In prion diseases (transmissible spongiform encephalopathies), what is the molecular basis for the species barrier, and how does variant CJD (vCJD) arising from bovine BSE overcome the normal human species barrier?

• A Normal prion protein (PrPC) has a species-specific glycosylation pattern; BSE-derived PrPSc cannot glycosylate human PrPC and therefore fails to template misfolding unless large infectious doses overwhelm detoxification enzymes
• B The species barrier exists because PrPSc from one species templates the misfolding of PrPC preferentially when the amino acid sequence of the host PrPC closely matches that of the infecting PrPSc; BSE prion is unusual because its PrPSc conformation has high fidelity for templating human PrPC misfolding despite primary sequence differences — likely because BSE arose from a common ancestor strain adaptable to multiple mammalian PrPC conformations ✓
• C vCJD arises only in patients with Met129Met genotype at codon 129 of PRNP, meaning the barrier is overcome exclusively by a host genetic susceptibility with no role for the strain conformation of BSE PrPSc
• D BSE overcomes the species barrier by incorporating host-derived lipids into the prion particle which enhance membrane binding and cellular uptake in humans, bypassing PrPC conformation-dependent templating

Explanation

The species barrier in prion disease is primarily determined by the compatibility between the three-dimensional conformation of the incoming PrPSc (the infectious template) and the primary amino acid sequence of the host's PrPC. When there is significant sequence divergence, the PrPSc template cannot efficiently misfold host PrPC. BSE prion has an unusual strain conformation that exhibits high promiscuity across species — it efficiently templates misfolding in cats, humans, and other mammals despite primary sequence differences. In humans, variant CJD (vCJD) has a distinctive PrPSc glycotype (diglycosylated PrPSc type 4, revealed by Western blot) identical to BSE strain, confirming BSE as the source. PRNP codon 129 Met/Met homozygosity greatly increases susceptibility and has been the genotype of all confirmed vCJD cases so far, representing a significant host modifying factor — but the strain conformation of BSE PrPSc is the primary determinant of barrier crossing.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.