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In Familial CJD, approximately 5-10% of all cases of TSE have now been found to result from one or another of more than two dozen different mutations in the gene that encodes the ‘prion’ protein. The translated portion of the gene has 253 codons, and mutations occur in many different regions, but the majority are found between codons 175 and 220, which in the encoded protein are composed of alpha helical domains. The preponderance of mutations in this region is not surprising because the change from a normal to an abnormal protein shape is associated with the conversion of alpha helices to beta sheets, and the mutated replacement of amino acids within the helices is thus well positioned to facilitate the conversion.
Environmentally-acquired cases of CJD are either ‘iatrogenic’ (meaning that they are unknowingly caused by physicians), or more recently, infectious due to the consumption of contaminated beef from cattle with BSE. Fortunately, the number of such cases remains comparatively small, and measures have been taken to prevent future infections. Nevertheless, because we did not immediately appreciate the causes of these acquired infections, and because the period of time elapsing between infection and illness can be so long, new cases of CJD continue to occur after longer and longer incubation periods from infections acquired during the 1970s and 1980’s.
Sporadic CJD occurs in a random distribution all over the world, and has no known genetic or environmental cause. It apparently arises from a one-in-a-million spontaneous conversion of a normal cellular protein (encoded by a gene on chromosome 20) into an abnormal 3-dimensional shape, or configuration. Once converted, the abnormal molecule is thought in some way to compel normal molecules of newly synthesized protein to assume the same abnormal configuration, leading to aggregated deposits in and around cells of the central nervous system, and causing neuronal degeneration and symptomatic disease. Sporadic CJD chiefly affects people between 50 and 75 years of age, but can occur as early as adolescence, and as late as the 9th decade of life (by which time it may be mistaken as 'senility').