Nucleotide sequence analysis of Aleutian mink disease parvovirus shows that multiple virus types are present in infected mink

Seroprevalence and Molecular Epidemiology of Aleutian Disease in Various Countries during 1972-2021: A Review and Meta-Analysis

Simple Summary

Aleutian disease is caused by the Aleutian mink disease virus and is one of the most serious infectious diseases that affect the family Mustelidae, including the American mink, wild European mink, weasels, badgers and other animal species, such as skunks, raccoons, dogs, cats and mice, as well as humans. Effective treatments and vaccines against Aleutian disease have not been developed to date. Prophylactic programs that focus on the identification and elimination of infected mink are one of the methods of controlling the negative outcomes of Aleutian disease. This article analyses the seroprevalence of Aleutian mink disease virus infections in American and European mink and other species around the world, and reviews recent knowledge relating to the molecular epidemiology of the Aleutian mink disease virus.

Abstract

Aleutian disease (AD) poses a serious threat to both free-ranging and farmed mink around the world. The disease is caused by the Aleutian mink disease virus (AMDV), which also poses a health risk for other members of the family Mustelidae, including wild mink, weasels, badgers and other animal species. This article analyses the seroprevalence of AMDV infections in mink and other species around the world, and reviews recent knowledge relating to the molecular epidemiology of the AMDV. Depending on the applied diagnostic technique and the country, the prevalence of anti-AMDV antibodies or AMDV DNA was established at 21.60–100.00% in farmed American mink, 0.00–93.30% in free-ranging American mink and 0.00–25.00% in European mink. Anti-AMDV antibodies or AMDV DNA were also detected in other free-living fur-bearing animals in Europe and Canada, where their prevalence was determined at 0.00–32.00% and 0.00–70.50%, respectively. This may indicate a potential threat to various animal species. AMDV strains are not clustered into genotypes based on the geographic origin, year of isolation or pathogenicity. The isolates that were identified on mink farms around the world originated from North America because American mink were introduced to Europe and Asia for breeding purposes and to restock natural populations.

Molecular epidemiology of Aleutian mink disease virus from fecal swab of mink in northeast China

Background

Aleutian mink disease parvovirus (AMDV) causes Aleutian mink disease (AMD), which is a serious infectious disease of mink. The aim of this study was to get a better understanding of the molecular epidemiology of AMDV in northeast China to control and prevent AMD from further spreading. This study for the first time isolated AMDV from fecal swab samples of mink in China.

Results

A total of 157/291 (54.0%) of the fecal swab samples were positive for AMDV. Of these, 23 AMDV positive samples were randomly selected for sequence alignment and phylogenetic analysis based on the acquired partial fragments of VP2 gene with the hypervariable region. Comparative DNA sequence analysis of 23 AMDV isolates with a reference nonpathogenic (AMDV-G) strain revealed 8.3% difference in partial VP2 nucleotide sequences. Amino acid alignment indicated the presence of several genetic variants, as well as one single amino acid residue deletion. The most concentrated area of variation was located in the hypervariable region of VP2 protein. According to phylogenetic analysis, the Chinese AMDV strains and the other reference AMDV strains from different countries clustered into three groups (clades A, B and C). Most of the newly sequenced strains were found to form a Chinese-specific group, which solely consisted of Chinese AMDV strains.

Conclusion

These findings indicated that a high genetic diversity was found in Chinese AMDV strains and the virus distribution were not dependent on geographical origin. Both local and imported AMDV positive species were prevalent in the Chinese mink farming population. The genetic evidence of AMDV variety and epidemic isolates have importance in mink farming practice.

Development of an EvaGreen-based real-time PCR assay for detection of Aleutian mink disease virus

The objective of this study was to develop a rapid, sensitive and specific EvaGreen (EG)-based real-time PCR assay capable of detecting Aleutian mink disease virus (AMDV) and to evaluate the reliability of the assay for analysis of blood or tissue samples. For this assay, a pair of primers was designed based on a nonstructural protein (NS)-encoding gene of AMDV, and the identity of PCR products was identified based on a melting temperature of 82.8°C. The EG-based real-time PCR assay did not detect canine distemper virus or mink enteritis virus, and the assay could be used to detect Chinese and American AMDV strains, in contrast to a commercial TaqMan kit that could only detect American AMDV strains. The amplification efficiencies of the EG assay were 104.8% for the Chinese strain and 94.4% for the American strain, and the detection limit was 1 copy/μL of AMDV plasmid or 3 pg/μL of viral DNA (Chinese strain). The intra- and inter-assay variation coefficients of melting temperature were all lower than 0.15%, confirming the high reproducibility of the assay. Forty-five clinical blood samples were simultaneously analyzed using the EG real-time PCR, TaqMan kit and conventional PCR, and the detection rates were 91.1%, 0.0% and 86.7%, respectively. Serum samples were also collected from the corresponding blood samples and tested using the counterimmunoelectrophoresis (CIEP) assay, where positive samples accounted for 24.4% of the 45 samples. In conclusion, EG-based real-time PCR is a rapid, sensitive, universal assay that can be effectively utilized as a reliable and specific tool for detection and quantitation of AMDV.

Molecular analysis of partial VP-2 gene amplified from rectal swab samples of diarrheic dogs in Pakistan confirms the circulation of canine parvovirus genetic variant CPV-2a and detects sequences of feline panleukopenia virus (FPV)

Background

The infection in dogs due to canine parvovirus (CPV), is a highly contagious one with high mortality rate. The present study was undertaken for a detailed genetic analysis of partial VP2 gene i.e., 630 bp isolated from rectal swab samples of infected domestic and stray dogs from all areas of district Faisalabad. Monitoring of viruses is important, as continuous prevalence of viral infection might be associated with emergence of new virulent strains.

Methods

In the present study, 40 rectal swab samples were collected from diarrheic dogs from different areas of district Faisalabad, Pakistan, in 2014–15 and screened for the presence of CPV by immunochromatography. Most of these dogs were stray dogs showing symptoms of diarrhea. Viral DNA was isolated and partial VP2 gene was amplified using gene specific primer pair Hfor/Hrev through PCR. Amplified fragments were cloned in pTZ57R/T (Fermentas) and completely sequenced. Sequences were analyzed and assembled by the Lasergene DNA analysis package (v8; DNAStar Inc., Madison, WI, USA).

