Characterisation of Toxoplasma gondii isolates using polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) of the non-coding Toxoplasma gondii (TGR)-gene sequences

Tracking transmission of the zoonosis Toxoplasma gondii

Toxoplasma gondii is a highly successful parasite that infects many host species and has colonised a wide range of habitats. Review of the parasite's life cycle demonstrates that it has become adapted to exploit multiple routes of transmission through a sexual cycle in the definitive host and asexually, through carnivory, and by vertical transmission. These alternative routes may operate synergistically to enhance transmission, but they might also provide a vehicle for selection leading to partitioning of strains in the environment. Genetic analysis has shown that parasite population structure varies globally. In South America, there is high strain diversity while in North America, Europe and Africa three clonal strain types predominate. This may imply a shift from sexual to asexual transmission. Mapping of the parasite genome has provided a wealth of markers for strain characterisation. Close genotyping of isolates gives evidence of multiple infection and recombination in natural populations and reveals differences in both the distribution and the phenotype of strains. More intensive epidemiological studies are now required to unravel the networks of transmission operating within defined habitats.

Diagnostic value of specific local antibody production and nucleic acid amplification technique-nested polymerase chain reaction (nPCR) in clinically suspected ocular toxoplasmosis

OBJECTIVE

The study was to evaluate the diagnostic efficacy of nested polymerase chain reaction (nPCR) using primers targeting B1 gene of Toxoplasma gondii (T. gondii) with Witmer Desmonts coefficient (WDC) technique in intraocular fluids of clinically suspected toxoplasma retino choroiditis (TRC) patients.

MATERIALS AND METHODS

Two hundred and seventy eight specimens from 189 patients (25 TRC patients and 164 controls) consisting of 189 serum samples and 89 intraocular fluids were included in the study. The clinical specimens were categorized into TRC patients (typical TRC lesion-group I & atypical TRC lesion-group II) and controls (voluntary blood donors-group III, patients undergoing uncomplicated cataract surgery-group IV, ocular inflammation of non-toxoplasma origin-group V). Detection of anti T. gondii IgG and IgM antibodies in serum samples and intraocular fluids were performed and WDC was calculated by the standard method. The standardized nPCR was applied on the 89 intraocular fluids.

RESULTS

Clinical diagnosis of TRC based on fundus examination was considered to be the "gold standard." Anti T. gondii IgG/IgM antibodies were detected in serum by ELISA in 95.6% of 25 clinically suspected TRC patients (gp I and II), 28% of gp III, 40.4% of gp IV, and in 58.3% of gpV. Witmer Desmont's coefficient was positive in 72.7% (16/22) and nPCR in 59.1% (13/22) of TRC patients (gp I and II). Both WDC and nPCR were negative in all the controls. The difference in sensitivity of WDC and nPCR was not statistically significant (p=0.5247).

CONCLUSIONS

Though both WDC and nPCR were reliable diagnostic techniques for ocular toxoplasmosis, nPCR is more acceptable because of the amount of specimen(s) required, rapidity, cost effectiveness, and direct evidence of T. gondii DNA in the intraocular fluids.

Genetic characteristics of the Korean isolate KI-1 of Toxoplasma gondii

Toxoplasma gondii tachyzoites were isolated from an ocular patient in the Republic of Korea and maintained in the laboratory (designated KI-1). In the present study, its genotype was determined by analyzing dense granule antigen 6 (GRA6) gene and surface antigen 2 (SAG2) gene as typing markers. Digestion of the amplification products of GRA6 and of the 5o and 3o ends of SAG2, respectively, with Mse I, Sau3A I, and Hha I, revealed that KI-1 is included in the genotype I, which includes the worldwide virulent RH strain. In addition, when the whole sequences of the coding regions of SAG1, rhoptry antigen 1 (ROP1), and GRA8 genes of KI-1 were compared with those of RH, minor nucleotide polymorphisms and amino acid substitutions were identified. These results show that KI-1 is a new geographical strain of T. gondii that can be included in the genotype I.

Structure analysis of two Toxoplasma gondii and Neospora caninum satellite DNA families and evolution of their common monomeric sequence

A family of repetitive DNA elements of approximately 350 bp-Sat350-that are members of Toxoplasma gondii satellite DNA was further analyzed. Sequence analysis identified at least three distinct repeat types within this family, called types A, B, and C. B repeats were divided into the subtypes B1 and B2. A search for internal repetitions within this family permitted the identification of conserved regions and the design of PCR primers that amplify almost all these repetitive elements. These primers amplified the expected 350-bp repeats and a novel 680-bp repetitive element (Sat680) related to this family. Two additional tandemly repeated high-order structures corresponding to this satellite DNA family were found by searching the Toxoplasma genome database with these sequences. These studies were confirmed by sequence analysis and identified: (1). an arrangement of AB1CB2 350-bp repeats and (2). an arrangement of two 350-bp-like repeats, resulting in a 680-bp monomer. Sequence comparison and phylogenetic analysis indicated that both high-order structures may have originated from the same ancestral 350-bp repeat. PCR amplification, sequence analysis and Southern blot showed that similar high-order structures were also found in the Toxoplasma-sister taxon Neospora caninum. The Toxoplasma genome database (http://ToxoDB.org ) permitted the assembly of a contig harboring Sat350 elements at one end and a long nonrepetitive DNA sequence flanking this satellite DNA. The region bordering the Sat350 repeats contained two differentially expressed sequence-related regions and interstitial telomeric sequences.

