I’m sitting in the Drexler Auditorium of the Buck Institute, where I’ve been working over the last six months. Today we’re being treated to an off-schedule “Special Institute Lecture” by Harvard’s Gregory Verdine. These are my notes about the talk; below, I’m paraphrasing Verdine’s words, not writing my own.
To fix it you have to find it: Repairing oxidative damage to DNA.
Mitochondrial metabolism generates oxygen radicals, which damage DNA and increase the risk of mutation. The primary oxidative adduct of guanine, 8-oxoG, differs by only two atoms from the original guanine, but this small difference is still enough to change the residue’s base-pairing characteristics (i.e., from G=C to 8-oxoG=A). 8-oxoG is repaired by the enzyme Ogg1, which displaces the damaged residue by covalently bonding to the DNA backbone.
Ogg1 has a tough job: oxoG-C base pairs are perfectly stable, and from the outside they look just like G-C pairs, so the detection of rare lesions within the genome poses a tremendous challenge. In crystallographic studies that exploited a catalytically dead Ogg1 enzyme (which can recognize but not cleave DNA), Verdine’s lab has shown that the binding of Ogg1 at a G-C base pair results in the extrusion of G or oxoG to the exterior of the double helix, setting the stage for repair — but how does Ogg1 find the lesion site in the first place? To answer this question, Verdine’s group visualized single molecules of Ogg1 diffusing in one dimension along a double helix. They observed that Ogg1 moves so quickly that it can’t be checking every G-C base pair along its path. Instead, Ogg1 (and other lesion-repair proteins) may exploit subtle rearrangements in the DNA backbones near lesion sites. The enzyme amplifies these local structural changes into substantial conformational changes, leading to base extrusion and starting the process of repair.
We want a new drug: Synthetic biologicals as novel pharmaceuticals
Small-molecule drugs have “good geography,” in the sense that they can cross cell membranes. However, they’re limited, in that they can only target proteins that engulf them (e.g., in hydrophobic pockets or active sites). Proteins, on the other hand, have much more diverse function, but terrible geography — they simply can’t get into cells, and they’re therefore useless for intracellular targets. Both classes of drug can attack (generously) only ~10% of prospective targets. Therefore, we need an entirely new class of drug: synthetic biologics like stapled peptides and RIPtides, which combine the bioavailability of small molecules with the functional diversity of proteins.
Stapled peptides are essentially the interaction domains of proteins, conformationally restrained in such a way that they still retain the active structure. (Think of a protein as a delivery system for an interaction domain, in which the non-interacting portions serve primarily to hold the ID in place.) An interaction domain alone would be too floppy to have a biological effect; conversely, the intact protein has the desired function but can’t cross the membrane. Solution: replace the main body of the protein with a hydrocarbon “staple” that keeps the interactive domain in the active conformation, without substantially increasing its size. Surprisingly, stapled peptides are taken up by cells via an energy-dependent active transport process, one upshot of which is that they don’t need to be uncharged and hydrophobic in order to cross the membrane. Drugs of this kind have already been used in animal studies to suppress leukemia by activating apoptotic factors in tumor cells.
Declaring open season on transcription factors
A brief concluding note: Transcription factors are among the most well-validated prospective targets, but they have historically been outside the scope of drug developers. TFs function primarily by protein-protein interactions that aren’t amenable to interference by small-molecule drugs. Recently, however, Verdine and others have been able to use synthetic biologicals to interfere with a specific oncogenic transcription factors.
My own comments
OK, so, not a lot of specific about either aging or cancer, but the idea of a novel class of pharmaceuticals that could be used to attack the “missing 80%” of validated prospective drug targets is still very exciting.