Smallpox was a disease caused by infection with one of two strains of Variola virus (Variola major and Variola minor) and a worldwide scourge that killed up 35% of the people it infected. Luckily, a vaccine was developed when Edward Jenner noticed milkmaids infected with cowpox did not contract smallpox. While Jenner was not the first to vaccinate against smallpox, his discovery and testing were spread to a wide audience and thus became the basis for the vaccination efforts that have eradicated the virus in our lifetime. Despite all the research on smallpox, not much is known about the evolution of the virus. Sequence data for the virus only span the last 50–60 years. However, recent efforts published in the New England Journal of Medicine uncovered a new source for examining the history of smallpox infection: mummies.
In 2004, five frozen mummies were discovered in the permafrost in Siberia and dated to late 17th to early 18th century. Because the communal grave was atypical and it seemed the five mummies were buried soon after death, researchers were curious about cause of death. Various tissue samples were taken from the mummies, and a tissue section from the lung of mummy #2, a female adult less than 23 years old, stained positive for iron, suggesting a hemorrhagic incident. Based on the hypothesis that mummy #2 suffered a sudden lethal infection, one of the causative agents considered was the smallpox Variola virus.
Biagini et al. were able to amplify three smallpox gene fragments from nucleic acid isolated from mummy #2’s teeth and lung tissue: hemagglutinin, DNA polymerase and 14kDa protein. DNA isolation and amplification occurred in two separate laboratories that had not previously handled pox viruses. To rule out possible amplification of modern smallpox particles, researchers attempted to amplify a 2,043bp region of viral DNA, which should be intact if there was modern viral DNA contamination but would be fragmented in DNA isolated from 300-year-old tissue. This PCR failed, suggesting the DNA template was fragmented due to age.
The PCR products from all three successful amplifications were cloned, sequenced and subjected to phylogenic analysis to see how they compare to sequences derived from smallpox virus in the last ~50 years. The modern strains of smallpox formed two clades, but sequences amplified from mummy #2 totaling 718bp formed a separate group called PoxSib during sequence comparison. The authors used Bayesian evolutionary analysis to determine the origin of the PoxSib smallpox strain, suggesting it could data back to 120A.D. Biagini et al. also hypothesized about PoxSib’s relationship to modern strains (it could be a direct ancestor or a strain that disappeared) and suggested PoxSib could have been involved in the 1714 smallpox epidemic.
Like the Andes mummy diagnosed with a lung infection using proteomic analysis, a preserved human body can provide historical information about an infectious agent and give contemporary humans a glimpse into the past and a better understanding of the evolution of causative agents of human diseases and viral epidemics.
Reference
- Biagini, P., Thèves, C., Balaresque, P., Géraut, A., Cannet, C., Keyser, C., Nikolaeva, D., Gérard, P., Duchesne, S. and Orlando, L. (2012). Variola Virus in a 300-Year-Old Siberian Mummy. New England Journal of Medicine 367 (21) 2057–2059. DOI: 10.1056/NEJMc1208124
Sara Klink
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