401
J. gen. Virol. (1987), 68, 401-407. Printed in Great Britain
Key words: serapie/incubation period/l mice
Genetic Control of Scrapie: Incubation Period and Plaque Formation
in I Mice
By R I C H A R D
I. C A R P , * R O G E R C. M O R E T Z , M I C H A E L
AND A L A N G. D I C K I N S O N J
NATELLI
New York State Institute Jbr Basic Research in Developmental Disabilities, 1050 Forest Hill
Road, Staten Island, New York 10314, U.S.A. and ~AFRC & MRC Neuropathogenesis Unit,
West Mains Road, Edinburgh EH9 3JF, U.K.
(Accepted 6 October 1986)
SUMMARY
The host component of control of scrapie incubation period in the mouse is
manifested largely through the action of the Sinc gene. Only one mouse strain (VM) has
been found that is pTp7 (prolonged incubation for ME7 agent) and two other strains
have been derived from VM. All other strains, designated s7s7, have a short incubation
for ME7. In the present study, the I strain was shown to fulfil the criteria that are
characteristic of mouse strains with the p7 allele of Sinc: (i) a comparatively long
incubation period for ME7 and a short incubation period for 22A, (ii) the incubation
period for F1 hybrid mice (s7s7 × p7p7) either fell between the incubation periods for
the parental strains (with ME7) or were longer than either parent (with 139A and 22A),
(iii) amyloid plaques occurred following injection of ME7 and 87V but not after 22A or
139A, (iv) lesion profiles for four scrapie strains were similar in I mice and p7p7 mouse
strains, and (v) injection of 87V led to disease in less than 300 days. Finally, allelism
tests using F~ hybrid mice (I x a pTp7 mouse strain) and progeny of backcrosses
between these F1 mice and I mice failed to reveal the segregation of additional major
genes affecting scrapie incubation period.
INTRODUCTION
The mouse gene Sinc, an acronym for scrapie incubation, controls the length of the incubation
period of all known strains of scrapie agent (Dickinson & Meikle, 1971). Almost all inbred
mouse strains carry the s7 allele of Sinc which leads to a comparatively short incubation period
for the ME7 scrapie strain. For inbred mice carrying the p7 allele the incubation period is
prolonged for ME7. Many scrapie strains have a similar pattern in terms of these two alleles, but
other strains (e.g. 22A and 87V) have an opposite pattern with a longer incubation period in s7
than in p7 inbred mouse strains (Dickinson & Meikle, 1969, 1971 ; Dickinson & Fraser, 1977;
Dickinson et al., 1984). Other genes are known to influence incubation period, but their effects
are trivial compared to Sinc; also they have never been found to alter the ranking of incubation
periods in different Sine genotypes (Outram, 1976; Kingsbury et al., 1983; Bruce & Dickinson,
1985). Scrapie strain-mouse strain combinations also differ in various aspects of the brain
lesions, including the presence and frequency of amyloid plaque formation.
The first documented p7 mouse strain (VM) was derived by selective inbreeding from a
colony of mice at the Moredun Institute (Edinburgh, U.K.) in which the s7 and p7 alleles were
segregating. Two additional inbred p7 strains (IM and MB) have been derived from the VM
strain by crossing and selective inbreeding. All other inbred mouse strains thus far tested are
homozygous for s7 (Carp et aL, 1985). In the current work, a commercially available inbred
mouse strain, I, has been shown to carry an allele of Sinc similar to or the same as p7.
0000°7355 © 1987 SGM
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Thu, 15 Jun 2017 19:38:11
402
R. I. CARP AND OTHERS
METHODS
Mice. Both male and female I mice and female C57BL mice were obtained from Jackson Laboratories (Bar
Harbor, Me., U.S.A.); I mice are sometimes listed as I/Ln. IM mice were maintained as an inbred strain in the
Institute for Basic Research in Developmental Disabilities animal colony by brother-sister matings and for these
experiments female mice were used. F1 hybrid mice were produced by crossing C57BL female and I male mice. In
a second series of experiments male and female I x IM F~ hybrid mice were used as well as mice produced from
the backcross of these F~ with I mice. All mice were injected within 1 to 3 weeks of weaning. Mice were
maintained in temperature (21 to 24 °C)- and humidity (40 to 50~)-controlled rooms, with a 12 h on, 12 h offcycle
of artificial light. Food and water were given ad l~bitum.
