This is an extended version of a shorter news article about the possibilities of a crucial gene for malaria transmission that Pieter published earlier this week.
A decade ago, hundreds of scientists finished the prestigious human genome project (HGP). The main task was to sequence the human genome, composed of DNA (deoxyribonucleic acid), the hereditary material that resides in cells of all organisms. In the early 1990’s, scientists started first by sequencing and comparing genomes of frequently investigated organisms, like E. coli gut bacteria, yeasts, worms and mice. Since these organisms are far apart in the evolutionary tree, the parts of DNA that are shared among these organisms are probably essential for life, delivering a framework for comparing and combining the sequence of bits of the human genome. Years later, in 2001, 90% of the human genome, or 3 billion base pairs, were sequenced. And in 2003, the sequencing of the human genome was completed.
Giant role of genes
One of the main goals of the Human genome project is to discover the sequence of genes. The human genome contains 20’000-25’000 protein coding genes which make up only 1,5% of the human genome. These genes are very important parts of DNA, for they are transcribed into mRNAs (messenger ribonucleic acid), which in turn are translated into one or more proteins (see figure below). Proteins are key molecules for cellular function and thus life. The particular order in which the bases are placed in the DNA of these genes, dictates what the protein will look like and how it is made. A gene is mutated if at least one base of its DNA sequence is changed, which could lead to changes in the way the protein is made, leading to the expression of nonfunctional proteins and even genetic diseases.
A well-known example is Duchenne muscular dystrophy, which is characterized by progressive muscle decline. This genetic disease is caused by a mutation in the dystrophin gene, leading to the absence of functional Dystrophin protein. Actually, all fields of medical and biological research in humans benefit from the HGP (such as cancer research).
Other genome projects
The HGP also has an indirect positive effect on combatting infectious diseases, for the project caused improvements in DNA sequencing methods and a reduction of costs. Therefore, many more genomes of organisms have been sequenced, including the one of the Plasmodium falciparum in 2002. The so-called P. falciparum genome project revealed that this dangerous malaria parasite contains about 5’300 protein coding genes.
What is malaria?
Malaria is a prevalent infectious disease that affects more than 200 million people worldwide. This disease is mainly caused by the transmission of a particularly dangerous malaria parasite, the Plasmodium falciparum protozoa. The transmission to the host occurs during mosquito bites of the Anopheles mosquito (see figure below). Shortly after infection, falciparum malaria can cause several symptoms in the host (usually human or cattle), including breathing problems, liver dysfunction, shock and even death. In fact, this terrible parasite accounts for 650’000 human deaths each year, which is more than 90% of global malaria deaths. In order to prevent the spread of the disease, several options are currently used, such as mosquito nets, rapid diagnostic tests and antimalarials. However, to significantly reduce death toll of this disease, a proper vaccine and hampering the P. falciparum development cycle seems two of the most promising options for the near future. More about the epidemiology, disease mechanisms and common treatments are described in Africa vs. malaria: the tables are turning.
Life cycle of the malaria parasite
The P. falciparums’ life cycle consists of roughly five different forms, namely sporozoites, merozoites, male or female gametocytes, zygotes and oocysts. When a person is bitten, P. falciparum parasites enter the bloodstream, travel to the liver to develop into merozoites (see figure below). Then, they go back to the blood stream and infect the red blood cells to reproduce, which cause terrible symptoms (mentioned above). During this stage, most new merozoites re-infect other red blood cells, continuing the cycle and progressing the disease. However, only up to 6% of the P. falciparum merozoites develop into male or female gametocytes. If these gametocytes are sucked up by another Anopheles mosquito (note 2), the male and female gametocytes fuse and become zygotes. These zygotes travel to the gut and become oocysts. There, the development of new sporozoites is induced and these parasites travel to the salivary gland to close the cycle. This article will focus on the development of merozoites into gametocytes, because understanding this rare but essential biological mechanism required for the infection of a new host, could be key in combating malaria infection.
What molecular mechanism is essential for the formation of P. falciparum gametocytes?
The central gene for the falciparum development cycle
Recently, Kafsack and colleagues unraveled the molecular mechanism that promotes the development of P. falciparum gametocytes. Central in the investigation are the pfap2-g gene and PfAP2-G protein, since Rovira-Graells and colleagues found different proteins that were increasingly expressed in early gametocyte development, including PfAP2-G. Because this protein in itself regulates expression of other genes, the researchers reasoned PfAP2-G is probably very important. P. falciparum laboratory grown P. falciparum parasites were used to investigate the gametocytes development. Some cells were adapted to express a mutated pfap2-g gene or not express pfap2-g at all.
This pfap2-g gene – which encodes for the PfAP2-G protein – turned out to be an interesting target for battling for three reasons. First of all, the more pfap2-g mRNA in merozoites (remember: the stage before gametocytes) the more gametocyte formation. Second, mutations or deletion of the pfap2-g gene in precursors of gametocytes prevent the formation of gametocytes. Third, the protein has a DNA-binding domain, which enables the protein to enhance the expression of early gametocyte-associated genes, including pfpeg4, pf11-1 and pfg27/25. This causes the formation of proteins that promote the development of gametocytes.
