general	prion	malaria
data mining	evolution	genomics	structure

PRION

Introduction

Prion diseases are uniquely manifest as spontaneous, inherited, and infectious maladies; they include Gerstmann-Sträussler-Scheinker (GSS) disease, fatal insomnia, and Creutzfeldt-Jakob disease (CJD). Most cases of CJD are sporadic with 10 to 15% being inherited. Although the infectious human prion diseases are most notorious, they account for less than 1% all prion disorders. Concern about these infectious disorders has been heightened by the identification of more than 100 young adults and teenagers who have developed new variant CJD (nvCJD) in Europe after exposure to bovine prions from cattle with bovine spongiform encephalopathy (BSE). Other infectious prion diseases include kuru among New Guinea natives caused by ritualistic cannibalism and iatrogenic CJD caused by prion contaminated cadaveric growth hormone and dura mater grafts.

The common etiologic agent in these diseases is a mis-folded form of the normal endogenous prion protein, PrPC, whose biological function is not clearly understood. The mis-folded and infectious form of the prion protein, PrPSc, has been shown to form aggregates in the cerebral tissue which are not effectively cleared. This results in cerebral damage and ultimately death.

Structural Studies of the Prion Protein

Identifying the structural characteristics of the pathological isoform, PrPSc, is central to understanding the mechanism of aggregation and is therefore key to developing potential therapeutics. While a partial three-dimensional structure of PrPC is known, the extreme insolubility of PrPSc makes it difficult to elucidate structure using classical techniques. However, with the help of electron-microscopy (EM) on two-dimensional prion crystals, recent advances have been made, providing low-resolution images of PrPSc.

The Cohen group aims to use molecular modelling to translate the low-resolution EM data into atomic-level three-dimensional models of PrPSc. This process relies on the combined use of experimental information, the analysis of related structures and computational methods. We hope that these approaches can help understand the biophysical mechanisms underlying prion disease, and other pathologies involving protein misfolding.

Developing Small Molecule Pharmacotherapies for Prion Disease

The Cohen group is actively researching small molecule therapies with the potential to treat prion diseases. We have used structure-based design and serendipitous methods to identify lead compounds capable of inhibiting PrPSc replication in a cell-based model of prion disease.

As an example of our computational drug design, we have used computational methods to identify small ligands capable of mimicking key residues on the surface of PrP. These four residues have been implicated in a binding interaction with an auxiliary to PrP conversion. Thus, small molecule mimics of these residues could potentially disrupt PrPSc replication by inhibiting the PrP-auxiliary interaction. Three-dimensional pharmacophores were derived based on the spatial and electronic features of these residues. An in silico screen of these pharmacophores against a compound library identified several dozen compounds which were subsequently assayed in vivo. Testing identified a non-toxic pyridine-based compound with micromolar activity against PrPSc replication. We employ classical medicinal chemistry techniques to derive a structure-activity relationship for our lead compounds. In this instance our medicinal chemistry efforts are continuing to improve the activity of pyridine-based compounds against PrPSc replication.

In contrast to our structure-based drug design projects we have also undertaken more serendipitous routes to lead compounds. For example, quinacrine was identified as a lead compound, active against PrPSc replication, following a screen of known psychoactive compounds. Quinacrine has recently entered clinical trials for the treatment of prion disease. Again, we are employing parallel synthesis techniques to build libraries of quinacrine analogs with the aim of improving the activity of this compound against PrPSc replication.