Inocula
Scrapie strains. The ME7 and 139A scrapie strains had been passaged serially by intracerebral (i.c.) injection
of C57BL female mice. At the time of clinical disease, brains were removed under sterile conditions and 10~o
homogenates prepared in phosphate-buffered saline containing Ca2÷ and Mg2÷ salts (PBS). The same procedure
was followed for the 22A and 87V strains which had been serially passaged in IM mice. All homogenates were
stored at - 70 °C prior to use.
New isolatesfrom scrapie sheep. Scrapie was transmitted to mice from two natural cases in Suffolksheep. One
sheep brain homogenate (10~) was passaged in IM mice (isolate C603), the other in C57BL mice (isolate C608).
Subsequent mouse-to-mousepassages were performed by i.c. injection of 5~ brain homogenates into the same
strain of mouse.
Homogenates (1 ~) prepared from the second mouse passage were analysed in terms of their relative incubation
periods in different strains of mice following i.c. injection.
Injection. For the established scrapie strains, homogenates of mouse brain were prepared in PBS at 10~o
concentration (w/v) and then stored at -70°C. Dilutions were prepared in PBS and mice were injected with
0.03 ml in the right cerebrum.
Incubation period. Mice were monitored weekly for clinical disease starting at 10 weeks post-infection by
observing their activity levels and competence on a set of parallel bars as previouslydescribed (Carp et al., 1984).
The scrapie incubation period was designated as the day after infection on which the mouse had shown clinical
symptoms for the 3rd consecutive week. At this stage they were within 1 to 4 weeks of death.
Histopathological evaluation. The technique for evaluation of vacuolation by lesion profiles has been described
previously (Fraser & Dickinson, 1973). Brains were removed and placed in 10~ neutral formalin for at least 2
days. Dehydration of brains, paraffin embedding and staining with haematoxylin and eosin were done by
standard procedures. Vacuolation was evaluated in nine positions of the grey matter as described (Fraser &
Dickinson, 1973). Amyloidplaque incidence at four slandard levels was assayed in sections stained with Masson's
trichrome.
RESULTS
Incubation periods for four scrapie strains in I mice and in representative s7 and p7 mouse
strains, C57BL and IM respectively, are shown in Table 1. Incubation periods for each of the
four scrapie strains in I mice were close to those seen in IM and quite different from those in
C57BL mice. The crucial comparison for assessing the action of Sinc is the relative incubation
period in s7s7 and pTp7 mice. In contrast, the absolute incubation period in any one genotype is
relatively uninformative on its own, because it is very dose-dependent and influenced to a minor
extent by genes other than Sinc (Outram, 1976; Kingsbury et al., 1983; Bruce & Dickinson,
1985). In the key comparisons, ME7 had a shorter incubation period and 22A a longer
incubation period in C57BL than in either IM or I mice. The 40 to 50 day difference between I
and IM mice with ME7 is of minor importance for the reasons just given. It is, for example, of
the same magnitude as the difference between R i l l and BALB/c mice, both of which carry the
s7 gene (Outram, 1976). Incubation periods for the 139A strain were similar in IM and I, and
significantly longer than in the s7 mouse strains. For 87V, incubation periods in both I and I M
mice were much shorter than for C57BL (Table 1).
Incubation periods in IM and C57BL mice were determined for 1 ~ homogenates prepared
from the second passage of isolates from two sheep, C603 and C608. For C603, the primary
isolation was in IM mice and the second passage also in IM mice, whereas for C608 primary
isolation and second passage were made in C57BL mice. The results (Table 2, experiment 1) show
that the C608 isolate had incubation period values similar to those for the ME7 scrapie strain,
whereas incubation periods for the C603 isolate were quite different, being shorter in the p7
strain (IM) than in the s7 strain (C57BL), results which are characteristic of the 22A group of
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Thu, 15 Jun 2017 19:38:11
Genetic control of scrapie
403
Table 1. Incubation periods of various scrapie strains in C57BL, I M and I mouse strains
Scrapie
strain
ME7
139A
22A
87V
Incubation period (days ±
c
*
C57BL(s7s7) IM (p7p7)
145 ± 1
300 ± 3
125 _+ 1
163 ± 1
356 ± 4
190 ± 1
>600*
274 ± 1
S.E.M.)
",
I
252 ± 3
162 -+ 3
184 ± 4
277 ± 2
* Following i.c. injection of 10% homogenates some C57BL mice became positive for clinical disease between
600 days and the end of their natural life span.
