Helminth genomics: The implications for human health

Abstract

More than two billion people (one-third of humanity) are infected with parasitic roundworms or flatworms, collectively known as helminth parasites. These infections cause diseases that are responsible for enormous levels of morbidity and mortality, delays in the physical development of children, loss of productivity among the workforce, and maintenance of poverty. Genomes of the major helminth species that affect humans, and many others of agricultural and veterinary significance, are now the subject of intensive genome sequencing and annotation. Draft genome sequences of the filarial worm Brugia malayi and two of the human schistosomes, Schistosoma japonicum and S. mansoni, are now available, among others. These genome data will provide the basis for a comprehensive understanding of the molecular mechanisms involved in helminth nutrition and metabolism, host-dependent development and maturation, immune evasion, and evolution. They are likely also to predict new potential vaccine candidates and drug targets. In this review, we present an overview of these efforts and emphasize the potential impact and importance of these new findings.

Figure 1. Montage of some of the major human helminth parasites, their developmental stages, and disease pathology.

(A) Microfilaria of Brugia malayi in a thick blood smear, stained with Giemsa (http://www.dpd.cdc.gov/dpdx/html/frames/a-f/filariasis/body_Filariasis_mic1.htm); the microfilaria is about 250 µm in length. (B) Patient with lymphedema of the left leg due to lymphatic filariasis (http://www.cdc.gov/ncidod/dpd/parasites/lymphaticfilariasis/index.htm). (C) Hookworm egg passed in the stool of an infected person; the microscopic egg, barrel-shaped with a thin wall, is about 70×40 µm in dimension. (D) longitudinal section through an adult hookworm attached to wall of small intestine, ingesting host blood and mucosal wall. The parasite is about 1 cm in length. (E) Eggs of Schistosoma mansoni. The egg is about 150×50 µm in dimension; the lateral spine is diagnostic for S. mansoni in comparison to the other human schistosome species. Fibrotic responses to schistosome eggs trapped in the intestines, liver, and other organs of the infected person are the cause of the schistosomiasis pathology and morbidity. (F) A pair of adult worms of the blood fluke Schistosoma mansoni; the more slender female worm resides in the gynecophoral canal of the thicker male. The worms are about 1.5 cm in length, and live for many years (http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Schistosomiasis_il.htm ).

Figure 2. Phylogeny of the major taxa of human helminths—nematodes and platyhelminths—as established by maximum likelihood (ML) analysis of 18S ribosomal RNA from 18 helminth species.

Sequences were aligned using ClustalX . The topology of the tree was derived from a consensus tree by neighbor-joining–based bootstrapping, its branch lengths were computed using a ML-based method, and it was rooted with the orthologue from the brewer's yeast, Saccharomyces cerevisiae. The branch lengths of the phylogenetic tree were computed using DNAML (PHYLIP package [94]) by allowing rate variation among sites. The headings Chromadorea, Enoplea, Trematoda, and Cestoda are major classes of the phyla Nematoda and Platyhelminthes. The GenBank accession numbers of aligned sequences are DQ118536.1 (Trichuris trichiura), AY851265.1 (Trichuris suis), AF036637.1 (Trichuris muris), AY497012.1 (Trichinella spiralis), U94366.1 (Ascaris lumbricoides), AF036587.1 (Ascaris suum), AF036588.1 (Brugia malayi), AJ920348.1 (Necator americanus), AJ920347.2 (Ancylostoma caninum), AF036597.1 (Nippostrongylus brasiliensis), X03680.1 (Caenorhabditis elegans), AF036605.1 (Strongyloides ratti), U81581.1 (Strongyloides ratti), AB453329.1 (Strongyloides ratti), AF279916.2 (Strongyloides stercoralis), AB453315.1 (Strongyloides stercoralis), M84229.1 (Strongyloides stercoralis), EU011664.1 (Saccharomyces cerevisiae), , U27015.1 (Saccharomyces cerevisiae), DQ157224.1 (Taenia solium), AF229852.1 (Clonorchis sinensis), Z11590.1 (Schistosoma japonicum), Z11976.1 (Schistosoma haematobium), U65657.1 (Schistosoma mansoni).

Figure 3. Some recent approaches to expressing transgenes in human helminths.

(A) Luciferase activity in Schistosoma mansoni larvae (schistosomules) after transduction with a pseudotyped retrovirus that expresses the luciferase reporter gene. Anti-luciferase antibody staining of schistosomules three days after exposure to pseudotyped lentivirus carrying the firefly luciferase transgene. Schistosomules examined by confocal laser microscopy; (i) bright field, (ii) fluorescence red channel, (iii) merged images. Control non-transformed worms showed only background levels of fluorescence (not shown; see – for relevant hypotheses and experimental methods). (B) Recent studies on transgenic Strongyloides stercoralis indicated that morphogenesis of the infective L3 stage larva requires the DAF-16 orthologue FKTF-1 . L3s of this parasitic nematode were transfected with plasmids carrying the transgene fktf-1b::gfp::fktf-1b and examined by fluorescence microscopy. (i, ii) Transgenic first-stage larvae express green fluorescent protein (GFP) in the procorpus (arrow) of the pharynx. (iii, iv) A first-stage larva (L1) expresses the GFP::FKTF-1b(wt) transgene in the hypodermis. (v, vi) An infective L3 expresses the GFP::FKTF-1b(wt) fusion protein in the hypodermis and in a narrow band in the pharynx (arrow). Scale bars, 10 µm. Adapted from .