DISEASES: Text mining and data integration of disease–gene associations
Sune Pletscher-Frankild, Albert Pallejà, Kalliopi Tsafou, Janos X Binder, Lars Juhl Jensen
Text mining is a flexible technology that can be applied to numerous different tasks in biology and medicine. We present a system for extracting disease–gene associations from biomedical abstracts. The system consists of a highly efficient dictionary-based tagger for named entity recognition of human genes and diseases, which we combine with a scoring scheme that takes into account co-occurrences both within and between sentences. We show that this approach is able to extract half of all manually curated associations with a false positive rate of only 0.16%. Nonetheless, text mining should not stand alone, but be combined with other types of evidence. For this reason, we have developed the DISEASES resource, which integrates the results from text mining with manually curated disease–gene associations, cancer mutation data, and genome-wide association studies from existing databases. The DISEASES resource is accessible through a user-friendly web interface at http://diseases.jensenlab.org/, where the text-mining software and all associations are also freely available for download.
A genomic map of the effects of linked selection in Drosophila
Eyal Elyashiv, Shmuel Sattath, Tina T. Hu, Alon Strustovsky, Graham McVicker, Peter Andolfatto, Graham Coop, Guy Sella
(Submitted on 23 Aug 2014)
Natural selection at one site shapes patterns of genetic variation at linked sites. Quantifying the effects of ‘linked selection’ on levels of genetic diversity is key to making reliable inference about demography, building a null model in scans for targets of adaptation, and learning about the dynamics of natural selection. Here, we introduce the first method that jointly infers parameters of distinct modes of linked selection, notably background selection and selective sweeps, from genome-wide diversity data, functional annotations and genetic maps. The central idea is to calculate the probability that a neutral site is polymorphic given local annotations, substitution patterns, and recombination rates. Information is then combined across sites and samples using composite likelihood in order to estimate genome-wide parameters of distinct modes of selection. In addition to parameter estimation, this approach yields a map of the expected neutral diversity levels along the genome. To illustrate the utility of our approach, we apply it to genome-wide resequencing data from 125 lines in Drosophila melanogaster and reliably predict diversity levels at the 1Mb scale. Our results corroborate estimates of a high fraction of beneficial substitutions in proteins and untranslated regions (UTR). They allow us to distinguish between the contribution of sweeps and other modes of selection around amino acid substitutions and to uncover evidence for pervasive sweeps in untranslated regions (UTRs). Our inference further suggests a substantial effect of linked selection from non-classic sweeps. More generally, we demonstrate that linked selection has had a larger effect in reducing diversity levels and increasing their variance in D. melanogaster than previously appreciated.
Escape from crossover interference increases with maternal age
Christopher L. Campbell, Nicholas A. Furlotte, Nick Eriksson, David Hinds, Adam Auton
(Submitted on 23 Aug 2014)
Recombination plays a fundamental role in meiosis, ensuring the proper segregation of chromosomes and contributing to genetic diversity by generating novel combinations of alleles. Using data derived from directUtoUconsumer genetic testing, we investigated patterns of recombination in over 4,200 families. Our analysis revealed a number of sex differences in the distribution of recombination. We find the fraction of male events occurring within hotspots to be 4.6% higher than for females. We confirm that the recombination rate increases with maternal age, while hotspot usage decreases, with no such effects observed in males. Finally, we show that the placement of female recombination events becomes increasingly deregulated with maternal age, with an increasing fraction of events appearing to escape crossover interference.
IPED2: Inheritance Path based Pedigree Reconstruction Algorithm for Complicated Pedigrees
Dan He, Zhanyong Wang, Laxmi Parida, Eleazar Eskin
(Submitted on 23 Aug 2014)
Reconstruction of family trees, or pedigree reconstruction, for a group of individuals is a fundamental problem in genetics. The problem is known to be NP-hard even for datasets known to only contain siblings. Some recent methods have been developed to accurately and efficiently reconstruct pedigrees. These methods, however, still consider relatively simple pedigrees, for example, they are not able to handle half-sibling situations where a pair of individuals only share one parent. In this work, we propose an efficient method, IPED2, based on our previous work, which specifically targets reconstruction of complicated pedigrees that include half-siblings. We note that the presence of half-siblings makes the reconstruction problem significantly more challenging which is why previous methods exclude the possibility of half-siblings. We proposed a novel model as well as an efficient graph algorithm and experiments show that our algorithm achieves relatively accurate reconstruction. To our knowledge, this is the first method that is able to handle pedigree reconstruction based on genotype data only when half-sibling exists in any generation of the pedigree.