In perspective
In summary, Kafsack and colleagues show the first gene – pfap2-g – and protein – PfAP2-G – that are absolutely essential for the P. falciparum’s development into gametocytes. In other words: PfAP2-G is the master regulator of sex in this organism. Although P. falciparum was already described in the 19th century, the P. falciparum genome project was necessary to enable the discovery of pfap2-g and elucidate the function of its corresponding protein.
Future directions
Malaria transmission can be reduced by hampering the reproduction of the P. falciparum parasites. Since only 1-6% of the merozoites develop into gametocytes in the control condition, this is a fragile process in the P. falciparum’s life cycle. Therefore, targeting the assembly of gametocytes by minimizing the amount of PfAP2-G proteins in merozoites is probably an interesting target for future therapy.
Minimizing the formation of functional PfAP2-G proteins in falciparum parasites can be achieved in three ways. (i) The formation can be completely blocked by changing or deleting the pfap2-g gene, as mentioned before, (2) the transcription of the pfap2-g gene can be reduced or (iii) the translation of pfap2-g mRNA can be limited.
To tackle the malaria transmission in the future, it is probably sensible to prevent the formation of gametocytes by PfAP2-G proteins. This can be achieved by ‘vaccinating’ Anopheles mosquitoes that carry P. falciparum parasites with lab-grown parasites with a dysfunctional pfap2-g gene. These parasites can compete with natural parasites for infection of future hosts. Another possibility to dramatically reduce the malaria transmission, is to decrease the formation of PfAP2-G proteins in malaria patients by minimizing the transcription of the pfap2-g gene or the pfap2-g mRNA in P. falciparum parasites. Of course, patients should also undergo conventional therapy, because malaria is not treated by preventing the formation of gametocytes.
Concluding remarks
To conclude, the human genome project accelerated the DNA research and thereby contributed to the discovery of an essential gene in the P. falciparum life cycle. This finding probably aids to the retreat of malaria in the near future.
Want to read more? Pieter also wrote about the epidemiology in Africa vs. Malaria and about an accidental finding that lead to a promising malaria vaccin.
Note 1: DNA is a double helix which consists of a sugarphosphate backbone and four different bases: adenosine (A), thymine (T), cytosine (C) and guanine (G). The bases pair with one another, creating a strong bond. A pairs with T and C with G.
Note 2: The Anopheles mosquito does not necessarily carry the Plasmodium falciparum parasites, The mosquito generally gets the parasite after a blood meal of an infected host. When it carries the parasite, the mosquito does not get malaria itself.
References:
Kafsack BF, Rovira-Graells N, Clark TG, Bancells C, Crowley VM, Campino SG, Williams AE, Drought LG, Kwiatkowski DP, Baker DA, Cortés A, & Llinás M (2014). A transcriptional switch underlies commitment to sexual development in malaria parasites. Nature, 507 (7491), 248-52 PMID: 24572369
Kåhrström CT (2014). Parasite development: master regulator of sex. Nature reviews. Microbiology, 12 (4) PMID: 24608336
Wells TN, Alonso PL, & Gutteridge WE (2009). New medicines to improve control and contribute to the eradication of malaria. Nature reviews. Drug discovery, 8 (11), 879-91 PMID: 19834482
Rovira-Graells N, Gupta AP, Planet E, Crowley VM, Mok S, Ribas de Pouplana L, Preiser PR, Bozdech Z, & Cortés A (2012). Transcriptional variation in the malaria parasite Plasmodium falciparum. Genome research, 22 (5), 925-38 PMID: 22415456
Su X, Hayton K, & Wellems TE (2007). Genetic linkage and association analyses for trait mapping in Plasmodium falciparum. Nature reviews. Genetics, 8 (7), 497-506 PMID: 17572690
Gardner, M., Hall, N., Fung, E., White, O., Berriman, M., Hyman, R., Carlton, J., Pain, A., Nelson, K., Bowman, S., Paulsen, I., James, K., Eisen, J., Rutherford, K., Salzberg, S., Craig, A., Kyes, S., Chan, M., Nene, V., Shallom, S., Suh, B., Peterson, J., Angiuoli, S., Pertea, M., Allen, J., Selengut, J., Haft, D., Mather, M., Vaidya, A., Martin, D., Fairlamb, A., Fraunholz, M., Roos, D., Ralph, S., McFadden, G., Cummings, L., Subramanian, G., Mungall, C., Venter, J., Carucci, D., Hoffman, S., Newbold, C., Davis, R., Fraser, C., & Barrell, B. (2002). Genome sequence of the human malaria parasite Plasmodium falciparum Nature, 419 (6906), 498-511 DOI: 10.1038/nature01097
human genome project, gene, malaria, block, transmission, gametocytes, mosquito, exterminate
