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In The Family That Couldn't Sleep, D.T. Max states that
Prion diseases are a fascinating medical mystery because they appear to be the only disease that takes on those three forms: genetic, infectious, and accidental ("sporadic").[1]
What a bizarre thing to say. Bizarre in part because prion "diseases", as the plural suggest, is a category of diseases rather than a single disease, and there is a near infinite set of categories that one could devise for diseases, based on molecular, cellular, genetic, physiological, anatomical, epidemiological and clinical variables. A great many of these categories will, of course, include individual diseases of each form. Max surely knows this: he himself introduces another disease category in the book -- Transmissible Spongiform Encephalopathies (TSEs). The TSE category overlaps with the prion disease category, but, as the name suggests, the TSEs are transmissible.
But bizarre, mostly, because the lack of truth in the statement stares one in the face.
The obvious example here is the disease category "cancer". We could call them "cell-cycle regulator diseases" if we wanted to illustrate that this category is based on similar molecular level criteria as prion diseases are. Cancers are clearly hereditary, familial ("genetic" is too simplistic[2]), and sporadic. But they can also fit into the "infectious" category, either in the form of viral oncogenesis (e.g. with HPV, famous for its role in cervical cancers), or in fully transmissible cancers, of which there are two known examples. One of these, Devil Facial Tumour Disease, was blogged about here.
But, to complicate things further, most instances of cancer can not even be neatly filed as genetic, sporadic, or transmissible. Indeed, few disease aetiologies can be pinned down so precisely. Rather, for every disease, a number of variables are interacting and playing a role. The cancer may have a familial trend, but it would not have occurred had the individual led a different lifestyle. You may live with HPV, but be deficient in the other variables that are required for cancer development. The Tasmanian Devil may even have a chance mutation in its major histocompatibility complex (the set of genes which raise flags to the immune system to inform it that the cell is a friendly one which should not be destroyed) which allows the immune system to recognise and combat the transmissible tumour.
Max is aware of this fact too, but I think he fails to pick up on its importance. In the book, he mentions that the incidence of variant CJD (the novel human prion disease thought to be caused by eating beef containing BSE prions) rose from three in 1995 to 27 in 2000, and then began falling again.[3] Rather than the numbers growing rapidly and continuously for many years, as initially projected, there were only a small number of deaths relative to the population of British meat-eaters. The reason? Most of those who died carried mutations which pre-disposed them to developing vCJD (the beef simply tipped them over the edge).[4] Of course, a vCJD epidemic has not yet been ruled out -- perhaps those who are not so pre-disposed merely have longer incubation times for the disease. If this turns out to be the case, how should be categorise vCJD? As a single disease caused by contaminated beef, or as two different diseases, classified according to genetics? Either way, to call it (and other "transmissible spongiform encephalopathies") transmissible, is an oversimplification.
Why do I labour this point so? Because I think it is another example of faulty "nature versus nurture" thinking: this prion disease is genetic; that prion disease was caused by contaminated food. There's just no need to categorise everything as one-or-the-other.
References
- ^ D.T. Max, 2006. The Family That Couldn't Sleep: A Medical Mystery. Random House.
- ^ There is an introduction to the differences here.
- ^ Azra C Ghani, Christl A Donnelly, Neil M Ferguson and Roy M Anderson: Updated projections of future vCJD deaths in the UK. BMC Infectious Diseases 2003, 3:4 doi
- ^ e.g. Cathepsin D -- Matthew T Bishop, Gabor G Kovacs, Pascual Sanchez-Juan and Richard SG Knight: Cathepsin D SNP associated with increased risk of variant Creutzfeldt-Jakob disease. BMC Medical Genetics 2008, 9:31 doi
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