Effect of tumor virus susceptibility alleles on the late-feathering gene K on early growth of White Leghorns

Intronic deletions of tva receptor gene decrease the susceptibility to infection by avian sarcoma and leukosis virus subgroup A

The group of avian sarcoma and leukosis virus (ASLV) in chickens contains six highly related subgroups, A to E and J. Four genetic loci, tva, tvb, tvc and tvj, encode for corresponding receptors that determine the susceptibility to the ASLV subgroups. The prevalence of ASLV in hosts may have imposed strong selection pressure toward resistance to ASLV infection, and the resistant alleles in all four receptor genes have been identified. In this study, two new alleles of the tva receptor gene, tva^r5 and tva^r6, with similar intronic deletions were identified in Chinese commercial broilers. These natural mutations delete the deduced branch point signal within the first intron, disrupting mRNA splicing of the tva receptor gene and leading to the retention of intron 1 and introduction of premature TGA stop codons in both the longer and shorter tva isoforms. As a result, decreased susceptibility to subgroup A ASLV in vitro and in vivo was observed in the subsequent analysis. In addition, we identified two groups of heterozygous allele pairs which exhibited quantitative differences in host susceptibility to ASLV-A. This study demonstrated that defective splicing of the tva receptor gene can confer genetic resistance to ASLV subgroup A in the host.

Single nucleotide polymorphism variants within tva and tvb receptor genes in Chinese chickens

Avian leukosis is an immunosuppressive neoplastic disease caused by avian leukosis viruses (ALV), which causes tremendous economic losses in the worldwide poultry industry. The susceptibility or resistance of chicken cells to subgroup A ALV and subgroup B, D, and E ALV are determined by the receptor genes tumor virus locus A (tva) and tumor virus locus B (tvb), respectively. Four genetic resistant loci (tva(r1), tva(r2), tva(r3), and tva(r4)) in tva receptor gene and a genetic resistant locus tvb(r) in the tvb receptor gene have been identified in inbred lines of White Leghorn. To evaluate the genetic resistance to subgroup A, B, D, and E ALV, genetic variations within resistant loci in tva and tvb genes were screened in Chinese local chicken breeds and commercial broiler lines. Here, the heterozygote tva(s1/r1) and the resistant genotype tva(r2/r2), tva(r3/r3), and tva(r4/r4) were detected in Chinese chickens by direct sequencing. The heterozygote tva(s1/r1) was detected in Huiyang Bearded chicken (HYBC), Rizhaoma chicken, and commercial broiler line 13 to 15 (CB13 to CB15), with the frequencies at 0.08, 0.18, 0.17, 0.25, and 0.15, respectively. The resistant genotype tva(r2/r2) was detected in Jiningbairi chicken (JNBRC), HYBC, and CB15, with the frequencies at 0.03, 0.08, and 0.06, respectively, whereas tva(r3/r3) and tva(r4/r4) were detected in 19 and 17 of the 25 Chinese chickens tested, with the average frequencies at 0.13 and 0.20, respectively. Furthermore, the resistant genotype tvb(r/r) was detected in JNBRC, CB07, CB12, CB14, and CB15 by pyrosequencing assay, with the frequencies at 0.03, 0.03, 0.11, 0.09, and 0.15, respectively. These results demonstrated that the potential for genetic improvement of resistance to subgroup A, B, D, and E ALV were great both in Chinese local chickens and commercial broilers. This study provides valuable insight into the selective breeding for chickens genetically resistant to ALV.

Methods for evaluating and developing commercial chicken strains free of endogenous subgroup E avian leukosis virus

The genome of nearly all chickens contains various DNA proviral insertions of retroviruses of subgroup E avian leukosis virus (ALVE). However, the elimination or control of ALVE gene expression is desirable to improve productivity, to improve resistance to avian leukosis virus (ALV)-induced tumours, and to develop safer live virus vaccines in chick embryos and cultured chick cells. Restriction fragment length polymorphism and polymerase chain reaction methods are used to define the presence of ALVE genes; and the expression of ALVE in chicken plasma or on cells, and the susceptibility of cells to ALVE is determined by flow cytometry using a specific (R2) antibody. ADOL line 0 chickens have been selected to be free of ALVE genes, while being resistant (i.e. lack receptors to ALVE), but susceptible to exogenous ALV (i.e. ALVA, ALVB, ALVC and ALVJ). To develop improved line 0-type chickens, ADOL line 0 was outcrossed to a commercial line that had one ALVE gene and evidence for ALVE resistance. Rous sarcoma virus (RSV) challenge was used to confirm resistance of F1 chickens to ALVE, and susceptibility of F2 breeders to ALVA and ALVB using test chicks produced by matings to line 7(2). Selected F2 breeders were resistant to ALVE, but susceptible to exogenous ALVA, ALVB, ALVC and ALVJ, based on challenge tests of progeny chick cells using an enzyme-linked immunosorbent assay. The new line, 0(1), has evidence for improved egg size, productivity, fertility and hatchability. Similar procedures may be used for development of productive ALVE free chicken lines with preferred ALV susceptibility traits.

