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Technical Advances |

From the Institut für Neuropathologie,*
Georg-August-Universität Göttingen, Göttingen; and
Bundesforschungsanstalt für Viruskrankheiten der
Tiere,
Tübingen, Germany
| Abstract |
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| Introduction |
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The definite diagnosis of prion diseases is based on histological investigations of brain tissue or the biochemical detection of the protease-resistant isoform of the prion protein (PrPSc).2 In idiopathic (sporadic) CJD PrPSc has been detected exclusively in tissues of the central nervous system. In sheep with scrapie and patients of nvCJD, however, PrPSc has been detected in the tonsils.3,4 In one case of nvCJD PrPSc was identified in the appendix 8 months before the onset of clinical disease.5
Three techniques with various inherent advantages and disadvantages are used for PrPSc-based diagnosis of CJD, BSE, and scrapie: 1) Immunohistochemistry, which is highly sensitive, shows superior anatomical resolution and can be performed on formalin-fixed and paraffin-embedded material. 2) The Western blot technique may be more sensitive and has the additional advantage of showing the different PrPSc types in CJD.6 Western blots require unfixed tissue and reveal no anatomical details. 3) The histoblot technique combines sensitive protein detection with anatomical tissue preservation. It has been used extensively in prion research; it seems useful for the detection of small amounts of PrPSc in brain tissue and is believed to be even more sensitive than Western blots.7,8 A definite disadvantage of the histoblot technique is its requirement for unfixed material; in the vast majority of routine and archival cases only formalin-fixed tissue is available in human and veterinary pathology.
We reasoned that by combining particular technical aspects of immunohistochemistry with the histoblot technique it might be possible to increase the sensitivity while preserving satisfactory anatomical detail. Here we present the results of this new technique, the paraffin-embedded tissue blot (PET blot), which has advantages over conventional immunohistochemical, Western blot, and histoblot techniques.
| Materials and Methods |
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Formalin-fixed and unfixed frozen brain tissue was taken from a large CJD surveillance study in Germany. In CJD cases the PrP gene (PRNP), including the polymorphism for methionine (M) and valine (V) at codon 129, was characterized.9 PrPSc types were investigated in 31 cases as described by Parchi et al.6 The PET blot technique was performed on 42 cases of sporadic CJD (sCJD) and included all variations of the polymorphism at codon 129 of PRNP, MM homozygotes (15 cases), MV heterozygotes (13 cases), and VV homozygotes (14 cases). All combinations of these three genotypes with PrPSc types 1 and 2 were investigated. The 24 controls were Alzheimer disease (12 cases), multiinfarct dementia (three cases), cerebral lymphoma (three cases), hypoxic encephalopathy (two cases), and one case each of bacterial meningitis, dementia with Lewy bodies, frontotemporal dementia, and Alper disease. For comparative studies the PET blot was used to investigate various prion diseases of animals. Weanling mice were intracerebrally infected with the ME7 scrapie strain and sacrificed sequentially at days 30, 60, 90, 120, and 150 and at the terminal stage 177 days after infection. In addition, a small number of brains from BSE-infected cattle and scrapie-infected sheep were investigated. Normal animal brains were used as controls.
PET Blot
Formalin-fixed brain tissue was cut into 2-mm-thick tissue blocks, decontaminated in concentrated formic acid for 1 hour, postfixed in 4% phosphate-buffered saline-buffered formalin for 48 hours following the protocol of Brown et al,10 and embedded in paraffin. Sections (57 µm) were cut on a microtome, placed in a water bath (55°C), collected on a prewetted 0.45-µm-pore nitrocellulose membrane (Bio-Rad, Richmond, CA), and dried for at least 30 minutes at 55°C. The nitrocellulose membrane was deparaffinized with xylene. Xylene was replaced with isopropanol, followed by stepwise rehydration. Tween 20 was added at a final concentration of 0.1% to the last rehydration step in distilled H2O. Membranes were dried and stored at room temperature for months without loss of quality of subsequent PrPSc staining.
After prewetting with TBST (10 mmol/L Tris-HCl, pH 7.8; 100 mmol/L NaCl; 0.05% Tween 20) digestion was performed with 250 µg/ml proteinase K (Boehringer) in PK-buffer (10 mmol/L Tris-HCl, pH 7.8; 100 mmol/L NaCl; 0.1% Brij 35) for 8 hours at 55°C. With this step the membrane-attached proteins were fixed to the membrane. After washing three times with TBST, the proteins on the membranes were denatured with 3 mol/L guanidine isothiocyanate in 10 mmol/L Tris-HCl (pH 7.8) for 10 minutes. Guanidine was washed out three times with TBST. Immunodetection was performed after preincubation in blocking solution (0.2% casein in TBST) for 30 minutes. The monoclonal antibodies 3F4 (1:2500),11 Gö138 (1:300),2 3B5 (1:50), and 12F10 (1:50)12 were used as primary antibodies in the blocking solution. Incubation was for at least 1 hour. After three washes in TBST, incubation for at least 1 hour was performed with an alkaline phosphatase-coupled rabbit anti-mouse antibody (Dako, Hamburg) at a dilution of 1:500. After five washes in TBST for 10 minutes, the membranes were adjusted to alkaline pH by incubating two times for 5 minutes in NTM (100 mmol/L Tris-HCl, pH 9.5; 100 mmol/L NaCl; 50 mmol/L MgCl2). The visualization of the antibody reaction was provided by formazan reaction using NBT/BCIP. Blots were evaluated with an Olympus dissecting microscope.
