Comprehensive identification and characterization of diallelic insertion-deletion polymorphisms in 330 human candidate genes

A biallelic multiple nucleotide length polymorphism explains functional causality at 5p15.33 prostate cancer risk locus

To date, single-nucleotide polymorphisms (SNPs) have been the most intensively investigated class of polymorphisms in genome wide associations studies (GWAS), however, other classes such as insertion-deletion or multiple nucleotide length polymorphism (MNLPs) may also confer disease risk. Multiple reports have shown that the 5p15.33 prostate cancer risk region is a particularly strong expression quantitative trait locus (eQTL) for Iroquois Homeobox 4 (IRX4) transcripts. Here, we demonstrate using epigenome and genome editing that a biallelic (21 and 47 base pairs (bp)) MNLP is the causal variant regulating IRX4 transcript levels. In LNCaP prostate cancer cells (homozygous for the 21 bp short allele), a single copy knock-in of the 47 bp long allele potently alters the chromatin state, enabling de novo functional binding of the androgen receptor (AR) associated with increased chromatin accessibility, Histone 3 lysine 27 acetylation (H3K27ac), and ~3-fold upregulation of IRX4 expression. We further show that an MNLP is amongst the strongest candidate susceptibility variants at two additional prostate cancer risk loci. We estimated that at least 5% of prostate cancer risk loci could be explained by functional non-SNP causal variants, which may have broader implications for other cancers GWAS. More generally, our results underscore the importance of investigating other classes of inherited variation as causal mediators of human traits.

Assessing Autosomal InDel Loci With Multiple Insertions or Deletions of Random DNA Sequences in Human Genome

Multiple mutational events of insertion/deletion occurring at or around InDel sites could form multi-allelic InDels and multi-InDels (abbreviated as MM-InDels), while InDels with random DNA sequences could imply a unique mutation event at these loci. In this study, preliminary investigation of MM-InDels with random sequences was conducted using high-throughput phased data from the 1000 Genomes Project. A total of 3,599 multi-allelic InDels and 6,375 multi-InDels were filtered with multiple alleles. A vast majority of the obtained MM-InDels (85.59%) presented 3 alleles, which implies that only one secondary insertion or deletion mutation event occurred at these loci. The more frequent presence of two adjacent InDel loci was observed within 20 bp. MM-InDels with random sequences presented an uneven distribution across the genome and showed a correlation with InDels, SNPs, recombination rate, and GC content. The average allelic frequencies and prevalence of multi-allelic InDels and multi-InDels presented similar distribution patterns in different populations. Altogether, MM-InDels with random sequences can provide useful information for population resolution.

Systematic Evaluation of a Novel 6-dye Direct and Multiplex PCR-CE-Based InDel Typing System for Forensic Purposes

Insertion/deletion (InDel) polymorphisms, combined desirable characteristics of both short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs), are considerable potential in the fields of forensic practices and population genetics. However, most commercial InDel kits designed based on non-Asians limited extensive forensic applications in East Asian (EAS) populations. Recently, a novel 6-dye direct and multiplex PCR-CE-based typing system was designed on the basis of genome-wide EAS population data, which could amplify 60 molecular genetic markers, consisting of 57 autosomal InDels (A-InDels), 2 Y-chromosomal InDels (Y-InDels), and Amelogenin in a single PCR reaction and detect by capillary electrophoresis, simultaneously. In the present study, the DNA profiles of 279 unrelated individuals from the Hainan Li group were generated by the novel typing system. In addition, we collected two A-InDel sets to evaluate the forensic performances of the novel system in the 1,000 Genomes Project (1KG) populations and Hainan Li group. For the Universal A-InDel set (UAIS, containing 44 A-InDels) the cumulative power of discrimination (CPD) ranged from 1-1.03 × 10-14 to 1-1.27 × 10-18, and the cumulative power of exclusion (CPE) varied from 0.993634 to 0.999908 in the 1KG populations. For the East Asia-based A-InDel set (EAIS, containing 57 A-InDels) the CPD spanned from 1-1.32 × 10-23 to 1-9.42 × 10-24, and the CPE ranged from 0.999965 to 0.999997. In the Hainan Li group, the average heterozygote (He) was 0.4666 (0.2366-0.5448), and the polymorphism information content (PIC) spanned from 0.2116 to 0.3750 (mean PIC: 0.3563 ± 0.0291). In total, the CPD and CPE of 57 A-InDels were 1-1.32 × 10-23 and 0.999965, respectively. Consequently, the novel 6-dye direct and multiplex PCR-CE-based typing system could be considered as the reliable and robust tool for human identification and intercontinental population differentiation, and supplied additional information for kinship analysis in the 1KG populations and Hainan Li group.

Structural and functional analysis of somatic coding and UTR indels in breast and lung cancer genomes

Insertions and deletions (Indels) represent one of the major variation types in the human genome and have been implicated in diseases including cancer. To study the features of somatic indels in different cancer genomes, we investigated the indels from two large samples of cancer types: invasive breast carcinoma (BRCA) and lung adenocarcinoma (LUAD). Besides mapping somatic indels in both coding and untranslated regions (UTRs) from the cancer whole exome sequences, we investigated the overlap between these indels and transcription factor binding sites (TFBSs), the key elements for regulation of gene expression that have been found in both coding and non-coding sequences. Compared to the germline indels in healthy genomes, somatic indels contain more coding indels with higher than expected frame-shift (FS) indels in cancer genomes. LUAD has a higher ratio of deletions and higher coding and FS indel rates than BRCA. More importantly, these somatic indels in cancer genomes tend to locate in sequences with important functions, which can affect the core secondary structures of proteins and have a bigger overlap with predicted TFBSs in coding regions than the germline indels. The somatic CDS indels are also enriched in highly conserved nucleotides when compared with germline CDS indels.

Genetics, its role in preventing the pandemic of coronary artery disease

Epidemiologists have claimed for decades that about 50% of predisposition for coronary artery disease (CAD) is genetic. Advances in technology made possible the discovery of hundreds of genetic risk variants predisposing to CAD. Multiple clinical trials have shown that cardiac events can be prevented by drugs to lower plasma low-density lipoprotein cholesterol (LDL-C). A major barrier to primary prevention is the lack of markers to identify those individuals at risk prior to the development of symptoms of the disease. Conventional risk factors are age-dependent, occurring mostly in the sixth or seventh decade, which is less than desirable for early primary prevention. A polygenic risk score, derived from the number of genetic risk variants predisposing to CAD inherited by an individual, has been evaluated in over 1 million individuals. The risk for CAD is stratified into high, intermediate, and low. Polygenic risk scores derived from retrospective genotyping of several clinical trials evaluating the effect of statin therapy or PCSK9 inhibitors show the genetic risk is reduced 40%-50% by decreasing plasma LDL-C. Prospective randomized placebo-controlled clinical trials document a 40%-50% reduction in cardiac events in individuals at high genetic risk associated with favorable lifestyle changes and increased physical activity. The polygenic risk score is not age-dependent and remains the same throughout life. Thus, the GRS is superior to conventional risk factors in identifying asymptomatic individuals at risk for CAD early in life for primary prevention. These results indicate clinical embracement of the GRS in primary prevention would be a paradigm shift in the treatment of the number one killer, CAD.

Genetic variation in the Mauritian cynomolgus macaque population reflects variation in the human population

The cynomolgus macaque is an important species for preclinical research, however the extent of genetic variation in this population and its similarity to the human population is not well understood. Exome sequencing was conducted for 101 cynomolgus macaques to characterize genetic variation. The variant distribution frequency was 7.81 variants per kilobase across the sequenced regions, with a total of 2,770,009 single nucleotide variants identified from 2,996,041 loci. A large portion (85.6%) had minor allele frequencies greater than 5%. Enriched pathways for genes with high genetic diversity (≥10 variants per kilobase) were those involving signaling peptides and immune response. Compared to human, the variant distribution frequency and nucleotide diversity in the macaque exome was approximately 4 times greater; however the ratio of non-synonymous to synonymous variants was similar (0.735 and 0.831, respectively). Understanding genetic variability in cynomolgus macaques will enable better interpretation and human translation of phenotypic variability in this species.

