Question

Background info: Leishmania is a parasite that causes a disfiguring infection that is treated by antimony. Antimony works because the parasites have a transporter protein that imports the chemical into parasite cells. Doctors found that many infections had become resistant to antimony, the standard treatment for this infection.When they sequenced the genomes of Leishmania from treatment-resistant Leishmania, scientists found that the gene encoding the protein transporter contained two inserted nucleotides.

Question: How would you explain to non-scientist patients how the failed treatment could be caused by dinucleotide insertion in the genome of Leishmania.

Answer 1

Antimony is a drug that is used to treat Leishmaniasis, a disease caused by the protozoa Leishmania. There are two forms of this protozoa- amastigote form which is intracellular in humans and promastigote form that is present in sandfly. The protozoan is an intracellular parasite of macrophages.

Antimony act on human form- the amastigote form. However, for the drug to act on humans, it has to first enter the cell. Antomony is produced in the pentavalent form. In this form, they are inactive. They enter the cell and then are converted to a active trivalent are used for conversion from inactive to active form via a non enzymatic method. Enzymes that can also reduce this compound are thiol dependent reductase enzyme produced by the parasite.

Drugs cannot directly enter the cell as the cell membrane will mostly prevent its entry. Transporter proteins are present embedded in the cell membrane that can bind the drug from outside the membrane. It then undergoes a change in shape and will release the drug in the intracellular side of the macrophage. Every protein is translated from the mRNA transcribed from the DNA sequence. If there is any mutation or change in the DNA sequence (composed of nucleotides), then the protein structure may also change.

Proteins are composed of sequence of amino acids liked together by peptide bonds. Each amino acid is coded by a sequence of three nucleotides in protein, known as a codon. Sequence of nucleotides is read in a sequential manner by the ribosome during protein synthesis, and is known as reading frame.

If there are two nucleotides (dinucleotides) are inserted in the DNA sequence (in genome), the reading frame of the mRNA will altered. This will lead to incorporation of different amino acids in the proteins sequence as the codon sequence is altered. The protein formed from this mRNA sequence, will be non functional as its primary structure is altered. This transporter protein, known as mutant protein will not be able to bind to the pentavalent form of antimony, hence, will not be able to transport the drug inside the macrophage. Hence, the macrophage will be resistant to the drug, thereby allowing the Leishmania parasite to grow continuously.