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Journal of Virology, May 2006, p. 4640-4642, Vol. 80, No. 9
0022-538X/06/$08.00+0     doi:10.1128/JVI.80.9.4640-4642.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

LETTER TO THE EDITOR

Newly Identified Families of Human Endogenous Retroviruses


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LETTER
 
Human endogenous retroviruses (HERVs) make up approximately 8.3% of the human genome (12). HERVs have previously been classified into 31 distinct families based upon sequence alignment of reverse transcriptase (RT) and envelope domains and subsequent phylogenetic analyses (1, 9, 16). Using the data mining program LTR_STRUC (13) in conjunction with conventional sequence homology techniques, we recently completed an analysis of chimpanzee long terminal repeat (LTR) retrotransposon families (unpublished data). Since LTR_STRUC searches for LTR retrotransposons based on structure (e.g., the presence of LTRs, target site duplications, tRNA binding sites, etc.) rather than homology, elements can be identified that go undetected in traditional BLAST searches. We identified nine chimpanzee LTR retrotransposon families that are orthologous to HERV families not previously identified. These nine newly discovered HERV families are described and characterized in this letter.

LTR retrotransposons and retroviruses are grouped into three major classes (14). Class I contains elements related to gammaretroviruses, class II elements are related to betaretroviruses, and class III elements are related to spumaviruses. The RT-based phylogeny indicates that all the newly identified HERVs described here are class I elements (Fig. 1). The detailed characteristics of each of the newly discovered HERV families are presented in Table 1 and Table 2. All are low-abundance families, being composed of only one to seven full-length members with low homology to previously identified HERVs. This may, in part, explain why they have not been previously identified. The newly discovered full-length elements are of standard HERV length (7,198 to 10,675 bp with 359- to 682-bp LTRs) and display typically sized target site duplications (4 or 5 bp). With the exception of a few mutated copies, the newly identified elements have the same canonical dinucleotides terminating the LTRs as previously characterized HERVs (TG/CA). Since LTR_STRUC can only identify elements having two LTRs, we conducted BLAST searches by using identified full-length elements as query sequences to identify solo LTRs and other fragmented elements. Consistent with what has been reported previously for other HERV families (15), we have found that each of the newly identified families is represented by significantly more solo LTRs and fragmented sequences than full-length elements (Table 1).


Figure 1
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  		FIG. 1. Unrooted RT-based neighbor-joining tree of human LTR retrotransposon families. The phylogenetic tree is built from the DNA sequence (using MEGA3 software [11]) of the RTs taken from all the members of the newly identified HERV families together with representative members of previously identified families. (*, human LTR retrotransposon family characterized in this study. {phi}, this element is present in the duplicated region of the genome on chromosome 15; as a result, six elements of this family of greater than 97% identity are present in the human genome.) Elements are grouped into families based on bootstrap values (shown for families newly identified in this report). Previously identified families of retrotransposons and retroviruses are included for comparison (GALV, gibbon ape leukemia virus [accession no. M26927]; PERV, porcine endogenous retrovirus [accession no. AF038601]; BaEV, baboon endogenous virus [accession no. X05470]; HFV, human foamy virus [accession no. Y07725]; FeFV, feline foamy virus [accession no. AJ223851]; HIV, human immunodeficiency virus [accession no. K03454]; MMTV, mouse mammary tumor virus [accession no. NC_001503]; RERV, rabbit endogenous retrovirus [accession no. AF480925]).


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  		TABLE 1. Representative elements of human LTR retrotransposon families characterized in this study


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  		TABLE 2. Characteristics of human LTR retrotransposon families identified in this study

Because HERV LTRs are synthesized from the same RNA template during reverse transcription, they are identical in sequence at the time of integration (2). Using a primate pseudogene nucleotide substitution rate of 0.16% divergence/million years (5, 8, 10), the relative integration time or age of any full-length HERV can be estimated from the level of sequence divergence existing between the element's 5' and 3' LTRs. Using this method, the estimated ages of the new families of HERVs described here range from 18.0 to 49.5 million years, indicating that members of these families have not been transpositionally active in the primate lineage since well before chimpanzees and humans diverged from a common ancestor (6 million years ago) (4). Although caution must be taken when using LTR divergence to estimate the ages of individual elements because of confounding processes such as recombination and conversion (e.g., see references 6 and 7), the method is able to provide useful age estimates, at least to a first approximation (e.g., see reference 3). Our estimated ages of the newly identified human elements fall within the median range of previously described families of HERVs (16). The possible contribution of these newly identified LTR retrotransposons to primate gene or genome evolution is currently under investigation.


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ACKNOWLEDGMENTS
 
This research was supported by a grant from the Georgia Institute of Technology Research Foundation.


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REFERENCES
 
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Nalini Polavarapu
Nathan J. Bowen
John F. McDonald*
School of Biology
Georgia Institute of Technology
310 Ferst Dr.
Atlanta, GA 30332-0230

* Phone: (404) 385-6631, Fax: (404) 894-0519, E-mail: john.mcdonald{at}biology.gatech.edu

Journal of Virology, May 2006, p. 4640-4642, Vol. 80, No. 9
0022-538X/06/$08.00+0     doi:10.1128/JVI.80.9.4640-4642.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.




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