Avian acute leukemia viruses

Unexpected diversity and expression of avian endogenous retroviruses

Endogenous retroviruses (ERVs) were identified and characterized in three avian genomes to gain insight into early retroviral evolution. Using the computer program RetroTector to detect relatively intact ERVs, we identified 500 ERVs in the chicken genome, 150 in the turkey genome, and 1,200 in the zebra finch genome. Previous studies suggested that endogenous alpharetroviruses were present in chicken genomes. In this analysis, a small number of alpharetroviruses were seen in the chicken and turkey genomes; however, these were greatly outnumbered by beta-like, gamma-like, and alphabeta proviruses. While the avian ERVs belonged to the same major groups as mammalian ERVs, they were more heterogeneous. In particular, the beta-like viruses revealed an evolutionary continuum with the gradual acquisition and loss of betaretroviral markers and a transition from beta to alphabeta and then to alpharetroviruses. Thus, it appears that birds may resemble a melting pot for early ERV evolution. Many of the ERVs were integrated in clusters on chromosomes, often near centromeres. About 25% of the chicken ERVs were in or near cellular transcription units; this is nearly random. The majority of these integrations were in the sense orientation in introns. A higher-than-random number of integrations were >100 kb from the nearest gene. Deep-sequencing studies of chicken embryo fibroblasts revealed that about 20% of the 500 ERVs were transcribed and translated. A subset of these were also transcribed in vivo in chickens, showing tissue-specific patterns of expression. IMPORTANCE Studies of avian endogenous retroviruses (ERVs) have given us a glimpse of an earlier retroviral world. Three different classes of ERVs were observed with many features of mammalian retroviruses, as well as some important differences. Many avian ERVs were transcribed and translated.

Reflections on the pathogenesis of diseases caused by the acute avian leukosis/sarcoma viruses with special reference to avian erythroblastosis

The various diseases that follow experimental infection with the acute and non-acute avian oncoviruses are discussed with special reference to the pathogenesis of avian erythroblastosis. One view, based on in vitro studies, sees erythroblastosis as the product of a failure in the differentiation of virus-infected stem cells to mature erythrocytes, as a result of cell 'transformation'. The results of some in vivo studies, however, point to a resemblance of the disease to a haemolytic anaemia, where cellular death is an important component. It seems probable that the disease is the result of transformation of cells of the erythroblastic series followed by the death of many of these cells due to influences that have not yet been determined. Determination of the causes of this cellular death may prove to be as important for our understanding of the problem of leukaemia as the work that has already been accomplished in explaining the causes of cell transformation. It is also suggested that the tendency of gs amino acid sequences of the avian leukosis viruses and mouse leukaemia viruses to form fusion proteins with a variety of proto-oncogenes may be part of a wider phenomenon, and that these sequences may fuse with other proteins, altering their properties. More work is required on the possibility that there is an undiscovered immunological component in the progression of the L/S diseases.

Altered structure and expression of c-myc in feline T-cell tumours

The c-myc gene is rearranged in a subset of feline T-cell lymphosarcomas. Detailed mapping of c-myc rearrangements showed that some result from feline leukaemia virus (FeLV) proviral integration within or upstream of c-myc, but one case involves a complex 3' alteration and amplification which is apparently not directly virus-induced. S1 nuclease mapping of RNA from normal cells using c-myc probes revealed two presumptive 5' ends, each corresponding to a promoter-like sequence (P1 and P2), and a major 3' discontinuity which mapped to the 3'-most of two possible polyadenylation signals. Analysis of RNA from a series of tumours revealed different modes of c-myc expression. All tumours produced P1 and P2 transcripts with apparently normal structure except for one case where an insertion in intron 1 displaced exon 1 sequences. The abundance ratio of P1/P2 transcripts varied considerably and was high in tumours which carry a rearrangement adjacent to c-myc, but some other T-cell tumours with no apparent myc alteration displayed an equally high ratio. However, a consistent feature was the lack of detectable RNA from normal c-myc alleles in tumours which express a rearranged c-myc allele or a transduced FeLV v-myc gene. We suggest that this may prove to be a useful indicator of the presence of an oncogenically active myc gene, whether this is a rearranged c-myc or transduced v-myc sequence.

