Genome-wide assessment of imprinted expression in human cells.

BACKGROUND: Parent-of-origin-dependent expression of alleles, imprinting, has been suggested to impact a substantial proportion of mammalian genes. Its discovery requires allele-specific detection of expressed transcripts, but in some cases detected allelic expression bias has been interpreted as imprinting without demonstrating compatible transmission patterns and excluding heritable variation. Therefore, we utilized a genome-wide tool exploiting high density genotyping arrays in parallel measurements of genotypes in RNA and DNA to determine allelic expression across the transcriptome in lymphoblastoid cell lines (LCLs) and skin fibroblasts derived from families. RESULTS: We were able to validate 43% of imprinted genes with previous demonstration of compatible transmission patterns in LCLs and fibroblasts. In contrast, we only validated 8% of genes suggested to be imprinted in the literature, but without clear evidence of parent-of-origin-determined expression. We also detected five novel imprinted genes and delineated regions of imprinted expression surrounding annotated imprinted genes. More subtle parent-of-origin-dependent expression, or partial imprinting, could be verified in four genes. Despite higher prevalence of monoallelic expression, immortalized LCLs showed consistent imprinting in fewer loci than primary cells. Random monoallelic expression has previously been observed in LCLs and we show that random monoallelic expression in LCLs can be partly explained by aberrant methylation in the genome. CONCLUSIONS: Our results indicate that widespread parent-of-origin-dependent expression observed recently in rodents is unlikely to be captured by assessment of human cells derived from adult tissues where genome-wide assessment of both primary and immortalized cells yields few new imprinted loci.

Global analysis of the impact of environmental perturbation on cis-regulation of gene expression.

Genetic variants altering cis-regulation of normal gene expression (cis-eQTLs) have been extensively mapped in human cells and tissues, but the extent by which controlled, environmental perturbation influences cis-eQTLs is unclear. We carried out large-scale induction experiments using primary human bone cells derived from unrelated donors of Swedish origin treated with 18 different stimuli (7 treatments and 2 controls, each assessed at 2 time points). The treatments with the largest impact on the transcriptome, verified on two independent expression arrays, included BMP-2 (t = 2h), dexamethasone (DEX) (t = 24 h), and PGE2 (t = 24 h). Using these treatments and control, we performed expression profiling for 18,144 RefSeq transcripts on biological replicates of the complete study cohort of 113 individuals (n(total) = 782) and combined it with genome-wide SNP-genotyping data in order to map treatment-specific cis-eQTLs (defined as SNPs located within the gene ± 250 kb). We found that 93% of cis-eQTLs at 1% FDR were observed in at least one additional treatment, and in fact, on average, only 1.4% of the cis-eQTLs were considered as treatment-specific at high confidence. The relative invariability of cis-regulation following perturbation was reiterated independently by genome-wide allelic expression tests where only a small proportion of variance could be attributed to treatment. Treatment-specific cis-regulatory effects were, however, 2- to 6-fold more abundant among differently expressed genes upon treatment. We further followed-up and validated the DEX-specific cis-regulation of the MYO6 and TNC loci and found top cis-regulatory variants located 180 kb and 250 kb upstream of the transcription start sites, respectively. Our results suggest that, as opposed to tissue-specificity of cis-eQTLs, the interactions between cellular environment and cis-variants are relatively rare (∼1.5%), but that detection of such specific interactions can be achieved by a combination of functional genomic approaches as described here.

Fine-scale variation and genetic determinants of alternative splicing across individuals.

Recently, thanks to the increasing throughput of new technologies, we have begun to explore the full extent of alternative pre-mRNA splicing (AS) in the human transcriptome. This is unveiling a vast layer of complexity in isoform-level expression differences between individuals. We used previously published splicing sensitive microarray data from lymphoblastoid cell lines to conduct an in-depth analysis on splicing efficiency of known and predicted exons. By combining publicly available AS annotation with a novel algorithm designed to search for AS, we show that many real AS events can be detected within the usually unexploited, speculative majority of the array and at significance levels much below standard multiple-testing thresholds, demonstrating that the extent of cis-regulated differential splicing between individuals is potentially far greater than previously reported. Specifically, many genes show subtle but significant genetically controlled differences in splice-site usage. PCR validation shows that 42 out of 58 (72%) candidate gene regions undergo detectable AS, amounting to the largest scale validation of isoform eQTLs to date. Targeted sequencing revealed a likely causative SNP in most validated cases. In all 17 incidences where a SNP affected a splice-site region, in silico splice-site strength modeling correctly predicted the direction of the micro-array and PCR results. In 13 other cases, we identified likely causative SNPs disrupting predicted splicing enhancers. Using Fst and REHH analysis, we uncovered significant evidence that 2 putative causative SNPs have undergone recent positive selection. We verified the effect of five SNPs using in vivo minigene assays. This study shows that splicing differences between individuals, including quantitative differences in isoform ratios, are frequent in human populations and that causative SNPs can be identified using in silico predictions. Several cases affected disease-relevant genes and it is likely some of these differences are involved in phenotypic diversity and susceptibility to complex diseases.

