DNA, gene, genome
The genome is the set of our genes. There are about 25000 different genes in our genome, which allows to synthesize all the proteins necessary for the functioning of cells and the body. The genome is carried by the DNA in our chromosomes, located in the cell nucleus. DNA consists of a sequence of building blocks or nucleotides which there are 4 kinds, A, T, G and C. It is the order of this nucleotide sequence that determines the genetic information.
Like a zipper, each string of nucleotides is facing another one. The two strings are complementary: an A is always facing a T, a C faces a G. This structure, known as the double helix, is a quality assurance for the cell.
A gene contains two regions:
- The “coding” region: the sequence of nucleotides, which determines the sequence of amino acids in the protein and thus structure and function;
- The regulatory regions: they determine in what cell, when, in response to what stimulus, and how much the protein must be synthesized.
To synthesize a protein, the coding part of the DNA of the gene is first copied into messenger RNA (mRNA). This one, which has only a single strand, leaves the cell nucleus. Ribosomal and transfer RNA assemble on the messenger RNA, read its nucleotide sequence and use that information to build the correct sequence of amino acids and produce the corresponding protein. By different mechanisms, not detailed here, a gene may encode not only one but a family of proteins.
Interfering RNA (RNAi) are RNA that interact with mRNA to prevent the synthesis of the corresponding protein. There are two classes of interfering RNA:
- micro-RNA, or miRNA, naturally encoded by our DNA to control the expression of other genes in our genome
- Small Interfering RNA, or siRNA, that are introduced artificially into cells
The two classes of interfering RNA share several characteristics:
- they are small (approximately 20 nucleotides)
- their sequence is complementary to that part of a messenger RNA cell with which they hybridize.
- In the cell, they are supported by a complex of several proteins called RISC, acronym for RNAi Induced Silencing Complex. All RISC/RNA interfering scans different messenger RNA molecules in the cell. Several situations can occur:
- If there is no homology between RNAi and scanned messenger RNA, it is normally translated into protein.
- If RNAi is a miRNA and that there is a partial homology between him and mRNA translation is blocked but the messenger RNA is not degraded
- If RNAi is a siRNA which nucleotide is perfectly homologous with a region of mRNA, it is cut by RISC. Cut messenger RNA is very quickly degraded and the corresponding protein cannot be synthesized.
- The interfering RISC + ARN complex is not destroyed by cutting and it can again scanner mRNA and start a cut.
If you know the sequence of the nucleotides of MRNA which codes for a protein that interests us, can synthesize a siRNA able to hybridize with this sequence and insert it into the cell. You can prevent the synthesis of the protein normally translated from messenger RNA.
The great power of RNA interference holds three of its properties:
- It is a very efficient mechanism because the interfering RNA “serves” several times
- It uses a natural essential machinery of the cell that is functional in all cells in all mammals.
- It is a highly specific process: now that we know the order of all nucleotides of our genome, it is quite easy to choose a sequence of siRNA that is specific of a gene being taken for target. This does of course not exempt to many controls to check that other genes are not disturbed by this siRNA, which can occur in some cases.
An innovative therapeutic tool
We know of many cases in which a protein is responsible of a disease. For example, viruses infect cells, inject their genome so that the infected cells produce the proteins they need to replicate. If the sequence of the genome of the virus is known, one can use RNA interference to prevent replication of the virus. In other diseases, including cancers, certain proteins are expressed, even though they should not be. There again, RNA interference can be used to inhibit the synthesis of these unwanted proteins.
One of the great strengths of RNA interference is its specificity. Because of the complementarity of the bases (A with T, G with C), a small interfering RNA makes the difference between two RNA messengers of closely related sequences, for example between a normal messenger RNA and a mutated mRNA produced by a cancerous cell. This specificity opens a way to develop treatments that reaching the cell sick without disturbing the other cells of the body and therefore devoid of side effects.
The systematic sequencing of the human genome has enabled us to know the complete sequence of the nucleotides in the DNA of our genes. It does not tell us what is the function of these genes. By inhibiting gene expression, RNA interference allows access to this information and determine what proteins whose presence is indispensable in a given cell process (division, migration, apoptosis…).