- Email: [email protected]
SCRAPIE-ASSOCIATED FIBRILS
36 R. G. ROHWER:
Letters
to
the Editor
SIR,-Recent advances in our understanding of scrapie and the scrapie-like diseases, Creutzfeldt-Jakob disease (CJD) and kuru, are not easy to follow for those not directly involved, and there seems to be a widespread misconception that the scrapie agent has been isolatedl’l and that it is a "prion".’ This is incorrect. As yet there is that can be clearly associated with this term. consistent story is emerging. An excellent candidate for the scrapie agent is "scrapie associated fibrils" (SAF), first described in 1981 by P. Merz and her colleagues.4SAF are structurally similar to but distinguishable from amyloid subunits and paired helical filaments4and resemble the filamentous viruses of plants and protists. The following evidence points to SAF as the likely repository of scrapie infectivity: no
structure
a
(1) SAF have been observed only in the infected tissues of scrapie and the other subacute spongiform virus encephalopathies (SSVE), CJD and kuru, and never in a comprehensive series of controls including patients with Alzheimer’s disease, amyotrophic lateral sclerosis, and chemical and 5 virological diseases which resemble SSVE (2) SAF concentration correlates well with infectivity titres. Fractions that are highly enriched for SAF also contain large amounts and the concentration of brain-recoverable SAF increases in time as infectivity titres increase.7 (3) The finding of SAF in the spleen ofscrapie-mfected miceindicates that the fibrils, while similar in appearance to the ultrastructural subunits of brain amyloid, are not simply products of the degenerating brain, as has been suspected for brain amyloid, and could be the casual agents of its degeneration products of the degenerating brain.
histopathologically. of infectivity, 1,6
SAF have been isolated in four separate laboratories4-6,8 and a has been achieved by H. Diringer and his
significant purification
group.6,9
In 1982 S. B. Prusiner proposed his "prion" hypothesis.3Later that year D. C. Bolton in his laboratory identified a low-molecular-weight protein. However, this protein, 10, 1 while having a structure hypothesised for the "prion", is not itself infectious. Latterly Prusiner et all have also reported SAF-like structures which they call "prion-rods" presuming them to be large aggregates of prions. However, the featureless, rod-like appearance of this material is just that expected for SAF visualised by rotary shadowing, and when negatively stained it is indistinguishable from
scrapie-specific,
SAF.’
.
Fractions highly enriched for SAF(or for "rods"’) are also enriched for the scrapie-specific, low-molecular-weight, noninfectious protein mentioned above. This protein has only been demonstrated after protease treatment of SAF-containing fractions followed by denaturation and electrophoresis. It is reasonable to expect that this protein may be a subunit of SAF. A picture is emerging of an SSVE-specific ultrastructural feature, the SAF, which has viral dimensions, is likely to be infectious, and from which a structural protein has probably been identified. Moreover, other work 13,14 has provided simple explanations for the apparent small size and resistance to inactivation reported for the scrapie agent. By comparison, the "prion" remains a vague, yet to be identified, structure now postulated to be larger than the scrapiespecific protein but much smaller than SAF. Prusiner’s laboratory has made important contributions to the scrapie story but their use of the conjectural term prion in describing and interpreting these results has served only to confuse the picture, and their use of "prion-rods" to describe what are almost certainly SAF is unwarranted. Laboratory of Central Nervous System Studies, National Institutes of Health, Bethesda, Maryland 20205, USA.