Table 2. Incubation periods of two sheep scrapie isolates in C57BL, I M and I mice
Sheep isolate
C608
C603
,k
Expt.
Mouse
strain
C57BL
IM
C57BL
I
no.
1
2
No. of
mice
6
4
5
3
~k
Incubation period
(days -+ S.~.M.)
151 ± 2
300 ± 8
145 ± 0
255 ± 4
No. of
mice
5
3
6
3
Incubation period
(days -+ S.E.M.)
318 -+ 9
240 -+ 4
287 -+ 11
220 -+ 3
Table 3. Incubation periods of ME7, 139A and 22A in C57BL, land C57BL x I F1 hybrid mice
Incubation period (days) in
f
"~
C57BL
I
~k
Scrapie strain
ME7
139A
22A
No. of
mice
32
20
12
Days _+ S.E.M.
145 ± 2
125 ± 1
356 -+ 4
C57BL x I
*
No. of
mice
10
7
9
Days ± S.E.M.
252 +3
162 +3
184 -+4
A
No. of
mice
30
14
11
Days + S.~.M.
200 -+ 2
176 -+ 3
484 -+ 21
scrapie strains (Outram, 1976). The comparative incubation period values for these sheep
isolates in C57BL and I mice were essentially similar to those seen in the C57BL and IM strains
(Table 2, experiment 2). The C603 strain had a shorter incubation period in I than in C57BL
mice whereas the C608 strain had a shorter incubation period in C57BL than in I mice. Thus, the
results for I mice were similar to those for the representative p7 strain, IM.
In previous studies of incubation periods in s7p7 F 1 hybrid mice, some scrapie strains had
values that were between those for the two parental mouse strains, whereas the F1 incubation
periods for other scrapie strains were longer than the values for either parental mouse strain
(Dickinson & Meikle, 1971). Incubation periods for ME7, 139A and 22A were analysed in
C57BL × I F1 mice and the results (Table 3) were compared to the incubation periods for the
same scrapie strains in the parental mouse strains. The incubation period for ME7, 200 days, fell
midway between the values obtained for the two parental strains, whereas the values for the
139A and 22A strains were higher in FI mice than in either of the parental strains. Similar
relationships have been reported for the incubation periods of these three scrapie strains in
C57BL, IM (and VM) and their F~ cross (Dickinson, 1975; Scott & Dickinson, 1985).
It next needed to be established whether the similarity of control of scrapie incubation period
i s due to the same gene in the I stock as in the IM and VM stocks of mice. Substantial evidence in
favour of this stems from the comparison of incubation periods in I x IM F~ mice with those in
the two parental strains (Table 4): for 22A the F 1 incubation was 187 +_ 3 days, for ME7 it was
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Thu, 15 Jun 2017 19:38:11
404
R. I. CARP AND OTHERS
o
Fig. 1. Fluorescence micrographs of thioflavin S-stained sections from I mice infected with ME7
(a, c, e) and 87V (b, d,f) scrapie agents: (a, b) thalamus, (c, d) corpus callosum, (e0c) cortex. Bar markers
represent 100 ~tm.
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Thu, 15 Jun 2017 19:38:11
405
Genetic" control of scrapie
Table 4. Incubation periods of ME7, 22A and 87V serapie strains in I x IM F1 miee and
F 1 x I backcross mice
Mouse strain
g
Scrapie
strain
ME7
22A
87V
g
IM
c - ~ - - - ~
No.
D a y s + S.~.M.
9
69
105
300 ___ 3
190 + 1
274 + 1
r
No.
I
x
~
D a y s + S.E.M.
10
9
4
252 + 3
184 + 4
277 + 2
"5
•
No.
F~
~
~
D a y s + S.E.M.
,
No.
11
11
10
293 + 4
182 + 3
300 + 5
6
9
7
Backcross
x
D a y s _+ S.E.M.
286 _+ 3
173 + 2
315 ___ 10
293 _+ 4 days, for 87V it was 300 _+ 5 days, and for 139A (data not in table) it was 170 _+ 3 days.
These are essentially similar to the I and IM incubation periods for these scrapie strains in the
non-contemporary experiment shown in Table 1. Backcrossing of the I x IM F~ mice to one of
the parental strains (I) gave no evidence that gene segregation relating to incubation period had
occurred and this is further evidence that the same gene is involved (Table 4).
