Activation of an endogenous retrovirus enhancer by insertion into a heterologous context

Falling fowl of the chicken reference genome: pitfalls of studying polymorphic endogenous retroviruses

High quality reference genomes have facilitated the study of endogenous retroviruses (ERVs). However, there are an increasing number of published works which assume the ERVs in reference genomes are universal; even those of evolutionarily recent integrations. Consequently, these studies fail to properly characterise polymorphic ERVs, and even propose biological functions for ERVs that may not actually be present in the genomes of interest. Here, I outline the pitfalls of three studies of chicken endogenous Avian Leukosis Viruses (ALVEs or "ev genes": the "original" ERVs), all confounded by the assumption that the reference genome provides a representative ALVE baseline.

Identification and characterisation of endogenous Avian Leukosis Virus subgroup E (ALVE) insertions in chicken whole genome sequencing data

Background

Endogenous retroviruses (ERVs) are the remnants of retroviral infections which can elicit prolonged genomic and immunological stress on their host organism. In chickens, endogenous Avian Leukosis Virus subgroup E (ALVE) expression has been associated with reductions in muscle growth rate and egg production, as well as providing the potential for novel recombinant viruses. However, ALVEs can remain in commercial stock due to their incomplete identification and association with desirable traits, such as ALVE21 and slow feathering. The availability of whole genome sequencing (WGS) data facilitates high-throughput identification and characterisation of these retroviral remnants.

Results

We have developed obsERVer, a new bioinformatic ERV identification pipeline which can identify ALVEs in WGS data without further sequencing. With this pipeline, 20 ALVEs were identified across eight elite layer lines from Hy-Line International, including four novel integrations and characterisation of a fast feathered phenotypic revertant that still contained ALVE21. These bioinformatically detected sites were subsequently validated using new high-throughput KASP assays, which showed that obsERVer was highly precise and exhibited a 0% false discovery rate. A further fifty-seven diverse chicken WGS datasets were analysed for their ALVE content, identifying a total of 322 integration sites, over 80% of which were novel. Like exogenous ALV, ALVEs show site preference for proximity to protein-coding genes, but also exhibit signs of selection against deleterious integrations within genes.

Conclusions

obsERVer is a highly precise and broadly applicable pipeline for identifying retroviral integrations in WGS data. ALVE identification in commercial layers has aided development of high-throughput diagnostic assays which will aid ALVE management, with the aim to eventually eradicate ALVEs from high performance lines. Analysis of non-commercial chicken datasets with obsERVer has revealed broad ALVE diversity and facilitates the study of the biological effects of these ERVs in wild and domesticated populations.

Quantitative evaluation of DNA methylation patterns for ALVE and TVB genes in a neoplastic disease susceptible and resistant chicken model

Chicken endogenous viruses, ALVE (Avian Leukosis Virus subgroup E), are inherited as LTR (long terminal repeat) retrotransposons, which are negatively correlated with disease resistance, and any changes in DNA methylation may contribute to the susceptibility to neoplastic disease. The relationship between ALVE methylation status and neoplastic disease in the chicken is undefined. White Leghorn inbred lines 7(2) and 6(3) at the ADOL have been respectively selected for resistance and susceptibility to tumors that are induced by avian viruses. In this study, the DNA methylation patterns of 3 approximately 6 CpG sites of four conserved regions in ALVE, including one unique region in ALVE1, the promoter region in the TVB (tumor virus receptor of ALV subgroup B, D and E) locus, were analyzed in the two lines using pyrosequencing methods in four tissues, i.e., liver, spleen, blood and hypothalamus. A significant CpG hypermethylation level was seen in line 7(2) in all four tissues, e.g., 91.86 +/- 1.63% for ALVE region2 in blood, whereas the same region was hemimethylated (46.16 +/- 2.56%) in line 6(3). CpG methylation contents of the ALVE regions were significantly lower in line 6(3) than in line 7(2) in all tissues (P < 0.01) except the ALVE region 3/4 in liver. RNA expressions of ALVE regions 2 and 3 (PPT-U3) were significantly higher in line 6(3) than in line 7(2) (P < 0.01). The methylation levels of six recombinant congenic strains (RCSs) closely resembled to the background line 6(3) in ALVE-region 2, which imply the methylation pattern of ALVE-region 2 may be a biomarker in resistant disease breeding. The methylation level of the promoter region in the TVB was significantly different in blood (P < 0.05) and hypothalamus (P < 0.0001), respectively. Our data disclosed a hypermethylation pattern of ALVE that may be relevant for resistance against ALV induced tumors in chickens.

The BARE-1 retrotransposon is transcribed in barley from an LTR promoter active in transient assays

The BARE-1 retrotransposon occurs in more than 10(4) copies in the barley genome. The element is bounded by long terminal repeats (LTRs, 1829 bp) containing motifs typical of retrotransposon promoters. These, the presence of predicted priming sites for reverse transcription, and the high conservation for all key functional domains of the coding region suggest that copies within the genome could be active retrotransposons. In view of this, we looked for transcription of BARE-1 within barley tissues and examined the promoter function of the BARE-1 LTR. We demonstrate here that BARE-1-like elements are transcribed in barley tissues, and that the transcripts begin within the BARE-1 LTR downstream of TATA boxes. The LTR can drive expression of reporter genes in transiently transformed barley protoplasts. This is dependent on the presence of a TATA box functional in planta as well. Furthermore, we identify regions within the LTR responsible for expression within protoplasts by deletion analyses of LTR-luc constructs. Similarities between promoter regulatory motifs and regions of the LTR were identified by comparisons to sequence libraries. The activity of the LTR as a promoter, combined with the abundance of BARE-1 in the genome, suggests that BARE-1 may retain the potential for propagation in the barley genome.