particles cause scrapie. Science 1982; 216: 136-44. 4. Merz PA, Somerville RA, Wisniewski HM, Iqbal K. Abnormal fibrils from scrapie infected brain. Acta Neuropathol (Berlin) 1981; 54: 63-74. 5. Merz PA, Rohwer RG, Kascsak R, Wisniewski HM, Somerville RA, Gibbs CJ Jr, Gajdusek DC. An infection specific particle from the unconventional slow virus diseases. Science (in press). 6. Diringer H, Gelderblom H, Hilmert H, Ozel M, Edelbluth C, Kimberlin R. Scrapie infectivity, fibrils and low molecular weight protein. Nature 1983; 306: 476-78. 7. Merz PA, Somerville RA, Wisniewski HM. Abnormal fibrils in scrapie and senile dementia of the Alzheimer type. In: Court LA, Cathala F, eds Virus non conventionnels et affections du système nerveux central. Paris: Masson, 1983: 259-81 8. Merz PA, Somerville RA, Wisniewski HM, Manuelidis L, Manuelidis EE. Scrapieassociated fibrils in Creutzfeldt-Jakob disease. Nature 1983; 306: 474-76. 9. Diringer H, Hilmert H, Simon D, Werner E, Ehlers B. Towards purification of the scrapie agent. Eur J Biochem 1983; 134: 555-60. 10. Bolton DC, McKinley MP, Prusiner SB. Identification ofa protein that purifies with the scrapie prion. Science 1982; 218: 1309-11. 11. McKinley MP, Bolton DC, Prusiner SB. A protease-resistant protein is a structural component of the scrapie prion. Cell 1983; 35: 57-62. 12. Prusiner SB, Bolton DC, Groth DF, Bowman KA, Cochran SP, McKinley MP Further purification and characterization of scrapie prions. Biochemistry 1982; 21: 6942-50. 13. Rohwer RG. Scrapie infectious agent is virus-like in size and susceptibility to inactivation. Nature 1984; 308: 658-62. 14. Rohwer RG. Virus-like sensitivity of the scrapie agent to heat inactivation. Science 1984; 223: 600-02. 3. Prusiner SB. Novel proteinaceous infectious
SCRAPIE-ASSOCIATED FIBRILS
Nevertheless,
REFERENCES—continued
ROBERT G. ROHWER
SB, McKinley MP, Bowman KA, Bolton DC, Bendheim PE, Groth DF, Glenner GG. Scrapie prions aggregate to form amyloid-like birefringent rods. Cell 1983; 35: 349-58. 2. Prusiner SB. Some speculations about prions, amyloid, and Alzheimer’s disease. N Engl J Med 1984; 310: 661-63.
CAPTOPRIL IN RHEUMATOID ARTHRITIS
SIR,-We read with interest the report by Dr Martin and colleagues (June 16, p 1325) of an uncontrolled study of captopril as a disease-suppressive agent in rheumatoid arthritis (RA). We are, however, concerned about the possible prescription for RA of this potent agent by physicians inexperienced in the use of convertingenzyme inhibitors. Though Martin et al saw only mild hypotension (enough, however, to lead to withdrawal in one patient) and indeed did not report erect or supine blood pressures in their patients, the addition of captopril to thiazide or loop diuretic therapy for indications other than hypertension would probably provoke severe hypotension. Sodium or volume depeletion (eg, after a gastrointestinal bleed or diarrhoea) would do the same. Thus, if captopril is to be prescribed for RA, clear warnings about concurrent medication and action in the event offluid loss must be given both to the patients and to other medical attendants. We cannot agree with Martin’s conclusions on toxicity. Though the strict claim that "the frequency of side-effects was lower than seen in many original clinical studies in which high doses were used" may be true, the occurrence ’of 2 transient rashes and 1 withdrawal due to a rash (ie, 3/16 patients or 19%) compares poorly with their cited average incidence of 12%, probably itself an overestimate compared with recent data.23 instances of dysgeusia in 15 patients again represents an incidence of 20%, compared with their cited average of 6%. The doses they used (75-150 mg) are certainly not high, but are higher than many physicians now advise and use for treatment of essential hypertension. Though all their patients had normal renal function, it must be emphasised that renal among patients with RA) requires a impairment (relatively common reduced dose of captopril.3 Two other points should be noted. The chemical structure of penicillamine they illustrate incorrectly shows a terminal -CH2. D-penicillamine is &bgr;,&bgr;-dimethy1cysteine. Also the structural, chemical, clinical, and toxicological similarities between thiols have penicillamine, sodium aurothiomalate, and many been discussed at length over the past few years.4-9The apparent importance of the - SH group has been repeatedly noted and, in 1981, Merlet et allo reported an uncontrolled study of captopril in 13 patients with RA, again claiming a beneficial effect. We would thus urge caution in the use of this valuable and potent agent for a condition for which it was not designed and for which it does not appear likely to be superior to other agents.