Results

The results with immunochromatography showed that 33/40 (82%) of dogs were positive for CPV. We were able to amplify a fragment of 630 bp from 25 samples. In 25 samples the sequences of CPV-2a were detected showing the amino acid substitution Ser297Ala and presence of amino acid (426-Asn) in partial VP2 protein. Interestingly the BLAST analysis showed the of feline panleukopenia virus (FPV) sequences in 3 samples which were already positive for new CPV-2a, with 99% sequence homology to other FPV sequences present in GenBank.

Conclusions

Phylogenetic analysis showed clustering of partial CPV-VP-2 gene with viruses from China, India, Japan and Uruguay identifying a new variant, whereas the 3 FPV sequences showed immediate ancestral relationship with viruses from Portugal, South Africa and USA. Interesting observation was that CPV are clustering away from the commercial vaccine strains. In this work we provide a better understanding of CPV prevailing in Pakistan at molecular level. The detection of FPV could be a case of real co-infection or a case of dual presence, due to ingestion of contaminated food.

Identification of a novel Aleutian mink disease virus B-cell epitope using a monoclonal antibody against VP2 protein

Aleutian mink disease virus (AMDV) is a parvovirus that causes an immune complex-mediated disease in minks. Capsid protein VP2 is a major structural viral protein and can be used to diagnose AMDV. In this study, a specific monoclonal antibody, 1M13, was produced against the AMDV VP2 protein (amino acids 291-502). A linear VP2-protein epitope was identified by subjecting a series of partially overlapping synthesized peptides to be enzyme-linked immunosorbent assay (ELISA) analysis. The results indicated that (386)HLQQNFSTRYIYD(398) was the minimal linear epitope that could be recognized by mAb 1M13. ELISA assays revealed that mink anti-AMDV sera could also recognize the minimal linear epitope. Sequence alignments demonstrated that the linear epitope is highly conserved among AMDV strains except (386)H and is less conserved among Raccoon dog amdovirus, Gray fox amdovirus, Red fox amdovirus, Bat parvovirus and Mink enteritis parvovirus. Taken together, the generation of this VP2-specific mAb with a defined linear peptide epitope may have potential applications in the development of suitable diagnostic techniques for AMDV.

A fast and robust method for whole genome sequencing of the Aleutian Mink Disease Virus (AMDV) genome

Aleutian Mink Disease Virus (AMDV) is a frequently encountered pathogen associated with commercial mink breeding. AMDV infection leads to increased mortality and compromised animal health and welfare. Currently little is known about the molecular evolution of the virus, and the few existing studies have focused on limited regions of the viral genome. This paper describes a robust, reliable, and fast protocol for amplification of the full AMDV genome using long-range PCR. The method was used to generate next generation sequencing data for the non-virulent cell-culture adapted AMDV-G strain as well as for the virulent AMDV-Utah strain. Comparisons at nucleotide- and amino acid level showed that, in agreement with existing literature, the highest variability between the two virus strains was found in the left open reading frame, which encodes the non-structural (NS1-3) genes. This paper also reports a number of differences that potentially can be linked to virulence and host range. To the authors' knowledge, this is the first study to apply next generation sequencing on the entire AMDV genome. The results from the study will facilitate the development of new diagnostic tools and can form the basis for more detailed molecular epidemiological analyses of the virus.

Driving forces behind the evolution of the Aleutian mink disease parvovirus in the context of intensive farming

Aleutian mink disease virus (AMDV) causes plasmacytosis, an immune complex-associated syndrome that affects wild and farmed mink. The virus can also infect other small mammals (e.g., ferrets, skunks, ermines, and raccoons), but the disease in these hosts has been studied less. In 2007, a mink plasmacytosis outbreak began on the Island of Newfoundland, and the virus has been endemic in farms since then. In this study, we evaluated the molecular epidemiology of AMDV in farmed and wild animals of Newfoundland since before the beginning of the outbreak and investigated the epidemic in a global context by studying AMDV worldwide, thereby examining its diffusion and phylogeography. Furthermore, AMDV evolution was examined in the context of intensive farming, where host population dynamics strongly influence viral evolution. Partial NS1 sequences and several complete genomes were obtained from Newfoundland viruses and analyzed along with numerous sequences from other locations worldwide that were either obtained as part of this study or from public databases. We observed very high viral diversity within Newfoundland and within single farms, where high rates of co-infection, recombinant viruses and polymorphisms were observed within single infected individuals. Worldwide, we documented a partial geographic distribution of strains, where viruses from different countries co-exist within clades but form country-specific subclades. Finally, we observed the occurrence of recombination and the predominance of negative selection pressure on AMDV proteins. A surprisingly low number of immunoepitopic sites were under diversifying pressure, possibly because AMDV gains no benefit by escaping the immune response as viral entry into target cells is mediated through interactions with antibodies, which therefore contribute to cell infection. In conclusion, the high prevalence of AMDV in farms facilitates the establishment of co-infections that can favor the occurrence of recombination and enhance viral diversity. Viruses are then exchanged between different farms and countries and can be introduced into the wild, with the rapidly evolving viruses producing many parallel lineages.

Amdoparvoviruses in small mammals: expanding our understanding of parvovirus diversity, distribution, and pathology

Many new viruses have been discovered recently, thanks in part to the advent of next-generation sequencing technologies. Among the Parvoviridae, three novel members of the genus Amdoparvovirus have been described in the last 4 years, expanding this genus that had contained a single species since its discovery, Aleutian mink disease virus. The increasing number of molecular and epidemiological studies on these viruses around the world also highlights the growing interest in this genus. Some aspects of amdoparvoviruses have been well characterized, however, many other aspects still need to be elucidated and the most recent reviews on this topic are outdated. We provide here an up-to-date overview of what is known and what still needs to be investigated about these scientifically and clinically relevant animal viruses.

Genetic characterization of Aleutian mink disease viruses isolated in China

Aleutian mink disease virus (AMDV) is a parvovirus that causes an immune complex mediated disease in minks. To understand the genetic characterization of AMDV in China, the genomic sequences of three isolates, ADV-LN1, ADV-LN2, and ADV-LN3, from different farms in the Northern China were analyzed. The results showed that the lengths of genomic sequences of three isolates were 4,543, 4,566, and 4,566 bp, respectively. They shared only 95.5-96.3 % nucleotide identity with each other. The nucleotide and amino acid homology of genome sequence between the Chinese isolates and European or American strains (ADV-G, ADV-Utah1, and ADV-SL3) were 92.4-95.0 % and 92.1-93.8 %, respectively. The amino acid substitutions randomly distributed in the genome, especially NS gene. ADV-LN1 strain had a 9-amino-acid deletion at amino acid positions 70 and 72-79 in the VP1 gene, comparing with ADV-G strain; ADV-LN2 and ADV-LN3 strains had 1-amino-acid deletion at amino acid positions 70 in the VP1. Some potential glycosylation site mutations in VP and NS genes were also observed. Phylogenetic analysis results showed that the three strains belonged to two different branches based on the complete coding sequence of VP2 gene. However, they all were in the same group together with the strains from United States based on the NS1 sequence. It indicated that Chinese AMDV isolates had genetic diversity. The origin of the ancestors of the Chinese AMDV strains might be associated with the American strains.

