The unique genetic variation within the O174L gene of Polish strains of African swine fever virus facilitates tracking virus origin
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More than ten years after the emergence of African swine fever (ASF) in Georgia, the disease continues to pose a serious threat not only to European countries, which are leading pork producers, but also to worldwide trade of swine. The front of the ASF zone is constantly expanding westward, and recently, the first cases were reported in Belgium, suggesting a crucial role of human activity in ASF spread, considering the exceptional resistance of the causal agent to environmental conditions [1, 2, 3, 4]. Moreover, the disease was confirmed in numerous pig herds in China, raising worldwide concern about an approaching ASF pandemic . In Poland, the disease continues to spread into new areas. At the end of 2017, ASF cases were confirmed in wild boars in the area surrounding Warsaw, indicating an unexpected incursion of the disease into a previously ASF-free area of the country. From 2014 to the end of 2018, a total of 3347 cases in wild boars and 213 outbreaks in pig herds were reported in Poland .
Previous reports on the genetic diversity of African swine fever virus (ASFV) have reported a low level of genetic variability among European isolates [7, 8, 9, 10, 11, 12]. Recent investigations of ASFV genotypes have been based on sequencing of the B646L and E183L genes and analysis of the intergenic region (IGR) between I73R and I329L genes [7, 8]. Another study of the molecular evolution of the EP402R and MGF505-2R genes of 67 Polish strains revealed minor genetic diversity within these genes, indicating slow but consistent molecular evolution in these regions .
Analysis of the whole genome sequences of seven Polish ASFV isolates revealed 14-nucleotide sequence variation within the O174L gene, which encodes the DNA polymerase beta-like protein . Analysis of 46 ASFV isolates collected from ASF cases in wild boars and outbreaks in pigs between 2014 and 2018 in Poland confirmed the presence of a specific mutation – a 14-nucleotide insertion that might be useful to distinguish closely related ASFV isolates belonging to the highly pathogenic genotype II. This might aid in achieving a better understanding of ASFV transmission routes and sources of infection in Eastern and Central European countries. In this brief report, we present the identification of a new ASFV genetic marker within the nucleotide sequences of some ASFV Polish isolates. A spatiotemporal analysis in combination with phylogenetic data partially allowed the spread of ASFV in Poland to be traced.
African swine fever isolates originating from Poland from 2014 to 2018, selected for a study of genetic variation within the O174L gene. Isolates containing a mutation resulting in the insertion of 14 nucleotides within the O174L gene are indicated by gray shading
An alignment of O174L nucleotide sequences of ASFV revealed the presence of an unique variation in 12 isolates originating from Poland. An insertion of 14 nucleotides (TCACTACTGAAAAA) was found, corresponding to a tandem repeat of nt 128282-128283 with reference to the Georgia 2007/1 strain. This insertion resulted in a shift to a different reading frame containing a premature stop codon. The wild-type and mutated proteins thus shared only 93% amino acid sequence identity. The disrupted O174L gene encodes a reparative DNA polymerase belonging to the family X DNA polymerases . The ASFV polymerase X is able to efficiently repair single-nucleotide DNA breaks by base-excision repair (BER) during viral infection . Previous studies have shown that deletion of the ASFV polymerase X gene results in accumulation of DNA damage with an increase of mutation frequency in swine macrophages . Therefore, this particular gene is essential for maintaining viral genetic information. However, the effect of the observed insertion on the function of the polymerase remains unknown and needs to be investigated.
Analysis of whole genome sequences of nine Polish ASFV isolates with confirmatory sequencing by the Sanger method revealed a new genetic marker within the O174L gene that might be useful for distinguishing closely related ASFV isolates in Poland. The data suggest that the simultaneous emergence of ASFV on domestic pig farms in various locations in Poland was probably due to multiple independent, unrelated disease outbreaks. However, an involvement of human activity in the spread of ASF in Poland, particularly into previously ASF-free and geographically distant areas could not be excluded, since the migratory activity of wild boars is very limited . The genetic variability within the O174L gene provides an opportunity for deeper insight into the spread of ASFV in Poland and provides evidence of a relationship between geographically separated outbreaks, as was the case with the emergence of ASFV in the region surrounding Warsaw, where the virus seems to be to have been imported from the region of Biała Podlaska. The predicted amino acid protein sequence differs moderately from the original ASFV Georgia 2007/1 strain and shows only 93% sequence identity, whereas the encoded DNA polymerase is highly conserved among various ASFV genotypes (Fig. 1). The duplication of the 14-nt long sequence seems to have occurred suddenly, since no intermediate sequences between original and mutated sequences were detected, indicating that this tandem repeat has a higher evolution rate that the whole ASFV genome, a phenomenon that has been observed with other DNA viruses . Genetic data like those presented in this report are essential for the study of ASFV evolution. However, an investigation of the effect of the variation described here on biology of the virus is still required.
The datasets generated and analyzed in the current study are available in the GenBank repository under accession numbers MH764305-MH764350.
Compliance with ethical standards
This study was supported by the National Science Centre (grant no. UMO-2016/21/D/NZ6/00974) and ‘KNOW’ (Leading National Research Centre) Scientific Consortium, ‘Healthy Animal - Safe Food’, a decision of the Ministry of Science and Higher Education, no. 05-1/KNOW2/2015.
Conflict of interest
The authors declare that they have no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
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