other
1. Prusiner
Dunn Laboratories, Medical Professorial Unit, St Bartholmew’s Hospital, London EC1A 7BE
P. L. DRURY D. PERRETT
Letters
to
the Editor
SIR,-Recent advances in our understanding of scrapie and the scrapie-like diseases, Creutzfeldt-Jakob disease (CJD) and kuru, are not easy to follow for those not directly involved, and there seems to be a widespread misconception that the scrapie agent has been isolatedl’l and that it is a "prion".’ This is incorrect. As yet there is that can be clearly associated with this term. consistent story is emerging. An excellent candidate for the scrapie agent is "scrapie associated fibrils" (SAF), first described in 1981 by P. Merz and her colleagues.4SAF are structurally similar to but distinguishable from amyloid subunits and paired helical filaments4and resemble the filamentous viruses of plants and protists. The following evidence points to SAF as the likely repository of scrapie infectivity: no
structure
a
(1) SAF have been observed only in the infected tissues of scrapie and the other subacute spongiform virus encephalopathies (SSVE), CJD and kuru, and never in a comprehensive series of controls including patients with Alzheimer’s disease, amyotrophic lateral sclerosis, and chemical and 5 virological diseases which resemble SSVE (2) SAF concentration correlates well with infectivity titres. Fractions that are highly enriched for SAF also contain large amounts and the concentration of brain-recoverable SAF increases in time as infectivity titres increase.7 (3) The finding of SAF in the spleen ofscrapie-mfected miceindicates that the fibrils, while similar in appearance to the ultrastructural subunits of brain amyloid, are not simply products of the degenerating brain, as has been suspected for brain amyloid, and could be the casual agents of its degeneration products of the degenerating brain.
histopathologically. of infectivity, 1,6
SAF have been isolated in four separate laboratories4-6,8 and a has been achieved by H. Diringer and his
significant purification
group.6,9
In 1982 S. B. Prusiner proposed his "prion" hypothesis.3Later that year D. C. Bolton in his laboratory identified a low-molecular-weight protein. However, this protein, 10, 1 while having a structure hypothesised for the "prion", is not itself infectious. Latterly Prusiner et all have also reported SAF-like structures which they call "prion-rods" presuming them to be large aggregates of prions. However, the featureless, rod-like appearance of this material is just that expected for SAF visualised by rotary shadowing, and when negatively stained it is indistinguishable from
scrapie-specific,
SAF.’
.
Fractions highly enriched for SAF(or for "rods"’) are also enriched for the scrapie-specific, low-molecular-weight, noninfectious protein mentioned above. This protein has only been demonstrated after protease treatment of SAF-containing fractions followed by denaturation and electrophoresis. It is reasonable to expect that this protein may be a subunit of SAF. A picture is emerging of an SSVE-specific ultrastructural feature, the SAF, which has viral dimensions, is likely to be infectious, and from which a structural protein has probably been identified. Moreover, other work 13,14 has provided simple explanations for the apparent small size and resistance to inactivation reported for the scrapie agent. By comparison, the "prion" remains a vague, yet to be identified, structure now postulated to be larger than the scrapiespecific protein but much smaller than SAF. Prusiner’s laboratory has made important contributions to the scrapie story but their use of the conjectural term prion in describing and interpreting these results has served only to confuse the picture, and their use of "prion-rods" to describe what are almost certainly SAF is unwarranted. Laboratory of Central Nervous System Studies, National Institutes of Health, Bethesda, Maryland 20205, USA.
