Sequence heterogeneity of TT virus and closely related viruses

TT virus (TTV) is a recently discovered infectious agent originally obtaine d from transfusion-related hepatitis, However, the causative link between t he TTV infection and liver disease remains uncertain, Recent studies demons trated that genome sequences of different TTV strains are significantly div ergent. To assess genetic heterogeneity of the Tm genome in more detail, a sequence analysis of PCR fragments (271 bp) amplified from open reading fra me 1 (ORF1) was performed., PCR fragments were amplified from 5 to 40% of s erum specimens obtained from patients with different forms of hepatitis who reside in different countries (e.g., China, Egypt, Vietnam, and the United States) and from normal human specimens obtained from U.S. residents, A to tal of 170 PCR fragments were sequenced and compared to sequences derived f rom the corresponding TTV genome region deposited in GenBank. Genotypes 2 a nd 3 were found to be significantly more genetically related than any other TTV genotype, Moreover, three sequences were shown to be almost equally re lated to both genotypes 2 and 3, These observations suggest a merger of gen otypes 2 and 3 into one genotype, 2/3, Additionally, five new groups of TTV sequences were identified, One group represents a new genotype, whereas th e other four groups were shown to be more evolutionary distant from all kno wn TTV sequences, The evolutionary distances between these four groups were also shown to be greater than between TTV genotypes. The phylogenetic anal ysis suggested that these four new genetic groups represent closely related yet different viral species. Thus,TTV exists as a "swarm" of at least five closely related but different viruses. These observations suggest a high d egree of genetic complexity within the TTV population, The finding of the a dditional TTV-related species should be taken into consideration when the a ssociation between TTV infections and human diseases of unknown etiology is studied.