|MadSci Network: Immunology|
How do viruses like HIV develop resistance to drugs?
The drugs used to treat HIV are “substrates” or molecules that enzymes of the HIV virus bind and act upon. The reverse transcriptase and the protease are the two main enzymes that are the targets of anti-HIV drugs. Resistance occurs when these enzymes evolve into a form that no longer binds the drug.
The reverse transcriptase is the enzyme responsible for copying the RNA which contains the genetic information of the virus. This enzyme binds the nucleotide molecules, dATP, dCTP, dGTP, and dTTP, and adds them to the newly synthesized DNA copy of the HIV genome. AZT, the first anti-AIDS drug, has a structure similar to dTTP. But AZT has an azido group consisting of three nitrogen atoms where dTTP has an OH. AZT can fit into the active site of the reverse transcriptase. Thus AZT can be taken up by the reverse transcriptase as if it was dTTP and AZT can be added to the new stand of DNA that is being synthesized. But the nitrogen on the AZT acts as a block to synthesis and no more nucleotides can be added to that particular strand of DNA. Thus the strand is effectively dead.
Note, there are also cellular DNA polymerases that use dTTP to make DNA for the cell. The cellular polymerases bind dTTP about 100 times better than AZT, so the cell is relatively safe from AZT. How does this happen?
As mentioned above, nucleotides are substrates for the reverse transcriptase. Substrates fit into enzymes like a hand fits into a glove. Enzymes are proteins. Like all proteins, enzymes are composed of amino acids and each enzyme is made of its own particular sequence of amino acids. The reverse transcriptase and the cellular DNA polymerase have some structural similarities, but their amino acid sequences are different. The amino acid differences in the substrate binding sites affect the sites in such a way that the reverse transcriptase binds and incorporates AZT more that the cellular polymerase.
How does HIV evade anti-HIV drugs? After AZT treatment, one of the first changes found in the HIV reverse transcriptase is that the amino acid lysine at position 70 is changed to an arginine. Reverse transcriptase with this change is 4 to 8 fold resistant to AZT. Then the threonine at position 215 is often found changed into a tyrosine. This adds another 10 fold resistance, for a total 64 fold resistance. With changes in its amino acid sequence, the reverse transcriptase evolves into a structure that binds and functions with dTTP and rejects AZT.
How does HIV create the resistant forms of its enzymes? The reverse transcriptase, the enzyme that copies the genome of HIV, is rather inaccurate. It makes mistakes in the DNA at a rate of about 3x10-5 per nucleotide. The HIV genome is about 10x10+3 nucleotides long, so 0.3 (about 1 in 3) will have a mutation. Every 3x10+4 viruses will give a chance that each nucleotide will be mutated. An infected person may produce 10x10+9 viruses a day, and that is enough to produce mutations at every site, and many different combinations of mutations. And mutations in the genes will be represented as changes in the enzymes the genes code for. This way it can be seen that a reverse transcriptase with a resistance mutation is bound to occur. Many mutated viruses will not be viable and after treatment the level of viruses falls rapidly. However, with just AZT, a resistant virus often appears within a year.
If two reverse transcriptase inhibitor drugs are given, the chance that the combination of mutations needed to produce resistance to both drugs will occur at the same time is proportionally smaller. Add an inhibitor to the protease (another enzyme that HIV needs for its life cycle), and a mutation in this enzyme also has to occur for the HIV to survive. This is called “triple therapy” or “HAART” (highly active anti-retroviral therapy). HAART can reduce the viral load to “undetectable” or below 20 viruses per ml. HAART increases the health and adds many years of life to someone infected with HIV, but even with this low level of virus they can still spread HIV infection.
In the past it was hoped that reducing the HIV to low levels would eventually eliminate the virus, but this hasn’t been found to happen. New drugs like Fuzeon, a fusion inhibitor, have been approved and drugs are being developed to block other targets. But they all interact with proteins, and the problem of HIV developing resistance persists.
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