Helicases as Drug Discovery Targets
Helicases – ATP-powered nucleic acid unwinding enzymes – are an interesting therapeutic target class, being pursued for both infectious diseases and cancer. The rationale for inhibiting helicases in viruses, bacteria, and cancer cells is similar: they are required for replication and/or repair of their respective genomes. A recent review from David Frick’s group at the University of Wisconsin, Milwaukee (J. Biomol Screening 18, 761-781) provides a nice overview of the history of helicases as drug targets and describes the challenges and recent successes in developing helicase inhibitors.
The idea of targeting helicases is relatively recent, only really gaining traction 10 years ago with the discovery of several potent antiviral compounds at Bayer and Boehringer-Ingelheim that inhibit an essential herpes simplex virus helicase complex. The first clinical success is just being realized with an Astellas compound, called amenamevir, that is proving effective against HSV strains that are resistant to Acyclovir, a DNA polymerase inhibitor that is currently used as first line therapy for HSV infections. While discovering similarly potent and specific inhibitors for other helicases has been quite challenging, considerable progress has been made in recent years with other viral enzymes including those from papillomaviruses, polyomaviruses, coronaviruses, the hepatitis c virus and various flaviviruses. In addition, both DnaB-like and RecBCD-like helicases are promising bacterial targets.
In humans, success targeting RecQ-like helicases has suggested they may be attractive targets for some cancers. There are five known human RecQ-like proteins: RecQ1, WRN, BLM, RecQ4, and RecQ5, which were first discovered through genetic links to autosomal recessive diseases including Werner Syndrome, a premature aging disease, and Bloom syndrome, which is characterized by short stature and predisposition to the development of cancer. RecQ-like helicases are vital components in DNA replication, recombination and repair, and multiple lines of evidence have suggested that inhibiting them could make cancer cells more sensitive to chemotherapy.
High-throughput assays used to identify helicase inhibitors function by measuring nucleic acid unwinding or ATP hydrolysis. Screens measuring DNA unwinding can be challenging as they tend to yield few hits, and many of the hits act by binding the nucleic acid substrate. Helicase ATP hydrolysis is measured by monitoring the appearance of ADP or phosphate. When using colorimetric phosphate assays, ATP must be removed or multiple reagents must be added in a precisely timed procedure. Assays which directly detect ADP eliminate the need for ATP removal/reagent addition and allow for a much simpler screening procedure. Bellbrook’s Transcreener ADP assay has been used in two recent studies: one from Scott Singleton’s group at the University of North Carolina, Chapel Hill targeting E. coli RecA, and another from David Frick’s lab at UW-Milwaukee focused on the targeting of a viral RNA helicase for Hepatitis C.