Phylogenetic analysis of the VP2 gene of Aleutian mink disease parvoviruses isolated from 2009 to 2011 in China

Aleutian mink disease parvovirus (AMDV) is a non-enveloped virus with a single-stranded DNA genome that causes a fatal, usually persistent immune complex disease in minks. In this study, a total of 18,654 serum samples were collected from minks that were farmed in China from 2009 to 2011. After testing by counter-current immunoelectrophoresis (CIE), the seroprevalence of AMDV was found to be 68.67 %. The results show that there is a serious epidemic among Chinese minks used for breeding. To gain detailed information regarding the molecular epidemiology of AMDV in China, nine strains of AMDV were isolated from mink samples that were collected from four of the primary mink farming areas in China. The full-length capsid protein VP2 gene from each strain was sequenced after PCR amplification, and a phylogenetic analysis was performed on the VP2 gene sequence, including the VP2 genes from the other 10 AMDV strains available in the GenBank database, which were submitted from the 1970s to 2009. The phylogenetic analysis showed that the AMDV isolates were divided into five independent clades. The Chinese AMDV strains were distributed among all five groups and showed a high level of genetic diversity. Over 50 % of the Chinese AMDV strains were classified into two clades that consisted only of isolates from China and that were distinct from AMDV strains found in other countries. This finding indicated that both local and imported ADMV species are prevalent in the Chinese mink farming population.

Diversity and stability of Aleutian mink disease virus during bottleneck transitions resulting from eradication in domestic mink in Denmark

Aleutian mink disease (plasmacytosis) virus (AMDV) in domestic mink (Neovison vison) has been subject to eradication in Denmark since 1976. In 2001, approximately 5% of Danish mink farms were still infected and all were located in the northern part of the peninsula of Jutland. In the present study a total of 274 Danish isolates of AMDV collected during the two seasons of 2004 and 2005 were characterized by partial sequencing of the coding region of the non-structural (NS) proteins. Older AMDV isolates from Denmark, available, were also included. The Danish isolates represent a very homogenous cluster compared with Swedish, Finnish and Dutch isolates and seem to represent a minor fraction of the genetic diversity previously found in Denmark. Stability of nucleotide deviations reveals that the purifying selection of bottlenecks imposed on the AMDV population in Denmark by the stamping out policy for more than 6 years exceeds the rate of mutation driven diversity. Among the isolates from farms in northern Jutland two distinct types could be identified and within each of them a number of sub-types which were all useful in tracking spread of infections. Infection at a farm the preceding season was a predisposing risk parameter for disease outbreak at a farm, and strain identity substantiates the suggestion that inadequate disinfection is involved in the recurrence of outbreaks. In cases of new introductions to farms it is indicated that contact including transport between farms played a most significant role.

Implementation and validation of a sensitive PCR detection method in the eradication campaign against Aleutian mink disease virus

Aleutian mink disease virus (AMDV) is a severe progressive disease causing multiple different clinical syndromes in mink. In Denmark, the disease is notifiable and under official control. The control programme, based on serological screening, has confined successfully AMDV to the northern part of Denmark. However, re-infections and new introductions of virus into farms require a confirmatory virological test to verify the positive test results of single animals and ultimately to investigate disease transmission. A one step PCR amplifying a 374-base fragment of the NS1 gene of AMDV was compared to the counter-current immune electrophoresis (CIE) routinely used in the serological screening programme. Mink organs (n=299) obtained from 55 recently infected farms and 8 non-infected farms from 2008 to 2010 were tested by PCR, and the results were found to have a high correlation with the serological status of the mink. The relative diagnostic sensitivity of the PCR was 94.7%, and the relative diagnostic specificity was 97.9% when read in parallel with the CIE. PCR positive samples were sequenced and phylogenetic analysis revealed high similarity within the analysed AMDV strains and to AMDV strains described previously.

Neuropathologic Features of Aleutian Disease in Farmed Mink in Ireland and Molecular Characterization of Aleutian Mink Disease Virus Detected in Brain Tissues

A neuropathologic survey was conducted on mink brains from the 5 licensed mink farms in Ireland. The survey was part of a transmissible spongiform encephalopathy surveillance study. Aleutian disease (AD) was present on 4 of the 5 farms (80%). Neuropathologic features of nonsuppurative meningoencephalitis were common in mink from the 4 affected farms but were absent in the mink from the fifth farm, which was free of AD. The meningoencephalitis was characterized by infiltrates of lymphocytes and plasma cells, which were present in meninges, perivascular spaces, and the brain parenchyma. Fibrinoid necrotizing arteritis was seen in 11 mink brains, all of which were obtained from a single farm. Aleutian mink disease virus (AMDV) sequences for the capsid protein VP2 were obtained from brain samples from all affected farms. Although containing previously unreported amino acid residues, similarities with European and North American isolates were observed in the hypervariable regions within VP2, suggesting Irish AMDV is related to those isolates. The predicted amino acid residues, suspected of conferring pathogenicity at certain positions of the VP2 sequence, were present in the viral nucleic acid sequences.

Molecular epidemiology of Aleutian mink disease virus in Finland

Aleutian mink disease virus (AMDV) is a parvovirus that causes an immune complex-mediated disease in minks. To gain a more detailed view of the molecular epidemiology of mink AMDV in Finland, we phylogenetically analysed 14 new Finnish strains from 5 farms and all 40 strains with corresponding sequences available in GenBank. A part of the major non-structural (NS1) protein gene was amplified and analysed phylogenetically. A rooted nucleotide tree was constructed using the maximum parsimony method. The strains described in this study showed 86-100% nucleotide identity and were nearly identical on each farm. The ratio of synonymous to non-synonymous substitutions was approximately 2.7, indicating a mild purifying selection. Phylogenetic analysis confirmed that AMDV strains form three groups (I-III), all of which contained Finnish strains. The tree inferred that the three lineages of AMDV have been introduced to Finland independently. The analysis suggested that AMDV strains do not cluster into genotypes based on geographical origin, year of isolation or pathogenicity. Based on these data, the molecular clock is not applicable to AMDV, and within this gene area no recombination was detected.