Histoblot
Ten-micron frozen sections were cut on a Reichart-Jung cryomicrotome, mounted on nitrocellulose membranes (0.45-µm pore size; Bio-Rad), and lysed in a buffer containing 10 mmol/L EDTA, 0.5% Na-desoxycholate, and 0.5% Nonidet P-40 in 10 mmol/L Tris-HCl and 100 mmol/L NaCl. Proteinase K digestion was performed at 55°C for 8 hours with 50 µg/ml proteinase K (Boehringer). Proteins were denatured using 3 mol/L guanidine isothiocyanate. Immunodetection was performed as described above.
Immunohistochemistry
Immunohistochemical staining of 2-µm microtome sections was performed after hydrolytic autoclaving in 2 mmol/L HCl at 121°C for 30 minutes.13 After incubation with the primary antibody (either Gö 138, 3F4 or 12F10) at 4°C for 18 hours a standard APAAP technique was used for detection. The slides were counterstained with hemalum.
Western Blot
Tissues were homogenized in 9 volumes of lysis buffer (100 mmol/L NaCl, 10 mmol/L EDTA, 0.5% Nonidet P-40, 0.5% sodium deoxycholate, 10 mmol/L Tris, pH 7.4). Aliquots were digested with proteinase K at a concentration of 50 µg/ml at 37°C for 1 hour. Digestion was terminated by the addition of phenylmethylsulfonyl fluoride (2 mmol/L final concentration). Samples were boiled in elec- trophoresis buffer (3% sodium dodecyl sulfate in 60 mmol/L Tris, pH 6.8) and run on a 12% sodium dodecyl sulfate-polyacrylamide gel. Each lane contained the equivalent of 1 mg of tissue or 0.25 µg total protein from the lysate. Western blotting was performed by semidry transfer to a nitrocellulose membrane (0.45 µm; Bio-Rad), and immunodetection was performed using the polyclonal antibody K72,14 followed by goat anti-rabbit IgG (Dako) coupled to alkaline phosphatase. Enzymatic activity was visualized using the CDP-Star chemiluminescent system (Tropix, Bedford, MA) as described by the manufacturer.
| Results |
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To get a better estimate of the sensitivity of this new technique, it
was compared to the histoblot technique, using a series of C57/Bl6 mice
infected with the mouse-adapted scrapie strain ME7 (Figure 2)
. The PET blot first showed a delicate
staining of the anterior part of the thalamus at day 30 after
infection. Starting at day 60, PrPSc was detected
in brainstem nuclei and the rostral cortex. The histoblot showed
delicate PrPSc depositions in the frontal cortex,
starting at day 60. Definite staining of the frontal cortex and
hippocampus was noted at day 90. A detailed anatomical comparison in
this series showed that in some areas a delicate staining was visible
in PET blots 30 days before it was noted in histoblots, and definite
and strong staining as a rule was also first visible in the PET blot 30
days before staining in the histoblot. Thus the PET blot is positive 30
days after infection, very early in the incubation time, and more than
140 days before the appearance of clinical signs 177 days after
infection. In addition, a better anatomical resolution can be achieved
with this new technique.
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| Discussion |
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The major advantages of the PET blot are its extremely high sensitivity (which, as we have shown, is superior to those of immunohistochemistry, Western blotting, and histoblotting), its satisfactory anatomical resolution, and its applicability to formalin-fixed and paraffin-embedded tissue. Thus archival neuropathological material can be studied. In addition, tissues can be decontaminated with formic acid after fixation in formalin, which makes tissue handling safer by several orders of magnitude.10
Although treatment of conventional histological sections for immunohistochemistry with proteinase K is possible, it is not routinely performed by many groups because this treatment has a very destructive effect on tissue sections on glass slides. Instead tissue pretreatment protocols such as hydrolytic autoclaving are used to destroy PrPC, and the remainder is assumed to represent denatured PrPSc. As a result immunohistochemistry without preceding treatment with proteinase K detects an ill-defined mixture of PrPSc and PrPC that may have escaped destruction by tissue pretreatment. The PET blot technique requires tissue pretreatment with high concentrations of proteinase K. As a consequence, only PrPSc is detected by this method. Conventional immunohistochemistry has a better microscopic resolution than the PET blot at the cellular and subcellular levels. However, the PET blots morphological resolution is superior to that of the histoblot technique.
The technique will be useful in neuropathological diagnosis of CJD,
particularly in brain biopsy diagnosis, detection of very small amounts
of PrPSc in tissues as observed in lymphoid
organs in nvCJD patients4
and scrapie in
sheep,3
and to complement conventional
immunohistochemistry, particularly in cases with equivocal results.
This may be of great interest because PrPSc has
been identified in the appendix of a nvCJD patient in the incubation
time months before the onset of clinical signs.5
As our
infection series in mice has shown (Figure 2)
, the PET blot may
facilitate the diagnosis of prion diseases in preclinical or early
clinical stages in peripheral tissue.
In experimental prion research the PET blot may prove to show more clearly the anatomical and temporal distribution of PrPSc deposition in the brain and peripheral tissues. The technique may be used to reinvestigate unclear neurodegenerative diseases from neuropathological archival material. We envisage that PrPSc deposition will be detected in cases where it has so far been difficult or impossible to show in human neurodegenerative diseases that belong to the prion disease group, and we suggest that PET blot analysis will be instrumental in detecting PrPSc in BSE in early stages of the disease or even early in the incubation time in various tissues.
| Acknowledgements |
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| Footnotes |
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Supported by a grant from the Federal Ministry of Health (Bundesgesundheitsministerium).
Accepted for publication October 2, 1999.
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114121 in scrapie-infected mouse neuroblastoma cells leads to trans-dominant inhibition of wild-type PrPSc accumulation. J Virol 1998, 72:1153-1159This article has been cited by other articles:
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