A Journey through Genetic Architecture and Predisposition of Coronary Artery Disease

Introduction

To halt the spread of coronary artery disease (CAD), the number one killer in the world, requires primary prevention. Fifty percent of all Americans are expected to experience a cardiac event; the challenge is identifying those at risk. 40 to 60% of predisposition to CAD is genetic. The first genetic risk variant, 9p21, was discovered in 2007. Genome-Wide Association Studies has since discovered hundreds of genetic risk variants. The genetic burden for CAD can be expressed as a single number, Genetic Risk Score (GRS). Assessment of GRS to risk stratify for CAD was superior to conventional risk factors in several large clinical trials assessing statin therapy, and more recently in a population of nearly 500,000 (UK Biobank). Studies were performed based on prospective genetic risk stratification for CAD. These studies showed that a favorable lifestyle was associated with a 46% reduction in cardiac events and programmed exercise, a 50% reduction in cardiac events. Genetic risk score is superior to conventional risk factors, and is markedly attenuated by lifestyle changes and drug therapy. Genetic risk can be determined at birth or any time thereafter.

Conclusion

Utilizing the GRS to risk stratify young, asymptomatic individuals could provide a paradigm shift in the primary prevention of CAD and significantly halt its spread.

Forensic characteristic and population structure dissection of Shaanxi Han population in the light of diallelic deletion/insertion polymorphism data

The genetic polymorphisms of diallelic deletion/insertion polymorphic (DIP) loci in the Shaanxi Han population are still not clearly characterized. Herein, allele frequencies and forensic application efficiencies for 30 diallelic DIP loci were investigated in 506 unrelated healthy Han individuals from Chinese Shaanxi province. Based on population data of the same 30 diallelic DIP loci, the genetic differentiations, hierarchical clustering relationships and population architectures among Shaanxi Han and other 50 populations were further dissected through genetic and bioinformatics analyses. Results indicated that most of the 30 diallelic DIP loci were relatively high polymorphisms in the Shaanxi Han population; and there were the genetically intimate relationships between Shaanxi Han and the East Asian populations. In summary, this study provided significant insights into genetic background of Shaanxi Han population, and the multiplex amplification of these 30 diallelic DIP loci was appropriate for forensic individual identification and population genetic research in Shaanxi Han population.

Whole genome detection of sequence and structural polymorphism in six diverse horses

The domesticated horse has played a unique role in human history, serving not just as a source of animal protein, but also as a catalyst for long-distance migration and military conquest. As a result, the horse developed unique physiological adaptations to meet the demands of both their climatic environment and their relationship with man. Completed in 2009, the first domesticated horse reference genome assembly (EquCab 2.0) produced most of the publicly available genetic variations annotations in this species. Yet, there are around 400 geographically and physiologically diverse breeds of horse. To enrich the current collection of genetic variants in the horse, we sequenced whole genomes from six horses of six different breeds: an American Miniature, a Percheron, an Arabian, a Mangalarga Marchador, a Native Mongolian Chakouyi, and a Tennessee Walking Horse, and mapped them to EquCab3.0 genome. Aside from extreme contrasts in body size, these breeds originate from diverse global locations and each possess unique adaptive physiology. A total of 1.3 billion reads were generated for the six horses with coverage between 15x to 24x per horse. After applying rigorous filtration, we identified and functionally annotated 17,514,723 Single Nucleotide Polymorphisms (SNPs), and 1,923,693 Insertions/Deletions (INDELs), as well as an average of 1,540 Copy Number Variations (CNVs) and 3,321 Structural Variations (SVs) per horse. Our results revealed putative functional variants including genes associated with size variation like LCORL gene (found in all horses), ZFAT in the Arabian, American Miniature and Percheron horses and ANKRD1 in the Native Mongolian Chakouyi horse. We detected a copy number variation in the Latherin gene that may be the result of evolutionary selection impacting thermoregulation by sweating, an important component of athleticism and heat tolerance. The newly discovered variants were formatted into user-friendly browser tracks and will provide a foundational database for future studies of the genetic underpinnings of diverse phenotypes within the horse.

The domesticated horse played a unique role in human history, serving not just as a source of dietary animal protein, but also as a catalyst for long-distance migration and military conquest. As a result, the horse developed unique physiological adaptations to meet the demands of both their climatic environment and their relationship with man. Although the completion of the horse reference genome allowed for the discovery of many genetic variants, the remarkable diversity across breeds of horse calls for additional effort to quantify the complete span of genetic polymorphism within this unique species. In this work, we present genome re-sequencing and variant detection analysis for six horses belonging to six different breeds representing different morphology, origins and vary in their physiological demands and response. We identified and annotated not just single nucleotide polymorphisms (SNPs), but also insertions and deletions (INDELs), copy number variations (CNVs) and structural variations (SVs). Our results illustrate novel sources of polymorphism and highlight potentially impactful variations for phenotypes of body size and conformation. We also detected a copy number loss in the Latherin gene that could be the result of an evolutionary selection affecting thermoregulation through sweating. Our newly discovered variants were formatted into easy-to-use tracks that can be easily accessed by researchers around the globe.

Detection of key gene InDels in TGF-β pathway and its relationship with growth traits in four sheep breeds

TGF-β signaling pathway plays an important role in regulating cell proliferation and differentiation, embryonic development, bone formation, etc. LTBP1, THBS1, SMAD4 and other genes are important members of TGF-β signaling pathway. LTBP1 binds to TGF-β, while THBS1 binds to LTBP1, which is an important signal transduction molecule in the TGF-β pathway. In order to explore the effects of the insertion/deletion variation of three genes (LTBP1, THBS1, SMAD4) in the TGF-β signaling pathway on the growth traits such as body length and body weight of sheep, a total of 625 healthy individuals from 4 breeds of the Tong sheep, Hu sheep, small-tail Han sheep and Lanzhou fat-tail sheep were identified and analyzed. In this study, we identified 4 InDel loci: one loci of LTBP1, two loci of THBS1, and one loci of SMAD4, respectively named as: InDel-1 (deletion 13 bp), InDel-2 (deletion 16 bp), InDel-3 (deletion 22 bp), InDel-4 (deletion 7 bp). Among the 4 analyzed breeds, association analysis showed that all new InDel polymorphisms were significantly associated with 10 different growth traits (p < 0.05), which may provide a theoretical basis for sheep breeding to accelerate the progression of marker-assisted selection in sheep breeding.

Prediction and management of CAD risk based on genetic stratification

Discovery of genetic risk variants for CAD and their assembly on a computerized microarray enables a genetic risk score (GRS) to be expressed as a single number. Utilizing this array, genetic risk stratification has been performed in over 1 million cases and controls. The genetic score based on one's DNA can be determined anytime from birth on and is independent of age and conventional risk factors. Utilizing the GRS, one can select those at highest risk and would benefit most from primary prevention. Clinical trials have shown that modifying lifestyle or using statin therapy reduces the risk for CAD by approximately 50%. The use of the GRS for primary prevention will have a transformative effect on preventing the spread of CAD.