Simultaneous persistence of multiple genome variants of human parvovirus B19

The species human parvovirus B19 (B19V) can be divided into three genotypes. In this study, we addressed the question as to whether infection of an individual is restricted to one genotype. As viral DNA is detectable in tissue for long times after acute infection, we examined 87 liver specimens from adults for the presence of B19V DNA. Fifty-nine samples were found to be positive, 32 of them for genotype 1, 27 for genotype 2 and four for genotype 3. In four samples, DNA of two genotypes was detected; samples from three individuals were positive for genotypes 1 and 2 and a sample from one individual was positive for genotypes 1 and 3. Surprisingly, significant sequence heterogeneity was observed at approximately 1 % of the nucleotides of the genotype 1 genomes from individuals with double genotype 1 and 2 infection. Controls using different enzymes for genome amplification and dilutions of the template verified that nucleotide heterogeneity was due to the presence of three or more genome variants of genotype 1. In summary, the evidence shows that individuals can be infected with two different genotypes, and B19V DNA can persist as a population of different genomes. The results may have implications for the understanding of the antiviral immune response and the development of vaccines against B19V.

Comparative analysis reveals frequent recombination in the parvoviruses

Parvoviruses are small single-stranded DNA viruses that are ubiquitous in nature. Infections with both autonomous and helper-virus dependent parvoviruses are common in both human and animal populations, and many animals are host to a number of different parvoviral species. Despite the epidemiological importance of parvoviruses, the presence and role of genome recombination within or among parvoviral species has not been well characterized. Here we show that natural recombination may be widespread in these viruses. Different genome regions of both porcine parvoviruses and Aleutian mink disease viruses have conflicting phylogenetic histories, providing evidence for recombination within each of these two species. Further, the rodent parvoviruses show complex evolutionary histories for separate genomic regions, suggesting recombination at the interspecies level.

High genetic diversity of the VP2 gene of a canine parvovirus strain detected in a domestic cat

This study reports the detection of co-infection by multiple CPV variants and the high genetic complexity of a CPV-2 strain detected in a domestic cat. The CPV variants selected by cloning the VP2 gene were sequenced, and genetic diversity and selection pressure were investigated. Comparison of the nucleotide sequences has evidenced 10 different viral populations, and, in the same animal, more CPV variants coexist. Our analysis excludes the possibility that the recombination events took place during infection and that negative selection acted on the VP2 gene. These findings confirm that CPV-2 shows high genetic heterogeneity resembling the quasispecies found in RNA viruses.

Parvovirus variation for disease: a difference with RNA viruses?

The Parvoviridae, a family of viruses with single-stranded DNA genomes widely spread from invertebrates to mammal and human hosts, display a remarkable evolutionary capacity uncommon in DNA genomes. Parvovirus populations show high genetic heterogeneity and large population sizes resembling the quasispecies found in RNA viruses. These viruses multiply in proliferating cells, causing acute, persistent or latent infections relying in the immunocompetence and developmental stage of the hosts. Some parvovirus populations in natural settings, such as carnivore autonomous parvoviruses or primate adeno associated virus, show a high degree of genetic heterogeneity. However, other parvoviruses such as the pathogenic B19 human erythrovirus or the porcine parvovirus, show little genetic variation, indicating different virus-host relationships. The Parvoviridae evolutionary potential in mammal infections has been modeled in the experimental system formed by the immunodeficient scid mouse infected by the minute virus of mice (MVM) under distinct immune and adaptive pressures. The sequence of viral genomes (close to 10(5) nucleotides) in emerging MVM pathogenic populations present in the organs of 26 mice showed consensus sequences not representing the complex distribution of viral clones and a high genetic heterogeneity (average mutation frequency 8.3 x 10(-4) substitutions/nt accumulated over 2-3 months). Specific amino acid changes, selected at a rate up to 1% in the capsid and in the NS2 nonstructural protein, endowed these viruses with new tropism and increased fitness. Further molecular analysis supported the notion that, in addition to immune pressures, the affinity of molecular interactions with cellular targets, as the Crml nuclear export receptor or the primary capsid receptor, as well as the adaptation to tissues enriched in proliferating cells, are major selective factors in the rapid parvovirus evolutionary dynamics.

Virulent variants emerging in mice infected with the apathogenic prototype strain of the parvovirus minute virus of mice exhibit a capsid with low avidity for a primary receptor

The mechanisms involved in the emergence of virulent mammalian viruses were investigated in the adult immunodeficient SCID mouse infected by the attenuated prototype strain of the parvovirus Minute Virus of Mice (MVMp). Cloned MVMp intravenously inoculated in mice consistently evolved during weeks of subclinical infection to variants showing altered plaque phenotypes. All the isolated large-plaque variants spread systemically from the oronasal cavity and replicated in major organs (brain, kidney, liver), in sharp contrast to the absolute inability of the MVMp and small-plaque variants to productively invade SCID organs by this natural route of infection. The virulent variants retained the MVMp capacity to infect mouse fibroblasts, consistent with the lack of genetic changes across the 220-to-335 amino acid sequence of VP2, a capsid domain containing main determinants of MVM tropism. However, the capsid of the virulent variants shared a lower affinity than the wild type for a primary receptor used in the cytotoxic infection. The capsid gene of a virulent variant engineered in the MVMp background endowed the recombinant virus with a large-plaque phenotype, lower affinity for the receptor, and productive invasiveness by the oronasal route in SCID mice, eventually leading to 100% mortality. In the analysis of virulence in mice, both MVMp and the recombinant virus similarly gained the bloodstream 1 to 2 days postoronasal inoculation and remained infectious when adsorbed to blood cells in vitro. However, the wild-type MVMp was cleared from circulation a few days afterwards, in contrast to the viremia of the recombinant virus, which was sustained for life. Significantly, attachment to an abundant receptor of primary mouse kidney epithelial cells by both viruses could be quantitatively competed by wild-type MVMp capsids, indicating that virulence is not due to an extended receptor usage in target tissues. We conclude that the selection of capsid-receptor interactions of low affinity, which favors systemic infection, is a major evolutionary process in the adaptation of parvoviruses to new hosts and in the cause of disease.

