to exert a major influence on the transcription and translation of messenger RNA (mRNA).
We have been using two types of non-coding RNA, siRNAs and ribozymes, in attempts to regulate gene expression
in order to cure dominantly inherited diseases and viral infections. Small interfering RNAs (siRNAs), are
short RNA duplexes that lead to the degradation of targeted messenger RNAs using a protein apparatus called
RISC. Ribozymes are RNA enzymes that cleave RNA molecules without the aid of proteins. Scientists in my group
are designing these small
RNAs to block the synthesis of mutant RNAs that lead to disease and to block the spread of human herpes virus.
Delivery of ncRNAs
To deliver the therapeutic RNA molecules we use Adeno-associated virus (AAV) vectors. These non-pathogenic
viruses are able to stably infect many tissues and cell types. AAV has been particularly useful in delivering
genes, including small RNAs, to many parts of the eye and brain. Unlike retroviruses, recombinant AAV does
not insert into chromosomes, disrupting genes, but remains stable as an extrachromosomal genetic element.
We can tailor the delivery of ribozymes and siRNAs by varying the capsid of the virus or by choosing a
cell-type specific promoter element. This selectivity allows us to provide a continuous supply of
ncRNA to specific cells without affecting neighboring tissues in which their production might be harmful.
Disease Targets
Our research has focused on developing therapies for dominantly inherited diseases of the central nervous
system. Dominant inheritance may indicate that a mutation confers some new toxic property to a protein or
it may mean that the resultant protein can no longer fit in as part of a multi-subunit complex. In either
case, inheritance of the gene from either parent leads to disease. We are working primarily with dominant
mutations that cause retinal degeneration in such diseases as retinitis pigmentosa. Our goal is to suppress
the synthesis of the defective proteins that cause photoreceptors to die, while at the same time to increase
the production of normal
versions of the same proteins. In rodent models of retinitis pigmentosa, this approach has been successful.
We are also collaborating with other investigators at the University of Florida to use this approach to treat neurodegenerative diseases such as Huntington Disease and Parkinson Disease. Aggregates of mis-folded protein are associated with neuronal damage in both diseases, and we hope that block the production of the mutant protein will be beneficial. We are using the same approach to try to combat Herpes Simplex Virus infections of the cornea. Reactivation of herpes infections leads to scarring of the cornea, and is the leading infectious cause of blindness. We have characterized some ribozymes that block herpes replication in animals. These ribozymes may be useful in cases in which conventional drug therapy has failed. Finally, we have used these same tools—AAV-delivered siRNAs and ribozymes—in efforts to develop animal models of ocular diseases including age related macular degeneration and Leber Hereditary Optic Neuropathy. These efforts have helped us understand the causes of tissue damage in these difficult diseases.
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