Betatorquevirus

Betatorquevirus is a genus of viruses in the family Anelloviridae, in group II in the Baltimore classification. The genus Betatorquevirus includes all "torque teno mini viruses" (TTMV), numbered from 1 to 38 as 38 species.

Taxonomy
The genus contains the following species:


 * Torque teno mini virus 1
 * Torque teno mini virus 2
 * Torque teno mini virus 3
 * Torque teno mini virus 4
 * Torque teno mini virus 5
 * Torque teno mini virus 6
 * Torque teno mini virus 7
 * Torque teno mini virus 8
 * Torque teno mini virus 9
 * Torque teno mini virus 10
 * Torque teno mini virus 11
 * Torque teno mini virus 12
 * Torque teno mini virus 13
 * Torque teno mini virus 14
 * Torque teno mini virus 15
 * Torque teno mini virus 16
 * Torque teno mini virus 17
 * Torque teno mini virus 18
 * Torque teno mini virus 19
 * Torque teno mini virus 20
 * Torque teno mini virus 21
 * Torque teno mini virus 22
 * Torque teno mini virus 23
 * Torque teno mini virus 24
 * Torque teno mini virus 25
 * Torque teno mini virus 26
 * Torque teno mini virus 27
 * Torque teno mini virus 28
 * Torque teno mini virus 29
 * Torque teno mini virus 30
 * Torque teno mini virus 31
 * Torque teno mini virus 32
 * Torque teno mini virus 33
 * Torque teno mini virus 34
 * Torque teno mini virus 35
 * Torque teno mini virus 36
 * Torque teno mini virus 37
 * Torque teno mini virus 38

Initial discovery
The discovery of TTMV, like the original Torque Teno virus (TTV) isolate, was accidental. After TTV was isolated in 1997 from a Japanese patient, primers were created to study TTV in more detail. TTV-specific primers used in PCR of human plasma samples yielded sequences that partially matched that of TTV, but were noticeably shorter. TTV was eventually understood to have a genome of 3.6–3.9 kb, while TTMV has a genome of 2.8–2.9 kb. Another TT-like virus later isolated in 2007, Torque teno midi virus or TTMDV, has a genome of 3.2 kb.

Genome and capsid
TTMV shares similar morphologic features with the other human anelloviruses. The capsid has a T=1 icosahedral symmetry. The virion does not have a lipid envelope and is thus "naked", making it an extremely simple virus. It is estimated that the virion is a little less than 30 nm in diameter.

The genome is circular and made of single-stranded DNA of negative polarity. It is 2.86–2.91 kilobases long. Anelloviruses are known for having 3 or 4 overlapping, nested open reading frames; TTMV has 3 ORF's that overlap. ORF-2 and ORF-3 overlap with ORF-1 at opposite ends. For TTMV, ORF-1 is about 663 residues and ORF-2 is about 91 residues long. There is a highly conserved 130-nt region just downstream of the TATA box.

Phylogeny & spread
TTMV is highly divergent. The first phylogenetic tree created from TTMV genomic sequences revealed a large cluster of strains; ORF-1 had divergences of over 42% at the nucleotide level and over 67% at the amino acid level.

TTMV is also highly prevalent, like other human anelloviruses. Subsequent research after it was discovered has yielded the prevalence of TTMV DNA among blood donors to be 48%–72%. It can likely infect a wide range of tissues, as it has been isolated from various body fluids and tissues, including saliva, feces, plasma/serum, PBMCs, bone marrow, spleen, pancreas, kidneys, nervous tissue, lymph nodes, semen, and cervical swabs. Its exact transmission mechanism is unknown, but is thought to be possible by blood-borne, sexual, fecal-oral, and respiratory routes.

Recent studies have shown that humans can have multiple infections of TT viruses.

Clinical
Though TTVs are potentially associated with dieases and their pathogenicity has been debated since their discovery, TTMV is not currently known as an explicit cause of any human disease. TTMV has been isolated from a number of parapneumonic empyema. However, its clinical significance remains unclear.