Enhanced cytoplasmic sequestration of the nuclear export receptor CRM1 by NS2 mutations developed in the host regulates parvovirus fitness

To investigate whether a DNA virus can evade passive immunotherapy with a polyclonal antiserum, we analyzed the protection of a neutralizing capsid antiserum against a lethal infection of the immunosuppressive strain of the parvovirus minute virus of mice (MVMi) in 42 immunodeficient mice over a period of 200 days. A few mice were effectively protected, but most developed a delayed lethal leukopenic syndrome during the treatment or weeks afterwards. Unexpectedly, viruses isolated from treated but also from control leukopenic mice showed no amino acid changes throughout the entire capsid coding region, although the viral populations were genetically heterogeneous, mainly in the second exon of the coding sequence of the NS2 nonstructural protein. The NS2 point amino acid changes (T88A, K96E, L103P, and L153 M) that were consistently selected in several mice clustered within the nuclear exportin CRM1 binding domain, in a reading frame that did not alter the overlapping NS1 coding region. These mutations endowed emerging viruses with an increased fitness that was demonstrable by their relative resistance to the neutralizing capsid antiserum in a postentry plaque-forming assay, the rapid overgrowth of a competing wild-type (wt) population in culture, and a larger yield of infectious particles. Mutant NS2 proteins interacted with a higher affinity and sequestered CRM1 in the perinuclear region of the cytoplasm more efficiently than the wt. Correspondingly this phenomenon, as well as the following timely ordered release of the NS1 nonstructural protein and the empty capsid from the nucleus to the cytoplasm, occurred markedly earlier in the infection cycle of the mutant viruses. We hypothesize that the enhanced cytoplasmic sequestration of CRM1 by the NS2 mutations selected in mice may trigger pleiotropic effects leading to an accelerated MVMi life cycle and thus to increased fitness. These results strengthen our earlier report on the rapid evolutionary capacity of this mammalian-specific DNA virus in vivo and indicate that the NS2-CRM1 interaction is an important determinant of parvovirus virulence that can be modulated in nature, hampering the effectiveness of passive antibody therapies in the long term.

Nucleotide sequence and polymerase chain reaction/restriction fragment length polymorphism analyses of Aleutian disease virus in ferrets in Japan

Two ferrets with spontaneous Aleutian disease (AD) were found in Japan. The diagnosis was verified by polymerase chain reaction (PCR) amplification of part of the capsid gene specific to AD virus (ADV). The nucleotide sequences (365 bp in length) of the amplified fragments from the 2 ferrets differed by a single nucleotide, producing an amino acid alteration. Compared with other types of ADV, these isolates had 96% sequence similarity to a published ferret ADV (FADV) in contrast to <91% homology to various types of mink ADV (MADV). The phylogenetic tree of ADVs indicates that these 2 isolates and the published FADV belong to the same genetic group and definitely are divergent from MADVs. The predicted amino acid sequence of the hypervariable segment in the capsid gene was conserved among the 3 types of FADV. These results indicated that the 2 isolates found in Japan were new DNA types of FADV and could have been derived from FADV(s). A restriction fragment length polymorphism (RFLP) method to distinguish the ferret types of ADV from the mink types of ADV was developed on the basis of differences in their nucleotide sequences. Digestion of the PCR products with Afal or ScaI provided different cleavage patterns for FADV and MADV. This PCR/RFLP analysis of the ADV capsid gene will be a valuable asset for diagnosis of this virus infection in ferrets.

Unusual, high genetic diversity of Aleutian mink disease virus

The genetic diversity of Aleutian mink disease virus (AMDV) was examined. Sequences obtained from 35 clinical samples were compared with five published sequences. An unusual, high genetic variability was revealed. Three phylogenetic subgroups of AMDV were identified, and the presence of more than one genotype at some farms was detected.

Epitope mapping of Aleutian mink disease parvovirus virion protein VP1 and 2

Six overlapping fragments of the Aleutian Mink Disease parvoVirus (AMDV) virion protein VP1 and 2 (VP1/2) gene were inserted into the expression vector pMAL-c2. Four of the clones carried large overlapping fragments covering the entire VP1/2 gene. The remaining two clones covered specifically chosen regions within the VP1/2 gene. Using a Western blotting detection system, sera from AMDV-infected mink were tested against the recombinant polypeptides. These studies showed reactions primarily directed against the two AMDV polypeptides ranging from amino acids 297 to 518. Weaker reactions against other regions of the VP1/2 were also observed. The small fusion protein designed to cover the presumed AMDV VP1/2 loop 4 was purified by affinity chromatography and used to develop solid-phase immunoassays. Twelve small synthetic peptides were constructed and used as inhibitors. A peptide covering amino acids S428 to T448 was shown to block the reactivity of a pool of positive mink sera, indicating the presence of one dominant linear epitope.

Construction of pathogenic molecular clones of Aleutian mink disease parvovirus that replicate both in vivo and in vitro

The ADV-G isolate of Aleutian mink disease parvovirus (ADV) replicates permissively in Crandell feline kidney (CRFK) cells but is nonpathogenic for mink, whereas the highly pathogenic ADV-Utah isolate is nonviable in CRFK cells. To assign control of host range in CRFK cells and pathogenicity to specific regions of the ADV genome, we constructed a full-length molecular clone chimeric between ADV-G and ADV-Utah. If either the map unit (MU) 54-65 (clone G/U-5) or MU 65-88 (clone G/U-7) sections were ADV-Utah, viability in CRFK cells was abolished, thus indicating that in vitro host range was controlled by two independent determinants: A in the MU 54-65 segment and B in the MU 65-88 segment. Determinant B could be divided into two subregions, B1 (MU 65-69) and B2 (MU 73-88), neither of which alone could inhibit replication in CRFK cells, an observation suggesting that expression of the B determinant required interaction between noncontiguous sequences. Adult mink of Aleutian genotype inoculated with G/U-8 or G/U-10 developed viremia, antiviral antibody, and histopathological changes characteristic of progressive Aleutian disease. The capsid sequences of G/U-8 and G/U-10 differed from ADV-G at five and four amino acid residues, respectively. Our results suggested that the host range and pathogenicity of ADV are regulated by sequences in the capsid protein gene.

