A retrovirus makes chicken eggshells blue

Araucana eggWhen you purchase chicken eggs at the market, they usually have white or brown shells. But some breeds of chicken produce blue or green eggs. The blue color is caused by insertion of a retrovirus into the chicken genome, which activates a gene involved in the production of blue eggs.

The Araucana, a chicken breed from Chile, and Dongxiang and Lushi chickens in China lay blue eggs. Blue eggshell color is controlled by an autosomal dominant gene: eggs produced by homozygote chickens are darker blue than those from heterozygotes. The gene causing blue eggshell color is called oocyan (O) and was previously mapped to the short arm of chromosome 1.

To further refine the location of the O gene, genetic crosses were performed using molecular markers on chromosome 1. The O gene was then located in a ~120 kb region which contained four genes. Only the SLCO1B3 was expressed in the uterus of Dongxiang chickens that produce blue eggs; it was not expressed in chickens that produce brown eggs.

Sequence analysis of the SLCO1B3 revealed that an endogenous avian retrovirus called EAV-HP has inserted just upstream of the gene. This insertion places a promoter sequence in front of the SLCO1B3 gene. As a consequence, the SLCO1B3 gene is transcribed. In chickens that produce brown eggs, no retrovirus is inserted before the SLCO1B3 gene, and no mRNA encoding the protein is produced.

The retrovirus insertion has occurred at different positions in the Chilean and Chinese chicken genomes. This observation indicates that the insertion arose independently during breeding of chicken strains several hundred years ago to produce blue egg layers. The chicken genome contains multiple copies of endogenous retroviruses, which can duplicate and move to other locations. We can assume that a random insertion upstream of the SLCO1B3 gene was selected for by breeding procedures that were aimed at producing blue egg-laying chickens.

The SLCO1B3 gene encodes a membrane transporter protein that mediates the uptake of a wide range of organic compounds into the cell. The blue eggshell color is produced by deposition of biliverdin on the eggshell as it develops in the uterus. Biliverdin is one component of bile salts, which are transported by SLCO1B3, providing a plausible hypothesis for the role of the protein in making blue eggshells.

Blue eggshell color is another example of the important roles that retroviruses have played in animal development. One other is the help provided by retroviruses in producing the placenta of mammals. Not all retroviral insertions are beneficial – integration next to an oncogene can lead to transformation and oncogenesis.

Spread of koala retrovirus in Australia

friendly-male-koalaThe Koala retrovirus (KoRV) continues to spread within Australia, according to results of a new analysis of a larger sample size from a wider geographical range than was previously studied.

Blood or tissue samples were collected from koalas in different regions of Australia, and polymerase chain reaction (PCR) was used to detect the presence of KoRV proviral DNA, a DNA copy of the retroviral genome integrated into host cell DNA. Most of the koalas from the Australian mainland were positive for KoRV proviral DNA (442/466; 94.8%). All samples from animals in Queensland and New South Wales were KoRV positive. In mainland Victoria 65 of 89 animals contained KoRV DNA (73%). On the Victorian islands prevalence of KoRV ranged from 0% on Philip Island (0/11) to 50% on Snake Island (6/12). On the previously KoRV-free Kangaroo Island (link), 24 of 162 animals (14.8%) were KoRV positive. These results suggest that KoRV initially entered the koala population in the north of Australia and has been slowly spreading to the south. There are also other potential explanations for the results: there may be differences in KoRV susceptibility in northern versus southern animals, and the rate of transmission might differ in the two areas.

The genome of Queensland koalas contain far more copies of KoRV per cell, 165, than animals in Victoria, which ranged from less than one to 1.5 copies per cell. The Queensland koalas are likely fully endogenized – that is, the integrated KoRV DNA is passed from parent to offspring in the germline, and hence every koala cell contains viral DNA. In contrast, in Victoria koalas KoRV has either recently entered the germline (1.5 copies/cell) or has not yet entered this state (<1 copy/cell). In animals with less than one proviral copy per cell, KoRV infection was likely acquired exogenously from one animal to another. The mode of transmission of KoRV among koalas is not known, but might involve animal-animal contact or arthropod transmission.

It seems likely that eventually all wild koalas will be endogenized by KoRV. Whether this process will impact the long-term survival of the species is not known, especially since the disease caused by KoRV infection is poorly understood.