Uncovering missed indels by leveraging unmapped reads

In current practice, Next Generation Sequencing (NGS) applications start with mapping/aligning short reads to the reference genome, with the aim of identifying genetic variants. Although existing alignment tools have shown great accuracy in mapping short reads to the reference genome, a significant number of short reads still remain unmapped and are often excluded from downstream analyses thereby causing nonnegligible information loss in the subsequent variant calling procedure. This paper describes Genesis-indel, a computational pipeline that explores the unmapped reads to identify novel indels that are initially missed in the original procedure. Genesis-indel is applied to the unmapped reads of 30 breast cancer patients from TCGA. Results show that the unmapped reads are conserved between the two subtypes of breast cancer investigated in this study and might contribute to the divergence between the subtypes. Genesis-indel identifies 72,997 novel high-quality indels previously not found, among which 16,141 have not been annotated in the widely used mutation database. Statistical analysis of these indels shows significant enrichment of indels residing in oncogenes and tumour suppressor genes. Functional annotation further reveals that these indels are strongly correlated with pathways of cancer and can have high to moderate impact on protein functions. Additionally, some of the indels overlap with the genes that do not have any indel mutations called from the originally mapped reads but have been shown to contribute to the tumorigenesis in multiple carcinomas, further emphasizing the importance of rescuing indels hidden in the unmapped reads in cancer and disease studies.

Conserved Critical Evolutionary Gene Structures in Orthologs

Unravelling gene structure requires the identification and understanding of the constraints that are often associated with the evolutionary history and functional domains of genes. We speculated in this manuscript with the possibility of the existence in orthologs of an emergent highly conserved gene structure that might explain their coordinated evolution during speciation events and their parental function. Here, we will address the following issues: (1) is there any conserved hypothetical structure along ortholog gene sequences? (2) If any, are such conserved structures maintained and conserved during speciation events? The data presented show evidences supporting this hypothesis. We have found that, (1) most orthologs studied share highly conserved compositional structures not observed previously. (2) While the percent identity of nucleotide sequences of orthologs correlates with the percent identity of composon sequences, the number of emergent compositional structures conserved during speciation does not correlate with the percent identity. (3) A broad range of species conserves the emergent compositional stretches. We will also discuss the concept of critical gene structure.

Effects of short indels on protein structure and function in human genomes

Insertions and deletions (indels) represent the second most common type of genetic variations in human genomes. Indels can be deleterious and contribute to disease susceptibility as recent genome sequencing projects revealed a large number of indels in various cancer types. In this study, we investigated the possible effects of small coding indels on protein structure and function, and the baseline characteristics of indels in 2504 individuals of 26 populations from the 1000 Genomes Project. We found that each population has a distinct pattern in genes with small indels. Frameshift (FS) indels are enriched in olfactory receptor activity while non-frameshift (NFS) indels are enriched in transcription-related proteins. Structural analysis of NFS indels revealed that they predominantly adopt coil or disordered conformations, especially in proteins with transcription-related NFS indels. These results suggest that the annotated coding indels from the 1000 Genomes Project, while contributing to genetic variations and phenotypic diversity, generally do not affect the core protein structures and have no deleterious effect on essential biological processes. In addition, we found that a number of reference genome annotations might need to be updated due to the high prevalence of annotated homozygous indels in the general population.

Genetics: Implications for Prevention and Management of Coronary Artery Disease

An exciting new era has dawned for the prevention and management of coronary artery disease (CAD) utilizing genetic risk variants. The recent identification of over 60 susceptibility loci for CAD confirms not only the importance of established risk factors, but also the existence of many novel causal pathways that are expected to improve our understanding of the genetic basis of CAD and facilitate the development of new therapeutic agents over time. Concurrently, Mendelian randomization studies have provided intriguing insights on the causal relationship between CAD-related traits, and highlight the potential benefits of long-term modifications of risk factors. Last, genetic risk scores of CAD may serve not only as prognostic, but also as predictive markers, and carry the potential to considerably improve the delivery of established prevention strategies. This review will summarize the evolution and discovery of genetic risk variants for CAD and their current and future clinical applications.

An Incomplete Understanding of Human Genetic Variation

Deciphering the genetic basis of human disease requires a comprehensive knowledge of genetic variants irrespective of their class or frequency. Although an impressive number of human genetic variants have been catalogued, a large fraction of the genetic difference that distinguishes two human genomes is still not understood at the base-pair level. This is because the emphasis has been on single-nucleotide variation as opposed to less tractable and more complex genetic variants, including indels and structural variants. The latter, we propose, will have a large impact on human phenotypes but require a more systematic assessment of genomes at deeper coverage and alternate sequencing and mapping technologies.

Discovery and genotyping of structural variation from long-read haploid genome sequence data

In an effort to more fully understand the full spectrum of human genetic variation, we generated deep single-molecule, real-time (SMRT) sequencing data from two haploid human genomes. By using an assembly-based approach (SMRT-SV), we systematically assessed each genome independently for structural variants (SVs) and indels resolving the sequence structure of 461,553 genetic variants from 2 bp to 28 kbp in length. We find that >89% of these variants have been missed as part of analysis of the 1000 Genomes Project even after adjusting for more common variants (MAF > 1%). We estimate that this theoretical human diploid differs by as much as ∼16 Mbp with respect to the human reference, with long-read sequencing data providing a fivefold increase in sensitivity for genetic variants ranging in size from 7 bp to 1 kbp compared with short-read sequence data. Although a large fraction of genetic variants were not detected by short-read approaches, once the alternate allele is sequence-resolved, we show that 61% of SVs can be genotyped in short-read sequence data sets with high accuracy. Uncoupling discovery from genotyping thus allows for the majority of this missed common variation to be genotyped in the human population. Interestingly, when we repeat SV detection on a pseudodiploid genome constructed in silico by merging the two haploids, we find that ∼59% of the heterozygous SVs are no longer detected by SMRT-SV. These results indicate that haploid resolution of long-read sequencing data will significantly increase sensitivity of SV detection.

Using high-throughput sequencing transcriptome data for INDEL detection: challenges for cancer drug discovery

INTRODUCTION

A cancer cell is a mosaic of genomic and epigenomic alterations. Distinct cancer molecular signatures can be observed depending on tumor type or patient genetic background. One type of genomic alteration is the insertion and/or deletion (INDEL) of nucleotides in the DNA sequence, which may vary in length, and may change the encoded protein or modify protein domains. INDELs are associated to a large number of diseases and their detection is done based on low-throughput techniques. However, high-throughput sequencing has also started to be used for detection of novel disease-causing INDELs. This search may identify novel drug targets.

AREAS COVERED

This review presents examples of using high-throughput sequencing (DNA-Seq and RNA-Seq) to investigate the incidence of INDELs in coding regions of human genes. Some of these examples successfully utilized RNA-Seq to identify INDELs associated to diseases. In addition, other studies have described small INDELs related to chemo-resistance or poor outcome of patients, while structural variants were associated with a better clinical outcome.

EXPERT OPINION

On average, there is twice as much RNA-Seq data available at the most used repositories for such data compared to DNA-Seq. Therefore, using RNA-Seq data is a promising strategy for studying cancer samples with unknown mechanisms of drug resistance, aiming at the discovery of proteins with potential as novel drug targets.

Assessment of PAX6 alleles in 66 families with aniridia

We report on PAX6 alleles associated with a clinical diagnosis of classical aniridia in 81 affected individuals representing 66 families. Allelic variants expected to affect PAX6 function were identified in 61 families (76 individuals). Ten cases of sporadic aniridia (10 families) had complete (8 cases) or partial (2 cases) deletion of the PAX6 gene. Sequence changes that introduced a premature termination codon into the open reading frame of PAX6 occurred in 47 families (62 individuals). Three individuals with sporadic aniridia (three families) had sequence changes (one deletion, two run-on mutations) expected to result in a C-terminal extension. An intronic deletion of unknown functional significance was detected in one case of sporadic aniridia (one family), but not in unaffected relatives. Within these 61 families, single nucleotide substitutions accounted for 30/61 (49%), indels for 23/61 (38%), and complete deletion of the PAX6 locus for 8/61 (13%). In five cases of sporadic aniridia (five families), no disease-causing mutation in the coding region was detected. In total, 23 unique variants were identified that have not been reported in the Leiden Open Variation Database (LOVD) database. Within the group assessed, 92% had sequence changes expected to reduce PAX6 function, confirming the primacy of PAX6 haploinsufficiency as causal for aniridia.

