Mad Cow Disease, technically known as Bovine Spongiform Encephalopathy (BSE), is one of a group of transmissible diseases that destroy brain tissue, collectively known as Transmissible Spongiform Encephalopathies (TSEs). TSEs are an unknown agent(s) that act by damaging the structure of brain proteins known as “prions” (PREE ons). In turn, these damaged prion proteins damage other normal prions and together, they build up to collectively destroy tissue in the brain stem, causing cavities to develop in the victim’s brain so it resembles swiss cheese in mammals. This neurodegenerative disease rapidly leads to death for the victims, of course.
In humans, this disease has long been known as Creutzfeldt-Jakob Disease (CJD) in honor of the two medical scientists who first described it in the literature. But is the agent that causes BSE in cattle the same one that causes CJD in humans? What are TSEs; genetic diseases, infectious agents, or sporatic mutations? Where did the first TSE come from? These basic questions about the nature of TSEs have remained unanswered for decades — until today, that is.
Today, a paper was released by the open-access journal, PLoS Pathogens, that was written by two veterinary scientists, Jürgen A. Richt and S. Mark Hall, from the United States Department of Agriculture. These scientists show that a rare pathogenic mutation underlies BSE that is identical to CJD, and further, their study also supports the hypothesis that BSE originated from a previously undetected TSE from contaminated cattle feed in the UK.
To do this work, the researchers isolated proteins from the brainstems of a variety of animals and humans that had been diagnosed with a TSE. They performed immunohistochemical studies on these proteins and confirmed the presence of an abnormal Prion Protein, PrPd, that characterizes Mad Cow Disease and other TSEs (figure 1);
Figure 1. Analysis of brainstem samples from BSE-infected animals employing various methods. (A) Hybrid Immunoblot Analysis using enriched samples: Lanes 1-5: monoclonal antibody 6H4 (raised against human PrP residues 144-152), lanes 8-12 monoclonal antibody P4 (raised against ovine PrP residues 89-104): 1 = sheep scrapie control, 2 mg; 2 = classical BSE (2003 U.S. BSE case), 2 mg; 3 = H-type BSE case (2004 U.S. BSE case), 2 mg; 4 = U.S. BSE Alabama case, 1 mg; 5 = U.S. BSE Alabama case, 2.5 mg; 6,7 = protein weight maker; 8 = U.S. BSE Alabama case, 2.5 mg; 9 = U.S. BSE Alabama case, 1 mg; 10 = H-type BSE case (2004 U.S. BSE case), 2 mg; 11 = classical BSE (2003 U.S. BSE case), 2 mg; 12 = sheep scrapie control, 2 mg. [larger view] (B) Immunohistochemistry of the U.S. BSE Alabama case (H-type BSE) using PrP-specific monoclonal antibody F99/97.6.1. Brainstem at the level of obex was examined. Bar = 35 µm. (C) Immunohistochemistry of a classical BSE case  using PrP-specific monoclonal antibody F99/97.6.1. Brainstem at the level of obex was examined. Spongiform changes are found in the area with highly PrPd-positive cells. Bar = 90 µm. [larger view].
In the above figure, 1A provided information regarding the molecular size and relative amount of this protein, while figures 1B and C revealed the location of these abnormal proteins in the brainstem tissues at death.
After confirming the presence of the abnormal proteins, the researchers then isolated DNA from each animal and sequenced the prion protein genes. To distinguish minor differences between these genes, they aligned all the DNA sequences next to each other and compared mutations in each (Figure 2);
Figure 2. Alignment of bovine, ovine, cervid and human Prnp sequences. (A) Nucleotide sequences. Standard single letter codes are used for nucleotides. Y = C or T; R = A or G; K =G or T; W= A or T. Boxed area indicates the 6th octapeptide-repeat of the bovine protein (U.S. BSE Alabama case and sequence AY335912). Additional Prnp sequences are as follows: AJ567986 (sheep), AF016227 (elk), Hsap M13899 (human, normal) and Hsap PRNPvar [human, variant; see ]. [larger view]. (B) Amino acid sequences. Standard IUPAC single letter codes are used for amino acids. Codon numbering refers to the most common six-copy octapeptide repeat allele for Bos Taurus. Boxed area indicates the 6th octapeptide repeat of the bovine protein [animals B14842  and AY335912]. AJ567986 (sheep), AF016227 (elk), Hsap M13899 (human, normal) and Hsap PRNPvar [human, variant; see ] each contain a 5 octapeptide repeat region in the protein. [larger view].
As a result of their work, the authors identified a novel mutation in the bovine prion protein, which they named E211K. As you can easily see in the above figure, this mutation is identical to the E200K pathogenic mutation in the human prion protein — the most common cause of genetic CJD in humans.
The study lends support the hypothesis that all three forms of TSEs in humans are also found in cattle: infectious, sporadic, and genetic. It provides additional support that the BSE epidemic may have originated from a genetic case of BSE in an individual cow. Of course, similar mutations can pop up in cattle herds throughout the world and might provide the raw genetic material for new epidemics to develop in the future, especially in countries where it is a common practice to feed slaughterhouse scraps to healthy animals that will enter the human food chain in the future.
Jürgen A. Richt, S. Mark Hall, David Westaway (2008). BSE Case Associated with Prion Protein Gene Mutation PLoS Pathogens, 4 (9) [PDF] DOI: 10.1371/journal.ppat.1000156.