The newly sequenced genome of a deadly parasitic amoeba shows that it has borrowed a surprising number of genes from bacteria. The sequence helps clear up questions about the evolution of the parasite and could help researchers develop new therapies and diagnostic tools.
Entamoeba histolytica infects up to 50 million people worldwide and causes amoebiasis--debilitating diarrhea and liver damage that kills an estimated 100,000 people per year. Among parasitic diseases, amoebiasis is second only to malaria in the numbers it affects and kills. Unlike other amoeba, which possess several types of intracellular compartments called organelles, E. histolytica contains only simple forms of two common organelles, the Golgi apparatus and endoplasmic reticulum. It also lacks the energy-generating mitochondria present in most living things and digests food with fermentation enzymes found only in bacteria and other very simple single-celled organisms. But researchers weren't sure if these were signs that the amoeba was primitive or if it had merely degenerated because of its parasitic lifestyle.
The complete genome of E. histolytica clears up the picture and offers some surprises. Led by scientists at The Institute for Genomic Research (TIGR) in Rockville, Maryland, and the Wellcome Trust Sanger Institute in the United Kingdom, the study indicates that the amoeba has snagged an astonishing 92 genes from bacteria in recent times, presumably by gobbling them up during its life in the human gut. A majority of these genes are involved in metabolism and presumably allow the amoeba to use bacterial metabolic pathways to adapt to the low oxygen environment of the gut. As a result, the amoeba could afford to loose its complex organelles.
The researchers also found unexpected complexity among the amoeba's nearly 10,000 genes. The organism has a huge suite of genes for receptor and signaling proteins, far more than in most similar organisms. This may help it cope with the varied environments of the human gut, says Neil Hall, a biologist who helped oversee the project at TIGR. "It seems to be more intelligent than we thought before," he says. The team reports its results today in Nature.
Entamoeba researcher Daniel Eichinger of New York University Medical Center in New York City predicts the findings will help investigators find more specific targets for antibiotics or vaccines.