BackgroundThe soil transmitted helminths are a group of parasitic worms responsible for extensive morbidity in many of the world’s most economically depressed locations. With growing emphasis on disease mapping and eradication, the availability of accurate and cost-effective diagnostic measures is of paramount importance to global control and elimination efforts. While real-time PCR-based molecular detection assays have shown great promise, to date, these assays have utilized sub-optimal targets. By performing next-generation sequencing-based repeat analyses, we have identified high copy-number, non-coding DNA sequences from a series of soil transmitted pathogens. We have used these repetitive DNA elements as targets in the development of novel, multi-parallel, PCR-based diagnostic assays.Methodology/Principal FindingsUtilizing next-generation sequencing and the Galaxy-based RepeatExplorer web server, we performed repeat DNA analysis on five species of soil transmitted helminths (Necator americanus, Ancylostoma duodenale, Trichuris trichiura, Ascaris lumbricoides, and Strongyloides stercoralis). Employing high copy-number, non-coding repeat DNA sequences as targets, novel real-time PCR assays were designed, and assays were tested against established molecular detection methods. Each assay provided consistent detection of genomic DNA at quantities of 2 fg or less, demonstrated species-specificity, and showed an improved limit of detection over the existing, proven PCR-based assay.Conclusions/SignificanceThe utilization of next-generation sequencing-based repeat DNA analysis methodologies for the identification of molecular diagnostic targets has the ability to improve assay species-specificity and limits of detection. By exploiting such high copy-number repeat sequences, the assays described here will facilitate soil transmitted helminth diagnostic efforts. We recommend similar analyses when designing PCR-based diagnostic tests for the detection of other eukaryotic pathogens.

PCR has recently been studied as a promising tool for monitoring the progress of efforts to eliminate lymphatic filariasis. PCR can be used to test concurrently at least 30 pools, with as many as 40 mosquitoes in each pool, for the presence of filarial larvae. The SspI PCR assay for the detection of Wuchereria bancrofti DNA in pools of mosquitoes has been used since 1994 in a variety of laboratories worldwide. During that time, the original assay has been modified in these different laboratories and no standardized assay currently exists. In an effort to standardize and improve the assay, a meeting was held on 15-16 November 2001, at Emory University in Atlanta, with representatives from most of the laboratories currently using the assay. The first round of testing was designed to test the four most promising methods for DNA extraction from pools of mosquitoes. Two of the four methods stood out as clearly the best and these will be now optimised and evaluated in two further rounds of testing.

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An inversion polymorphism of the filamin and emerin genes at the tip of the long arm of the human X-chromosome serves as the basis of an investigative laboratory in which students learn something new about their own genomes. Long, nearly identical inverted repeats flanking the filamin and emerin genes illustrate how repetitive elements can lead to alterations in genome structure (inversions) through nonallelic homologous recombination. The near identity of the inverted repeats is an example of concerted evolution through gene conversion. While the laboratory in its entirety is designed for college level genetics courses, portions of the laboratory are appropriate for courses at other levels. Because the polymorphism is on the X-chromosome, the laboratory can be used in introductory biology courses to enhance understanding of sex-linkage and to test for Hardy-Weinberg equilibrium in females. More advanced topics, such as chromosome interference, the molecular model for recombination, and inversion heterozygosity suppression of recombination can be explored in upper-level genetics and evolution courses. DNA isolation, restriction digests, ligation, long PCR, and iPCR provide experience with techniques in molecular biology. This investigative laboratory weaves together topics stretching from molecular genetics to cytogenetics and sex-linkage, population genetics and evolutionary genetics.

Little is currently known about the rates at which non-allelic homologous recombination (NAHR) occurs. However, most current research suggests that NAHR is rare. Previous work by Small, et al (1998), examined an inversion polymorphism on the long arm of the X-chromosome, involving two genes (FLNA and EMD), and determined the frequency of the two gene arrangements in a group of European individuals. Here we quantify the rate at which the causal NAHR, in inverted repeats flanking the FLNA and EMD genes, occurs in meiosis using digital PCR of sperm samples, with male cheek cells as controls. NAHR was documented in all samples, including the cheek cell samples at a mean recombination rate of 1.8%, indicating that NAHR occurs much more frequently than initially believed, and appears to be occurring in mitosis. The increase in NAHR frequency in spermatogenesis is not significant leaving in question NAHR occurrence in meiosis. This study reveals a more accurate way to quantitate NAHR, serving as an important first step in better understanding various NAHR-associated diseases. Author SummaryWe sought to more accurately quantitate and characterize NAHR at a site at the end of the long arm of the X chromosome that contains a set of inverted repeats flanking two genes, filamin and emerin. We determined that NAHR is happening far more frequently than previously thought, and in this case unequally, depending on the direction of the inversion. We speculate on the possibility of local adaptation playing a role in this. These high-resolution results were obtained by modifying a previously published assay which can be easily adapted to other inversions. This could be especially helpful in studying those NAHR inversions related to disease.