Harris discusses novel target to promote HIV eradication
By Jeffrey Stumpf
HIV therapeutics are currently capable of suppressing virus replication to virtually undetectable levels and enabling patients to lead near-normal lives. However, in a talk April 16 at NIEHS, cancer geneticist Reuben Harris, Ph.D., reminded attendees of the obvious reason to continue studying cellular processes that could be targets for novel therapeutic strategies.
“We haven’t cured HIV, yet,” Harris stated. “It is still a big problem around the globe for human health and economically.” Harris spoke as part of the Laboratory of Molecular Carcinogenesis (LMG) Fellows Invited Guest Lecture Series. NIEHS fellow Kathleen Richter, Ph.D., and her colleagues in the Somatic Hypermutation Group hosted Harris.
As an associate professor at the University of Minnesota, Harris has maintained a long-standing interest in DNA replication and mechanisms that increase mutation. Studying immune defenses to the virus that causes AIDS satisfies his intellectual curiosity with the possibility of improving human health.
“The long term goal is a cure. The short term goal is to learn a lot about the molecular mechanisms and the details about how these proteins inhibit virus replication, and how the virus fights back,” Harris explained.
The battle for HIV replication: Human APOBEC versus HIV Vif
Harris focused his talk on the role of the family of APOBEC proteins in rendering the HIV virus non-infectious. APOBEC deaminates deoxycytosines in single-stranded DNA, thus converting them into deoxyuracils and leading to mutations that can inactivate the virus. HIV counters the attack by the APOBECs using the Vif protein, which signals the ubiquitin ligase cascade to target the degradation of APOBEC proteins.
There are many APOBEC proteins that can potentially inhibit viral replication. APOBEC3G is known to inhibit HIV infectivity in cultured cells. Using new viral vector techniques to ablate genes in human cell lines, Harris showed that APOBEC3D, 3F, and 3H, in addition to APOBEC3G, also contribute to the human defense against HIV infection.
Reconstituting the degradation of the APOBEC proteins by Vif remained elusive until recently, according to Harris. The in vitro studies were lacking a cofactor, which Harris demonstrated was the human protein CBFbeta. HIV is thought to recruit the CBFbeta protein to aid the stability, or proper folding, of the HIV Vif protein.
To mutate or not to mutate
Immune responses and therapies against HIV are stymied by the unrelenting mutation rate that renders the viral genome a moving target. The relationship between the mutation-promoting APOBECs and Vif exemplifies the delicate balance of how mutations can destroy the virus or make it more difficult to control. Harris mentioned the idea of targeting APOBEC proteins for a novel HIV therapy, but asked whether it was better to intensify the mutagenic process or deter it.
On one hand, inhibiting proteins in the APOBEC degradation pathway, as Harris points out, may unleash a mutation load that the virus could not overcome. “I would call it therapy by hypermutation,” proposed Harris. “If you can stop degradation of APOBEC proteins, you would have a very high level of virus mutations.”
On the other hand, Harris postulated that eliminating APOBEC completely may stagnate the viral genome, making it more vulnerable to natural and therapy-based defenses. “One could imagine inhibiting APOBECs as a novel strategy,” Harris surmised. “If you decrease viral mutation rates by taking away all of APOBECs contribution, you may have a smaller swarm of viruses that the immune response could effectively clear.”
Following the latter choice, Harris announced the first class of APOBEC3G inhibitors from small molecule screens. “The molecules bind to the pocket adjacent to the active site and covalently react with the cytosine to prevent deamination,” Harris reported. “The inhibitor is like a shield around the cytosine.” Future studies will determine whether therapy by hypomutation will become a reality in the fight against HIV infection.
(Jeffrey Stumpf, Ph.D., is a research fellow in the NIEHS Laboratory of Molecular Genetics Mitochondrial DNA Replication Group.)