Protective Effect of Val129-PrP against Bovine Spongiform Encephalopathy but not Variant Creutzfeldt-Jakob Disease
Natalia Fernández-Borges,1 Juan Carlos Espinosa,1 Alba Marín-Moreno, Patricia Aguilar-Calvo, Emmanuel A. Asante, Tetsuyuki Kitamoto, Shirou Mohri, Olivier Andréoletti, Juan María Torres
Bovine spongiform encephalopathy (BSE) is the only known zoonotic prion that causes variant Creutzfeldt-Jakob disease (vCJD) in humans. The major risk determinant for this disease is the polymorphic codon 129 of the human prion protein (HuPrP), where either methionine (Met129) or valine (Val129) can be encoded. To date, all clinical and neuropathologically confirmed vCJD cases have been Met129 homozygous, with the exception of 1 recently reported Met/Val heterozygous case. Here, we found that transgenic mice homozygous for Val129 Hu-PrP show severely restricted propagation of the BSE prion strain, but this constraint can be partially overcome by adaptation of the BSE agent to the Met129 Hu-PrP. In addition, the transmission of vCJD to transgenic mice homozygous for Val129 Hu-PrP resulted in a prion with distinct strain features. These observations may indicate increased risk for vCJD secondary transmission in Val129 Hu-PrP–positive humans with the emergence of new strain features.
We report a detailed comparison of the transmission properties of BSE and vCJD prions among humanized transgenic mice with different PRNP codon 129 genotypes. Because a high expression level of PrP in transgenic mice directly influences prion disease susceptibility and incubation time, these transgenic mice have an advantage over knock-in mice for evaluating these features in the different human PrP genotypes. In addition, the 3 mouse models used in our study have equivalent PrP expression levels, making them suitable for studying comparative susceptibilities across the different PRNP codon 129 genotypes.
In previous reports, we demonstrated that Met129 homozygous individuals might be susceptible to a sheep or goat BSE agent to a higher degree than to cattle BSE and that these agents might transmit with molecular and neuropathological properties indistinguishable from those of vCJD (31). In this study, we wanted to extend these results to the other human PRNP genotypes: Met/Val129 and Val/ Val129. We gained a different perspective when several BSE isolates adapted to different species inoculated in TgVal129 mice showed an apparent lack of transmission. In addition, almost all inoculated TgMet/Val129 mice did not transmit BSE; this finding supports the interpretation by Wadsworth et al. that human PrP Val129 severely restricts propagation of the BSE prion strain (27).
An unexpected result of this study was the finding that 1 BSE isolate from a goat (Ca-BSE/Go) was clinically transmitted to 1 of 10 TgMet/Val129 mice and subclinically transmitted to TgVal129 mice. This particular isolate is characterized by a high infectious titer (35) that could explain the potential for this inoculum to overcome the restriction on BSE prions to propagate in TgVal129 mice.
Although cattle BSE did not transmit to TgMet/Val129 mice directly, adaptation of the BSE agent to human PrP Met129 sequence and subsequent inoculation of the resultant vCJD prions to TgMet/Val129 mice produced a 100% attack rate. However, we did not detect clinical prion disease, supporting a slower rate of vCJD conversion compared with that among TgMet129 mice. This slow but potential conversion rate in TgMet/Val129 mice correlates well with the single vCJD case of a human carrying the PrP Met/Val129 genotype (22) and with the description of subclinical secondary transmissions through human vCJD–infected tissues (4–7,47).
TgVal129 mice challenged with Hu-vCJD2 did not produce detectable brain PrPres and clinical signs, in spite of the overexpression of HuPrP-Val129 and the use of the more efficient intracerebral route of infection. However, subclinical infection in these TgVal129 mice was demonstrated in BoPrP-Tg110 mice. These data suggest that PrP Val129 could sustain a very slow and limited vCJD conversion rate that is consistent with the detection of PrPres in tonsils and appendixes of asymptomatic PrP Val129 persons (23–25). Previous studies of other transgenic mice expressing PrP Val129 have also shown a low transmission efficiency of vCJD (2,27,30).
The fluctuating subclinical transmissibility of both vCJD inocula in TgVal129 mice (negative for Hu-vCJD1 and positive for Hu-vCJD2 ) might be caused by differences in prion titer between inocula. This assessment was strengthened after the transmission of both vCJDs to TgMet129 mice, in which a shorter incubation period was observed in animals inoculated with Hu-vCJD2 . A certain variability in subclinical transmissibility and incubation time between different vCJD isolates is not uncommon, as has been previously reported (2,27,30), suggesting that a Val129 transmission barrier can only be overcome with highly infectious vCJD isolates.
The dramatic changes in the susceptibility of TgVal129 mice (Table 3) challenged with vCJD isolates first passaged in TgMet129 mice suggest an apparent increase in titer of both vCJD prion isolates; however, adaptation of the inocula to the new host mouse cannot be disregarded as being partly responsible for this increased susceptibility. We observed a 100% infection rate, but without clinical signs of prion disease. We observed similar transmission features when we passaged vCJD in TgMet/Val129 mice. In addition, the apparent PrPVal129 restricted propagation of cattle BSE and BSE from other species was completely abolished after its adaptation to human PrPMet129.
Although PrP overexpression and the inoculation route can affect transmission efficiency, our results and those previously reported in both overexpressing and knock-in transgenic mice (2,27,30) suggest that the Val129 PrP variant could sustain a very slow and limited vCJD conversion rate, and is unable to completely prevent vCJD transmission. Biochemical and neuropathological features of vCJD transmission to TgVal129 mice showed substantial differences compared to TgMet129 or TgMet/ Val129 mice. Similar to previous reports (2,27,28,48), a type 5 PrPSc associated with very weak and diffuse PrP plaques without a florid morphology was the hallmark among these mice. In addition, our demonstration of previously unreported type 5 PrPSc in brain samples of vCJDchallenged knock-in Ki-Hu129V/V mice (30) establishes that the evolution of type 5 PrPSc associated with the transmission of vCJD prions to the Val129 genotype is not an artifact of PrP overexpression. This finding further reinforces the specific biochemical features of vCJD when transmitted to the human-PrP Val129 sequence.
Extrapolation of results from prion transmission studies based on transgenic mice has to be done with caution, especially when human susceptibility to prions is analyzed. However, our results clearly indicate that PrPVal129 individuals are highly resistant to transmission of cattle BSE or BSE passaged in other species. Also, PrPVal129 individuals might be susceptible to infection with human-passaged BSE (vCJD) prions, and the propagated agents might transmit with molecular and neuropathological properties distinguishable from those of type 4 PrPres. Although the resultant type 5 PrPSc shares the same fragment sizes as those of type 2 PrPSc, the 2 PrPSc types can be distinguished by the predominance of the diglycosylated glycoform associated with type 5 PrPSc. Overall, our results indicate that human Val129-PrP polymorphic variant is a strong molecular protector against BSE zoonotic transmission but fails to prevent human-to-human vCJD transmission. Because potential late-onset vCJD cases could appear in the population (49,50) these findings underline the need for continued investigation of all forms of human prion disease.
''Overall, our results indicate that human Val129-PrP polymorphic variant is a strong molecular protector against BSE zoonotic transmission but fails to prevent human-to-human vCJD transmission. Because potential late-onset vCJD cases could appear in the population (49,50) these findings underline the need for continued investigation of all forms of human prion disease.''
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