The pattern of vacuolation intensity at nine positions in the brain is termed the lesion profile
(Fraser & Dickinson, 1973; Fraser, 1976). The appearance of the profiles for ME7, 22A, 87V
and 139A were similar in I and IM mice. The differences were no greater than those seen within
other groupings of p7p7 or s7s7 mouse strains (Fraser, 1971).
An analysis of plaque formation in I mice yielded results that were similar to those seen in IM
mice and other p7 inbred strains. The 22A and 139A strains failed to cause plaque formation in
either I or IM mice. In contrast, ME7 and 87V produced extensive plaque formation in I mice.
In Fig. 1, sections of thalamus, corpus callosum and cortex are shown for ME7 (Fig. 1 a, c, e) and
87V (Fig. lb, d,f). The extent of plaque formation in I mice injected with 87V and ME7 was
similar to that seen in IM mice injected with these strains. The results for the 139A and 87V
strains in ! mice cannot be used to distinguish the Sine genotypes since 139A does not form
plaques in mouse strains carrying either the s7 or p7 allele and 87V produces plaques in both. In
contrast, the complete absence of plaques in 22A-inJected I mice and the presence of extensive
plaque formation in ME7-injected I mice is different from results obtained in s7s7 strains in
which 22A causes plaques and ME7 produces few if any.
DISCUSSION
The incubation period characteristics for all the scrapie strains tested clearly show that I mice
carry an allele of Sine either identical to p7 or a new allele very similar to it. In comparison to
data obtained with s7 mouse strains such as C57BL, the relatively long incubation period for
ME7 and the relatively short incubation period for 22A is the criterion that establishes the
similarity of I mice to a p7 strain. The close similarity is confirmed by the patterns of dominance
and overdominance displayed in F1 mice (I x C57BL and IM x C57BL) with scrapie strains
ME7, 22A and 139A. The incubation period data obtained with I x IM FI hybrid mice were
similar with each scrapie strain to those observed in the parental stocks. This is strong evidence
that the same gene, namely Sine and not a p7-1ike mutant at a different locus, is operative.
Further evidence on this point is provided by the fact that incubation periods for the progeny of
a backcross between I and the I x IM F1 mice yielded incubation periods identical to those seen
in both I and IM mice.
All other data support the above conclusion. First, 87V caused disease in I mice in less than
300 days, whereas in s7 strains 87V does not cause disease before at least 600 days, even after i,e.
injection with 10 ~ brain homogenate. Second, there was extensive amyloid plaque formation in
ME7-injected I mice, a pathological lesion found in ME7-p7p7 but absent or at very low
incidence in ME7-s7s7 combinations. Also, 22A did not produce plaques in I mice, whereas this
strain does produce plaques in mice carrying s7. Third, the lesion profiles for I mice with ME7,
22A, 87V and 139A were broadly similar to those seen in p7 strains and quite distinct from s7
strains. Finally, the relationship of incubation periods of two new sheep isolates were similar in I
compared to C57BL as in IM compared to C57BL mice.
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Thu, 15 Jun 2017 19:38:11
406
R . I, C A R P
AND OTHERS
In a previous study (Kingsbury et al., 1983) four I mice (referred to as ILN/J), injected more
than 200 days previously with 'Chandler agent', died suddenly without any neurological
symptoms characteristic of scrapie. No histopathological findings for these mice were given
although they were said to have died from scrapie. In the present study, I mice at the clinical
stage of scrapie showed typical scrapie symptoms, with locomotor and coordination signs the
earliest seen (Carp et al., 1984). After their third consecutive positive score, the I mice tended to
die more quickly than other strains, usually within 1 or 2 weeks.
Extensive searches of available inbred lines of mice and a few randomly bred colonies
(Outram, 1976; Carp et al., 1985 ; A. G. Dickinson, unpublished) had hitherto failed to find Sinc
alleles other than s7, apart from the initial discovery of p7 in a randomly bred colony which had
been used since the 1940s for work with scrapie (Dickinson & Meikle, 1971). The original p7
allele and that in the I mice must have arisen as independent mutations because the stocks
involved have had completely separate histories. The I strain originated in 1926 in the U.S.A.
(Festing, 1979); the VM strain, produced as a carrier of the p7 allele, was inbred in Edinburgh,
U.K. from randomly bred animals stemming from the Institute of Animal Genetics in
Edinburgh, which has never imported the I strain.
