Transcription and expression of Plasmodium falciparum histidine-rich proteins in different stages and strains: implications for rapid diagnostic tests

Abstract

Background: Although rapid diagnostic tests (RDTs) for Plasmodium falciparum infection that target histidine rich protein 2 (PfHRP2) are generally sensitive, their performance has been reported to be variable. One possible explanation for variable test performance is differences in expression level of PfHRP in different parasite isolates.

Methods: Total RNA and protein were extracted from synchronised cultures of 7 P. falciparum lines over 5 time points of the life cycle, and from synchronised ring stages of 10 falciparum lines. Using quantitative real-time polymerase chain reaction, Western blot analysis and ELISA we investigated variations in the transcription and protein levels of pfhrp2, pfhrp3 and PfHRP respectively in the different parasite lines, over the parasite intraerythrocytic life cycle.

Results: Transcription of pfhrp2 and pfhrp3 in different parasite lines over the parasite life cycle was observed to vary relative to the control parasite K1. In some parasite lines very low transcription of these genes was observed. The peak transcription was observed in ring-stage parasites. Pfhrp2 transcription was observed to be consistently higher than pfhrp3 transcription within parasite lines. The intraerythrocytic lifecycle stage at which the peak level of protein was present varied across strains. Total protein levels were more constant relative to total mRNA transcription, however a maximum 24 fold difference in expression at ring-stage parasites relative to the K1 strain was observed.

Conclusions: The levels of transcription of pfhrp2 and pfhrp3, and protein expression of PfHRP varied between different P. falciparum strains. This variation may impact on the detection sensitivity of PfHRP2-detecting RDTs.

Figure 1. Transcription of pfhrp2 (A) and pfhrp3(B) normalised to trna over the intraerythrocytic life cycle of parasites (with varying proportions of ring stage).

Figure 2. Quantity of pfhrp2 (A) and pfhrp3 (B) transcripts in 5 P. falciparum lines over the intraerythrocytic life cycle (with varying proportion of ring stage) relative to K1.

Figure 3. Quantity of pfhrp2 transcription relative to pfhrp3 transcription in 5 parasite strains at varying proportion of ring stage.

Figure 4. Quantity of pfhrp2 transcripts (relative to K1) in samples with 80–100% ring stage for 15 P. falciparum lines.

Figure 5. Expression of PfHRPs in different P. falciparum lines over the intraerythrocytic life cycle.

(A) Quantity of HRP protein expression in parasites with varying proportion of ring stage of 5 P. falciparum lines. (B) Protein expression values relative to K1 for PfHRP in samples with 80–100% ring stage for 15 P. falciparum lines. (C) Quantity of HRP protein expression in parasites with varying proportion of ring stage for 5 P. falciparum lines normalised to K1. (D) Comparison of total pfhrp2 transcription and total expression of PfHRP across the intraerythrocytic life cycle for 5 parasite lines.