Mark Purdey:"This work has identified that a package of toxic environmental factors underpins the pathogenesis of TSEs, where the simultaneous exposure to a copper deficient food chain and rogue radioactive metal pollutants brings about the malformation of the prion protein..."
I was disappointed not to find any references/bibliography in this article, but his papers (mostly published in Medical Hypotheses so hidden behind a subscription) are archived on the site for further reading.
Here's George Monbiot's article from 2000 (also without references). This is the paper noted by Monbiot that shows some effects of manganese replacing copper in binding to the prion protein, and this is a paper published a year later that shows effects of copper itself binding to the prion protein.
There are also a few papers that suggest a role for RNA or viral intermediaries in the conversion of prion protein to the pathogenic form.