GS Simmons, PR Young, JJ Hanger, K Jones, D Clarke, JJ McKeed, J Meersa. 2012. Prevalence of koala retrovirus in geographically diverse populations in Australia. Austr. Vet. J. 90(10):404-9.

XMRV is a recombinant virus from mice

recombinant xmrvThe novel human retrovirus XMRV has been associated with prostate cancer and chronic fatigue syndrome. The nucleotide sequence of XMRV isolated from humans indicates that the virus is nearly identical with XMRV produced from a human prostate tumor cell line called 22Rv1. This cell line was derived by passage of human prostate tumor tissue in nude mice. Sequence analyses reveal that the genomes of these mouse strains contain two different proviral DNAs related to XMRV. These viral genomes recombined to produce XMRV that has been isolated from humans.

XMRV was originally isolated from a human prostate cancer in 2006, and subsequently associated with ME/CFS. The human cell line 22Rv1, which was established from a human prostate tumor (CWR22), produces infectious XMRV. An important question is whether XMRV was present in the original prostate tumor, or was obtained by passage through nude mice. To answer this question, DNA from various passages of the prostate tumor in nude mice (called xenografts), and the mouse strains used to passage the tumor, were analyzed for the presence of XMRV proviral DNA.

Early-passage xenografts did not contain XMRV, but mouse cells found in them did contain two related proviruses called PreXMRV-1 and PreXMRV-2. The 3’-3211 nucleotides of PreXMRV-1, and both LTRs, are identical to XMRV save for two nucleotide differences. The genomic 5’-half of XMRV and PreXMRV-1 differs by 9-10%. PreXMRV-1 is defective for replication due to mutations in genes encoding the gag and pol proteins. PreXMRV-2 does not contain obvious mutations that would prevent the production of infectious viruses. The gag-pro-pol and a part of the env region of this viral genome is identical to that of XMRV save for two base differences; the LTRs and the remainder of the genome differ by 6-12% from XMRV.

Comparison of the sequences of PreXMRV-1 and PreXMRV-2 indicates that recombination between the two viral genomes led to the formation of XMRV. When the sequences of PreXMRV-1 and −2 are used to construct the recombinant XMRV, the resulting virus differs by only 4 nucleotides from the consensus XMRV sequence derived from all human isolates reported to date.

The nude mice used for passage of the original prostate tumor were likely the NU/NU and Hsd strains. Neither mouse strain contains XMRV proviral DNA, but both contain PreXMRV-1 and PreXMRV-2 proviral DNA.

These data demonstrate that XMRV was not present in the original CWR22 prostate tumor, but arose by recombination of PreXMRV-1 and PreXMRV-2 between 1993-1996. When the original prostate tumor was implanted into nude mice, some of the mice harbored both pre-XMRV-1 and −2 endogenous proviruses, which recombined to form XMRV. The authors believe that XMRV originating from the CRWR22 xenografts, the22Rv1 cell line, or other related cell lines has contaminated all human samples positive for the virus. In addition, they suggest that PCR assays for XMRV may actually detect PreXMRV-1 and −2 or other endogenous viral DNA from contaminating mouse DNA.

Another possibility to explain the origin of XMRV is that it arose in mice and can infect humans. If this is true, then XMRV would have to be present in the nude mice used to passage the CWR22 human prostate tumor. No evidence for an XMRV provirus was found in 12 different nude mouse strains, including two used to passage the CWR22 tumor. Furthermore, a screen of 89 inbred and wild mice failed to reveal the presence of proviral XMRV DNA. Hence the authors conclude:

…that XMRV arose from a recombination event between two endogenous MLVs that took place around 1993-1996 in a nude mouse carrying the CWR22 PC xenograft, and that all of the XMRV isolates reported to date are descended from this one event.

It is possible that XMRV produced during passage of CWR22 in nude mice subsequently infected humans. Because XMRV arose between 1993-1996, this scenario could not explain cases of prostate cancer and chronic fatigue syndrome that arose prior to that date.