Expression of Aleutian mink disease parvovirus capsid proteins in defined segments: localization of immunoreactive sites and neutralizing epitopes to specific regions

The capsid proteins of the ADV-G isolate of Aleutian mink disease parvovirus (ADV) were expressed in 10 nonoverlapping segments as fusions with maltose-binding protein in pMAL-C2 (pVP1, pVP2a through pVP2i). The constructs were designed to capture the VP1 unique sequence and the portions analogous to the four variable surface loops of canine parvovirus (CPV) in individual fragments (pVP2b, pVP2d, pVP2e, and pVP2g, respectively). The panel of fusion proteins was immunoblotted with sera from mink infected with ADV. Seropositive mink infected with either ADV-TR, ADV-Utah, or ADV-Pullman reacted preferentially against certain segments, regardless of mink genotype or virus inoculum. The most consistently immunoreactive regions were pVP2g, pVP2e, and pVP2f, the segments that encompassed the analogs of CPV surface loops 3 and 4. The VP1 unique region was also consistently immunoreactive. These findings indicated that infected mink recognize linear epitopes that localized to certain regions of the capsid protein sequence. The segment containing the hypervariable region (pVP2d), corresponding to CPV loop 2, was also expressed from ADV-Utah. An anti-ADV-G monoclonal antibody and a rabbit anti-ADV-G capsid antibody reacted exclusively with the ADV-G pVP2d segment but not with the corresponding segment from ADV-Utah. Mink infected with ADV-TR or ADV-Utah also preferentially reacted with the pVP2d sequence characteristic of that virus. These results suggested that the loop 2 region may contain a type-specific linear epitope and that the epitope may also be specifically recognized by infected mink. Heterologous antisera were prepared against the VP1 unique region and the four segments capturing the variable surface loops of CPV. The antisera against the proteins containing loop 3 or loop 4, as well as the anticapsid antibody, neutralized ADV-G infectivity in vitro and bound to capsids in immune electron microscopy. These results suggested that regions of the ADV capsid proteins corresponding to surface loops 3 and 4 of CPV contain linear epitopes that are located on the external surface of the ADV capsid. Furthermore, these linear epitopes contain neutralizing determinants. Computer comparisons with the CPV crystal structure suggest that these sequences may be adjacent to the threefold axis of symmetry of the viral particle.

Identification of a DNA segment in ferret Aleutian disease virus similar to a hypervariable capsid region of mink Aleutian disease parvovirus

A 401bp DNA fragment of ferret Aleutian disease virus (ADV) was amplified using PCR primers spanning a hypervariable region of mink ADV capsid sequence. The amplified fragment was 88-89% homologous to the same region of previously known sequence of three different strains of mink ADV, however, as low as 54% homology was observed when compared with a 39bp segment known as hypervariable region. Within the predicted 13 amino acid hypervariable region, the ferret ADV sequence differed at 6 positions from the wild type mink Utah1 strain. Three amino acids Gln289, Glu293 and Thr295 in this region were common to the pathogenic ferret ADV, mink Utah1 and ADVK strains, but differed from the cell culture adapted nonpathogenic mink strain ADVG suggesting that these three conserved residues may have some functional significance.

The relationship between capsid protein (VP2) sequence and pathogenicity of Aleutian mink disease parvovirus (ADV): a possible role for raccoons in the transmission of ADV infections

Aleutian mink disease parvovirus (ADV) DNA was identified by PCR in samples from mink and raccoons on commercial ranches during an outbreak of Aleutian disease (AD). Comparison of DNA sequences of the hypervariable portion of VP2, the major capsid protein of ADV, indicated that both mink and raccoons were infected by a new isolate of ADV, designated ADV-TR. Because the capsid proteins of other parvoviruses play a prominent role in the determination of viral pathogenicity and host range, we decided to examine the relationship between the capsid protein sequences and pathogenicity of ADV. Comparison of the ADV-TR hypervariable region sequence with sequences of other isolates of ADV revealed that ADV-TR was 94 to 100% related to the nonpathogenic type 1 ADV-G at both the DNA and amino acid levels but less than 90% related to other pathogenic ADVs like the type 2 ADV-Utah, the type 3 ADV-ZK8, or ADV-Pullman. This finding indicated that a virus with a type 1 hypervariable region could be pathogenic. To perform a more comprehensive analysis, the complete VP2 sequence of ADV-TR was obtained and compared with that of the 647-amino-acid VP2 of ADV-G and the corresponding VP2 sequences of the pathogenic ADV-Utah, ADV-Pullman, and ADV-ZK8. Although the hypervariable region amino acid sequence of ADV-TR was identical to that of ADV-G, there were 12 amino acid differences between ADV-G and ADV-TR. Each of these differences was at a position where other pathogenic isolates also differed from ADV-G. Thus, although ADV-TR had the hypervariable sequence of the nonpathogenic type 1 ADV-G, the remainder of the VP2 sequence resembled sequences of other pathogenic ADVs. Under experimental conditions, ADV-TR and ADV-Utah were highly pathogenic and induced typical AD in trios of both Aleutian and non-Aleutian mink, whereas ADV-Pullman was pathogenic only for Aleutian mink and ADV-G was noninfectious. Trios of raccoons experimentally inoculated with ADV-TR and ADV-Utah all became infected with ADV, but only a single ADV-Pullman-inoculated raccoon showed evidence of infection. Furthermore, none of the ADV isolates induced pathological findings of AD in raccoons. Finally, when a preparation of ADV-TR prepared from infected raccoon lymph nodes was inoculated into mink and raccoons, typical AD was induced in Aleutian and non-Aleutian mink, but raccoons failed to show serological or pathological evidence of infection. These results indicated that raccoons can become infected with ADV and may have a role in the transmission of virus to mink but that raccoon-to-raccoon transmission of ADV is unlikely.

Parvovirus B19 infection

Human parvovirus B19, discovered in 1974, is a single-stranded DNA virus which causes erythema infectiosum, arthralgia, aplastic crisis in patients with red cell defects, chronic anaemia in immunocompromised patients, and fetal hydrops. Seroprevalence in developed countries is 2-10% in children less than 5 years, 40-60% in adults more than 20 years, and 85% or more in those over 70 years. The virus may be transmitted by the respiratory route and by transfusion of infected blood and blood products. After an incubation period of six to eight days, viraemia occurs, during which reticulocyte numbers fall dramatically resulting in a temporary drop in haemoglobin of 1 g/dl in a normal person. Clearance of viraemia is dependent on development of specific antibody to the B19 structural proteins, VP1 and VP2. The red cell receptor for the virus is blood group P antigen. Diagnosis in immunocompetent persons depends on detection of specific IgM in serum. Diagnosis in immunocompromised persons depends on detection of B19 antigen or DNA in serum. There is no specific treatment for B19 infection; however, human normal immunoglobulin may be used as a source of specific antibody in chronically infected persons. A recombinant parvovirus B19 vaccine is under development.

