Background
China has been severely afflicted by falciparum malaria. In 1954, falciparum malaria cases totaled about 4.18 million in the whole country. With active implementation of malaria control measures for more than 50 years, considerable success has been achieved 1. However, up to the present, falciparum malaria is still an important public health problem in China, especially in Yunnan and Hainan provinces, with 3,240 cases reported from the two provinces in 2006, accounting for 93.4% of the total reported cases in the country 2.
The plant Artemisia annua (Asteraceae) has been used for more than 2,000 years in Chinese traditional medicine for the treatment of febrile illnesses, including malaria. Artemisia annua contains artemisinin, which was isolated in 1972, and since that time its efficacy against malaria has been amply demonstrated 34. Following increased chloroquine (CQ), piperaquine (PQ) and pyrimethamine-sulphadoxine (SP) resistance, artemisinin and its derivatives gradually became the mainstay of falciparum malaria therapy in China. Before the World Health Organization (WHO) urged pharmaceutical companies to end the marketing and sale of artemisinin for monotherapy in 2006 5, many drug manufacturers in China distributed artemisinin alone to treat malaria; indiscriminate use of artemisinin derivatives was common practice. Therefore, whether resistance to artemisinins has developed in China is a cause for concern.
To this day, clinically relevant artemisinin resistance has not yet been documented, but it was found that the susceptibility of Plasmodium falciparum to artemisinin derivatives was declining in China, as IC50 for artesunate in 1999 were 3.3 times of that in 1988 6. Laboratory studies have shown that genetically stable and transmissible artemisinin-resistant rodent malaria parasite could be selected through prolonged exposure of drug-sensitive lines to low and increasing levels of artemisinins 7. It is likely a matter of time before clinical artemisinin resistance is observed in China.
Recent studies provide compelling evidences that artemisinins act by selectively inhibiting PfATPase6 protein, the only SERCA-type Ca2+-ATPase in the P. falciparum genome, believed to be the primary target for artemisinins 89. A subsequent study in French Guyana showed that S769N PfATPase6 mutation was associated with raised artemether IC50, and suggested that the mutation may be used in molecular monitoring of artemisinin resistance to complement continuing in-vitro surveillance 10. So far, no studies on this molecular marker have been carried out in China. The main purpose of the present study was to investigate whether there are any changes at codon S769N of the PfATPase6 locus in field samples from the country following the long-term use of artemisinins. A nested PCR-sequencing assay was developed to analyse the mutation of PfATPase6 S769N among P. falciparum field isolates collected from Hainan and Yunnan provinces, China.
Methods
Samples
Plasmodium falciparum field isolates were collected in 2006–2007 from two provinces. 27 samples were collected from Hainan province and 74 samples were from Yunnan province (Figure 1). The two provinces contributed more than 90% of the total falciparum malaria cases in the whole country in 2006 2. Parasite samples were collected from patients with uncomplicated P. falciparum infections before drug treatment. Diagnosis was carried out by microscopic examination of Giemsa-stained thick blood films. For each sample, approximately 20 μL of finger-prick blood was spotted on a piece of 3 MM filter paper (Whatman, Maidstone, UK) and air-dried. The dried filter paper samples were stored in individual zipper plastic bags with dryer at -20°C until DNA extraction. This study was reviewed and approved by the ethics committee of the National Institute of Parasitic Diseases, Chinese Center for Disease Control and Prevention.
<p>Figure 1</p>
Map of China showing the locations (shadow) where parasite samples were collected
Map of China showing the locations (shadow) where parasite samples were collected.
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DNA extraction
Extraction of DNA from bloodspots on filter paper was carried out by the Chelex-100 (Bio-Rad Laboratories, Hercules, CA) boiling method described by Wooden and others 11 with some modifications described by Pearce and others 12. The quality of DNA samples was tested by OD 260/280 measurement.
Nested PCR-sequencing assay
The fragment of pfATPase6 was amplified by nested PCR with primer F1 (5'-CAAATAAGAAGGATAAATCACCA-3') and primer R1(5'-TCATAAATACACGTATACCAGCC-3') (94°C for 3 min; then 94°C for 40 s, 46°C for 1 min, and 72°C for 1 min for 35 cycles; followed by a final extension step at 72°C for 10 min), and then primer F2 (5'-AAAATAAATACCACATCAACACAT-3') and primer R2 (5'-TCAATAATACCTAATCCACCTAAA-3') (94°C for 3 min; then 94°C for 40 s, 45°C for 1 min, and 72°C for 1 min for 35 cycles; followed by a final extension step at 72°C for 10 min). The 50 μL PCR mix contained primers at 0.25 μmol/L final concentration, 2 mmol/L MgCl2, 240 μmol/L of each deoxynucleoside triphosphate, and 2 U Taq polymerase. Template DNA (1 μL) was introduced to outer reaction mixtures. 1 μL outer PCR product was introduced into a 50 μL inner amplification mixture. Secondary PCR products were resolved by electrophoresis on 1% agarose gels and visualized by staining with ethidium bromide. Sequencing reactions were carried out using ABI PRISM Big Dye terminator v3.1 Cycle Sequencing kit (Applied Biosystems, CA, USA) as specified by the manufacturer's protocol. The sequences of the amplicons were aligned with the published data of the NCBI database by BLAST analysis.
Discussion
In this study, by using nested PCR-sequencing assay, no PfATPase6 S769N mutation was found in any P. falciparum isolates collected from China. The finding is consistent with the results from the in vitro microtests 1314 and the high (more than 96%) cure rates of artemisinin combination therapies (ACTs) treatment for falciparum malaria in China 1516. Accordingly, until now there is no sufficient evidence for assuming that artemisinin resistance has occurred in China. However, stable resistance to artemisinins has been induced in the laboratory, and the declining susceptibility of P. falciparum to artemisinin derivatives has been reported in the country 6. Artemisinin drugs are believed as the most effective and the last hope in the near future in the battle against multidrug-resistant malaria, if resistance to artemisinins develops and spreads, it will be the most devastating event in the history of malaria control 17. Therefore, it is wise to phase out artemisinin monotherapy in China; the use of ACTs recommended by WHO as first line treatment for uncomplicated malaria will either prevent, or at least delay, the development of artemisinin resistance.
Although evidences from laboratory and field studies have suggested that PfATPase6 protein is the primary target for artemisinins, and a specific S769N mutation in the gene was significantly associated with the reduced efficacy of artemisinins 8910. It should be pointed out that one laboratory study could not identify such mutation in artemisinin-resistant Plasmodium chabaudi 7. Field studies carried out in Tanzania and Brazil also could not detect the S769N PfATPase6 mutation in the analysed samples 1819. Thus, whether this mutation can be widely used as a reliable marker for artemisinin resistance in epidemiological studies needs further validation.