Genetic factors implicated in asymptomatic malaria infections

Abstract

Plasmodium falciparum malaria infections can be classified as either asymptomatic or symptomatic which includes uncomplicated malaria, severe malaria, cerebral malaria, and naïve malaria. In symptomatic cases, individuals infected with the parasites display symptoms associated with malaria. In contrast, asymptomatic cases do not show any classical symptoms and, therefore, act as reservoirs of the parasite. The persistence of P. falciparum parasites in an asymptomatic malaria individual plays a significant role in maintaining transmission. However, the molecular mechanisms underlying P. falciparum's persistence in asymptomatic infections remain largely unknown. This study hypothesised that changes in the parasite's gene expression during asymptomatic infections might enhance their adaptability and fitness and therefore contribute to their persistence in the host. To investigate this hypothesis, a transcriptome analysis was done to identify P. falciparum's genetic factors that are associated with asymptomatic infections. Whole blood RNA profiles from 25 field samples (15 asymptomatic (ASM) and ten uncomplicated malaria (UM) samples) were analyzed. In addition, publicly available transcriptome datasets from whole blood samples of 35 individuals with asymptomatic malaria (ASM, n=11), uncomplicated malaria (UM, n = 12), and naïve-malaria (NM, n = 12) were included for differential gene expression analysis, long noncoding RNAs expression analysis, and single nucleotide polymorphisms analysis within the virulent genes of P. falciparum. Data analysis revealed 755 differentially expressed genes (DEGs) between asymptomatic carriers and individuals with uncomplicated malaria, and 1773 DEGs between NM and ASM. Among the top differentially expressed genes were numerous genes coding for proteins of unknown functions (PUFs). The ontology analysis of DEGs revealed activated pathways linked to host-parasite interactions, including antigenic variation, immune evasion, crystalloid, apoplast, and binding processes and supressed pathways associated with carbon metabolism. Additionally, the analysis identified 28 immune evasion genes associated with host-parasite and symbiotic interactions, such as cell adhesion, evasion of the host immune response, modulation by symbionts of host cellular processes, and responses to biotic and external biotic stimuli. Moreover, 237 differentially expressed noncoding RNAs were detected when comparing asymptomatic carriers (ASM) to those with uncomplicated malaria (UM). Among these, five RNAs were observed to interact with six immune evasion genes, indicating a potential role in modulating the immune response during asymptomatic infections. The analysis of var gene family expression did not reveal statistically significant differences in the expression levels of the var groups. However, two genes, CUFF.75 and CUFF.203, were upregulated in asymptomatic infections compared to uncomplicated malaria infections. These findings demonstrate that P. falciparum establishes asymptomatic infections by suppressing the central carbon metabolism and expressing immune evasion genes, which mediate sequestration to avoid clearance and adapt to the host's defenses, resulting in long-lasting chronic infections. Additionally, the study identifies potential biomarkers for detection of asymptomatic malaria. These findings provide novel insights into P. falciparum genetic factors that confer a fitness advantage during asymptomatic infections.