Resolving microsatellite genotype ambiguity in populations of allopolyploid and diploidized autopolyploid organisms using negative correlations between alleles

Resolving microsatellite genotype ambiguity in populations of allopolyploid and diploidized autopolyploid organisms using negative correlations between alleles

Lindsay V Clark, Andrea Drauch Schreier

A major limitation in the analysis of genetic marker data from polyploid organisms is non-Mendelian segregation, particularly when a single marker yields allelic signals from multiple, independently segregating loci (isoloci). However, with markers such as microsatellites that detect more than two alleles, it is sometimes possible to deduce which alleles belong to which isoloci. Here we describe a novel mathematical property of codominant marker data when it is recoded as binary (presence/absence) allelic variables: under random mating in an infinite population, two allelic variables will be negatively correlated if they belong to the same locus, but uncorrelated if they belong to different loci. We present an algorithm to take advantage of this mathematical property, sorting alleles into isoloci based on correlations, then refining the allele assignments after checking for consistency with individual genotypes. We demonstrate the utility of our method on simulated data, as well as a real microsatellite dataset from a natural population of octoploid white sturgeon (Acipenser transmontanus). Our methodology is implemented in the R package polysat version 1.4.

Are Genetic Interactions Influencing Gene Expression Evidence for Biological Epistasis or Statistical Artifacts?

Are Genetic Interactions Influencing Gene Expression Evidence for Biological Epistasis or Statistical Artifacts?

Alexandra Fish, John A. Capra, William S Bush

Interactions between genetic variants, also called epistasis, are pervasive in model organisms; however, their importance in humans remains unclear because statistical interactions in observational studies can be explained by processes other than biological epistasis. Using statistical modeling, we identified 1,093 interactions between pairs of cis-regulatory variants impacting gene expression in lymphoblastoid cell lines. Factors known to confound these analyses (ceiling/floor effects, population stratification, haplotype effects, or single variants tagged through linkage disequilibrium) explained most of these interactions. However, we found 15 interactions robust to these explanations, and we further show that despite potential confounding, interacting variants were enriched in numerous regulatory regions suggesting potential biological importance. While genetic interactions may not be the true underlying mechanism of all our statistical models, our analyses discover new signals undetected in standard single-marker analyses. Ultimately, we identified new complex genetic architectures regulating 23 genes, suggesting that single-variant analyses may miss important modifiers.

The weighting is the hardest part: on the behavior of the likelihood ratio test and score test under weight misspecification in rare variant association studies

The weighting is the hardest part: on the behavior of the likelihood ratio test and score test under weight misspecification in rare variant association studies

Camelia Claudia Minica, Giulio Genovese, Dorret I. Boomsma, Christina M. Hultman, René Pool, Jacqueline M. Vink, Conor V. Dolan, Benjamin M. Neale

Rare variant association studies are gaining importance in human genetic research with the increasing availability of exome/genome sequence data. One important test of association between a target set of rare variants (RVs) and a given phenotype is the sequence kernel association test (SKAT). Assignment of weights reflecting the hypothesized contribution of the RVs to the trait variance is embedded within any set-based test. Since the true weights are generally unknown, it is important to establish the effect of weight misspecification in SKAT. We used simulated and real data to characterize the behavior of the likelihood ratio test (LRT) and score test under weight misspecification. Results revealed that LRT is generally more robust to weight misspecification, and more powerful than score test in such a circumstance. For instance, when the rare variants within the target were simulated to have larger betas than the more common ones, incorrect assignment of equal weights reduced the power of the LRT by ~5% while the power of score test dropped by ~30%. Furthermore, LRT was more robust to the inclusion of weighed neutral variation in the test. To optimize weighting we proposed the use of a data-driven weighting scheme. With this approach and the LRT we detected significant enrichment of case mutations with MAF below 5% (P-value=7E-04) of a set of highly constrained genes in the Swedish schizophrenia case-control cohort of 4,940 individuals with observed exome-sequencing data. The score test is currently widely used in sequence kernel association studies for both its computational efficiency and power. Indeed, assuming correct specification, in some circumstances the score test is the most powerful test. However, our results showed that LRT has the compelling qualities of being generally more robust and more powerful under weight misspecification. This is a paramount result, given that, arguably, misspecified models are likely to be the rule rather than the exception in the weighting-based approaches.

