CLONING OF A VIRULENCE FACTOR OF ENTAMOEBA-HISTOLYTICA - PATHOGENIC STRAINS POSSESS A UNIQUE CYSTEINE PROTEINASE GENE

Cysteine proteinases are hypothesized to be important virulence factor s of Entamoeba histolytica, the causative agent of amebic dysentery an d liver abscesses. The release of a histolytic cysteine proteinase fro m E. histolytica correlates with the pathogenicity of both axenic stra ins and recent clinical isolates as determined by clinical history of invasive disease, zymodeme analysis, and cytopathic effect. We now sho w that pathogenic isolates have a unique cysteine proteinase gene (ACP 1). Two other cysteine proteinase genes (ACP2, ACP3) are 85% identical to each other and are present in both pathogenic and nonpathogenic is olates. ACP1 is only 35 and 45% identical in sequence to the two genes found in all isolates and is present on a distinct chromosome-size DN A fragment. Presence of the ACP1 gene correlates with increased protei nase expression and activity in pathogenic isolates as well as cytopat hic effect on a fibroblast monolayer, an in vitro assay of virulence. Analysis of the predicted amino acid sequence of the ACP1 proteinase g ene reveals homology with cysteine proteinases released by activated m acrophages and invasive cancer cells, suggesting an evolutionarily con served mechanism of tissue invasion. The observation that a histolytic cysteine proteinase gene is present only in pathogenic isolates of E. histolytica suggests that this aspect of virulence in amebiasis is ge netically predetermined.