A cis-acting regulatory variant in the IL2RA locus.

The mechanism for the association of type 1 diabetes (T1D) with IL2RA remains to be clarified. Neither of the two distinct, transmission-disequilibrium confirmed loci mapping to this gene can be explained by a coding variant. An effect on the levels of the soluble protein product sIL-2RA has been reported but its cause and relationship to disease risk is not clear. To look for an allelic effect on IL2RA transcription in cis, we examined RNA from 48 heterozygous lymphocyte samples for differential allele expression. Of the 48 samples, 32 showed statistically significant allelic imbalance. No known single nucleotide polymorphism (SNP) had perfect correlation with this transcriptional effect but the one that showed the most significant (p = 1.6 x 10(-5)) linkage disequilibrium with it was the SNP rs3118470. We had previously shown rs3118470 to confer T1D susceptibility in a Canadian dataset, independently of rs41295061 as the major reported locus (p = 5 x 10(-3), after accounting for rs41295061 by conditional regression). Lower IL2RA levels consistently originated from the T1D predisposing allele. We conclude that an as yet unidentified variant or haplotype, best marked by rs3118470, is responsible for this independent effect and increases T1D risk through diminished expression of the IL-2R, likely by interfering with the proper development of regulatory T cells.

Global patterns of cis variation in human cells revealed by high-density allelic expression analysis.

Cis-acting variants altering gene expression are a source of phenotypic differences. The cis-acting components of expression variation can be identified through the mapping of differences in allelic expression (AE), which is the measure of relative expression between two allelic transcripts. We generated a map of AE associated SNPs using quantitative measurements of AE on Illumina Human1M BeadChips. In 53 lymphoblastoid cell lines derived from donors of European descent, we identified common cis variants affecting 30% (2935/9751) of the measured RefSeq transcripts at 0.001 permutation significance. The pervasive influence of cis-regulatory variants, which explain 50% of population variation in AE, extend to full-length transcripts and their isoforms as well as to unannotated transcripts. These strong effects facilitate fine mapping of cis-regulatory SNPs, as demonstrated by dissection of heritable control of transcripts in the systemic lupus erythematosus-associated C8orf13-BLK region in chromosome 8. The dense collection of associations will facilitate large-scale isolation of cis-regulatory SNPs.

Allele-specific chromatin remodeling in the ZPBP2/GSDMB/ORMDL3 locus associated with the risk of asthma and autoimmune disease.

Common SNPs in the chromosome 17q12-q21 region alter the risk for asthma, type 1 diabetes, primary biliary cirrhosis, and Crohn disease. Previous reports by us and others have linked the disease-associated genetic variants with changes in expression of GSDMB and ORMDL3 transcripts in human lymphoblastoid cell lines (LCLs). The variants also alter regulation of other transcripts, and this domain-wide cis-regulatory effect suggests a mechanism involving long-range chromatin interactions. Here, we further dissect the disease-linked haplotype and identify putative causal DNA variants via a combination of genetic and functional analyses. First, high-throughput resequencing of the region and genotyping of potential candidate variants were performed. Next, additional mapping of allelic expression differences in Yoruba HapMap LCLs allowed us to fine-map the basis of the cis-regulatory differences to a handful of candidate functional variants. Functional assays identified allele-specific differences in nucleosome distribution, an allele-specific association with the insulator protein CTCF, as well as a weak promoter activity for rs12936231. Overall, this study shows a common disease allele linked to changes in CTCF binding and nucleosome occupancy leading to altered domain-wide cis-regulation. Finally, a strong association between asthma and cis-regulatory haplotypes was observed in three independent family-based cohorts (p = 1.78 x 10(-8)). This study demonstrates the requirement of multiple parallel allele-specific tools for the investigation of noncoding disease variants and functional fine-mapping of human disease-associated haplotypes.

Tissue effect on genetic control of transcript isoform variation.

Current genome-wide association studies (GWAS) are moving towards the use of large cohorts of primary cell lines to study a disease of interest and to assign biological relevance to the genetic signals identified. Here, we use a panel of human osteoblasts (HObs) to carry out a transcriptomic survey, similar to recent studies in lymphoblastoid cell lines (LCLs). The distinct nature of HObs and LCLs is reflected by the preferential grouping of cell type-specific genes within biologically and functionally relevant pathways unique to each tissue type. We performed cis-association analysis with SNP genotypes to identify genetic variations of transcript isoforms, and our analysis indicates that differential expression of transcript isoforms in HObs is also partly controlled by cis-regulatory genetic variants. These isoforms are regulated by genetic variants in both a tissue-specific and tissue-independent fashion, and these associations have been confirmed by RT-PCR validation. Our study suggests that multiple transcript isoforms are often present in both tissues and that genetic control may affect the relative expression of one isoform to another, rather than having an all-or-none effect. Examination of the top SNPs from a GWAS of bone mineral density show overlap with probeset associations observed in this study. The top hit corresponding to the FAM118A gene was tested for association studies in two additional clinical studies, revealing a novel transcript isoform variant. Our approach to examining transcriptome variation in multiple tissue types is useful for detecting the proportion of genetic variation common to different cell types and for the identification of cell-specific isoform variants that may be functionally relevant, an important follow-up step for GWAS.