Conservation of the c-myc coding sequence in transduced feline v-myc genes

We have cloned the normal feline c-myc locus and determined the nucleotide sequence of all three exons. The feline c-myc gene shows close homology to other mammalian c-myc genes, particularly human c-myc. The feline and human sequences are colinear within the open reading frame for the putative c-myc product but show insertions and deletions relative to each other outside this domain. We have also analyzed a cloned FeLV provirus, CT4, which contains the host-derived myc gene. In this provirus the v-myc sequences are located at the 3' end of the pol gene, replacing pol and env sequences. Nucleotide sequence analysis of CT4 shows an open reading frame for a v-myc gene product which may be expressed without fusion to any viral protein sequences. This contrasts with another FeLV v-myc (LC), in which myc and gag sequences were found to be fused. Unlike previously identified avian v-myc genes, the feline v-myc genes contain exon 1-derived sequences, but these have been truncated or internally deleted. The FeLV CT4 v-myc sequence shows very few coding changes relative to c-myc and the FeLV LC v-myc coding sequence is unchanged relative to c-myc apart from fusion to gag. These results are discussed in relation to the mechanism of transduction and activation of myc by FeLV.

Isolation of an MH2 retrovirus mutant temperature sensitive for macrophage but not fibroblast transformation

A conditional mutant of the MH2 avian retrovirus, termed ts41MH2, was isolated. Unlike wtMH2, ts41MH2 permitted transformed macrophages to differentiate during a 5- to 7-day temperature shift from 37 to 42 degrees C. Mutant-infected cells incubated at 42 degrees C exhibited a flattened morphology and then fused to form giant multinucleated cells that closely resembled normal macrophage maturation in vitro. These differentiated cells reacted strongly with a myeloid-macrophage-specific monoclonal antibody. The process of differentiation was inhibited when ts41MH2-transformed nonproducer clones were superinfected before the temperature shift with the myc gene-containing MC29 or OK10 viruses. By contrast, no inhibition was observed in clones superinfected with the MH2-PA200 virus that contains only the mil gene. The mutant also demonstrated a reduced oncogenic potential relative to that of wtMH2 when it was inoculated intravenously into young birds. However, in contrast to the results obtained with hematopoietic cells, none of the five fibroblast transformation parameters tested for ts41MH2 were altered from those induced by wtMH2. These results suggest that the mutation in ts41MH2 is located in a region of myc required for macrophage transformation, but not required for fibroblast transformation.

V-myc- and c-myc-encoded proteins are associated with the nuclear matrix

A series of extraction procedures were applied to avian nuclei which allowed us to define three types of association of v-myc- and c-myc-encoded proteins with nuclei: (i) a major fraction (60 to 90%) which is retained in DNA- and RNA-depleted nuclei after low- and high-salt extraction, (ii) a small fraction (1%) released during nuclease digestion of DNA in intact nuclei in the presence of low-salt buffer, and (iii) a fraction of myc protein (less than 10%) extractable with salt or detergents and found to have affinity for both single- and double-stranded DNA. Immunofluorescence analysis with anti-myc peptide sera on cells extracted sequentially with nucleases and salts confirmed the idea that myc proteins were associated with a complex residual nuclear structure (matrix-lamin fraction) which also contained avian nuclear lamin protein. Dispersal of myc proteins into the cytoplasm was found to occur during mitosis. Both c-myc and v-myc proteins were associated with the matrix-lamin, suggesting that the function of myc may relate to nuclear structural organization.

V-myc- and c-myc-encoded proteins are associated with the nuclear matrix

A series of extraction procedures were applied to avian nuclei which allowed us to define three types of association of v-myc- and c-myc-encoded proteins with nuclei: (i) a major fraction (60 to 90%) which is retained in DNA- and RNA-depleted nuclei after low- and high-salt extraction, (ii) a small fraction (1%) released during nuclease digestion of DNA in intact nuclei in the presence of low-salt buffer, and (iii) a fraction of myc protein (less than 10%) extractable with salt or detergents and found to have affinity for both single- and double-stranded DNA. Immunofluorescence analysis with anti-myc peptide sera on cells extracted sequentially with nucleases and salts confirmed the idea that myc proteins were associated with a complex residual nuclear structure (matrix-lamin fraction) which also contained avian nuclear lamin protein. Dispersal of myc proteins into the cytoplasm was found to occur during mitosis. Both c-myc and v-myc proteins were associated with the matrix-lamin, suggesting that the function of myc may relate to nuclear structural organization.