SETD5-Coordinated Chromatin Reprogramming Regulates Adaptive Resistance to Targeted Pancreatic Cancer Therapy.

Molecular mechanisms underlying adaptive targeted therapy resistance in pancreatic ductal adenocarcinoma (PDAC) are poorly understood. Here, we identify SETD5 as a major driver of PDAC resistance to MEK1/2 inhibition (MEKi). SETD5 is induced by MEKi resistance and its deletion restores refractory PDAC vulnerability to MEKi therapy in mouse models and patient-derived xenografts. SETD5 lacks histone methyltransferase activity but scaffolds a co-repressor complex, including HDAC3 and G9a. Gene silencing by the SETD5 complex regulates known drug resistance pathways to reprogram cellular responses to MEKi. Pharmacological co-targeting of MEK1/2, HDAC3, and G9a sustains PDAC tumor growth inhibition in vivo. Our work uncovers SETD5 as a key mediator of acquired MEKi therapy resistance in PDAC and suggests a context for advancing MEKi use in the clinic.

Inhibition of hepatitis C viral RNA-dependent RNA polymerase by α-P-boranophosphate nucleotides: exploring a potential strategy for mechanism-based HCV drug design.

Improved treatments for chronic HCV infections remain a challenge, and new chemical strategies are needed to expand the current paradigm. The HCV RNA polymerase (RdR(P)) has been a target for antiviral development. For the first time we show that the boranophosphate (BP) modification increases the substrate efficiency of ATP analogs into HCV NS5BΔ55 RdRP-catalyzed RNA. Boranophosphate nucleotides contain a borane (BH3) group substituted for a non-bridging phosphoryl oxygen of a normal phosphate group, resulting in a class of modified isoelectronic DNA and RNA mimics capable of modulating the reading and writing of genetic information. We determine that HCV NS5BΔ55, being a stereospecific enzyme, incorporates the Rp isomer of both ATPαB and the two boranophosphate analogs: 2'-O-methyladenosine 5'-(α-P-borano) triphosphate (2'-OMe ATPαB, 5a) and 3'-deoxyadenosine 5'-(α-P-borano) triphosphate (3'-dATPαB, 5b). The R(p) diastereomer of ATPαB (6), having no ribose modifications, was found to be a slightly better substrate than natural ATP, showing a 42% decrease in the apparent Michaelis-Menten constant (K(m)). The IC50 of both 2'-O-Me and 3'-deoxy ATP was decreased with the boranophosphate modification up to 16-fold. This "borano effect" was further confirmed by determining the steady-state inhibitory constant (K(i)), showing a comparable potency shift (21-fold). These experiments also indicate that the boranophosphate analogs 5a and 5b inhibit HCV NS5B through a competitive mode of inhibition. This evidence, together with previous crystal structure data, further supports the idea that HCV NS5B (in a similar manner to HIV-1 RT) discriminates against the 3'-deoxy modification via lost interactions between the 3'-OH on the ribose and the active site residues, or lost intramolecular hydrogen bonding interactions between the 3'-OH and the pyrophosphate leaving group during phosphoryl transfer. To our knowledge, these data represent the first time a phosphate modified NTP has been studied as a substrate for HCV NS5B RdRP.