Polymerase chain reaction (PCR), which has solved manyforensic puzzles, now has contributed to findings that,according to paleopathologist Arthur Aufderheide, "provide themost specific evidence possible for the pre-Columbian presenceof human tuberculosis in the New World."
PCR helped determine that a middle-aged woman found deepin a sandy grave in southern Peru succumbed to primarypulmonary tuberculosis. Carbon dating established her time ofdeath at 1,000 years ago, give or take 44 years.
Aufderheide's paper, titled "Identification of Mycobacteriumtuberculosis DNA in a pre-Columbian Peruvian mummy,"appears in today's issue of the Proceedings of the NationalAcademy of Sciences (PNAS).
Experts differ sharply on whether Europeans during the Age ofDiscovery exported TB to the Americas or whether it existedthere prior their arrival. Those favoring TB's pre-Columbianpresence pointed to lesions and bacterial remains in excavatedhuman tissues that looked like TB.
"Those against reasoned that soon after Europeans came here,American Indians experienced widespread outbreaks of TB,"said molecular biologist and biochemist Wilmar Salo, who, likeAufderheide, is a member of the University of Minnesotamedical faculty. "They concluded that these indigenous people,having never been exposed to the infection, were particularlysusceptible."
But "there might be other reasons to explain why they had TBoutbreaks," Salo, a co-author of today's PNAS report, toldBioWorld. "For one thing, their living conditions changed, theywere forced into more crowded situations, they were on therun and so on PP all of which might have contributed to TB."
On expeditions to the Atacama Desert of southern Peru ("whichhasn't seen rains in thousands of years," Salo observed),Aufderheide in the 1980s discovered burial sites with literallythousands of bodies spontaneously mummified by the extremearidity rather than reduced to skeletons. "He must haveautopsied hundreds," said Salo.
Among them was the 40- to 45-year-old female with "a grossmorphology suggestive of primary pulmonary tuberculosis."That suggestion was confirmed by sequencing and amplifyingbacterial DNA recovered from her right lung and adjacentlymph node.
The modern M. tuberculosis genome contains a 1,361-base-pairmobile genetic repeat segment, Salo explained, "present inanywhere from 10 to 16 copies, depending upon the clinicalisolate." It correlates highly with clinically diagnosed TB.Because the sequence seems to be highly conserved, hetargeted a 97-bp segment of it for nested-PCR amplification.Digestion duly yielded two fragments, 42 and 55 bprespectively.
To prove that they were dealing with truly ancient nucleic acidand not some modern contaminant, the Minnesota team wenton to probe bigger and bigger fragments. At 800 bp, themummified DNA no longer amplified, but its moderncounterpart did, confirming the authenticity of the millennium-old specimen.
What this repeat segment encodes -- if anything -- nobodyknows, said Salo, not even in the modern pathogen.
Adding confirmation to TB's pre-Columbian presence is notnecessarily the major question, he observed. "Aufderheide isinterested in diseases that plagued humans in the distant past,why they ebbed and flowed, their success and failurethroughout human history," he said.
That ancient epidemiology has lessons for the present, Saloemphasized. "Right now, for example, TB is on the rise in oursociety," he said. "Many people blame this on HIV andmounting bacterial resistance to antibiotics. Not many realizethat prior to the advent of AIDS and antibiotics, the incidenceof TB was already decreasing." Thanks to PCR, he added, it willperhaps be possible to track the spread of infections in ancientcultures.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.