Ancestors of the two species probably separated from each other nearly 81 million years ago—long before the origin of humans—when they inhabited bird nests. Today, house dust mites are blamed for causing allergic reactions in more than 65 million people worldwide and thrive in the mattresses, sofas and carpets of even the cleanest homes. Hamid, Klimov and their colleagues examined genetic variation in the group 1 allergen gene from samples of the two mite species collected in the U.S. and Pakistan. The group 1 allergen gene encodes for the most important allergy-causing protein in house dust mites. An inactive form of this protein is used in clinics worldwide as part of the standard skin-prick test for allergies. Though the test can be inaccurate if it does not include local genetic variants of the allergy-causing protein, geographical variation in group 1 allergen proteins has not been extensively studied in the U.S., Klimov said. "We need to have a better idea about the diversity of allergenic proteins around the world, and particularly in the United States," he said. In genetic sequences from American house dust mites (Dermatophagoides farinae), the researchers observed mutations at 14 positions along the length of the group 1 allergen gene. All but one of the mutations are "silent," meaning they occur at the DNA level without changing the amino acid structure of the protein. Only mutations at the protein level have medical significance because they can change allergenic properties. "The most unexpected result was the finding that a previously unknown mutation occurred at the active site of the protein at position 197," Klimov said. "This was a rare mutation, found in only a single population of house dust mite in South Asia. "Our analysis indicates that this mutation might alter the enzyme activity of the protein. But allergenic properties, immune response and cross-reactivity of the protein are unknown at this time," he said. "Follow-up experiments to elucidate these issues are underway in our lab." Hamid is in the Department of Zoology at the Pir Mehr Ali Shah Arid Agriculture University in Rawalpindi, Pakistan. The other authors of the PLOS ONE paper are Muhammad Inam of the University of the Punjab in Lahore, Pakistan; Farhana Riaz Chaudhary of the Pir Mehr Ali Shah Arid Agriculture University; and Barry OConnor of the U-M Department of Ecology and Evolutionary Biology. The research was supported by the U.S. National Science Foundation, the Higher Education Commission and the International Research Support Initiative Program in
Pakistan, the Ministry of Education and Science of the Russian Federation, and the U.S. National Pediculosis Association. Pavel Klimov: http://bit.ly/1zwoeSL