Molecular properties, biology, and clinical implications of TT virus, a recently identified widespread infectious agent of humans

Abstract

TT virus (TTV) was first described in 1997 by representational difference analysis of sera from non-A to non-G posttransfusion hepatitis patients and hence intensively investigated as a possible addition to the list of hepatitis-inducing viruses. The TTV genome is a covalently closed single-stranded DNA of approximately 3.8 kb with a number of characteristics typical of animal circoviruses, especially the chicken anemia virus. TTV is genetically highly heterogeneous, which has led investigators to group isolates into numerous genotypes and subtypes and has limited the sensitivity of many PCR assays used for virus detection. The most remarkable feature of TTV is the extraordinarily high prevalence of chronic viremia in apparently healthy people, up to nearly 100% in some countries. The original hypothesis that it might be an important cause of cryptogenic hepatitis has not been borne out, although the possibility that it may produce liver damage under specific circumstances has not been excluded. The virus has not yet been etiologically linked to any other human disease. Thus, TTV should be considered an orphan virus.

FIG. 1

Strategy used for TTV discovery and characterization. Representational difference analysis was performed on paired sera from patient TT with posttransfusion non-A to -G hepatitis . By cloning and sequencing over 97% of the entire genome and testing its sensitivity to various nucleases (i.e., DNase I, RNase I, mung bean nuclease, and restriction endonuclease NdeI), the TTV genome was deemed to be single-stranded and linear DNA, similar to that of parvoviruses . By extending the genome toward its 5′ and 3′ ends, the genome was found to be circular, thus resembling that of circoviruses , .

FIG. 2

Genomic organization of TTV showing the circular, single-stranded DNA of negative polarity encapsidated into the virion. Nucleotide positions refer to the TA278 isolate. Shaded boxes indicate ORF1, ORF2, and ORF3, all present in the plus strand complementary to the virion DNA. ORF3 has been positioned downstream of ORF1, according to the structure in reference . ORF2 starts from the first ATG (positioned at nt 107), although, most probably, translation initiates at the second in-frame ATG (located at nt 263), which is conserved among all isolates. The dotted segments of the UTR identify potentially coding portions. The CAV-like amino acid motif and 36-nt stretch are indicated as in references and , respectively. HVR are those in reference . The N22 region is the first tract of TTV DNA identified , which has been extensively used for molecular epidemiology surveys and genotyping. The potential stem-loop structures indicated in UTR are conserved among TTV isolates . The GC-rich tract also contains several potential stem-loop structures, which, however, vary considerably in shape among different isolates.

FIG. 3

Phylogenetic relationships of TTV with the animal circoviruses CAV, porcine circovirus (PCV), and psittacine beak and feather disease virus (BFDV). The phylogenetic tree was constructed by comparing the entire deduced products of the ORFs that contain Rep protein motifs, namely, ORF1 (TTV), VP1 (CAV), ORF1 (PCV), V1 (BFDV), and VP1 (parvovirus B19), using the last as outgroup. TTV isolates are grouped based on N22 segment analysis , . The Fitch-Margoliash algorithm was used by randomizing the input order of sequences. Genetic distances were calculated by using the Poisson correction distance (PoissonP) as provided by DAMBE software (version 3.7.48) . Bootstrap values above 70 out of 100 are shown at the main branch points. The tree was drawn by using the Treeview program (version 1.5.2) . The bar indicates the number of amino acid substitutions per site. All sequences are available in the GenBank database and are indicated by their accession number or, whenever possible, isolate name.