Population split time estimation and X to autosome effective population size differences inferred using physically phased genomes
Shiya Song, Elzbieta Sliwerska, Jeffrey M Kidd
Haplotype resolved genome sequence information is of growing interest due to its applications in both population genetics and medical genetics. Here, we assess the ability to correctly reconstruct haplotype sequences using fosmid pooled sequencing and apply the sequences to explore historical population relationships. We resolved phased haplotypes of sample NA19240, a trio child from the Yoruba HapMap collection using pools of a total of 521,783 fosmid clones. We phased 93% of heterozygous SNPs into haplotype-resolved blocks, with an N50 size of 318kb. Using trio information from HapMap, we linked adjacent blocks together to form paternal and maternal alleles, producing near-to-complete haplotypes. Comparison with 33 individual fosmids sequenced using capillary sequencing shows that our reconstructed sequence haplotypes have a sequence error rate of 0.005%. Utilizing fosmid-phased haplotypes from a Yoruba, a European and a Gujarati sample, we analyzed population history and inferred population split times. We date the initial split between Yoruba and out of African populations to 90,000-100,000 years ago with substantial gene flow occurring until nearly 50,000 years ago, and obtain congruent results with the autosomes and the X chromosome. We estimate that the initial split between European and Gujarati population occurred around 45,000 years ago and gene flow ended around 28,000 years ago. Analysis of X vs autosome inferred effective population sizes reveals distinct epochs in which the ratio of the effective number of males to females changes. We find a period of female bias during the ancestral human lineage up to 1 million years ago and a short period of male bias in Yoruba lineage from 160-400 thousand years ago. We demonstrate the construction of haplotype sequences of sufficient completeness and accuracy for population genetic analysis. As experimental and analytic methods improve, these approaches will continue to shed new light on the history of populations.
Sources of PCR-induced distortions in high-throughput sequencing datasets
Justus M Kebschull, Anthony M Zador
PCR allows the exponential and sequence specific amplification of DNA, even from minute starting quantities. Today, PCR is at the core of the most successful DNA sequencing technologies and is a fundamental step in preparing DNA samples for high throughput sequencing. Despite its importance, we have little comprehensive understanding of the biases and errors that PCR introduces into pools of DNA molecules. Understanding PCRs imperfections and their impact on the amplification of different sequences in a complex mixture is particularly important for a proper understanding of high-throughput sequencing data. We examined the effects of bias, stochasticity, template switches and polymerase errors introduced during PCR on sequence representation in next-generation sequencing libraries. Using Illumina sequencing results of a pool of diverse PCR amplicons with a defined structure, we searched for signatures of each process. We further developed quantitative models for each process and compared predictions of these models to our experimental data. We find that PCR stochasticity is the major force skewing sequence representation after amplification of a pool of unique DNA amplicons. PCR errors become very common in later cycles of PCR but have little impact on the overall sequence distribution as they are confined to small copy numbers. PCR template switches are rare and confined to low copy numbers. Our results will have particular relevance to single cell sequencing, in which sequences are represented by only one or a few molecules.
Variation of nonsynonymous/synonymous rate ratios at HLA genes over time and phylogenetic context
Bárbara D Bitarello, Rodrigo dos Santos Francisco, Diogo Meyer
Many HLA loci show an excess of nonsynonymous (dN) with respect to synonymous (dS) substitutions at codons of the antigen recognition site (ARS), a hallmark of adaptive evolution. However, it remains unclear how these changes are distributed over time and across branches of the HLA phylogeny. In particular, although HLA alleles can be assigned to functionally and phylogenetically defined groups (“lineages”), a test for differences in ω (ω = dN/dS) within and between lineages is lacking. We analysed variation of ω across divergence times and phylogenetic contexts (placement of branches in the phylogeny). We found a significant positive correlation between ω at ARS codons and divergence time, and that branches between lineages have higher ω than those within lineages. The excess of nonsynonymous hanges between lineages attained significance when we used non-ARS codons to account for the fact that, even under purifying selection, ω is inflated for recently diverged alleles. Although less intensely selected, within-lineage variation at ARS codons bears evidence of selection, in the form of higher ω than those of non-ARS codons. Our results show that ω ratios of class I HLA genes vary over time, and are higher in branches connecting alleles from distinct lineages. These results suggest that although within-lineage variation bears evidence of balancing selection, the between-lineage changes have been more intensely selected. Our findings indicate the importance of considering the effect of timescale when analysing ω values over a wide spectrum of divergences, and the value of using additional markers (in our case the tightly linked non-ARS codons) to account for the temporal dynamics of ω.