Identification of EFIV, a stable factor present in many avian cell types that transactivates sequences in the 5' portion of the Rous sarcoma virus long terminal repeat enhancer

We define a protein complex present in avian nuclear extracts that interacts with the Schmidt-Ruppin strain of the Rous sarcoma virus (RSV) long terminal repeat (LTR) between positions -197 and -168 relative to the transcriptional start site. We call this complex EFIV and demonstrate that the EFIV protein(s) is present in several avian cell types examined, including B cells (S13 and DT40), T cells (MSB), and chicken embryo fibroblasts. We also report that the EFIV binding site activates transcription of reporter constructs after transfection into avian B cells and chicken embryo fibroblasts, demonstrating that the EFIV region constitutes a functional transactivator sequence. By chemical interference footprinting and mutational analyses we define the EFIV binding site as including the sequence GCAACATG, which is present in two copies between positions -197 and -168, as well as sequences that lie between the two repeats. Electrophoretic mobility shift competition experiments suggest that the EFIV protein(s) may be related to members of the CCAAT/enhancer-binding protein family of transcription factors that interact with different regions of the RSV and the avian leukosis virus (ALV) LTRs. However, as defined by differences in sensitivity to protein synthesis inhibitors and footprinting patterns, EFIV is clearly distinct from these previously defined LTR binding factors. In addition, the finding that EFIV binding activity is stable in B cells indicates either that the lability of all 5' LTR binding activities is not required for B-cell transformation by the ALV/RSV family of viruses or that nonacute transforming viruses that include an RSV LTR may use a mechanism to effect cellular transformation different from that proposed for ALV.

Functional and defective components of avian endogenous virus long terminal repeat enhancer sequences

Oncogenic avian retroviruses, such as Rous sarcoma virus (RSV) and the avian leukosis viruses, contain a strong enhancer in the U3 portion of the proviral long terminal repeat (LTR). The LTRs of a second class of avian retroviruses, the endogenous viruses (ev) lack detectable enhancer activity. By creating ev-RSV hybrid LTRs, we previously demonstrated that, despite the lack of independent enhancer activity in the ev U3 region, ev LTRs contain sequences that are able to functionally replace essential enhancer domains from the RSV enhancer. A hypothesis proposed to explain these data was that ev LTRs contain a partial enhancer that includes sequences necessary but not sufficient for enhancer activity and that these sequences were complemented by RSV enhancer domains present in the original hybrid constructs to generate a functional enhancer. Studies described in this report were designed to define sequences from both the ev and RSV LTRs required to generate this composite enhancer. This was approached by generating additional ev-RSV hybrid LTRs that exchanged defined regions between ev and RSV and by directly testing the requirement for specific motifs by site-directed mutagenesis. Results obtained demonstrate that ev enhancer sequences are present in the same relative location as upstream enhancer sequences from RSV, with which they share limited sequence similarity. In addition, a 67-bp region from the internal portion of the RSV LTR that is required to complement ev enhancer sequences was identified. Finally, data showing that CArG motifs are essential for high-level activity, a finding that has not been previously demonstrated for retroviral LTRs, are presented.

CArG, CCAAT, and CCAAT-like protein binding sites in avian retrovirus long terminal repeat enhancers

A strong enhancer element is located within the long terminal repeats (LTRs) of exogenous, oncogenic avian retroviruses, such as Rous sarcoma virus (RSV) and the avian leukosis viruses. The LTRs of a second class of avian retroviruses, the endogenous viruses (evs), lack detectable enhancer function, a property that correlates with major sequence differences between the LTRs of these two virus groups. Despite this lack of independent enhancer activity, we previously identified sequences in ev LTRs that were able to functionally replace essential enhancer domains from the RSV enhancer with which they share limited sequence similarity. To identify candidate enhancer domains in ev LTRs that are functionally equivalent to those in RSV LTRs, we analyzed and compared ev and RSV LTR-specific DNA-protein interactions. Using this approach, we identified two candidate enhancer domains and one deficiency in ev LTRs. One of the proposed ev enhancer domains was identified as a CArG box, a motif also found upstream of several muscle-specific genes, and as the core sequence of the c-fos serum response element. The RSV LTR contains two CArG motifs, one at a previously identified site and one identified in this report at the same relative location as the ev CArG motif. A second factor binding site that interacts with a heat-stable protein was also identified in ev LTRs and, contrary to previous suggestions, appears to be different from previously described exogenous virus enhancer binding proteins. Finally, a deficiency in factor binding was found within the one inverted CCAAT box in ev LTRs, affirming the importance of sequences that flank CCAAT motifs in factor binding and providing a candidate defect in the ev enhancer.