Population genomics in a disease targeted primary cell model.

The common genetic variants associated with complex traits typically lie in noncoding DNA and may alter gene regulation in a cell type-specific manner. Consequently, the choice of tissue or cell model in the dissection of disease associations is important. We carried out an expression quantitative trait loci (eQTL) study of primary human osteoblasts (HOb) derived from 95 unrelated donors of Swedish origin, each represented by two independently derived primary lines to provide biological replication. We combined our data with publicly available information from a genome-wide association study (GWAS) of bone mineral density (BMD). The top 2000 BMD-associated SNPs (P < approximately 10(-3)) were tested for cis-association of gene expression in HObs and in lymphoblastoid cell lines (LCLs) using publicly available data and showed that HObs have a significantly greater enrichment (threefold) of converging cis-eQTLs as compared to LCLs. The top 10 BMD loci with SNPs showing strong cis-effects on gene expression in HObs (P = 6 x 10(-10) - 7 x 10(-16)) were selected for further validation using a staged design in two cohorts of Caucasian male subjects. All 10 variants were tested in the Swedish MrOS Cohort (n = 3014), providing evidence for two novel BMD loci (SRR and MSH3). These variants were then tested in the Rotterdam Study (n = 2090), yielding converging evidence for BMD association at the 17p13.3 SRR locus (P(combined) = 5.6 x 10(-5)). The cis-regulatory effect was further fine-mapped to the proximal promoter of the SRR gene (rs3744270, r(2) = 0.5, P = 2.6 x 10(-15)). Our results suggest that primary cells relevant to disease phenotypes complement traditional approaches for prioritization and validation of GWAS hits for follow-up studies.

Targeted screening of cis-regulatory variation in human haplotypes.

Regulatory cis-acting variants account for a large proportion of gene expression variability in populations. Cis-acting differences can be specifically measured by comparing relative levels of allelic transcripts within a sample. Allelic expression (AE) mapping for cis-regulatory variant discovery has been hindered by the requirements of having informative or heterozygous single nucleotide polymorphisms (SNPs) within genes in order to assign the allelic origin of each transcript. In this study we have developed an approach to systematically screen for heritable cis-variants in common human haplotypes across >1,000 genes. In order to achieve the highest level of information per haplotype studied, we carried out allelic expression measurements by using both intronic and exonic SNPs in primary transcripts. We used a novel RNA pooling strategy in immortalized lymphoblastoid cell lines (LCLs) and primary human osteoblast cell lines (HObs) to allow for high-throughput AE. Screening hits from RNA pools were further validated by performing allelic expression mapping in individual samples. Our results indicate that >10% of expressed genes in human LCLs show genotype-linked AE. In addition, we have validated cis-acting variants in over 20 genes linked with common disease susceptibility in recent genome-wide studies. More generally, our results indicate that RNA pooling coupled with AE read-out by second generation sequencing or by other methods provides a high-throughput tool for cataloging the impact of common noncoding variants in the human genome.

Systematic assessment of the human osteoblast transcriptome in resting and induced primary cells.

Osteoblasts are key players in bone remodeling. The accessibility of human primary osteoblast-like cells (HObs) from bone explants makes them a lucrative model for studying molecular physiology of bone turnover, for discovering novel anabolic therapeutics, and for mesenchymal cell biology in general. Relatively little is known about resting and dynamic expression profiles of HObs, and to date no studies have been conducted to systematically assess the osteoblast transcriptome. The aim of this study was to characterize HObs and investigate signaling cascades and gene networks with genomewide expression profiling in resting and bone morphogenic protein (BMP)-2- and dexamethasone-induced cells. In addition, we compared HOb gene expression with publicly available samples from the Gene Expression Omnibus. Our data show a vast number of genes and networks expressed predominantly in HObs compared with closely related cells such as fibroblasts or chondrocytes. For instance, genes in the insulin-like growth factor (IGF) signaling pathway were enriched in HObs (P = 0.003) and included the binding proteins (IGFBP-1, -2, -5) and IGF-II and its receptor. Another HOb-specific expression pattern included leptin and its receptor (P < 10(-8)). Furthermore, after stimulation of HObs with BMP-2 or dexamethasone, the expression of several interesting genes and pathways was observed. For instance, our data support the role of peripheral leptin signaling in bone cell function. In conclusion, we provide the landscape of tissue-specific and dynamic gene expression in HObs. This resource will allow utilization of osteoblasts as a model to study specific gene networks and gene families related to human bone physiology and diseases.