Epigenetic Regulation Of Plasmodium Falciparum Var Gene Family

The variant antigen Plasmodium falciparum erythrocyte membrane protein1(PfEMP1), which is expressed on the surface of P. falciparum infected red blood cells,is a critical virulence factor for malaria. Each parasite posseses60antigenicallydistinct var genes that each codes for a different PfEMP1protein. During infection theclonal parasite population expresses only one gene at a time before switching to theexpression of a new variant antigen as an immune evasion mechanism to avoid thehost antibody response. The mechanism by which59of the60var genes are silencedremains largely unknown.Lubin Jiang and coworkers showed that knocking out the Plasmodiumfalciparum variant-silencing SET2gene which encodes an orthologue of Drosophilamelanogaster ASh1and controls histone H3lysine36trimethylation (H3K36me3) onvar genes, results in the transcription of virtually all var genes in the single parasitenuclei and their expression as proteins on the surface of individual infected red bloodcells. PfSET2-dependent H3K36me3is present along the entire gene body, includingthe transcription start site, to silence var genes.In our study, a different phenotypehas been observed by PfSET2knockdowninstead of knockout. As a result of over-expression of exogenous truncates of PfSET2,the endogenetic PfSET2functional ration decreased, which leads to the tri-methylation of H3K36of var genes relatively partial failure. The RT-PCR of vargenes showed that the original “on” var gene and var2csa are both “on” in our study,which disrupts the mutually exclusive gene expression of var genes. The multi-copyvar gene family encodes the primary antigenic and virulence determinant of themalaria parasite Plasmodium falciparum. var genes are mutually exclusivelytranscribed, with switching between active genes resulting in antigenic variation andthe perpetuation of long-term, chronic infections. Switching appears to be coordinatedto result in timely activation of individual genes leading to sequential waves ofantigenically distinct parasite populations, however the molecular basis for thiscoordination is unknown. Here we show that var2csa, an unusual and highly conserved gene, occupies a unique position within the switching hierarchy. Inductionof switching through the destabilization of var specific chromatin using both geneticand chemical methods repeatedly led to the rapid and exclusive activation of var2csa.This activation was specific to the conserved var2csa locus on chromosome12anddid not extend to a second copy located at a different chromosomal position. Thesedata provide the first insights into the mechanisms by which var gene switching iscoordinated as well the first example of a pharmacological agent that can disruptantigenic variation by malaria parasites.var genes are composed of a long hypervariable exon1, followed by an about1kbintron that possesses promoter activity, and finally a short, conserved exon2. And varintrons contain simultaneous bidirectional promoter activities that transcribe sense andanti-sense noncoding RNAs. Several studies have shown that the presence of theintron promoter is required for silencing and proper recognition of a var promoter.Based on the other eukaryote model, we hypothesize ncRNAs transcripted by varintron play an important role for the var genes regulation. Hence, in our study, wefocused on creating var genes ncRNAs producing episomal plasmid, which couldoffer molecular biology tools for var genes ncRNAs function research, and tried toidentify the essential elements in5′UTR for ncRNAs transcription.