Genetic diversity in the non-structural gene of parvovirus B19 detected by single-stranded conformational polymorphism assay (SSCP) and partial nucleotide sequencing

A homologous region in the parvovirus B19 non-structural gene (B19 nt 1399-1682) was examined in 50 samples from patients with a wide variety of B19-related disease from various countries by PCR amplification, single-stranded conformational polymorphism (SSCP) assay and nucleotide sequencing. Five SSCP types were confirmed by nucleotide sequence analysis. Of a total of 6 mutations, all were silent. Types 3 and 4 accounted for 92% of strains. There was no correlation between genome type and either clinical illness or patient age. However, there was a correlation between SSCP type and country of origin. Type 3 strains predominated in Japan (18/26) and the UK (6/8), whereas type 4 predominated in the USA (9/12). Notably, type 3 strains also predominated among females (14/18), whereas there were approximately equal numbers of strain types 3 (7/17) and 4 (8/17) among males; an observation which remains unexplained. Within the Japanese group, although type 3 strains predominated overall, strains isolated from 1981 to 1987 consisted of types 1 (2/15), 2 (1/15), 3 (8/15), and 4 (4/15), whereas strains isolated from 1990 to 1994 consisted almost entirely of type 3 (10/11).

Parvovirus B19 infection--persistence and genetic variation

53 patients with acute B19 infection were studied; symptoms at acute infection were rash and arthralgia (n = 26), rash (n = 7), arthralgia (n = 16), aplastic crisis (n = 3), and intrauterine fetal death (n = 1). These patients were followed for 26-85 months (mean 57 months) and re-assessed for persistent symptoms, anti-B19 antibodies, and B19 DNA. At follow-up, 7 individuals were positive for serum B19 DNA, compared with none of the controls (2-tailed p value = 0.016). All 7 of those persistently infected were women, 3 of whom had symptoms; 1 had a chronic haemolytic anaemia (initial presentation was aplastic crisis); 1 had persistent arthralgia in both knees (initial presentation was bilateral knee arthralgia); and 1 had arthralgia in one knee and chronic fatigue syndrome (initial presentation was bilateral arthralgia in knees and shoulders). For the 7 persistently infected patients, serum from the time of diagnosis of acute B19 infection was available for 4, all of which contained B19 DNA. With single-stranded conformational polymorphism (SSCP) assay of these 11 PCR products, identical SSCP types were demonstrated in 5 of 7 follow-up isolates. In 2 of the 4 cases for which both acute and follow-up PCR product was available, the SSCP type of the follow-up product was different from that of the acute product. Two B19 virus types were demonstrated in one patient (with persistent arthralgia and chronic fatigue syndrome) at follow-up assessment.

Sequence comparison of the non-structural genes of four different types of Aleutian mink disease parvovirus indicates an unusual degree of variability

The present work shows that at least four different sequence types of Aleutian mink disease parvovirus (ADV) are present in ADV isolates from mink. We here report the nucleotide sequences of these four types of ADV from nucleotide 123 to 2208 (map unit 3 to 46). This part of the genome encodes three non-structural (NS) proteins of ADV. Comparison of the deduced amino acid sequences of these NS proteins showed that the ADV proteins are much less conserved than the NS proteins from other members of the autonomous group of parvoviruses. In general, we found that the middle region of the ADV NS-1 protein was relatively well conserved among the types, while both the amino- and carboxy-terminal ends of the protein had higher amino acid variability. Interestingly, the putative NS-3 protein from type 3 ADV is truncated in the carboxy-terminal end. The molecular evolutionary relationship among the four types of ADV was examined. This analysis, taken together with the unusually high degree of variability of the ADV types, indicates that the ADV infection in mink is likely to be an old infection compared to the other parvovirus infections or, alternatively, that ADV accumulates sequence changes much faster than other parvoviruses.

Acute interstitial pneumonia in mink kits inoculated with defined isolates of Aleutian mink disease parvovirus

The present study addressed the causal role of Aleutian mink disease parvovirus (ADV) in acute interstitial pneumonia in mink kits. All the examined isolates of ADV caused interstitial pneumonia in newborn kits, although the severity of disease and the mortality varied. These findings indicate that ADV is the direct causal agent of this disease in mink kits and that cofactors, which could have been present in the original ADV-K isolate, do not play a role. Acute interstitial pneumonia characterized by hypertrophy and hyperplasia of alveolar type II cells, intranuclear viral inclusions, interstitial edema, and hyaline membrane formation was experimentally reproduced in mink kits infected as newborns with five different isolates of ADV. Four hundred forty-nine newborn mink kits were included in the study, of which 247 were necropsied. The lesions caused by the different isolates were indistinguishable by histopathologic examination, but the incidence (50-100%) and severity (mortality of 30-100%, n = 218) of disease among the mink kits varied. Also, the content of ADV antigens in the lungs of infected kits varied among the groups. According to these features, the examined isolates could be placed in groups of high and low virulence. ADV-K, ADV-Utah I, and ADV-DK were in a highly virulent group producing a mortality of 90-100% (n = 110) in mink inoculated as newborns. ADV-GL and ADV-Pullman belonged to a group of low virulence, with an incidence of clinical disease of 50-70% and a mortality of approximately 30-50% (n = 118) in kits inoculated as newborns. The mortality in the control group receiving a mock inoculum was around 12% (n = 34). The period from infection to development of fatal disease varied from approximately 12 days for the highly virulent isolates up to around 20 days for the isolates of low virulence. The 107 mink kits that survived inoculation with ADV as newborns developed lesions typical of classical Aleutian disease irrespective of the ADV isolate used. The lesions consisted of chronic immune complex-mediated glomerulonephritis and infiltrations with mononuclear cells, including plasma cells in lung, liver, spleen, kidney, mesenteric lymph node, and intestine. Surviving kits also had hypertrophy of the bronchus-associated lymphoid tissue and focal subpleural, intraalveolar accumulations of large cells with foamy cytoplasm, so-called lipid pneumonia.

Characterization of chimeric full-length molecular clones of Aleutian mink disease parvovirus (ADV): identification of a determinant governing replication of ADV in cell culture

The ADV-G strain of Aleutian mink disease parvovirus (ADV) is nonpathogenic for mink but replicates permissively in cell culture, whereas the ADV-Utah 1 strain is highly pathogenic for mink but replicates poorly in cell culture. In order to relate these phenotypic differences to primary genomic features, we constructed a series of chimeric plasmids between a full-length replication-competent molecular clone of ADV-G and subgenomic clones of ADV-Utah 1 representing map units (MU) 15 to 88. After transfection of the plasmids into cell culture and serial passage of cell lysates, we determined that substitution of several segments of the ADV-Utah 1 genome (MU 15 to 54 and 65 to 73) within an infectious ADV-G plasmid did not impair the ability of these constructs to yield infectious virus in vitro. Like ADV-G, the viruses derived from these replication-competent clones caused neither detectable viremia 10 days after inoculation nor any evidence of Aleutian disease in adult mink. On the other hand, other chimeric plasmids were incapable of yielding infectious virus and were therefore replication defective in vitro. The MU 54 to 65 EcoRI-EcoRV fragment of ADV-Utah 1 was the minimal segment capable of rendering ADV-G replication defective. Substitution of the ADV-G EcoRI-EcoRV fragment into a replication-defective clone restored replication competence, indicating that this 0.53-kb portion of the genome, wholly located within shared coding sequences for the capsid proteins VP1 and VP2, contained a determinant that governs replication in cell culture. When cultures of cells were studied 5 days after transfection with replication-defective clones, rescue of dimeric replicative form DNA and single-stranded progeny DNA could not be demonstrated. This defect could not be complemented by cotransfection with a replication-competent construction.

