A team of scientists reports that 'mad cow'-like diseases have not been a major force in human history, nor have been cannibalistic rituals that are known to be associated with disease transmission.
In a new study published by the journal Genome Research, Prof. Jaume Bertranpetit at the Universitat Pompeu Fabra and his colleagues used a fresh set of genetic data to show that balancing selection associated with cannibalism has not been a major selective driving force on the prion protein gene, as has been recently proposed.
The study also has important implications for researchers using a specific class of DNA markers called SNPs (single nucleotide polymorphisms) for examining genetic associations with diseases or for evaluation of historical patterns of human migration.
The prion protein gene (PRNP) encodes a protein that can abnormally fold and amass in brain tissues to cause fatal neurodegenerative diseases such as mad cow disease. These diseases, cumulatively known as transmissible spongiform encephalopathies (TSEs), include CJD (Creutzfeldt-Jakob disease) and kuru in humans. Kuru is confined to a human population in Papua-New Guinea and is transmitted by cannibalism at ritualistic mortuary feasts.
A high-profile study published nearly three years ago suggested that individuals who were heterozygous for a common polymorphism in the PRNP gene were relatively resistant to the disease. Over time, homozygotes who participated in the cannibalistic rituals purportedly decreased in number due to their increased susceptibility to kuru. This indicated that cannibalism conferred an effect of balancing selection on the PRNP gene throughout human history.
Bertranpetit and his colleagues sequenced 2,378 base pairs of the PRNP gene in 174 individuals; in addition, they genotyped two SNPs from the PRNP gene in 1000 individuals from populations worldwide. They identified 28 different haplotypes - or combinations of DNA variants - in the PRNP gene. This data was used to assess the ages of the mutations, to identify geographic patterns of variation, and to evaluate selective forces that have potentially influenced these patterns.
"In contrast to the previous study, which concluded that variation in the PRNP gene was strongly skewed toward intermediate frequency variants, our results showed that there was, in fact, a deficit of intermediate frequency variants," says Bertranpetit. "Our results are consistent with a complex history of episodic or fluctuating selection, including positive selection, purifying selection, and possibly even short periods of balancing selection."
On a more technical note, the study cautions researchers involved in SNP-based population genetics studies. The work is among the first to empirically demonstrate how SNP ascertainment can introduce a strong bias in population genetics studies and seriously distort the conclusions. Bertranpetit and his colleagues point out that at a time when a flood of ascertained SNP data is being generated, it is essential that SNP ascertainment be taken into consideration in data analyses.
The first author on the study is Dr. Marta Soldevila, who completed her Ph.D. at the Universitat Pompeu Fabra and performed a substantial part of the sequencing work at DeCODE Genetics (Reykjavik, Iceland).