PExFInS: An Integrative Post-GWAS Explorer for Functional Indels and SNPs

Expression quantitative trait loci (eQTLs) mapping and linkage disequilibrium (LD) analysis have been widely employed to interpret findings of genome-wide association studies (GWAS). With the availability of deep sequencing data of 423 lymphoblastoid cell lines (LCLs) from six global populations and the microarray expression data, we performed eQTL analysis, identified more than 228 K SNP cis-eQTLs and 21 K indel cis-eQTLs and generated a LCL cis-eQTL database. We demonstrate that the percentages of population-shared and population-specific cis-eQTLs are comparable; while indel cis-eQTLs in the population-specific subsection make more contribution to gene expression variations than those in the population-shared subsection. We found cis-eQTLs, especially the population-shared cis-eQTLs are significantly enriched toward transcription start site. Moreover, the National Human Genome Research Institute cataloged GWAS SNPs are enriched for LCL cis-eQTLs. Specifically, 32.8% GWAS SNPs are LCL cis-eQTLs, among which 12.5% can be tagged by indel cis-eQTLs, suggesting the fundamental contribution of indel cis-eQTLs to GWAS association signals. To search for functional indels and SNPs tagging GWAS SNPs, a pipeline Post-GWAS Explorer for Functional Indels and SNPs (PExFInS) has been developed, integrating LD analysis, functional annotation from public databases, cis-eQTL mapping with our LCL cis-eQTL database and other published cis-eQTL datasets.

Performance evaluation of indel calling tools using real short-read data

BACKGROUND

Insertion and deletion (indel), a common form of genetic variation, has been shown to cause or contribute to human genetic diseases and cancer. With the advance of next-generation sequencing technology, many indel calling tools have been developed; however, evaluation and comparison of these tools using large-scale real data are still scant. Here we evaluated seven popular and publicly available indel calling tools, GATK Unified Genotyper, VarScan, Pindel, SAMtools, Dindel, GTAK HaplotypeCaller, and Platypus, using 78 human genome low-coverage data from the 1000 Genomes project.

RESULTS

Comparing indels called by these tools with a known set of indels, we found that Platypus outperforms other tools. In addition, a high percentage of known indels still remain undetected and the number of common indels called by all seven tools is very low.

CONCLUSION

All these findings indicate the necessity of improving the existing tools or developing new algorithms to achieve reliable and consistent indel calling results.

Comparison and evaluation of two exome capture kits and sequencing platforms for variant calling

Background

To promote the clinical application of next-generation sequencing, it is important to obtain accurate and consistent variants of target genomic regions at low cost. Ion Proton, the latest updated semiconductor-based sequencing instrument from Life Technologies, is designed to provide investigators with an inexpensive platform for human whole exome sequencing that achieves a rapid turnaround time. However, few studies have comprehensively compared and evaluated the accuracy of variant calling between Ion Proton and Illumina sequencing platforms such as HiSeq 2000, which is the most popular sequencing platform for the human genome. The Ion Proton sequencer combined with the Ion TargetSeq™ Exome Enrichment Kit together make up TargetSeq-Proton, whereas SureSelect-Hiseq is based on the Agilent SureSelect Human All Exon v4 Kit and the HiSeq 2000 sequencer.

Results

Here, we sequenced exonic DNA from four human blood samples using both TargetSeq-Proton and SureSelect-HiSeq. We then called variants in the exonic regions that overlapped between the two exome capture kits (33.6 Mb). The rates of shared variant loci called by two sequencing platforms were from 68.0 to 75.3 % in four samples, whereas the concordance of co-detected variant loci reached 99 %. Sanger sequencing validation revealed that the validated rate of concordant single nucleotide polymorphisms (SNPs) (91.5 %) was higher than the SNPs specific to TargetSeq-Proton (60.0 %) or specific to SureSelect-HiSeq (88.3 %). With regard to 1-bp small insertions and deletions (InDels), the Sanger sequencing validated rates of concordant variants (100.0 %) and SureSelect-HiSeq-specific (89.6 %) were higher than those of TargetSeq-Proton-specific (15.8 %).

Conclusions

In the sequencing of exonic regions, a combination of using of two sequencing strategies (SureSelect-HiSeq and TargetSeq-Proton) increased the variant calling specificity for concordant variant loci and the sensitivity for variant loci called by any one platform. However, for the sequencing of platform-specific variants, the accuracy of variant calling by HiSeq 2000 was higher than that of Ion Proton, specifically for the InDel detection. Moreover, the variant calling software also influences the detection of SNPs and, specifically, InDels in Ion Proton exome sequencing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1796-6) contains supplementary material, which is available to authorized users.

BatAlign: an incremental method for accurate alignment of sequencing reads

Structural variations (SVs) play a crucial role in genetic diversity. However, the alignments of reads near/across SVs are made inaccurate by the presence of polymorphisms. BatAlign is an algorithm that integrated two strategies called 'Reverse-Alignment' and 'Deep-Scan' to improve the accuracy of read-alignment. In our experiments, BatAlign was able to obtain the highest F-measures in read-alignments on mismatch-aberrant, indel-aberrant, concordantly/discordantly paired and SV-spanning data sets. On real data, the alignments of BatAlign were able to recover 4.3% more PCR-validated SVs with 73.3% less callings. These suggest BatAlign to be effective in detecting SVs and other polymorphic-variants accurately using high-throughput data. BatAlign is publicly available at https://goo.gl/a6phxB.

Characteristics of de novo structural changes in the human genome

Small insertions and deletions (indels) and large structural variations (SVs) are major contributors to human genetic diversity and disease. However, mutation rates and characteristics of de novo indels and SVs in the general population have remained largely unexplored. We report 332 validated de novo structural changes identified in whole genomes of 250 families, including complex indels, retrotransposon insertions, and interchromosomal events. These data indicate a mutation rate of 2.94 indels (1-20 bp) and 0.16 SVs (>20 bp) per generation. De novo structural changes affect on average 4.1 kbp of genomic sequence and 29 coding bases per generation, which is 91 and 52 times more nucleotides than de novo substitutions, respectively. This contrasts with the equal genomic footprint of inherited SVs and substitutions. An excess of structural changes originated on paternal haplotypes. Additionally, we observed a nonuniform distribution of de novo SVs across offspring. These results reveal the importance of different mutational mechanisms to changes in human genome structure across generations.

A genetic basis for coronary artery disease

CAD and cancer account for over one-half of all deaths in the world. It is claimed that the 21st century is the last century for CAD. This is, in part, because CAD is preventable based on randomized, placebo-controlled clinical trials, which show modifying known risk factors such as cholesterol is associated consistently with 40-60% reduction in morbidity and mortality from CAD. Comprehensive prevention will require modifying genetic risk factors that are claimed to account for 40-60% of predisposition to CAD. The 21st century is meeting this challenge with over 50 genetic risk variants discovered and replicated in large genome-wide association studies involving over 200,000 cases and controls. Similarly, 157 genetic variants have been discovered that regulate plasma lipids including, LDL-C, HDL-C, triglycerides, and total cholesterol. A major finding from these studies is that only 15 of the 50 genetic variants for CAD act through known risk factors. Hence, the pathogenesis of CAD in addition to cholesterol and other known risk factors is due to various other factors, many of which remain unknown. Secondly, genes regulating the plasma triglyceride levels are strongly associated with the pathogenesis of CAD. Thirdly, Mendelian randomization studies show no protection from genes that increase plasma HDL cholesterol. This is contrary to current opinion. These genetic risk variants have provided new targets for the development of novel therapies to prevent CAD. Already a new and potent drug has been developed targeting PCSK9, which is in phase 3 clinical trials and shows great efficacy and safety for prevention of CAD. The 21st century is looking very bright for the prevention of CAD.