Comparison of promoter activity in Aleutian mink disease parvovirus, minute virus of mice, and canine parvovirus: possible role of weak promoters in the pathogenesis of Aleutian mink disease parvovirus infection

Aleutian mink disease parvovirus (ADV) infection causes both acute and chronic disease in mink, and we have previously shown that it is the level of viral gene expression that determines the disease pattern. To study the gene regulation of ADV, we have cloned the P3 ADV and P36 ADV promoters in front of a reporter gene, the chloramphenicol acetyltransferase (CAT) gene, and analyzed these constructs by transient transfection in a feline kidney cell line and mouse NIH 3T3 cells. The genes for ADV structural proteins (VP1 and VP2) and the nonstructural proteins (NS-1, NS-2, and NS-3) were cloned into a eukaryotic expression vector, and their functions in regulation of the P3 ADV and P36 ADV promoters were examined in cotransfection experiments. The ADV NS-1 protein was able to transactivate the P36 ADV promoter and, to a lesser degree, the P3 ADV promoter. Constitutive activities of the P3 ADV and P36 ADV promoters were weaker than those of the corresponding promoters from the prototypic parvovirus minute virus of mice (MVM) and canine parvovirus (CPV). Also, the level of transactivation of the P36 ADV promoter was much lower than those of the corresponding P38 MVM and P38 CPV promoters transactivated with MVM NS-1. Moreover, the ADV NS-1 gene product could transactivate the P38 MVM promoter to higher levels than it could transactivate the P36 ADV promoter, while the P36 ADV promoter could be transactivated by MVM NS-1 and ADV NS-1 to similar levels. Taken together, these data indicated that cis-acting sequences in the P36 ADV promoter play a major role in determining the low level of transactivation observed. The P3 ADV and P4 MVM promoters could be transactivated to some degree by their respective NS-1 gene products. However, in contrast to the situation for the late promoters, switching NS-1 proteins between the two viruses was not possible. This finding may indicate a different mechanism of transactivation of the early promoters (P3 ADV and P4 MVM) compared with the late (P36 ADV and P38 MVM) promoters. In summary, the constitutive levels of expression from the ADV promoters are weaker than the levels from the corresponding promoters of MVM and CPV. Moreover, the level of NS-1-mediated transactivation of the late ADV promoter is impaired compared with the level of transactivation of the late promoters of MVM and CPV.(ABSTRACT TRUNCATED AT 400 WORDS)

cis-acting sequences in the Aleutian mink disease parvovirus late promoter important for transcription: comparison to the canine parvovirus and minute virus of mice

We are currently investigating the regulation of transcription of the Aleutian mink disease parvovirus (ADV). ADV causes a chronic immune complex-mediated condition known as classical Aleutian disease, characterized by slow viral replication. This slow replication is an intrinsic property of ADV and distinguishes it from the more prototypic parvoviruses such as minute virus of mice (MVM) and canine parvovirus (CPV). We have previously suggested a role for the weak ADV promoters in the slow replication and thereby the absence of acute cytopathology and instead establishment of persistent ADV infection with progressive immune complex-mediated chronic lesions. In this study, we have mapped the cis-acting sequences around the ADV P36 promoter responsible for both constitutive transcription and transactivation mediated by the nonstructural protein 1. The mapping was performed by using endpoint deletions of the ADV P36 promoter and by making chimeras between the ADV P36 and MVM P38 promoters. We found the weak constitutive activity of the ADV P36 promoter to be caused by suboptimal promoter proximal sequences, while the low level of transactivation was caused mainly by an upstream region including sequences with homology to the transactivation responsive element (tar) of the H-1 parvovirus (M.-L. Gu, F.-X. Chen, and S. L. Rhode, Virology 187:10-17, 1992). We also found the corresponding regions in the MVM and CPV P38 promoters to be important for transactivation of these promoters by making 5' deletions of the promoter region. In addition, it was found that MVM tar-like and upstream sequences could transfer high nonstructural protein 1 responsiveness to the ADV promoter even though the distance between the tar-like element and the TATA box was significantly changed. On the basis of comparative data for ADV, MVM, CPV, and H-1, a new clustered motif (TTGGTT) is proposed to be the responsive cis-acting element for transactivation. Homology comparison of the specific transcriptional elements of the ADV P36, MVM P38, and CPV P38 promoters suggests that few, but crucial, changes in the ADV P36 promoter and upstream region are responsible for the weak constitutive activity and low level of transactivation of the ADV P36 promoter.

Identification of alternatively spliced mRNAs encoding potential new regulatory proteins in cattle infected with bovine leukemia virus

The polymerase chain reaction was used to detect and characterize low-abundance bovine leukemia virus (BLV) mRNAs. In infected cattle we could detect spliced mRNA with a splice pattern consistent with a Tax/Rex mRNA, as well as at least four alternatively spliced RNAs. Two of the alternatively spliced mRNAs encoded hitherto unrecognized BLV proteins, designated RIII and GIV. The Tax/Rex and alternatively spliced mRNAs could be detected at their highest levels in BLV-infected cell cultures; the next highest levels were found in samples from calves experimentally infected at 6 weeks postinoculation. Alternatively spliced mRNAs were also expressed, albeit at lower levels, in naturally infected animals; they were detected by a nested polymerase chain reaction. Interestingly, the GIV mRNA was specifically detected in naturally infected cows with persistent lymphocytosis and in two of five calves at 6 months after experimental infection with BLV. Furthermore, the calf with the strongest signal for GIV had the highest lymphocyte counts. These data may suggest a correlation between expression of the GIV product and development of persistent lymphocytosis. Some of the donor and acceptor sites in the alternatively spliced mRNAs were highly unusual. The biological mechanisms and significance of such a choice of unexpected splice sites are currently unknown.