particles cause scrapie. Science 1982; 216: 136-44. 4. Merz PA, Somerville RA, Wisniewski HM, Iqbal K. Abnormal fibrils from scrapie infected brain. Acta Neuropathol (Berlin) 1981; 54: 63-74. 5. Merz PA, Rohwer RG, Kascsak R, Wisniewski HM, Somerville RA, Gibbs CJ Jr, Gajdusek DC. An infection specific particle from the unconventional slow virus diseases. Science (in press). 6. Diringer H, Gelderblom H, Hilmert H, Ozel M, Edelbluth C, Kimberlin R. Scrapie infectivity, fibrils and low molecular weight protein. Nature 1983; 306: 476-78. 7. Merz PA, Somerville RA, Wisniewski HM. Abnormal fibrils in scrapie and senile dementia of the Alzheimer type. In: Court LA, Cathala F, eds Virus non conventionnels et affections du système nerveux central. Paris: Masson, 1983: 259-81 8. Merz PA, Somerville RA, Wisniewski HM, Manuelidis L, Manuelidis EE. Scrapieassociated fibrils in Creutzfeldt-Jakob disease. Nature 1983; 306: 474-76. 9. Diringer H, Hilmert H, Simon D, Werner E, Ehlers B. Towards purification of the scrapie agent. Eur J Biochem 1983; 134: 555-60. 10. Bolton DC, McKinley MP, Prusiner SB. Identification ofa protein that purifies with the scrapie prion. Science 1982; 218: 1309-11. 11. McKinley MP, Bolton DC, Prusiner SB. A protease-resistant protein is a structural component of the scrapie prion. Cell 1983; 35: 57-62. 12. Prusiner SB, Bolton DC, Groth DF, Bowman KA, Cochran SP, McKinley MP Further purification and characterization of scrapie prions. Biochemistry 1982; 21: 6942-50. 13. Rohwer RG. Scrapie infectious agent is virus-like in size and susceptibility to inactivation. Nature 1984; 308: 658-62. 14. Rohwer RG. Virus-like sensitivity of the scrapie agent to heat inactivation. Science 1984; 223: 600-02. 3. Prusiner SB. Novel proteinaceous infectious
SCRAPIE-ASSOCIATED FIBRILS
Nevertheless,
REFERENCES—continued
ROBERT G. ROHWER
SB, McKinley MP, Bowman KA, Bolton DC, Bendheim PE, Groth DF, Glenner GG. Scrapie prions aggregate to form amyloid-like birefringent rods. Cell 1983; 35: 349-58. 2. Prusiner SB. Some speculations about prions, amyloid, and Alzheimer’s disease. N Engl J Med 1984; 310: 661-63.
CAPTOPRIL IN RHEUMATOID ARTHRITIS
SIR,-We read with interest the report by Dr Martin and colleagues (June 16, p 1325) of an uncontrolled study of captopril as a disease-suppressive agent in rheumatoid arthritis (RA). We are, however, concerned about the possible prescription for RA of this potent agent by physicians inexperienced in the use of convertingenzyme inhibitors. Though Martin et al saw only mild hypotension (enough, however, to lead to withdrawal in one patient) and indeed did not report erect or supine blood pressures in their patients, the addition of captopril to thiazide or loop diuretic therapy for indications other than hypertension would probably provoke severe hypotension. Sodium or volume depeletion (eg, after a gastrointestinal bleed or diarrhoea) would do the same. Thus, if captopril is to be prescribed for RA, clear warnings about concurrent medication and action in the event offluid loss must be given both to the patients and to other medical attendants. We cannot agree with Martin’s conclusions on toxicity. Though the strict claim that "the frequency of side-effects was lower than seen in many original clinical studies in which high doses were used" may be true, the occurrence ’of 2 transient rashes and 1 withdrawal due to a rash (ie, 3/16 patients or 19%) compares poorly with their cited average incidence of 12%, probably itself an overestimate compared with recent data.23 instances of dysgeusia in 15 patients again represents an incidence of 20%, compared with their cited average of 6%. The doses they used (75-150 mg) are certainly not high, but are higher than many physicians now advise and use for treatment of essential hypertension. Though all their patients had normal renal function, it must be emphasised that renal among patients with RA) requires a impairment (relatively common reduced dose of captopril.3 Two other points should be noted. The chemical structure of penicillamine they illustrate incorrectly shows a terminal -CH2. D-penicillamine is &bgr;,&bgr;-dimethy1cysteine. Also the structural, chemical, clinical, and toxicological similarities between thiols have penicillamine, sodium aurothiomalate, and many been discussed at length over the past few years.4-9The apparent importance of the - SH group has been repeatedly noted and, in 1981, Merlet et allo reported an uncontrolled study of captopril in 13 patients with RA, again claiming a beneficial effect. We would thus urge caution in the use of this valuable and potent agent for a condition for which it was not designed and for which it does not appear likely to be superior to other agents.
other
1. Prusiner
Dunn Laboratories, Medical Professorial Unit, St Bartholmew’s Hospital, London EC1A 7BE
P. L. DRURY D. PERRETT