Molecular analysis of the congopain gene family.

Abstract

Animal trypanosomosis is a major constraint in livestock production in Sub-Saharan Africa. With the emergence of resistance against trypanocidal drugs, the cost and environmental concerns raised by vector control, and the challenge of antigenic variation in vaccine development, alternative control measures are being sought. An anti-disease strategy, whereby the immune response or chemotherapy is aimed towards pathogenic factors rather than the parasite itself, constitutes such a novel approach. Congopain is the major cysteine protease in Trypanosoma congolense, and upon release in the bloodstream of infected cattle, acts as a pathogenic factor. It is therefore an attractive candidate for an anti-disease vaccine. It was hence deemed necessary to investigate the variability of congopain-like cysteine proteases before attempting to design drugs and vaccines based on the inhibition of congopain. Most congopain-like cysteine protease genes of T. congolense exist in a single locus of 12-14 copies organised as tandem repeats of 2 kb gene units. A gene unit library of 120 clones was constructed out of several cosmid clones selected in a previous study that contained various lengths of the congopain locus. Some 24 gene unit clones were sequenced, and it was found that congopain genes cluster in three sub-families, named CP1 (8 clones), CP2 (12 clones) and CP3 (4 clones). The latter most characteristically shows a substitution of the active site cysteine by a serine. Isoform specific primers were designed and used to verify the proportions of the three isoforms (one third CP1, half CP2 and a sixth CP3) in the remaining clones of the library. Since this first study was conducted in one isolate, IL 3000, the results were subsequently validated in a large array of isolates, of T. congolense, as well as T. vivax and T. brucei subspecies, by a PCR approach. Finally, to gain access to copies of congopain genes that are not present in the locus, but rather scattered in the genome, an attempt was made to construct a 2 kb size-restricted genomic library. Only 206 clones could be produced, of which a mere 8 coded for congopain-like proteases. The fact that 7 out of 8 of these clones belong to CP3 (thought to be inactive) suggested a cloning artefact, possibly related to the activity of the cloned proteases.

Overall, all congopain genes appear very conserved in a given species, with 87-99% identity at protein level. The pre- and pro-region were the most conserved, while the catalytic domain was the most variable, especially around the active site cysteine, with frequent replacement by a serine residue, and in one instance by phenylalanine. The histidine residue of the catalytic triad was also substituted by either a serine or a tyrosine in some instances. The proenzyme cleavage site sequence was also variable, with APEA being the predominant N-terminal sequence. RT-PCR analyses indicated that CP1, CP2 and CP3 mRNA are all present in the bloodstream forms of T. congolense, showing that these variants are likely to be expressed. The conclusion of this study is that, given the high overall conservation of congopain genes in the genome, for the purpose of anti-disease vaccine, it is likely that a single immunogen will suffice to raise antibody able to inhibit all circulating congopain-like cysteine proteases. For chemotherapy however, a more in-depth enzymatic characterisation of the mutants, involving functional recombinant expression, will have to be undertaken.