Identification of a truncated splice variant of IL-18 receptor alpha in the human and rat, with evidence of wider evolutionary conservation

Interleukin-18 (IL-18) is a pro-inflammatory cytokine which stimulates activation of the nuclear factor kappa beta (NF-κB) pathway via interaction with the IL-18 receptor. The receptor itself is formed from a dimer of two subunits, with the ligand-binding IL-18Rα subunit being encoded by the IL18R1 gene. A splice variant of murine IL18r1, which has been previously described, is formed by transcription of an unspliced intron (forming a ‘type II’ IL18r1 transcript) and is predicted to encode a receptor with a truncated intracellular domain lacking the capacity to generate downstream signalling. In order to examine the relevance of this finding to human IL-18 function, we assessed the presence of a homologous transcript by reverse transcription-polymerase chain reaction (RT-PCR) in the human and rat as another common laboratory animal. We present evidence for type II IL18R1 transcripts in both species. While the mouse and rat transcripts are predicted to encode a truncated receptor with a novel 5 amino acid C-terminal domain, the human sequence is predicted to encode a truncated protein with a novel 22 amino acid sequence bearing resemblance to the ‘Box 1’ motif of the Toll/interleukin-1 receptor (TIR) domain, in a similar fashion to the inhibitory interleukin-1 receptor 2. Given that transcripts from these three species are all formed by inclusion of homologous unspliced intronic regions, an analysis of homologous introns across a wider array of 33 species with available IL18R1 gene records was performed, which suggests similar transcripts may encode truncated type II IL-18Rα subunits in other species. This splice variant may represent a conserved evolutionary mechanism for regulating IL-18 activity.

Genome-wide characterization of insertion and deletion variation in chicken using next generation sequencing

Insertion and deletion (INDEL) is one of the main events contributing to genetic and phenotypic diversity, which receives less attention than SNP and large structural variation. To gain a better knowledge of INDEL variation in chicken genome, we applied next generation sequencing on 12 diverse chicken breeds at an average effective depth of 8.6. Over 1.3 million non-redundant short INDELs (1-49 bp) were obtained, the vast majority (92.48%) of which were novel. Follow-up validation assays confirmed that most (88.00%) of the randomly selected INDELs represent true variations. The majority (95.76%) of INDELs were less than 10 bp. Both the detected number and affected bases were larger for deletions than insertions. In total, INDELs covered 3.8 Mbp, corresponding to 0.36% of the chicken genome. The average genomic INDEL density was estimated as 0.49 per kb. INDELs were ubiquitous and distributed in a non-uniform fashion across chromosomes, with lower INDEL density in micro-chromosomes than in others, and some functional regions like exons and UTRs were prone to less INDELs than introns and intergenic regions. Nearly 620,253 INDELs fell in genic regions, 1,765 (0.28%) of which located in exons, spanning 1,358 (7.56%) unique Ensembl genes. Many of them are associated with economically important traits and some are the homologues of human disease-related genes. We demonstrate that sequencing multiple individuals at a medium depth offers a promising way for reliable identification of INDELs. The coding INDELs are valuable candidates for further elucidation of the association between genotypes and phenotypes. The chicken INDELs revealed by our study can be useful for future studies, including development of INDEL markers, construction of high density linkage map, INDEL arrays design, and hopefully, molecular breeding programs in chicken.

Simple sequence repeats in the national longitudinal study of adolescent health: an ethnically diverse resource for genetic analysis of health and behavior

Simple sequence repeats (SSRs) are one of the earliest available forms of genetic variation available for analysis and have been utilized in studies of neurological, behavioral, and health phenotypes. Although findings from these studies have been suggestive, their interpretation has been complicated by a variety of factors including, among others, limited power due to small sample sizes. The current report details the availability, diversity, and allele and genotype frequencies of six commonly examined SSRs in the ethnically diverse, population-based National Longitudinal Study of Adolescent Health. A total of 106,743 genotypes were generated across 15,140 participants that included four microsatellites and two di-nucleotide repeats in three dopamine genes (DAT1, DRD4, DRD5), the serotonin transporter, and monoamine oxidase A. Allele and genotype frequencies showed a complex pattern and differed significantly between populations. For both di-nucleotide repeats we observed a greater allelic diversity than previously reported. The availability of these six SSRs in a large, ethnically diverse sample with extensive environmental measures assessed longitudinally offers a unique resource for researchers interested in health and behavior.

Impact of human pathogenic micro-insertions and micro-deletions on post-transcriptional regulation

Small insertions/deletions (INDELs) of ≤21 bp comprise 18% of all recorded mutations causing human inherited disease and are evident in 24% of documented Mendelian diseases. INDELs affect gene function in multiple ways: for example, by introducing premature stop codons that either lead to the production of truncated proteins or affect transcriptional efficiency. However, the means by which they impact post-transcriptional regulation, including alternative splicing, have not been fully evaluated. In this study, we collate disease-causing INDELs from the Human Gene Mutation Database (HGMD) and neutral INDELs from the 1000 Genomes Project. The potential of these two types of INDELs to affect binding-site affinity of RNA-binding proteins (RBPs) was then evaluated. We identified several sequence features that can distinguish disease-causing INDELs from neutral INDELs. Moreover, we built a machine-learning predictor called PinPor (predicting pathogenic small insertions and deletions affecting post-transcriptional regulation, http://watson.compbio.iupui.edu/pinpor/) to ascertain which newly observed INDELs are likely to be pathogenic. Our results show that disease-causing INDELs are more likely to ablate RBP-binding sites and tend to affect more RBP-binding sites than neutral INDELs. Additionally, disease-causing INDELs give rise to greater deviations in binding affinity than neutral INDELs. We also demonstrated that disease-causing INDELs may be distinguished from neutral INDELs by several sequence features, such as their proximity to splice sites and their potential effects on RNA secondary structure. This predictor showed satisfactory performance in identifying numerous pathogenic INDELs, with a Matthews correlation coefficient (MCC) value of 0.51 and an accuracy of 0.75.

Correlated occurrence and bypass of frame-shifting insertion-deletions (InDels) to give functional proteins

Short insertions and deletions (InDels) comprise an important part of the natural mutational repertoire. InDels are, however, highly deleterious, primarily because two-thirds result in frame-shifts. Bypass through slippage over homonucleotide repeats by transcriptional and/or translational infidelity is known to occur sporadically. However, the overall frequency of bypass and its relation to sequence composition remain unclear. Intriguingly, the occurrence of InDels and the bypass of frame-shifts are mechanistically related - occurring through slippage over repeats by DNA or RNA polymerases, or by the ribosome, respectively. Here, we show that the frequency of frame-shifting InDels, and the frequency by which they are bypassed to give full-length, functional proteins, are indeed highly correlated. Using a laboratory genetic drift, we have exhaustively mapped all InDels that occurred within a single gene. We thus compared the naive InDel repertoire that results from DNA polymerase slippage to the frame-shifting InDels tolerated following selection to maintain protein function. We found that InDels repeatedly occurred, and were bypassed, within homonucleotide repeats of 3–8 bases. The longer the repeat, the higher was the frequency of InDels formation, and the more frequent was their bypass. Besides an expected 8A repeat, other types of repeats, including short ones, and G and C repeats, were bypassed. Although obtained in vitro, our results indicate a direct link between the genetic occurrence of InDels and their phenotypic rescue, thus suggesting a potential role for frame-shifting InDels as bridging evolutionary intermediates.

