Molecular identification of avian leukosis virus subgroup E loci and tumor virus B locus in Chinese indigenous chickens

Genetic variation at the TVB locus in Turkish and Iranian native chicken breeds

Avian leukosis viruses (ALVs) are retroviruses that can induce tumors, as well as reduce growth rate and productivity in chicken. The tumor virus B (TVB) locus encodes the cellular receptor that mediates an infection through ALVB, ALVD, and ALVE subgroups of ALV. Two single nucleotide polymorphisms (SNPs) at nucleotide positions 172 and 184 of the autosomal TVB locus account for the three alleles, TVB*S1, TVB*S3, and TVB*R. The receptor encoded by TVB*R allele prevents viral entry into the cell of ALVB, ALVD, or ALVE subgroups. In this study, both SNPs at the TVB locus of 2 Turkish and 7 Iranian native chicken breeds were investigated by using a PCR-RFLP technique to determine genotypes and gene frequencies at TVB locus. The frequency of TVB*S1, and TVB*S3 alleles changed from 0.857 to 1.000, and from 0.022 to 0.143, respectively, in all studied breeds. The TVB*R allele for resistance to ALV-related viruses was not determined. As a result, it can be concluded that Turkish and Iranian native chickens are strongly susceptible to the infection of ALV-related viruses.

Altered expression of the mismatch repair genes in DF-1 cells infected with the avian leukosis virus subgroup A

The absence or deficiency of DNA mismatch repair (MMR) activity results in microsatellite instability (MSI) in cancer. The avian leukosis virus (ALV) causes neoplastic disease in chickens. In this study, the status of MMR, MSI, the cell cycle and apoptosis were detected in DF-1 cells after avian leukosis virus subgroup A infection. Flow cytometry analysis results indicated that there was no significant difference in cell apoptosis between the control and infected groups. The percentage of cells in S and G2 phases were increased in the infected group. MSI and mutation of MSH2 and MLH1 gene exons were absent in DF-1 cells after infection. Levels of MSH2 and MLH1 mRNA were dramatically increased in DF-1 cells after infection. These results demonstrated that ALV RAV-1 infection may promote the expression of MSH2 and MLH1 genes rather than resulting in gene mutations. Mismatch repair functions were normal and may be have relationships with the arrest of S phase and G2 phase.

Fighting fire with fire: endogenous retrovirus envelopes as restriction factors

A considerable portion of vertebrate genomes are made up of endogenous retroviruses (ERVs). While aberrant or uncontrolled ERV expression has been perceived as a potential cause of disease, there is mounting evidence that some ERVs have become integral components of normal host development and physiology. Here, we revisit the longstanding concept that some of the gene products encoded by ERVs and other endogenous viral elements may offer to the host protection against viral infection. Notably, proteins produced from envelope (env) genes have been shown to act as restriction factors against related exogenous retroviruses in chickens, sheep, mice, and cats. Based on the proposed mode of restriction and the domain architecture of known antiretroviral env, we argue that many more env gene-derived restriction factors await discovery in vertebrate genomes, including the human genome.

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.