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RNA Splicing and its Possibilities in Disease Treatment


The genetic code is a sophisticated system of organization for the blueprints of all life. In order to make this system operate smoothly, several molecular machines are employed. There are copying machines for DNA and RNA, repair machines, and editing machines. In nature, these machines work in a highly synchronized routine to copy, transcribe, translate, and synthesize genetic material. Once these machines are isolated, their power can be harnessed and used to help humans in ways such as disease fighting. Two of the most important machines that can be used in disease fighting are spliceosomes and ribozymes. These machines are involved in RNA processing and can be used to remove harmful portions of the genetic code, helping to fight diseases such as cancer and HIV.
RNA processing takes place in the cell nucleus after transcription of DNA produces a complementary pre-mRNA strand. The initial DNA strand and therefore the pre-mRNA molecules are about 8,000 nucleotides long, but only 1,200 nucleotides are needed to make an average protein (Campbell 2005). This means that much of the pre-mRNA strand contains non-coding segments and therefore must be removed. The process by which these non-coding segments are removed is RNA splicing. The non-coding segments are called intervening sequences or introns, since they lie between coding segments. The other segments are called exons, since they will leave the nucleus and probably be expressed in the gene and translated into polypeptides.
(Ussery 2000)
At each end of the intron, the RNA strand is marked by small nuclear ribonucleoproteins (snRNPs) composed of RNA and protein. Several snRNPs combine with other proteins to form the spliceosome, the main molecular machine involved in RNA splicing. The spliceosome connects to the pre-mRNA strand and cuts the strand on each side of the intron. The two ends of the exons are then joined, creating a new strand of only exons. In some situations, a different splicing machine can be used. Rybozymes are RNA molecules that function as enzymes and can splice RNA without the need for other proteins or RNA. These ribozymes make up the RNA of the introns and actually catalyze their own excision from the strand (Campbell 2005).
(Campbell 2005)
While these intron segments might seem unnecessary, they actually play an important role in the genetic process. Some introns have regulatory roles in the cell, and some can actually affect gene activity. However, the most important reason for having introns is that they allow one gene to code for multiple polypeptides (Campbell 2005). Depending on which segments are considered introns and which are considered exons, the gene can be processed in multiple ways. This process is called alternative RNA splicing and is the reason that an organism can produce many more proteins than it has genes. By harnessing the power of RNA splicing machines, introns can be directed and used in a new form of human gene therapy.
While gene therapy is still in its early stages, the theory has been around for nearly twenty years. Clinical trials of gene therapy beginning in 1990 and 2000 have shown to improve the conditions of patients with severe combined immunodeficiency (SCID) (Campbell 2005). The basic idea of gene therapy is to replace defective genes with viable ones by means of a vector. One common vector is a retrovirus. The retrovirus is made harmless to humans and then a copy of the desired allele is transplanted into it. This retrovirus is then inserted into somatic cells that multiply throughout the patients life, such as bone marrow cells. The retrovirus then inserts the modified RNA into the cells, causing them to produce DNA that contain the desired allele (Campbell 2005). Although this process has showed some success in the past, it is still being engineered for efficiency and safety.







(morefocus 2008)
Recently, several new approaches to gene therapy have been researched that involve the use of RNA splicing machines. One of these methods is called spliceosome-mediated RNA trans-splicing, or SMaRT. Instead of trying to replace the entire defective gene, this method attempts to repair only the part of the mRNA strand that contains the mutation (Mitchell 2008). First, an RNA strand that is engineered to bind to the intron near the mutated exon is introduced. The RNA strand binds to the intron, so that both the defective exon and the new correct exon are attached. Spliceosomes then remove the intron and the mutated exon, leaving the engineered exon in the strand. The repaired RNA strand then proceeds to the ribosome to be translated into a normal functioning protein (Mitchell 2008).
Another novel method of gene therapy makes use of a certain type of rybozyme called group I introns. By freezing this molecular machine in the middle of its work cycle, Purdue University researchers have been able to examine the structure of this intron and its binding sites. Once the intron is understood, it can be used to splice out defective genes so that normal genes can take their place (Purdue 2005). Because these introns cut themselves out of the RNA and reconnect the ends, researchers hope to be able to direct these introns to work on mutated genes. Humans do not have group one introns, but many diseases do. By targeting group I introns using a method called targeted trans-splicing, introns can by used to cut out defective genes (Purdue 2005).
Another form of intron, the group II intron, is also being researched as a possible means for disease fighting. These introns are inserted directly in to the target DNA and reverse transcribed into the genome. Researchers aim to manipulate group II introns to disrupt the HIV-1 coreceptor CCR5 gene, thus providing a possible treatment for HIV disease (Guo, Karberg et al. 2000). Because these introns remain active in the genome, HIV resistance has the potential to be more effective and longer lasting than previous methods.
These RNA splicing machines can be employed in a variety of ways to help fight diseases such as HIV and cancers. By studying how theses machines work, scientists gain a better understanding of how these machines can be used for human goals. Even though this technology is relatively new, there are already many methods available for using theses machines and more are being researched constantly. Spliceosomes, Group I introns, and Group II introns are just a few of the many types of gene therapy that have been discovered. Some of these methods have already been used to treat patients with amazingly positive results. Soon, these techniques will become more refined and sophisticated, possibly providing a cure for some of the world’s most deadly diseases.




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Mitchell, A. (2008). "New Approaches to Gene Therapy." Genetic Science Learning Center Retrieved March 5, 2008, from http://learn.genetics.utah.edu/units/genetherapy/gtapproaches/.

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