Analysis of the SAG5 locus reveals a distinct genomic organisation in virulent and avirulent strains of Toxoplasma gondii

We have recently characterised, in the virulent strain RH of Toxoplasma gondii, three glycosylphosphatidylinositol-anchored surface antigens related to SAG1 (p30) and encoded by highly homologous, tandemly arrayed genes named SAG5A, SAG5B and SAG5C. In the present study, we compared the genomic organisation of the SAG5 locus in strains belonging to the three major genotypes of T. gondii. Southern blot analysis using a SAG5-specific probe produced two related but distinct hybridisation patterns, one exclusive of genotype I virulent strains, the other shared by avirulent strains of either genotype II or genotype III. To understand the molecular bases of this intergenotypic heterogeneity, we cloned and sequenced the SAG5 locus in the genotype II strain Me49. We found that in this isolate the SAG5B gene is missing, with SAG5A and SAG5C laying contiguously. This genomic arrangement explains the hybridisation profiles observed for all the avirulent strains examined and indicates that the presence of SAG5B is a distinctive trait of genotype I. Furthermore, we identified two novel SAG1-related genes, SAG5D and SAG5E, mapping respectively 1.8 and 4.0 kb upstream of SAG5A. SAG5D is transcribed in tachyzoites and encodes a polypeptide of 362 amino acids sharing 50% identity with SAG5A-C, whereas SAG5E is a transcribed pseudogene. We also evaluated polymorphisms at the SAG5 locus by comparing the coding regions of SAG5A-E from strains representative of the three archetypal genotypes. In agreement with the strict allelic dimorphism of T. gondii, we identified two alleles for SAG5D, whereas SAG5A, SAG5C and SAG5E were found to be three distinct nucleotide variants. The higher intergenotypic polymorphism of SAG5A, SAG5C and SAG5E suggests that these genes underwent a more rapid genetic drift than the other members of the SAG1 family. Finally, we developed a new PCR-restriction fragment length polymorphism method based on the SAG5C gene that is able to discriminate between strains of genotype I, II and III by a single endonuclease digestion.

Laboratory passage and characterization of an isolate of Toxoplasma gondii from an ocular patient in Korea

Toxoplasma gondii tachyzoites were isolated from the blood of an ocular patient, and have been successfully passaged in the laboratory, for over a year, by peritoneal inoculation in mice. The isolated parasite was designated the Korean Isolate-1 (KI-1) and its characteristics were compared with those of the RH strain, a wellknown virulent strain originating from a child who suffered from encephalitis. The morphology, pathogenicity, infectivity and cell culture characteristics of the KI-1 were similar to those of the RH strain. Both RH and KI-1 antigens were detected by an anti-T. gondii monoclonal antibody (mAb), Tg563, against the major surface protein SAG1 (30 kDa), whereas no reaction was observed against an anti-Neospora caninum mAb, 12B4. The KI-1 was confirmed as an isolate of T. gondii. A long-term laboratory maintenance and characterization of a local T. gondii isolate is reported for the first time in the Republic of Korea.

MGE-PCR: a novel approach to the analysis of Toxoplasma gondii strain differentiation using mobile genetic elements

The position of mobile genetic elements (MGE) within eukaryotic genomes is often highly variable and we have exploited this phenomenon to develop a novel approach to strain differentiation in Toxoplasma gondii. Two PCR based strategies were designed in which specific primers were used to amplify T. gondii MGE's revealing information on element size and positional variation. The first PCR strategy involved the use of a standard two primer PCR while the second strategy used a single specific primer in a step-up PCR protocol. This approach was applied to T. gondii reference strains which were either acute virulent or avirulent to mice. The use of a standard two primer PCR reaction revealed the presence of a virulence related marker in which all avirulent strains possessed an additional 688 bp band. The single primer PCR strategy demonstrated that all virulent strains had identical banding patterns suggesting invariance within this group of strains. However, all avirulent strains had different banding patterns indicating the presence of a number of individual lineages within this group. The applicability and sensitivity of MGE-PCR in epidemiological studies was demonstrated by direct amplification of T. gondii from sheep tissue samples. All sheep isolates, tested in this way, gave identical banding patterns suggesting the presence of an endemic Toxoplasma strain on this farm.