10.04.2019: Humans possess a variety of highly effective innate immunity factors. These include the so-called APOBEC3 proteins, which play an important role in the immune activity against retroviruses. A team of researchers from Vetmeduni Vienna, studying the mouse virus MMTV in a mouse model, has now for the first time been able to explain which mechanisms the retroviruses use to evade attack by the host immune system. This discovery is of enormous relevance for humans and could lead to a better understanding and treatment of the immune system disease (AIDS) caused by HIV.

Innate immunity in humans is an important factor in the body’s daily defence against viral and other pathogens. One class of these innate immunity factors consists of members of the APOBEC3 family. APOBEC3 belongs to a group of enzymes called cytidine deaminases that are capable of inducing mutations in the genome of viral attackers.

These enzymes exhibit a particularly high level of activity with retroviruses like the human immunodeficiency virus type 1 (HIV-1) or the mouse mammary tumour virus (MMTV), a retrovirus that can cause tumours of the mammary glands among certain mouse strains. The APOBEC3 enzymes exert their activity by mutating cytidines during reverse transcription. Reverse transcription is an important process in the retroviral life cycle, in which viral DNA is created from viral RNA.

It all started with a scientific mystery

Retroviruses have developed a variety of mechanisms to neutralize the APOBEC3 enzymes. These include special accessory proteins such as Vif, which is found in HIV-1 and can block APOBEC3, or Bet, which is capable of preventing contact with the viral genome during reverse transcription. Most retroviruses, however, do not encode a Vif- or Bet-like accessory protein. The question as to how they protect themselves against APOBEC3 therefore remained a mystery.

Mechanism of retroviral defence discovered

The study, published in PLOS Pathogens by a research team from Vetmeduni Vienna, delivers a first possible explanation. Project leader Stanislav Indik from the Institute of Virology at Vetmeduni Vienna: “In our work, we describe a new mechanism that the majority of retroviruses use, especially those that do not produce Vif- or Bet-like proteins.”

Explanation thanks to mouse model

This mechanism is described by the researchers in a mouse model using MMTV. As APOBEC3 proteins attack an intermediate product of reverse transcription (single-stranded DNA), they can exert their antiviral function only within a limited amount of time – specifically, while the DNA remains in the single-stranded form before the second DNA strand is synthesized. “We found that MMTV contains a reverse transcriptase (RT) that catalyzes the DNA synthesis at a high rate, which prevents APOBEC3 proteins from inducing hypermutation of the MMTV genome. When the kinetics of reverse transcription is reduced, the result is a higher sensitivity of the retroviruses to APOBEC3 proteins and an increased frequency of mutations,” says Indik.

Findings of great relevance for humans, for example, in the treatment of HIV

The increased frequency of the mutations observed by the researchers is of great importance for the understanding and medicinal treatment of retroviral diseases such as AIDS. The findings suggest that conditions which slow down the process of reverse transcription – this includes the use of antiretroviral drugs in low/suboptimal concentration – actually increase the risk of viral mutations.

Effectively preventing new resistance through mutations

The authors of the study therefore recommend: “HIV-1 patients should maintain a high level of antiviral drugs, i.e., RT inhibitors, in their bloodstream. When the blood concentration of these drugs falls, the number of HIV-1 mutations increases as a result of the immune response with APOBEC3 enzymes. This leads to the emergence of resistant viral mutants, causing the patients to develop AIDS and, ultimately, die.” Against this background, the present work provides an important contribution to explain which mechanisms can contribute to the establishment of drug-resistant mutations in patients – and how this could be effectively prevented.

The article “A high rate of polymerization during synthesis of mouse mammary tumor virus DNA alleviates hypermutation by APOBEC3 proteins” by Benedikt Hagen, Martin Kraase, Ivana Indikova and Stanislav Indik was published in PLOS Pathogens.