Malaria remains one of the most deadly infections to humans worldwide. It is caused by several species in the genus Plasmodium, with Plasmodium falciparum being the most virulent. The malaria parasite’s complex lifecycle is initiated by an infectious bite from a female anopheline mosquito, injecting sporozoites into the bloodstream and leading to the invasion of liver cells. The subsequent growth of asexual parasites within red blood cells (RBCs) is responsible for pathology. A small percentage of infected RBCs transforms into male and female gametocytes, which can be transmitted to the mosquito vector. After fusion of gametes within the midgut, development proceeds at various sites in the mosquito ending with the invasion of the salivary glands. Upon the vector’s next bloodmeal the life cycle is completed with the injection of salivary gland sporozoites.
While malaria is among the longest studied afflictions of humans, progress towards new therapeutics and vaccines has generally been slow. The past decade has seen significant advances in the fundamental understanding of the parasite’s biology, which in turn has opened new and promising avenues for novel anti-malarial development. This resurgence has been brought about in large part by technological advances that have enhanced the ability to genetically manipulate the parasite [1
] as well as through the insights provided by the whole genome sequencing of many species and strains of Plasmodium
]. A major benefit of genome sequencing has been the ability to assay gene transcription on a genome-wide basis through the use of DNA microarrays, which have allowed probing questions to be asked regarding the transcriptional status at distinct life stages, differences in gene expression between strains, in response to changes in environmental conditions or drug perturbations. Microarrays can also be used to find genetic alterations such as copy number variations or single nucleotide polymorphisms. Despite these possible uses, the DNA microarray has not been broadly utilized for several reasons including the large amount of biological material required, technological impracticality for many research settings, and relatively high cost.
The first DNA microarrays for P. falciparum
were generated using either sheared genomic DNA [7
] or cDNAs [8
] and were used to compare differences across asexual blood stages. In the absence of the genome sequence, follow up of potential gene expression differences was pursued on a candidate basis by sequencing of the material spotted on the array. With the completion of the P. falciparum
strain 3D7 reference genome [3
], oligonucleotide-based arrays quickly followed and provided an unprecedented in-depth view of transcriptional changes during asexual development in red blood cells [9
]. Subsequent studies determined that other strains (HB3, Dd2) exhibited similar transcriptional programs in vitro
] and ultimately these general observations were extended to transcriptional profiles of patient isolates by either profiling parasite mRNA abundance directly from blood [12
] or after short-term ex vivo
]. Such transcriptional signatures from non-culture adapted parasites have identified subtle and important differences that are the source of on-going research. Other in vitro
studies have struggled to identify variations in transcriptional profiles under environmental perturbations [15
], although larger scale efforts examining growth perturbations under dozens of conditions have yielded associations between genes based on transcriptional covariation [18
]. More detailed studies have examined transcriptional changes associated with differences in cellular adhesion and antigenic variation [15
]. Ultimately, one of the real powers of DNA microarray analysis will be to characterize transcriptional responses to targeted genetic alterations [23
Since these early studies, other stages of P. falciparum
development have also been explored using DNA microarrays including sporozoites [25
] and various stages of gametocyte maturation [23
]. Furthermore, several efforts have utilized DNA microarrays for genome-wide transcriptional analysis in the rodent models of malaria, Plasmodium berghei
and Plasmodium yoelii
. From these efforts, there are reports characterizing ookinete development in P. berghei
] and Mikolajczek et al
have examined oocyst vs.
salivary gland sporozoites in P. yoelii
]. The liver stage of human malaria parasites has been difficult to analyse transcriptionally. However, microarray studies of P. yoelii,
or P. berghei
liver stage infections have characterized both the parasite [31
] and host transcriptional programs during this developmental stage [32
In light of the many insights already gained from this relatively small number of genome-wide experiments, improving the performance and increasing the availability of Plasmodium DNA microarray to the research community remains a worthwhile effort.