14 course questions and answers
Note: Question 1 is included in Section 3.
(a) Identify and carefully explain any errors in the following statement: 'Prion diseases such as BSE and vCJD are caused by mutation of the PrP gene from the PrPC allele to the PrPSc allele.'
(b) Rewrite this statement correctly.
(a) The main error in the statement is that there are no such thing as alleles of the PrP gene known as PrPC and PrPSc and therefore one cannot mutate into the other. Through transcription and translation, many genes produce proteins. Most genes exist in different versions or alleles. Mutation can cause one allele to change into another allele. In turn, this can cause a protein to be produced that has a slightly different sequence of amino acids. The PrP gene codes for the PrP protein. Different alleles of this gene produce different versions of the PrP protein which differ in their sequence of amino acids.
However, PrPC and PrPSc refer to different conformations that a PrP protein can adopt irrespective of its amino acid sequence. PrPC is the 'normal' conformation of the PrP protein and PrPSc is the conformation that results in prion diseases, such as BSE and vCJD. A PrPC molecule can adopt the PrPSc conformation either spontaneously or as a result of interaction with another molecule already in the PrPSc conformation.
(b) A correct version of the statement would be: 'Prion diseases such as BSE and vCJD are caused when the “normal” version of the PrP protein, coded for by the PrP gene and known as PrPC, takes on an alternative conformation, when it is known as PrPSc'.
In prion diseases, the production of disease-causing PrPSc protein in a cell has been described as a 'chain reaction'. (a) Identify two ways in which the PrPSc molecule that initiates the 'chain reaction' could have arrived in the cell. (b) How are further PrPSc molecules produced in the cell? (c) How does the production of PrPSc molecules give rise to the symptoms of prion diseases such as BSE and vCJD?
(a) The PrPSc molecule that starts the 'chain reaction' could have been produced within the cell by a 'normal' PrPC molecule spontaneously changing into the PrPSc conformation. Alternatively, it could have arrived in the cell from elsewhere. In the latter case, the PrPSc molecule could have been released by another cell in the same animal or it could have been produced by another animal and transferred from animal to animal in food.
(b) Further PrPSc molecules are produced through interaction between individual PrPSc molecules and PrPC molecules synthesised within the cell. This interaction somehow causes the PrPC molecules to adopt the PrPSc conformation.
(c) PrPSc molecules have a tendency to collect together as insoluble deposits which eventually kill brain cells. PrPSc molecules taken up by other cells can cause the 'chain reaction' to start in these cells too. The symptoms of prion diseases become apparent when large numbers of brain cells have been killed in this way.
(a) In 2003, epidemiologists at Imperial College London estimated the likely number of deaths from vCJD in the UK by the year 2080. What was their prediction? (b) Suppose that the number of deaths from vCJD in the UK by 2080 turned out to be (say) 3000. To what extent would this invalidate the prediction? (c) Critically discuss the possible reasons for the discrepancy between prediction and eventual outcome.
(a) The 2003 Imperial College prediction was as follows: best estimate, 200; upper and lower 95% confidence limits, 7000 and 10 respectively.
(b) Even 3000 vCJD deaths by 2080 are fewer than the upper 95% confidence limit. Nevertheless, this number of deaths is much higher than the best estimate of 200 deaths. So, although 3000 vCJD deaths does not invalidate the prediction, such a high number does call into question the assumptions upon which it was based.
(c) Even though the best estimate was 200, there could have been 3000 deaths by chance without undermining the assumptions. However, it could well be that one or more of the assumptions built into the team's model were wrong. An important assumption was that all the victims became infected by eating contaminated meat. It is already possible that some may have acquired their vCJD through blood transfusion. Another important assumption was that only people homozygous at triplet 129 of the PrP gene for methionine (MM) were susceptible to vCJD. It is already known that at least one person heterozygous at this triplet (MV) acquired vCJD. People homozygous for valine (VV) may also be susceptible.
(a) What was the ruminant feed ban and when was it introduced? (b) What was the scientific basis of the ban?
(a) The ruminant feed ban, introduced in July 1988, banned the inclusion in feed for ruminant animals (e.g. cattle, sheep and goats) of proteins (other than in milk) derived from ruminant animals.
(b) It was believed that BSE was transmitted as a consequence of the incorporation in cattle concentrates of MBM contaminated with some kind of biological agent (later identified as PrPSc protein) that had not been inactivated by the recently altered rendering process. Banning the inclusion of proteins from all ruminant animals in the feed given to other ruminant animals should have stopped the cycle of BSE transmission.
(a) What evidence is there that the ruminant feed ban did not work as effectively as it should have? (b) In what way(s) did the ban not work as intended?
(a) The number of new BSE cases reported annually grew until 1992 and then continued at a high level for several years after that (Figure 1). Although the relatively long incubation period of BSE makes interpretation of these data somewhat difficult, cases of BSE in cattle born after the introduction of the ban - particularly after controls were tightened in 1996 (Section 10) - provide incontrovertible evidence that either the ban was not working effectively or that cattle feed was not the only way in which animals could contract BSE.
(b) To some extent the ban was ignored on farms, for instance to use up old feed. There must also have been accidental contamination - both on farms and during manufacture - of feed intended for ruminants with feed intended for other animals (e.g. chickens) from which ruminant-derived materials had not been banned. Some SBO-containing material was exported as fertiliser and then reimported as feed.
To what extent was banning cattle SBO and staining it blue an example of the precautionary principle in practice?
Application of the precautionary principle involves going beyond available knowledge of risk so as to err on the side of caution. SBO derived from cattle with BSE was believed to be a relatively rich source of PrPSc protein (though not as rich as the brain and spinal cord), and had therefore been banned from food for human consumption. However, although SBO from scrapie-infected sheep has been proved to be infective, only nervous tissue has been found to be infective in the case of BSE (Section 4). So banning cattle SBO in the first place might be regarded as an application of the precautionary principle. Having decided on the SBO ban, staining it blue can be seen as an enforcement measure.
(a) What cures for vCJD had been developed at the time of writing (early 2005)?
(b) What issues are raised by attempts to find a cure for vCJD?
(a) None. A few treatments had been tried that were claimed to slow down - or even stop - progress of vCJD in some patients. These treatments were based on medicines developed, tested and approved as treatments for other conditions (e.g. quinacrine for malaria and PPS for urinary tract infections).
(b) If it were known that many thousands of people are likely to develop vCJD over the coming decades, then the huge investment needed to develop a cure for the disease might be justified. However, if only about 200 people are likely to develop vCJD - with about three-quarters of these having died already - then most people would agree that there are higher medical priorities for this investment. Trying possible treatments for vCJD on an ad hoc basis on small numbers of terminally ill patients also raises issues related to informed consent and to the objective evaluation of treatments.