Homonucleotide repeats are exceptionally prone to insertions and/or deletions of bases (InDels). However, unless they occur in a multiplicity of 3 bases, InDels disrupt the reading frame and are thus expected to be purged from coding regions. Homonucleotide repeats, however, are also vulnerable to slippage by RNA polymerases and the ribosome. Using laboratory evolution techniques, we systematically mapped the occurrence of InDels within a given gene, before and after selection. Our data indicate that frame-shifting InDels were frequently bypassed to give functional proteins at surprisingly high frequencies. Further, we found a strict correlation between the repeat length, the frequency of occurrence of InDels at the DNA level, and the likelihood of bypass by transcriptional/translational slippage. Our results suggest that frame-shifting InDels might comprise functional evolutionary intermediates, and an effective mean of sequence divergence (e.g. when an adjacent InDel restores the frame, resulting in altered sequence and, potentially, in an altered protein structure).

Strong heterogeneity in mutation rate causes misleading hallmarks of natural selection on indel mutations in the human genome

Elucidating the mechanisms of mutation accumulation and fixation is critical to understand the nature of genetic variation and its contribution to genome evolution. Of particular interest is the effect of insertions and deletions (indels) on the evolution of genome landscapes. Recent population-scaled sequencing efforts provide unprecedented data for analyzing the relative impact of selection versus nonadaptive forces operating on indels. Here, we combined McDonald-Kreitman tests with the analysis of derived allele frequency spectra to investigate the dynamics of allele fixation of short (1-50 bp) indels in the human genome. Our analyses revealed apparently higher fixation probabilities for insertions than deletions. However, this fixation bias is not consistent with either selection or biased gene conversion and varies with local mutation rate, being particularly pronounced at indel hotspots. Furthermore, we identified an unprecedented number of loci with evidence for multiple indel events in the primate phylogeny. Even in nonrepetitive sequence contexts (a priori not prone to indel mutations), such loci are 60-fold more frequent than expected according to a model of uniform indel mutation rate. This provides evidence of as yet unidentified cryptic indel hotspots. We propose that indel homoplasy, at known and cryptic hotspots, produces systematic errors in determination of ancestral alleles via parsimony and advise caution interpreting classic selection tests given the strong heterogeneity in indel rates across the genome. These results will have great impact on studies seeking to infer evolutionary forces operating on indels observed in closely related species, because such mutations are traditionally presumed homoplasy-free.

PyroHMMvar: a sensitive and accurate method to call short indels and SNPs for Ion Torrent and 454 data

MOTIVATION

The identification of short insertions and deletions (indels) and single nucleotide polymorphisms (SNPs) from Ion Torrent and 454 reads is a challenging problem, essentially because these techniques are prone to sequence erroneously at homopolymers and can, therefore, raise indels in reads. Most of the existing mapping programs do not model homopolymer errors when aligning reads against the reference. The resulting alignments will then contain various kinds of mismatches and indels that confound the accurate determination of variant loci and alleles.

RESULTS

To address these challenges, we realign reads against the reference using our previously proposed hidden Markov model that models homopolymer errors and then merges these pairwise alignments into a weighted alignment graph. Based on our weighted alignment graph and hidden Markov model, we develop a method called PyroHMMvar, which can simultaneously detect short indels and SNPs, as demonstrated in human resequencing data. Specifically, by applying our methods to simulated diploid datasets, we demonstrate that PyroHMMvar produces more accurate results than state-of-the-art methods, such as Samtools and GATK, and is less sensitive to mapping parameter settings than the other methods. We also apply PyroHMMvar to analyze one human whole genome resequencing dataset, and the results confirm that PyroHMMvar predicts SNPs and indels accurately.

AVAILABILITY AND IMPLEMENTATION

Source code freely available at the following URL: https://code.google.com/p/pyrohmmvar/, implemented in C++ and supported on Linux. .

Misleading phylogenetic inferences based on single-exemplar sampling in the turtle genus Pseudemys

Reconstructing species trees for clades containing weakly delimited or incorrectly identified taxa is one of the most serious challenges facing systematists because building phylogenetic trees is generally predicated on correctly identifying species membership for the terminals in an analysis. A common practice, particularly in large-scale phylogenetic analyses, is to use single-exemplar sampling under the implicit assumption that the resulting phylogenetic trees will be poorly supported if the sampled taxa are not good species. We examine this fundamental assumption in the North American turtle genus Pseudemys, a group of common, widely distributed freshwater turtles whose species boundaries and phylogenetic relationships have challenged systematists for over half a century. We sequenced 10 nuclear and three mitochondrial genes from the nine currently recognized species and subspecies of Pseudemys using geographically-widespread sampling of each taxon, and analyzed the resulting 86-individual data set using population-genetic and phylogenetic methods. We found little or no evidence supporting the division of Pseudemys into its currently recognized species/subspecies. Rather, our data strongly suggest that the group has been oversplit and contains fewer species than currently recognized. Even so, when we conducted 100 replicated, single-exemplar phylogenetic analyses of these same nine taxa, most Bayesian trees were well resolved, had high posterior probabilities, and yet returned completely conflicting topologies. These analyses suggest that phylogenetic analyses based on single-exemplar sampling may recover trees that depend on the individuals that are sampled, rather than the underlying species tree that systematists assume they are estimating. Our results clearly indicate that final resolution of Pseudemys will require an integrated analysis of morphology and historical biogeographic data coupled with extensive geographic sampling and large amounts of molecular data, and we do not recommend taxonomic changes based on our analyses. If our 100-tree resampling experiments generalize to other taxa, they suggest that single-exemplar phylogenies should be interpreted with caution, particularly for groups where species are shallowly diverged or inadequately delimited.

Genomics in cardiovascular disease

A paradigm shift toward biology occurred in the 1990s and was subsequently catalyzed by the sequencing of the human genome in 2000. The cost of deoxyribonucleic acid (DNA) sequencing has gone from millions to thousands of dollars with sequencing of one's entire genome costing only $1,000. Rapid DNA sequencing is being embraced for single gene disorders, particularly for sporadic cases and those from small families. Transmission of lethal genes such as associated with Huntington's disease can, through in vitro fertilization, avoid passing it on to one's offspring. DNA sequencing will meet the challenge of elucidating the genetic predisposition for common polygenic diseases, especially in determining the function of the novel common genetic risk variants and identifying the rare variants, which may also partially ascertain the source of the missing heritability. The challenge for DNA sequencing remains great, despite human genome sequences being 99.5% identical, the 3 million single nucleotide polymorphisms responsible for most of the unique features add up to 40 to 60 new mutations per person which, for 7 billion people, is 300 to 400 billion mutations. It is claimed that DNA sequencing has increased 10,000-fold while information storage and retrieval only 16-fold. The physician and health user will be challenged by the convergence of 2 major trends, whole genome sequencing, and the storage/retrieval and integration of the data.

