Estimation of scrapie nucleic acid MW from standard curves for virus sensitivity to ionizing radiation

Inactivation of viral and prion pathogens by gamma-irradiation under conditions that maintain the integrity of human albumin

BACKGROUND AND OBJECTIVES

The administration of therapeutic plasma protein concentrates has been associated with the real risk of transmitting viral diseases and the theoretical risks of prion transmission. Our objective was to determine if gamma-irradiation can inactivate viral or prion infectivity without damaging a protein biotherapeutically.

MATERIALS AND METHODS

Human albumin 25% solution, spiked with four model viruses (including porcine parvovirus) or with brain homogenate from scrapie-infected hamsters, was gamma-irradiated at constant low-dose rates and assayed for viral and prion infectivity or for albumin integrity.

RESULTS

At a radiation dose of 50 kGy, viruses were inactivated by >/= 3.2 to >/= 6.4 log10 and scrapie by an estimated 1.5 log10, whereas albumin was only moderately aggregated and fragmented.

CONCLUSIONS

gamma-Irradiation can preferentially inactivate viral and prion pathogens without excessive damage to albumin structure.

Structural studies of the scrapie prion protein using mass spectrometry and amino acid sequencing

The only component of the infectious scrapie prion identified to date is a protein designated PrPSc. A posttranslational process converts the cellular PrP isoform (PrPC) into PrPSc. Denatured PrPSc was digested with endoproteases, and the resulting fragments were isolated by HPLC. By both mass spectrometry and Edman sequencing, the primary structure of PrPSc was found to be the same as that deduced from the PrP gene sequence, arguing that neither RNA editing nor protein splicing feature in the synthesis of PrPSc. Mass spectrometry also was used to search for posttranslational chemical modifications other than the glycosylinositol phospholipid anchor attached to the C-terminus and two Asn-linked oligosaccharides already known to occur on both PrPSc and PrPC. These results contend that PrPSc molecules do not differ from PrPC at the level of an amino acid substitution or a posttranslational chemical modification; however, we cannot eliminate the possibility that a small fraction of PrPSc is modified by an as yet unidentified posttranslational process or that PrPC carries a modification that is removed in the formation of PrPSc. It seems likely that PrPSc differs from PrPC in its secondary and tertiary structure, but the possibility of a tightly bound, disease-specific molecule which purifies with PrPSc must also be considered.

A 'unified theory' of prion propagation

There is now very persuasive evidence that the transmissible agent for spongiform encephalopathies such as scrapie, consists of a modified form of the normal host protein PrPc, devoid of any nucleic acid. On the other hand, because there are many different strains of scrapie agent with distinct phenotypes which can be propagated in animals homozygous for the PrPc gene, it has been suggested that a nucleic acid must be a component of the agent. Can the two views be reconciled?

Molecular biology and transgenetics of prion diseases

Considerable progress has been made deciphering the role of an abnormal isoform of the prion protein (PrP) in scrapie of animals and Gerstmann-Sträussler syndrome (GSS) of humans. Some transgenic (Tg) mouse (Mo) lines that carry and express a Syrian hamster (Ha) PrP gene developed scrapie 75 d after inoculation with Ha prions; non-Tg mice failed to show symptoms after greater than 500 d. Brains of these infected Tg(HaPrP) mice featured protease-resistant HaPrPSc, amyloid plaques characteristic for Ha scrapie, and 10(9) ID50 units of Ha-specific prions upon bioassay. Studies on Syrian, Armenian, and Chinese hamsters suggest that the domain of the PrP molecule between codons 100 and 120 controls both the length of the incubation time and the deposition of PrP in amyloid plaques. Ataxic GSS in families shows genetic linkage to a mutation in the PrP gene, leading to the substitution of Leu for Pro at codon 102. Discovery of a point mutation in the Prp gene from humans with GSS established that GSS is unique among human diseases--it is both genetic and infectious. These results have revised thinking about sporadic Creutzfeldt-Jakob disease, suggesting it may arise from a somatic mutation. These findings combined with those from many other studies assert that PrPSc is a component of the transmissible particle, and the PrP amino acid sequence controls the neuropathology and species specificity of prion infectivity. The precise mechanism of PrPSc formation remains to be established. Attempts to demonstrate a scrapie-specific nucleic acid within highly purified preparations of prions have been unrewarding to date. Whether transmissible prions are composed only of PrPSc molecules or do they also contain a second component such as small polynucleotide remains uncertain.

The search for scrapie agent nucleic acid

Despite decades of research, the identity of the scrapie agent has remained elusive. Recent studies have discovered much about the influence of the host genome upon scrapie infection, yet relatively little is known about the causative agent itself. The predominant hypothesis in the scrapie field (the prion hypothesis) argues that the disease is the result of an infectious protein and that nucleic acid is not required for infection. Biological studies of the scrapie agent, however, suggest that a nucleic acid may be involved in the disease. Sensitive molecular biology techniques have yet to identify this putative nucleic acid.

Prion liposomes

Images

Scrapie prion liposomes and rods exhibit target sizes of 55,000 Da

Scrapie is a degenerative neurologic disease in sheep and goats which can be experimentally transmitted to laboratory rodents. Considerable evidence suggests that the scrapie agent is composed largely, if not entirely, of an abnormal isoform of the prion protein (PrPSc). Inactivation of scrapie prions by ionizing radiation exhibited single-hit kinetics and gave a target size of 55,000 +/- 9000 mol wt. The inactivation profile was independent of the form of the prion. Scrapie agent infectivity in brain homogenates, microsomal fractions, detergent-extracted microsomes, purified amyloid rods, and liposomes exhibited the same inactivation profile. Our data are consistent with the hypothesis that the infectious particle causing scrapie contains approximately 2 PrPSc molecules.