At present, the function of the Sinc gene in uninfected animals remains unknown, as does the
chemical nature of any product for which it may code. Furthermore, the mechanism of action of
the putative Sinc gene product, in determining the phenotypic expression seen following
infection with various scrapie strains, is still a matter for speculation. It is known that the gene
controls incubation period, regardless of the route of infection and that it acts throughout the
incubation period, rather than only on the primary infective events (Dickinson & Fraser, 1977).
It has been postulated that there are a finite number of replication sites for scrapie agent, coded
by Sinc, and variation between the alleles presumably relates to the specificity of these sites for
different strains of scrapie (Dickinson & Outram, 1979). The nature of the scrapie agent is still
unknown, apart from substantial evidence that it has an independent genome which can mutate
(Bruce & Dickinson, 1979; Dickinson et al., 1984; Scott & Dickinson, 1985). A small
nucleic acid with only regulatory functions and protected by host-specified components is a
plausible model for the agent (the virino hypothesis: Dickinson & Outram, 1983). The
specificity of Sinc for different scrapie strains could relate to specific interactions with sequences
of the agent genome. One possibility, though not particulary likely, is that agent replication is at
the level of the host chromosome (Dickinson & Meikle, 1969). If this were so, the possibility has
had to be entertained that a mutation from s7 to p7 in Edinburgh might have been induced
directly by scrapie agent. One important outcome of the present work is that it excludes this
explanation.
It has recently been reported (Carlson et al., 1986), using restriction fragment length
polymorphism analysis of I mice, that the gene coding for the precursor to the protein (PrP)
found in scrapie-associated fibrils (SAF) (Merz et al., 1981 ; Bolton et al., 1982; Oesch et al.,
1985; Chesebro et al., 1985; Robakis et al., 1986) is on the same chromosome as Sinc. It will be
important for understanding the control of scrapie agent replication to establish whether the
SAF protein determinant and Sinc are part of a closely linked, organized genetic system or even
a single gene with the precursor of the SAF protease-resistant protein as the Sinc product.
The existence of additional inbred strains carrying the p7 allele in a genetic background
different from that in VM (and its derivatives, IM and MB) will assist in investigation of the
mechanism of Sinc action in controlling the overall dynamics of agent replication. Some strains
of scrapie (e.g. 87V) have a high amyloid plaque incidence in I mice, which will assist in cerebral
amyloid investigations. Finally, the I strain can be used as a substitute for VM or IM mice in
strain-typing new scrapie isolates.
The authors to thank Mrs Adele Monaco for her excellent assistance in preparation of the manuscript and Ms
Sharon Callahan for her technical expertise. This work was supported in part by grant NS21349-02.
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Thu, 15 Jun 2017 19:38:11
Genetic control o f scrapie
407
REFERENCES
BOLTON, D. C., McKINLEY,M. P~ & PRUSINER,S. B. (1982). Identification of a protein that purifies with the scrapie
prion. Science 218, 1309-1311.
BRUCE, M. E. ~ DICKINSON, A. ~. (1979). Biological stability of different classes of scrapie agent. In Slow
Transmissible Diseases of the Nervous System, vol. 2, pp. 71 86. Edited by S. B. Prusiner & W. J. Hadlow. New
York: Academic Press.
BRUCE, M. E. & DICKINSON,A. G. (1985). Genetic control of amyloid plaque production and incubation period in
scrapie-infected mice. Journal of Neuropathology and Experimental Neurology 44, 285-294.
CARLSON, G. A,, GOODMAN, P., KINGSBURY, D. T. & PRUSINER, S. B. (1986). Scrapie incubation time and prion protein
genes are linked. Clinical Research 34, no. 2, 675A.
CARP, R. I., CALL~N, S. ~., SERSEN,E. A. & MORETZ, R. C. (1984). Preclinical changes in weight of scrapie-infected
mice as a function of scrapie agent-mouse strain combination. Intervirology 21, 61-69.
CARP, R. I., MERZ, P. A., MORETZ, R. C., SOMERVILLE, R. A., CALLAHAN, S. M. & WlSNIEWISK1, H. M. (1985). B i o l o g i c a l
properties of scrapie: an unconventional slow virus. In Subviral Pathogens of Plants and Animals: Viroids and
Prions, pp. 425 464. Edited by K. Maramorosch & J. J. McKelvey. New York: Academic Press.
