Taking Lessons from the CCR5Δ32 Mutation for Patient Treatment
Posted Nov 20 2009 10:00pm
I’m Lindsay Sween, and welcome to this installment of the AIDS Pandemic blog and podcast.
Human immunodeficiency virus type 1 (HIV-1) invades a CD4+ (T4) cell through the attachment of the viral protein gp120 to its primary cellular receptor, CD4, and to a transmembrane chemokine coreceptor, usually CCR5 or CXCR4. Agrawal et al. (2007) explain that the removal of 32 base pairs from the CCR5 gene results in the CCR5Δ32 mutation, which produces a shortened, nonfunctional protein that cannot act as a coreceptor due to the fact that it is no longer expressed on the cell membrane. Thus, individuals homozygous for the CCR5 mutation (also known as CCR5 -/- individuals) are extremely resistant to contracting HIV-1, while heterozygous people (aka CCR5+/- people) express fewer CCR5 proteins on the surface of their lymphocytes than wild type individuals, which slows the transition of HIV infection to AIDS. The CCR5Δ32 mutation confers HIV-1 resistance by two mechanisms: the mutated protein cannot be expressed on the lymphocyte surface, and it actively downregulates CXCR4 coreceptor production by causing the formation of heterodimers between CCR5 and CXCR4 proteins that then get trapped in the endoplasmic reticulum.
As explained by Nazari and Joshi (2008), individuals with the CCR5Δ32 mutation appear perfectly healthy in all other areas of their immune systems, which seems to indicate that the CCR5 chemokine receptor is not absolutely essential for immune function. Thus, with no selective pressure against the CCR5Δ32 mutation, Agrawal et al. (2007) report that Caucasians carry the mutation relatively frequently, with about 1% of individuals being homozygous for the mutated allele and approximately 10% of the population being heterozygous. Individuals of purely African or Asian descent, however, almost entirely lack the CCR5Δ32 mutation.
Figure 1. The CCR5Δ32 mutation results in a nonfunctional protein that cannot serve as a cell surface coreceptor for M-tropic (aka CCR5-tropic or R5) HIV viral isolates and, thus, confers some resistance to HIV-1 infection. The immune cells are still fully receptive to T-tropic (aka CXCR4-tropic or X4) viral isolates, which could bind to their coreceptor, CXCR4 (aka fusin), and transmit HIV-1 infection. From: Samson, Michel. “Human immunodeficiency virus (HIV).” Access Science Online. McGraw-Hill. .
There is now a new antiretroviral drug called maraviroc, which was approved by the U.S. Food and Drug Administration U.S. Food and Drug Administration in August 2007 and mimics the natural CCR5Δ32 mutation by acting as an antagonist for the CCR5 receptor and preventing the viral envelope protein gp120 from binding to it. Lieberman-Blum et al. (2008) report the results of two Phase IIb/III clinical trials, MOTIVATE 1 and 2, in which the effects of treatment with 300 mg of maraviroc once or twice daily were compared to placebo treatment in patients who were already being treated with HAART and still had primarily R5 HIV-1 infection. Maraviroc was found to decrease viral load by a greater percentage than placebo. Of the patients receiving maraviroc once or twice daily, 43.2% and 45.5%, respectively, had virus particle counts of less than 50 copies per milliliter, as opposed to 16.7% of patients in the placebo group. After the 48 weeks of the studies, patients demonstrated average viral load reductions of -1.68 log10 copies/mL for the once daily group and -1.84 log10 copies/mL for the twice daily group compared to -0.78 log10 copies/mL for the control group.
Figure 2. Most patients given maraviroc once or twice daily had lower HIV-1 viral loads and higher CD4 cell counts at the end of 48 weeks and had a long time period until treatment failure than did patients taking placebo. From: Gulick, R.M., Lalezari, J., Goodrich, J., Clumeck, N., DeJesus, E., Horban, A., Nadler, J., Clotet, B., Karlsson, A., Wohlfeiler, M., Montana, J.B., McHale, M., Sullivan, J., Ridgway, C., Felstead, S., Dunne, M.W., van der Ryst, E., Mayer, H. 2008. Maraviroc for Previously Treated Patients with R5 HIV-1 Infection. The New England Journal of Medicine 359: 1429-1441.
As would be predicted by the absence of adverse health problems in individuals lacking functional CCR5 receptors due to the CCR5Δ32 mutation, maraviroc produced few severe side effects for the immune system by blocking the CCR5 surface protein. According to Lieberman-Blum et al. (2008), 21 of 426 (4.9%) individuals taking maraviroc and 11 of 209 (5.3%) individuals taking placebo had poor health outcomes that lead them to stop taking their medication and quit the trials. Most patients (92.3%) reported at least one side effect, which included upper respiratory illness, cough, fever, and abdominal pain. The primary concern with the use of antiretroviral drugs that block the CCR5 receptor is that the HIV virus will evolve into X4 or R5X4 variants that will then evade the drug’s action. For the individuals who were not benefitted by maraviroc, 54.4% of the once-daily patients and 55.2% of the twice-daily patients demonstrated virus that had changed from the R5 strains to either X4 or R5X4 strains. When the researchers performed phylogenetic analyses of the viral envelope proteins in these strains, however, they found that the new X4 or R5X4 strains had developed from preexisting viral particles of these strains that had been missed in the screening process before the beginning of the drug trials and had not resulted from R5 mutation during the course of drug treatment. Thus, these clinical trials suggest that maraviroc could be a good possibility for “salvage therapy” for those HIV+ individuals who have experienced treatment failures in the current categories of HIV/AIDS medications. More studies are still needed, however, to determine the long-term effects of antagonizing the CCR5 receptor.
The CCR5Δ32 genetic mutation and the recent research investigating it and its therapeutic implications are very relevant topics given the fact that the HIV/AIDS pandemic is one of the greatest public health concerns in the world, especially in developing nations. As cited in Lieberman-Blum et al. (2008), the Joint United Nations Programme on HIV/AIDS and the World Heath Organization report that as of 2007 33.2 million people worldwide were HIV+, and 2.5 million of those cases were new infections. In addition, the virus’s high mutation rate makes viral resistance to current antiretroviral medications a growing problem for disease treatment. The research into the CCR5Δ32 mutation aided scientists in developing the new class of antiretroviral drugs known as CCR5 antagonists. Furthermore, most new infections of HIV-1 are caused by R5 (also known as CCR5-tropic or macrophage-tropic) viral isolates. Thus, gene therapy involving the complete downregulation of CCR5 by the CCR5Δ32 mutation inserted into cells via viral vectors could one day prevent transmission of HIV by removing the coreceptor in the semen-receiving individual. Through the CCR5Δ32 mutation, evolution and natural selection may have unwittingly supplied we humans with a very powerful weapon in the fight against the HIV/AIDS pandemic.
For more information, please see Agrawal, L., Jin, Q., Altenburg, J., Meyer, L., Tubiana, R., Theodorou, I., Alkhatib, G. 2007. CCR5Δ32 Protein Expression and Stability Are Critical for Resistance to Human Immunodeficiency Virus Type 1 In Vivo. Journal of Virology 81: 8041-8049.
Lieberman-Blum, S.S., Fung, H.B., Bandres, J.C. 2008. Maraviroc: A CCR5-Receptor Antagonist for the Treatment of HIV-1 Infection. Clinical Therapeutics 30: 1228-1250.
Nazari, R., Joshi, S. 2008. CCR5 as Target for HIV-1 Gene Therapy. Current Gene Therapy 8: 264-272.