How can these findings be reconciled with the published evidence that sera of ME/CFS patients from the 1980s contain antibodies to XMRV? Those antibodies were not shown to be directed specifically against XMRV, and therefore cannot be used to prove that XMRV circulated in humans prior to 1993-96. Furthermore, in the absence of clear isolation of an infectious virus, antibody tests alone have proven highly unreliable for identification of new viruses.

Where do these findings leave the hypothesis that XMRV is the etiologic agent of prostate cancer and ME/CFS? All published sequences of human XMRV isolates are clearly derived by recombination of PreXMRV-1 and −2. The finding of human XMRV isolates that are not derived from PreXMRV-1 and −2 would leave a role for XMRV in human disease. As of this writing, no such XMRV isolates have been reported in the scientific literature.

Update: A second paper has also been published in Science Express today entitled “No evidence of murine-like gammaretroviruses in CFS patients previously identified as XMRV-infected”. Editors of the journal Science have asked the authors to retract their 2009 paper linking XMRV infection with chronic fatigue syndrome. The authors have refused.

T. Paprotka, K. A. Delviks-Frankenberry, O. Cingoz, A. Martinez, H.-J. Kung, C.G. Tepper, W-S Hu, M. J. Fivash, J.M. Coffin, & V.K. Pathak (2011). Recombinant origin of the retrovirus XMRV. Science Express

TWiV 123: Contaminated prostates, absolute truth, and bleached worms

42Hosts: Vincent Racaniello, Alan Dove, and Rich Condit

On episode #123 of the podcast This Week in Virology, Vincent, Alan, and Rich talk about XMRV integration sites in prostate tumor DNA, the decline effect and scientific method, and the first virus of Caenorhabditis nematodes.

Click the arrow above to play, or right-click to download TWiV #123 (67 MB .mp3, 93 minutes).

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Retroviral integration and the XMRV provirus

XMRVA strong argument that the novel human retrovirus XMRV is not a laboratory contaminant is the finding that viral DNA is integrated in chromosomal DNA of prostate tumors. Why does this result constitute such strong proof of viral infection?

Establishment of an integrated copy of the viral genome – the provirus – is a critical step in the life cycle of retroviruses. Proviral DNA is transcribed by cellular RNA polymerase II to produce the viral RNA genome and the mRNAs required to complete the replication cycle. Without proviral DNA, retroviral replication cannot proceed.

To produce proviral DNA, the retroviral RNA genome is converted to a double-stranded DNA by the viral enzyme reverse transcriptase. This step occurs in the cytoplasm. Specific and efficient insertion of the viral DNA into the host cell DNA is catalyzed by a viral enzyme called integrase. This enzyme recognizes and nicks the two ends of viral DNA, and the new 3′-ends are then joined covalently to the host DNA at staggered nicks made by integrase.

The image below shows some of the characteristic features of retroviral integration. A the top is the unintegrated linear DNA of avian sarcoma/leukosis virus produced by reverse transcription. Upon completion of integration, two base pairs (AA•TT) are lost from both termini, and a 6-bp target site in host DNA (pink) is duplicated on either side of the proviral DNA. This target site varies in length from 4 to 6 bp among different retroviruses. The proviral DNA (middle) ends with the conserved 5′-T G…C A-3′ sequence. The provirus serves as a template for the production of the viral RNA genome (bottom).

To identify XMRV proviral DNA, genomic DNA was isolated from prostate tumors, and DNA was amplified using a primer that annealed in the viral env gene, near the right-hand LTR. Nucleotide sequence analyses of amplified DNAs from 14 9 different patients showed the expected viral CA sequence followed by human DNA. However, the other cardinal sign of retroviral integration – duplication of host DNA sequences flanking the integration site – could not be confirmed, because only the right-hand integration site was studied.

The isolation of the entire proviral DNA, including both flanking integration sites, from patients with prostate cancer or chronic fatigue syndrome would be additional evidence that XMRV is a virus that infects humans.

Kim, S., Kim, N., Dong, B., Boren, D., Lee, S., Das Gupta, J., Gaughan, C., Klein, E., Lee, C., Silverman, R., & Chow, S. (2008). Integration Site Preference of Xenotropic Murine Leukemia Virus-Related Virus, a New Human Retrovirus Associated with Prostate Cancer Journal of Virology, 82 (20), 9964-9977 DOI: 10.1128/JVI.01299-08