CHESEBRO, B., RACE, R., WEHRLY, K., NISHIO, J., BLOOM, M., LECHNER, D., BERGSTROM, S., ROBBINS, K., MAYER, L.,
KEITn, J. M., GARON, C. 8, nAASE,A. (1985). Identification of scrapie prion protein-specific m R N A in scrapieinfected and uninfected brain. Nature, London 315, 331-333.
DICKINSON, A. G. (1975). Host-pathogen interactions in scrapie. Genetics 79, 387-395.
DICKINSON, A. G. & FRASER, H. (1977). Scrapie pathogenesis in inbred mice: an assessment of host control and
response involving many strains of agent~ In Slow Virus Infections of the Central Nervous System, pp. 3 14.
Edited by V. ter Meulen & M. Katz. New York: Springer-Verlag.
DICKINSON,A. G. & MEIKLE,V. M. H. (1969). A comparison of some biological characteristics of the mouse-passaged
scrapie agents, 22A and ME7. Genetical Research 13, 213-225.
DICKINSON, A. G. & MEIKLE, V. M. H. (1971). Host genotype and agent effects in scrapie incubation: change in allelic
interaction with different strains of agent. Molecular and General Genetics 112, 73-79.
DICKINSON, A. G. & OUTRAM, ~. W. (1979). The scrapie replication site hypothesis and its implications for
pathogenesis. In Slow Transmissible Diseases of the Nervous System, vo]. 2, pp. 13-32. Edited by S. B. Prusiner
& W. J. Hadlow. New York: Academic Press.
DICKINSON, A. G. & OUTRAM,G. W. (1983). Operational limitations in the characterization of the infective units of
scrapie. In Virus Non Conventionnels et Affections du Syst~me Nerveux Central, pp. 3-16. Edited by L. A. Court
& F. Cathala. Paris: Masson.
DICKINSON, A. G., BRUCE, M. E., FRASER, H. & K1MBERL1N, R. H. (1984). Scrapie strain differences: the implications of
stability and mutation. In Proceedings of Workshop on Slow Transmissible Diseases, pp. 105 118. Tokyo:
Japanese Ministry of Health and Welfare.
FESTING, M. F. W. (1979). Inbred Strains in Biomedical Research. London: Macmillan.
FRASER,H. (1971). Scrapie : the experimental disease in inbred strains of mice. Ph.D. thesis, University of Edinburgh.
FRASER,R. (1976). The pathology of natural and experimental scrapie. In Slow Virus Diseases of Animals and Man,
pp. 267-305. Edited by R. FI. Kimberlin. Amsterdam: North-Holland.
FRASER, H. & DICKINSON,A. G. (1973). Scrapie in mice: agent-strain differences in the distribution and intensity of
grey matter vacuolation. Journal of Comparative Pathology 83, 29-40.
KINGSBURY, D. T., KASPER, K. C., STITES, D. P., WATSON, J. D., HOGAN, R. N. & PRUSINER, S. B. (1983). Genetic control
of scrapie and Creutzfeldt-Jakob disease in mice. Journal of Immunology 131,491-496.
MERZ, P. A., SOMERVILLE, R. A., WISNIEWSKI, H. M. & IQBAL, K. (1981). Abnormal fibrils from scrapie-infected brain.
Aeta neuropathologiea 54, 63-74.
OESCH, B., WESTAWAY, D~, WALCHLI, M~, McKINLEY, M. P., KENT, S. B. H., AEBERSOLD, R., BARRY, R. A., TEMPST, P.,
TEPLOW, D. B., HOOD, L. E., PRUSINER, S. B. & WEISSMANN, C. (1985). A cellular gene encodes scrapie PrP 27-30
protein. Cell 40, 735-746.
OUTRAM, G. W. (1976). The pathogenesis of scrapie in mice. In Slow Virus Diseases of Animals and Man, pp. 325358. Edited by R. H. Kimberlin. Amsterdam: North-Holland.
ROBAKIS, N. K., SAWH, P. R., WOLFE, G. C., RUBENSTEIN, R., CARP, R. I. & INNIS. M. A. (1986). Isolation of a cDN A clone
encoding the leader peptide of prion protein and expression of the homologous gene in various tissues.
Proceedings of the National Academy of Sciences, U.S.A. 83, 6377-6381.
SCOTT, J. S. & DICKINSON, A. G. (1985), A view of dementia from the standpoint of unconventional infections.
Gerontologia biomedica acta 1, 127-150.
(Received 17 July 1986)
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Thu, 15 Jun 2017 19:38:11