Genotyping of Plasmodium vivax by minisatellite marker and its application in differentiating relapse and new infection

Background Plasmodium vivax malaria is a major public health problem in India. Control of vivax malaria is challenging due to various factors including relapse which increase the burden significantly. There is no well studied marker to differentiate relapse from reinfection. This creates hindrance in search for anti-relapse medicines. The genomic study of minisatellite can help in characterization of relapse and new infection of vivax malaria. Methods Eighty-eight samples of P. vivax were collected from malaria clinic. All the 14 chromosomes of P. vivax were scanned for minisatellite marker by Tandem Repeat Finder software Version 4.07b. Minisatellite marker CH1T1M13779 from chromosome one was applied for genotyping in 88 samples of P. vivax including 2 recurrence cases. Results Whole genome of P. vivax was scanned and found to have one hundred minisatellite markers. CH1T1M13779 minisatellite marker from chromosome-1 was used for amplification in 88 samples of P. vivax. Of 66 amplified samples, 14 alleles were found with varied allele frequency. The base size of 280 (13.63 %) 320 bp (13.63 %) and 300 bp (16.66 %) showed the predominant allele in the P. vivax population. Genotyping of two paired samples (day 0 and day relapse) could demonstrate the presence of relapse and reinfection. Conclusion The CH1T1M13779 can be potential minisatellite marker which can be used to differentiate between relapse and new infection of P. vivax strain.


Background
Malaria is a major global public health problem [1]. Plasmodium vivax is the most prevalent malaria parasite in Asia [2]. Relapsing nature of P. vivax poses a challenge for malaria elimination. Relapse rates of 5-40 % have been reported from India [3]. Relapses are known to occur at different time intervals, ranged from 1 month to 1 year. The long-term relapsing malaria has been reported from India [3][4][5]. It is difficult to differentiate relapse from reinfection and the existing methods have limitations.
An attempt was made to evaluate the utility of a newly designed minisatellite marker for differentiating relapse from reinfection.

Sample collection
The blood samples from confirmed vivax malaria patients were collected by finger prick method aseptically from malaria clinic of National Institute of Malaria Research, New Delhi (Fig. 1). The diagnosis of malaria species was confirmed by microscopic method. Two to three drops of blood were also collected on 3 mm filter paper (Whatman International Ltd., Maidstone, UK) for DNA isolation. Altogether, 88 samples of P. vivax were collected. Patients were treated as per National Drug Policy of India [22]. Of 88 P. vivax cases, two reported back to malaria clinic after a lapse of nine and 11 months, while two patients from same family (father and son) reported on the same date. The ethics committee of National Institute of Malaria Research approved this study and written informed consent was obtained from the patients/guardians.

DNA isolation and PCR amplification
The genomic DNA of P. vivax was extracted from the blood samples collected on the filter paper using QIAamp DNA Blood Mini Kit (Qiagen, Hilden, Germany) as per manufacturer's instructions. PCR amplification of CH1T1M13779 was carried out in the final volume of 25 µl reaction contained 1× Buffer     (Fig. 3). The base size of 280 bp in 13.63 %, 320 bp in 13.63 % and 300 bp in 16.66 % showed the predominant allele in P. vivax population ( Table 1).

Validation of minisatellite in case control study
Minisatellite marker CH1T1M13779 was used for case study in relapse, new infection and transmission pattern of P. vivax. Genotyping of two samples collected on day 0 and after 9 months from one patient from Dwarka region showed the identical genotype (Fig. 4, Lane 1 and 2). Both the samples from this patient showed same strain of P. vivax which caused the relapse after 9 months ( Table 2). Genotyping of another patient based on samples collected on day 0 and after a lapse of 11 months from Vijay Enclave showed two different genotypes (Fig. 4, Lane 3 and 4). Different genotype from same patient showed new infection due to different strain of P. vivax ( Table 2). Genotyping of two patients from same family (father and son) who reported at the clinic on same day from Shahabad showed identical genotype (Fig. 4, Lane 5 and 6). The identical genotype of both the patients showed transmission of one P. vivax strain in the family (Table 2).

Discussion
The control and elimination of P. vivax is a major challenge due to the relapses. Relapses need to be differentiated from reinfection for knowing true efficacy of anti-relapse medicines during clinical trials. Current techniques to differentiate relapse from reinfections have limitations. The technique PCR-RFLP is used for allelic discrimination and polymorphism analysis in the genes such as Pvmsp3α, msp1 [24,25], sequencing of microsatellite [4], capillary electrophoresis-based heteroduplex tracking assay [25,26]. Recently, deep sequencing technique has been used for differentiation of relapse and reinfection [27].
These techniques are time consuming, expensive and need well established molecular laboratory. In the present study, whole genome of P. vivax was scanned and designed the simple, rapid, cost effective technique for differentiation of relapse and reinfection. The newly designed minisatellite marker CH1T1M13779 is highly polymorphic in nature and showed 14 alleles with varied allele frequency. The base size of 280 bp (13.63 %), 320 bp (13.63 %) and 300 bp (16.66 %) were predominant allele in P. vivax population.
The paired samples from one patient (day 0 and day recurrence at nine months) were analysed by minisatellite marker CH1T1M13779. Samples from another patient collected on day 0 and 11 months showed two different genotypes demonstrating that it was new infection due to different strain of P. vivax. Thus, this minisatellite marker could help to differentiate the relapse from new infection of P. vivax. Genotyping data of P. vivax by minisatellite marker supports the hypothesis of Kim et al. [10] and Koepfli et al. [28].
Genetic diversity of P. vivax has significant impact on malaria transmission [29]. The identical genotype of P. vivax in a father and son showed that transmission occurred due to same strain. Thus, the marker may also be helpful in identifying the origin and transmission of parasite in family or locality. Genotyping by the minisatellite marker can also help to differentiate P. vivax relapse from reinfection and thus help to determine efficacy of antirelapse medicines. This preliminary data shows that there is need to further strengthen data using more number of minisatellite markers in more paired P. vivax sample for statistical conclusions. This technique is rapid and cost effective as compared to the PCR-RFLP, sequencing, gene scanning and heteroduplex tracking assay.
Further, the study of polyclonal infections of P. vivax by the capillary electrophoresis-based heteroduplex

Conclusion
Study provided preliminary evidence of identifying the relapse and new infection by newly designed minisatellite marker CH1T1M13779. Further studies are needed to validate the same.
Author contributions NV, RD designed the research, DS, RD collected the samples, RD performed experiment and analysed the results, RD, ARA, RCD wrote the paper. All authors were involved in edited the manuscript and approved the manuscript.