The origin, evolution, and functional impact of short insertion-deletion variants identified in 179 human genomes

Short insertions and deletions (indels) are the second most abundant form of human genetic variation, but our understanding of their origins and functional effects lags behind that of other types of variants. Using population-scale sequencing, we have identified a high-quality set of 1.6 million indels from 179 individuals representing three diverse human populations. We show that rates of indel mutagenesis are highly heterogeneous, with 43%–48% of indels occurring in 4.03% of the genome, whereas in the remaining 96% their prevalence is 16 times lower than SNPs. Polymerase slippage can explain upwards of three-fourths of all indels, with the remainder being mostly simple deletions in complex sequence. However, insertions do occur and are significantly associated with pseudo-palindromic sequence features compatible with the fork stalling and template switching (FoSTeS) mechanism more commonly associated with large structural variations. We introduce a quantitative model of polymerase slippage, which enables us to identify indel-hypermutagenic protein-coding genes, some of which are associated with recurrent mutations leading to disease. Accounting for mutational rate heterogeneity due to sequence context, we find that indels across functional sequence are generally subject to stronger purifying selection than SNPs. We find that indel length modulates selection strength, and that indels affecting multiple functionally constrained nucleotides undergo stronger purifying selection. We further find that indels are enriched in associations with gene expression and find evidence for a contribution of nonsense-mediated decay. Finally, we show that indels can be integrated in existing genome-wide association studies (GWAS); although we do not find direct evidence that potentially causal protein-coding indels are enriched with associations to known disease-associated SNPs, our findings suggest that the causal variant underlying some of these associations may be indels.

The search for genetic modifiers of disease severity in the β-hemoglobinopathies

Sickle cell disease (SCD) and β-thalassemia, two monogenic diseases caused by mutations in the β-globin gene, affect millions of individuals worldwide. These hemoglobin disorders are characterized by extreme clinical heterogeneity, complicating patient management and treatment. A better understanding of this patient-to-patient clinical variability would dramatically improve care and might also guide the development of novel therapies. Studies of the natural history of these β-hemoglobinopathies have identified fetal hemoglobin levels and concomitant α-thalassemia as important modifiers of disease severity. Several small-scale studies have attempted to identify additional genetic modifiers of SCD and β-thalassemia, without much success. Fortunately, improved knowledge of the human genome and the development of new genomic tools, such as genome-wide genotyping arrays and next-generation DNA sequencers, offer new opportunities to use genetics to better understand the causes of the many complications observed in β-hemoglobinopathy patients. Here I discuss the most important factors to consider when planning an experiment to find associations between β-hemoglobinopathy-related complications and DNA sequence variants, with a focus on how to successfully perform a genome-wide association study. I also review the literature and explain why most published findings in the field of SCD modifier genetics are likely to be false-positive reports, with the goal to draw lessons allowing investigators to design better genetic experiments.

Species boundaries and phylogenetic relationships in the critically endangered Asian box turtle genus Cuora

Turtles are currently the most endangered major clade of vertebrates on earth, and Asian box turtles (Cuora) are in catastrophic decline. Effective management of this diverse turtle clade has been hampered by human-mediated, and perhaps natural hybridization, resulting in discordance between mitochondrial and nuclear markers and confusion regarding species boundaries and phylogenetic relationships among hypothesized species of Cuora. Here, we present analyses of mitochondrial and nuclear DNA data for all 12 currently hypothesized species to resolve both species boundaries and phylogenetic relationships. Our 15-gene, 40-individual nuclear data set was frequently in conflict with our mitochondrial data set; based on its general concordance with published morphological analyses and the strength of 15 independent estimates of evolutionary history, we interpret the nuclear data as representing the most reliable estimate of species boundaries and phylogeny of Cuora. Our results strongly reiterate the necessity of using multiple nuclear markers for phylogeny and species delimitation in these animals, including any form of DNA "barcoding", and point to Cuora as an important case study where reliance on mitochondrial DNA can lead to incorrect species identification.

The rhesus macaque is three times as diverse but more closely equivalent in damaging coding variation as compared to the human

Background

As a model organism in biomedicine, the rhesus macaque (Macaca mulatta) is the most widely used nonhuman primate. Although a draft genome sequence was completed in 2007, there has been no systematic genome-wide comparison of genetic variation of this species to humans. Comparative analysis of functional and nonfunctional diversity in this highly abundant and adaptable non-human primate could inform its use as a model for human biology, and could reveal how variation in population history and size alters patterns and levels of sequence variation in primates.

Results

We sequenced the mRNA transcriptome and H3K4me3-marked DNA regions in hippocampus from 14 humans and 14 rhesus macaques. Using equivalent methodology and sampling spaces, we identified 462,802 macaque SNPs, most of which were novel and disproportionately located in the functionally important genomic regions we had targeted in the sequencing. At least one SNP was identified in each of 16,797 annotated macaque genes. Accuracy of macaque SNP identification was conservatively estimated to be >90%. Comparative analyses using SNPs equivalently identified in the two species revealed that rhesus macaque has approximately three times higher SNP density and average nucleotide diversity as compared to the human. Based on this level of diversity, the effective population size of the rhesus macaque is approximately 80,000 which contrasts with an effective population size of less than 10,000 for humans. Across five categories of genomic regions, intergenic regions had the highest SNP density and average nucleotide diversity and CDS (coding sequences) the lowest, in both humans and macaques. Although there are more coding SNPs (cSNPs) per individual in macaques than in humans, the ratio of d_N/d_S is significantly lower in the macaque. Furthermore, the number of damaging nonsynonymous cSNPs (have damaging effects on protein functions from PolyPhen-2 prediction) in the macaque is more closely equivalent to that of the human.

Conclusions

This large panel of newly identified macaque SNPs enriched for functionally significant regions considerably expands our knowledge of genetic variation in the rhesus macaque. Comparative analysis reveals that this widespread, highly adaptable species is approximately three times as diverse as the human but more closely equivalent in damaging variation.

More single-nucleotide mutations surround small insertions than small deletions in primates

Early studies have shown that single-nucleotide mutation rates increase close to insertions and deletions, but it is not fully understood how natural selection shapes genome-wide patterns of indels and their nearby single-nucleotide mutations. In this study, we find that, in primates, more single-nucleotide mutations surround small insertions than small deletions. This pattern affects <150 base pair (bp) sequences close to indels and persists under different genomic properties, such as exon/intron/intergenic contexts, repeated/nonrepeated sequences, replication timing, recombination rates, indel density, and guanine-cytosine (GC) content. We propose two different, but not mutually exclusive, hypothetical mechanisms to explain the pattern. One mechanism is that the sequence context preferring insertion formation may also favor nucleotide substitutions. Another mechanism is related to a hypothesis in which indel heterozygosity tends to increase nearby nucleotide substitution rates. It means that if insertions spend more time in heterozygotes, insertions may accumulate more surrounding single-nucleotide changes. In conclusion, we characterize a special genome-wide evolutionary pattern for indels and nearby single-nucleotide changes. This pattern may be driven by natural selection and bias primates' genome evolution and phenotypic variations.

Characterizing linkage disequilibrium and evaluating imputation power of human genomic insertion-deletion polymorphisms

Background

Indels are an important cause of human variation and central to the study of human disease. The 1000 Genomes Project Low-Coverage Pilot identified over 1.3 million indels shorter than 50 bp, of which over 890 were identified as potentially disruptive variants. Yet, despite their ubiquity, the local genomic characteristics of indels remain unexplored.

Results

Herein we describe population- and minor allele frequency-based differences in linkage disequilibrium and imputation characteristics for indels included in the 1000 Genomes Project Low-Coverage Pilot for the CEU, YRI and CHB+JPT populations. Common indels were well tagged by nearby SNPs in all studied populations, and were also tagged at a similar rate to common SNPs. Both neutral and functionally deleterious common indels were imputed with greater than 95% concordance from HapMap Phase 3 and OMNI SNP sites. Further, 38 to 56% of low frequency indels were tagged by low frequency SNPs. We were able to impute heterozygous low frequency indels with over 50% concordance. Lastly, our analysis also revealed evidence of ascertainment bias. This bias prevents us from extending the applicability of our results to highly polymorphic indels that could not be identified in the Low-Coverage Pilot.