The next 20 years of genome research

The next 20 years of genome research

Michael Schatz

The last 20 years have been a remarkable era for biology and medicine. One of the most significant achievements has been the sequencing of the first human genomes, which has laid the foundation for profound insights into human genetics, the intricacies of regulation and development, and the forces of evolution. Incredibly, as we look into the future over the next 20 years, we see the very real potential for sequencing more than one billion genomes, bringing with it even deeper insights into human genetics as well as the genetics of millions of other species on the planet. Realizing this great potential, though, will only be achieved through the integration and development of highly scalable computational and quantitative approaches can keep pace with the rapid improvements to biotechnology. In this perspective, we aim to chart out these future technologies, anticipate the major themes of research, and call out the challenges ahead. One of the largest shifts will be in the training used to prepare the class of 2035 for their highly interdisciplinary world.

Learning quantitative sequence-function relationships from high-throughput biological data

Learning quantitative sequence-function relationships from high-throughput biological data

Gurinder S Atwal, Justin B Kinney

Understanding the transcriptional regulatory code, as well as other types of information encoded within biomolecular sequences, will require learning biophysical models of sequence-function relationships from high-throughput data. Controlling and characterizing the noise in such experiments, however, is notoriously difficult. The unpredictability of such noise creates problems for standard likelihood-based methods in statistical learning, which require that the quantitative form of experimental noise be known precisely. However, when this unpredictability is properly accounted for, important theoretical aspects of statistical learning which remain hidden in standard treatments are revealed. Specifically, one finds a close relationship between the standard inference method, based on likelihood, and an alternative inference method based on mutual information. Here we review and extend this relationship. We also describe its implications for learning sequence-function relationships from real biological data. Finally, we detail an idealized experiment in which these results can be demonstrated analytically.

Optimizing error correction of RNAseq reads

Optimizing error correction of RNAseq reads

Matthew D MacManes

Motivation: The correction of sequencing errors contained in Illumina reads derived from genomic DNA is a common pre-processing step in many de novo genome assembly pipelines, and has been shown to improved the quality of resultant assemblies. In contrast, the correction of errors in transcriptome sequence data is much less common, but can potentially yield similar improvements in mapping and assembly quality. This manuscript evaluates several popular read-correction tool’s ability to correct sequence errors commonplace to transcriptome derived Illumina reads. Results: I evaluated the efficacy of correction of transcriptome derived sequencing reads using using several metrics across a variety of sequencing depths. This evaluation demonstrates a complex relationship between the quality of the correction, depth of sequencing, and hardware availability which results in variable recommendations depending on the goals of the experiment, tolerance for false positives, and depth of coverage. Overall, read error correction is an important step in read quality control, and should become a standard part of analytical pipelines. Availability: Results are non-deterministically repeatable using AMI:ami-3dae4956 (MacManes EC 2015) and the Makefile available here:

Mixed Models for Meta-Analysis and Sequencing

Mixed Models for Meta-Analysis and Sequencing

Brendan Bulik-Sullivan

Mixed models are an effective statistical method for increasing power and avoiding confounding in genetic association studies. Existing mixed model methods have been designed for “pooled” studies where all individual-level genotype and phenotype data are simultaneously visible to a single analyst. Many studies follow a “meta-analysis” design, wherein a large number of independent cohorts share only summary statistics with a central meta-analysis group, and no one person can view individual-level data for more than a small fraction of the total sample. When using linear regression for GWAS, there is no difference in power between pooled studies and meta-analyses \cite{lin2010meta}; however, we show that when using mixed models, standard meta-analysis is much less powerful than mixed model association on a pooled study of equal size. We describe a method that allows meta-analyses to capture almost all of the power available to mixed model association on a pooled study without sharing individual-level genotype data. The added computational cost and analytical complexity of this method is minimal, but the increase in power can be large: based on the predictive performance of polygenic scoring reported in \cite{wood2014defining} and \cite{locke2015genetic}, we estimate that the next height and BMI studies could see increases in effective sample size of $\approx$15\% and $\approx$8\%, respectively. Last, we describe how a related technique can be used to increase power in sequencing, targeted sequencing and exome array studies. Note that these techniques are presently only applicable to randomly ascertained studies and will sometimes result in loss of power in ascertained case/control studies. We are developing similar methods for case/control studies, but this is more complicated.