Genetic mutations increase the risk of tuberculosis

Researchers at the University of São Paulo and the Necker Hospital of Paris shed light on how nine different genetic defects can prevent the body’s immune system from fighting infections caused by mycobacteria.

By Karina Toledo

Agência FAPESP – In partnership with a group from the Necker Hospital of Paris, researchers from the University of São Paulo (USP) are studying how nine different genetic mutations can prevent the immune system from adequately fighting infections caused by mycobacteria, among them, tuberculosis and Hansen’s disease.

Although through different routes, the genetic defects studied lead to the malfunction of what is known as the NADPH oxidase system (nicotinamide adenine dinucleotide phosphate oxidase) – responsible in some defense cells for the production of reactive oxygen species, which are essential for combating this type of pathogen.

According to the researchers, the discovery opens the door to new treatments. “There are already medicines capable of activating the NADPH oxidase system and stimulating the immune system to fight this type of infection. Based on the new evidence, we can think about starting clinical trials with these drugs,” said Antonio Condino Neto, professor in the Immunology Department of the Biomedical Sciences Institute of USP and coordinator of the FAPESP-funded study.

The partnership with France – carried out under the scope of a cooperation agreement between FAPESP and the Institut National de la Santé et de la Recherche Médicale (Inserm) – began when the group led by Frenchman Jean-Laurent Casanova discovered a mutation in the CYBB gene, which is responsible for encoding a protein called gp91phox, in two families of carriers of an atypical form of tuberculosis.

Casanova’s group had already discovered seven other mutations associated with the genetic susceptibility to infections caused by mycobacteria. However, none of these mutations was found in the two families studied. By sequencing the entire exome – the portion of the genome that contains information for protein production – in these patients, the scientists discovered the defect in the CYBB gene.

“Up until that time, the scientific literature had associated mutations in the CYBB gene only with the development of chronic granulomatous disease (CGD), a type of primary immunodeficiency that our group at USP has studied for many years. Because Casanova was intrigued at discovering how the genetic mutation of CGD could cause this atypical form of tuberculosis, he invited us to become part of the team,” said Condino Neto.

During the doctoral studies of Carolina Prando, conducted under the advisorship of Condino Neto at the University of Campinas (Unicamp), the group demonstrated that the CYBB gene mutation in the case of these two families impaired the functioning of the NADPH oxidase system in macrophages.

“The genetic defect destabilized the gp91phox protein, and, therefore, the NADPH oxidase system – responsible for the microbicidal activity inside macrophages – did not work. In this case, the macrophage manages to phagocytize the bacterium but is not able of killing it because the macrophage is incapable of producing reactive oxygen species,” Condino Neto explained.

The findings of the study were released in an article published in 2011 in the journal Nature Immunology. “We were able to show that defects in the CYBB gene, depending on the type of mutation, could result in chronic granulomatous disease as well as in a genetic susceptibility to mycobacteria,” stated Condino Neto.

The researcher explained that the difference is that instead of affecting only macrophages, CGD affects the microbicidal capacity of all leukocytes, causing an immunodeficiency that is much more severe.

Link found

During the doctoral studies of Walmir Cutrim Aragão Filho and Edgar Borges de Oliveira Junior, both FAPESP fellows under the advisorship of Condino Neto, the group further studied the role that reactive oxygen species play in combating tuberculosis and other mycobacterial infections.

For the NADPH oxidase system of the defense cells to function correctly, Condino Neto explained, five proteins need to be working in synergy: two in the membrane and three in the cytoplasm. When one of them undergoes a mutation, a functional defect occurs in the system and interrupts the production of reactive oxygen species.

The CYBB gene mutation, however, is the only one of the nine discoveries by Casanova that directly affects a protein of the NADPH oxidase system. “During Aragão Filho’s doctoral studies, we demonstrated that the other mutations that affect cytosine interferon gamma (IFNγ) receptors also interfere in the functioning of the NADPH oxidase system. In this case, the defect in the release of oxygen reactive species is accentuated. Until then, no one knew what the link between all of these mutations was,” Condino Neto said.

The research findings were released in an article published in Scandinavian Journal of Immunology.

Now a post-doc, Oliveira Júnior has received FAPESP funding to investigate the functional mechanisms of a mutation in the GATA2 gene, which also causes susceptibility to mycobacterial infections. The findings will be published soon.

New therapies

Based on the evidence regarding the fundamental role that the NDPH oxidase system plays in fighting mycobacteria, Condino Neto defends the use of substances such as cytosine IFNγ – which is a drug already commercially available – to treat tuberculosis, a hypothesis that will be tested in future clinical trials.

“As an immunologist, I think the path for treating this disease is not to develop ever stronger antibiotics but rather to develop new medications capable of stimulating the immune system to make it able to kill the mycobacterium,” explained the researcher.

The study “Human genetics in mycobacterial infections: new molecular genetic defects involved in Mendelian susceptibility to mycobacterial infections” was conducted with funding from FAPESP between April 2011 and March 2013.