Conclusions

Although further scope exists to improve the imputation of low frequency indels, our study demonstrates that there are already ample opportunities to retrospectively impute indels for prior genome-wide association studies and to incorporate indel imputation into future case/control studies.

Exploring the implications of INDELs in neuropsychiatric genetics: challenges and perspectives

The decade passed after publishing the Human Genome first draft faced an enormous growth at the understanding of the genomic variation among different subjects, populations, and groups of patients. Single nucleotide polymorphisms (SNPs) and insertion or deletions (INDELs) have been increasingly recognized as a major type of genetic variations, with potential impact in protein activities and gene expression changes observed in complex genetic traits, like neuropsychiatric diseases. INDELs represent the second most common class of variations after SNPs, but there is still an important gap between the number of INDELs reported and the actual knowledge about the functional implications of such variations. There are approximately 10 million SNPs already reported, and the human populations are expected to collectively harbor at least 1.6-2.5 million INDELs. One of the major challenges is to find better platforms to screen for INDELs in a high throughput manner. The discordance in between the data from different studies might be explained by the diverse approaches employed to sequence the genomes with variable platforms. Short INDEL variations increased the scope of genetic markers in human genetic diseases, and various studies showed that common microdeletions and smaller INDELs might be highly associated with neuropsychiatric diseases such as schizophrenia, autism, mental retardation, and Alzheimer disease. The rapidly increasing amount of resequencing, genotyping, and personal genome data generated by large-scale genetic human projects require the development of integrated bioinformatics tools able to efficiently manage and analyze these genetic data. Our group is currently dealing with different approaches that might optimize sequencing and bioinformatics analyses of short INDELs to broaden our research capabilities of identifying those intriguing genetic variations. Hopefully, INDELs might become a new trend in association studies in neuropsychiatric genetics since so far the level of significant and positive associations with the standard SNPs reported presents limited predictive application.

A probabilistic method for the detection and genotyping of small indels from population-scale sequence data

MOTIVATION

High-throughput sequencing technologies have made population-scale studies of human genetic variation possible. Accurate and comprehensive detection of DNA sequence variants is crucial for the success of these studies. Small insertions and deletions represent the second most frequent class of variation in the human genome after single nucleotide polymorphisms (SNPs). Although several alignment tools for the gapped alignment of sequence reads to a reference genome are available, computational methods for discriminating indels from sequencing errors and genotyping indels directly from sequence reads are needed.

RESULTS

We describe a probabilistic method for the accurate detection and genotyping of short indels from population-scale sequence data. In this approach, aligned sequence reads from a population of individuals are used to automatically account for context-specific sequencing errors associated with indels. We applied this approach to population sequence datasets from the 1000 Genomes exon pilot project generated using the Roche 454 and Illumina sequencing platforms, and were able to detect a significantly greater number of indels than reported previously. Comparison to indels identified in the 1000 Genomes pilot project demonstrated the sensitivity of our method. The consistency in the number of indels and the fraction of indels whose length is a multiple of three across different human populations and two different sequencing platforms indicated that our method has a low false discovery rate. Finally, the method represents a general approach for the detection and genotyping of small-scale DNA sequence variants for population-scale sequencing projects.

AVAILABILITY

A program implementing this method is available at http://polymorphism.scripps.edu/~vbansal/software/piCALL/

Natural genetic variation caused by small insertions and deletions in the human genome

Human genetic variation is expected to play a central role in personalized medicine. Yet only a fraction of the natural genetic variation that is harbored by humans has been discovered to date. Here we report almost 2 million small insertions and deletions (INDELs) that range from 1 bp to 10,000 bp in length in the genomes of 79 diverse humans. These variants include 819,363 small INDELs that map to human genes. Small INDELs frequently were found in the coding exons of these genes, and several lines of evidence indicate that such variation is a major determinant of human biological diversity. Microarray-based genotyping experiments revealed several interesting observations regarding the population genetics of small INDEL variation. For example, we found that many of our INDELs had high levels of linkage disequilibrium (LD) with both HapMap SNPs and with high-scoring SNPs from genome-wide association studies. Overall, our study indicates that small INDEL variation is likely to be a key factor underlying inherited traits and diseases in humans.

The rhox homeobox gene cluster is imprinted and selectively targeted for regulation by histone h1 and DNA methylation

Histone H1 is an abundant and essential component of chromatin whose precise role in regulating gene expression is poorly understood. Here, we report that a major target of H1-mediated regulation in embryonic stem (ES) cells is the X-linked Rhox homeobox gene cluster. To address the underlying mechanism, we examined the founding member of the Rhox gene cluster-Rhox5-and found that its distal promoter (Pd) loses H1, undergoes demethylation, and is transcriptionally activated in response to loss of H1 genes in ES cells. Demethylation of the Pd is required for its transcriptional induction and we identified a single cytosine in the Pd that, when methylated, is sufficient to inhibit Pd transcription. Methylation of this single cytosine prevents the Pd from binding GA-binding protein (GABP), a transcription factor essential for Pd transcription. Thus, H1 silences Rhox5 transcription by promoting methylation of one of its promoters, a mechanism likely to extend to other H1-regulated Rhox genes, based on analysis of ES cells lacking DNA methyltransferases. The Rhox cluster genes targeted for H1-mediated transcriptional repression are also subject to another DNA methylation-regulated process: Xp imprinting. Remarkably, we found that only H1-regulated Rhox genes are imprinted, not those immune to H1-mediated repression. Together, our results indicate that the Rhox gene cluster is a major target of H1-mediated transcriptional repression in ES cells and that H1 is a candidate to have a role in Xp imprinting.

Small insertions and deletions (INDELs) in human genomes

In this review, we focus on progress that has been made with detecting small insertions and deletions (INDELs) in human genomes. Over the past decade, several million small INDELs have been discovered in human populations and personal genomes. The amount of genetic variation that is caused by these small INDELs is substantial. The number of INDELs in human genomes is second only to the number of single nucleotide polymorphisms (SNPs), and, in terms of base pairs of variation, INDELs cause similar levels of variation as SNPs. Many of these INDELs map to functionally important sites within human genes, and thus, are likely to influence human traits and diseases. Therefore, small INDEL variation will play a prominent role in personalized medicine.

Light whole genome sequence for SNP discovery across domestic cat breeds

BACKGROUND

The domestic cat has offered enormous genomic potential in the veterinary description of over 250 hereditary disease models as well as the occurrence of several deadly feline viruses (feline leukemia virus--FeLV, feline coronavirus--FECV, feline immunodeficiency virus--FIV) that are homologues to human scourges (cancer, SARS, and AIDS respectively). However, to realize this bio-medical potential, a high density single nucleotide polymorphism (SNP) map is required in order to accomplish disease and phenotype association discovery.

DESCRIPTION

To remedy this, we generated 3,178,297 paired fosmid-end Sanger sequence reads from seven cats, and combined these data with the publicly available 2X cat whole genome sequence. All sequence reads were assembled together to form a 3X whole genome assembly allowing the discovery of over three million SNPs. To reduce potential false positive SNPs due to the low coverage assembly, a low upper-limit was placed on sequence coverage and a high lower-limit on the quality of the discrepant bases at a potential variant site. In all domestic cats of different breeds: female Abyssinian, female American shorthair, male Cornish Rex, female European Burmese, female Persian, female Siamese, a male Ragdoll and a female African wildcat were sequenced lightly. We report a total of 964 k common SNPs suitable for a domestic cat SNP genotyping array and an additional 900 k SNPs detected between African wildcat and domestic cats breeds. An empirical sampling of 94 discovered SNPs were tested in the sequenced cats resulting in a SNP validation rate of 99%.

CONCLUSIONS

These data provide a large collection of mapped feline SNPs across the cat genome that will allow for the development of SNP genotyping platforms for mapping feline diseases.