Development of an alloantiserum (R2) that detects susceptibility of chickens to subgroup E endogenous avian leukosis virus

An alloantiserum, termed R2, specifically agglutinates red blood cells (RBC) from line 100B chickens that are susceptible to avian leukosis viruses (ALV) belonging to subgroups B and E, but does not agglutinate RBC from congenic inbred line 7(2) chickens that are resistant to ALV B and E. The R2 antigen was also detected on lymphocytes and thrombocytes. Using chickens from a special cross, it was found that R2 reactivity requires that the chickens must: (1) be susceptible to infection by ALV-E; and (2) express a viral envelope gene with subgroup E specificity. With R2 antiserum, a nearly perfect association was observed between agglutination and susceptibility to ALV-B in F2 chickens containing endogenous viral genes ev2 and/or ev3. These results support earlier evidence that ALV-B and ALV-E share receptors. Moreover, the R2 antiserum was shown to neutralize ALV-E. The R2 antigen showed Mendelian segregation in chickens of a commercial White Leghorn strain-cross containing ev3, ev6 and ev9. However, commercial chickens with or without the R2 antigen did not differ in susceptibility to lymphoid leukosis induction or immune response on infection with ALV of subgroup A for complex reasons we discuss.

Genetic variation at the tumour virus B locus in commercial and laboratory chicken populations assessed by a medium-throughput or a high-throughput assay

The tumour virus B (TVB) locus encodes cellular receptors mediating infection by three subgroups of avian leukosis virus (B, D, and E). Three major alleles, TVB*S1, TVB*S3, and TVB*R, have been described. TVB*S1 encodes a cellular receptor mediating infection of subgroups B, D, and E. TVB*S3 encodes the receptor for two subgroups, B and D, and TVB*R encodes a dysfunctional receptor that does not permit infection by any of the subgroups, B, D, or E. Genetic diversity at the TVB locus of chickens was investigated in both layer and broiler commercial pure lines and laboratory lines. Genotyping assays were developed for both medium-throughput and high-throughput analysis. Of the 36 broiler lines sampled, 14 were fixed for the susceptible allele TVB*S1. Across all broiler lines, 83% of chickens were typed as TVB*S1/*S1, 3% as TVB*R/*R, and 14% as TVB*S1/*R. In the egg-layer lines, five of the 16 tested were fixed for TVB*S1/*S1. About 44% of egg-layers were typed as TVB*S1/*S1, 15% as TVB*R/*R, with the rest segregating for two or three of the alleles. In the laboratory chickens, 60% were fixed for TVB*S1/*S1, 6% for TVB*S3/*S3, 14% for TVB*R/*R, and the rest were heterozygotes (TVB*S1/*S3 or TVB*S1/*R). All commercial pure lines examined in this study carry the TVB*S1 allele that sustains the susceptibility to avian leukosis viruses B, D, and E. More importantly, the TVB*R allele was identified in multiple populations, thus upholding the opportunities for genetic improvement through selection.

Development and validation of a PCR-RFLP assay to evaluate TVB haplotypes coding receptors for subgroup B and subgroup E avian leukosis viruses in White Leghorns

The cellular receptor of subgroup B avian leukosis virus (ALVB) is encoded by a gene at the tumour virus B (TVB) locus. TVB alleles encode specific receptors permitting infection by exogenous ALVB or avian leukosis virus subgroup D (ALVD) as well as endogenous avian leukosis virus subgroup E (ALVE), and thus susceptibility is dominant to resistance. Two single nucleotide polymorphisms at the TVB locus have been reported distinguishing three TVB alleles (TVB*S1, TVB*S3 and TVB*R). We have developed a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay using the two single nucleotide polymorphisms to define three observed allelic haplotypes and to identify the six possible TVB genotypes consisting of the three haplotypes in defined laboratory strains of chickens. One additional potential allelic haplotype and four genotypes were also briefly discussed. Chickens from parents heterozygous for different TVB alleles were challenged with Rous sarcoma viruses of subgroup ALVB and ALVE to induce wing-web tumours. Tumour incidences were evaluated between chickens of the genotypes determined with this newly developed PCR-RFLP assay. Importantly, chickens typed with this assay as TVB*S3/*S3 were resistant to infection by ALVE only, and those TVB*R/*R were resistant to both ALVE and ALVB. Furthermore, a vast majority of chickens with the susceptible TVB*S1/- genotypes developed a tumour. This PCR-RFLP assay enables a relatively rapid assessment of all six anticipated TVB genotypes in experimental strains of chickens undergoing segregation for TVB*S1, TVB*S3, and TVB*R alleles. This non-infectious assay should be further evaluated for the capacity to select and breed commercial chickens for genetic resistance to infections by ALVB, ALVD and ALVE.