The cardiac transcription factor (TF) gene NKX2-5 has been associated with electrocardiographic (EKG) traits through genome-wide association studies (GWASs), but the extent to which differential binding of NKX2-5 at common regulatory variants contributes to these traits has not yet been studied. We analyzed transcriptomic and epigenomic data from induced pluripotent stem cell-derived cardiomyocytes from seven related individuals, and identified ~2,000 single-nucleotide variants associated with allele-specific effects (ASE-SNVs) on NKX2-5 binding. NKX2-5 ASE-SNVs were enriched for altered TF motifs, for heart-specific expression quantitative trait loci and for EKG GWAS signals. Using fine-mapping combined with epigenomic data from induced pluripotent stem cell–derived cardiomyocytes, we prioritized candidate causal variants for EKG traits, many of which were NKX2-5 ASE-SNVs. Experimentally characterizing two NKX2-5 ASE-SNVs (rs3807989 and rs590041) showed that they modulate the expression of target genes via differential protein binding in cardiac cells, indicating that they are functional variants underlying EKG GWAS signals. Our results show that differential NKX2-5 binding at numerous regulatory variants across the genome contributes to EKG phenotypes.
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All iPSC lines are available through the WiCell Research Institute (www.wicell.org; NHLBI Next Gen Collection). All genomic data are available through the database of Genotypes and Phenotypes (accessions phs000924 (RNA-Seq, ChIP-Seq, ATAC-Seq and Hi-C) and phs001325 (whole-genome-sequenced SNV and copy number variation genotypes)) and National Center for Biotechnology Information BioProject PRJNA285375. Processed data files are available through Gene Expression Omnibus accessions GSE125540 and GSE133833.
Custom-written code is available via GitHub (https://github.com/frazer-lab/NKX2-5_ASE_iPSC-CM).
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This work was supported in part by California Institute for Regenerative Medicine grant (CIRM) GC1R-06673-B, NIH grants HG008118 and HL107442, and National Science Foundation grant 1728497. P.B. was supported by the Swiss National Science Foundation Postdoc Mobility fellowships P2LAP3-155105 and P300PA-167612. W.W.Y.G. was supported by the NHLBI under award number HL142151. C.D. was supported in part by the UCSD Genetics Training Program through an institutional training grant from the NIGMS under award number GM008666 and the CIRM Interdisciplinary Stem Cell Training Program at UCSD II (TG2-01154). Library preparation and sequencing services were conducted by K. Jepsen and M. Khosroheidari at the UCSD IGM Genomics Center, supported by NIH grant CA023100. N.S. was supported by NIH grants HL116747 and HL141989. K.J.G. was supported by NIH grant DK114650 and ADA grant 1-17-JDF-027. W.M., F.Y. and M.G.R. were supported by NIH grants DK018477 and DK039949. M.G.R. is a HHMI investigator. We are thankful to C.-A. Yen and N. Spann for assistance with the ChIP-Seq experiments, and to A. Schmitt for the Hi-C data. We thank A. Aguirre for performing immunofluorescence. We thank E. Farley and K. Olson for help with reporter assays. We thank many colleagues for helpful comments.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Integrated supplementary information
Supplementary Figure 1 Characterization of iPSC-CMs by flow cytometry, immunofluorescence and gene expression.
(a) Percentage of cells positive for the cardiomyocyte-specific marker TNNT2 analyzed by flow cytometry on iPSC-CMs generated in this study and collected at either day 15 (n = 15 independent samples) or day 25 and purified with L-lactate selection (n = 12 independent samples). The day 15 sample that was lactate purified is indicated. Plot lines indicate median, lower and upper quartiles. (b) Confocal microscopy of immunofluorescence for TNNT2 and MYL7 in one replicate of iPSCORE_2_1 day 15 iPSC-CMs at two different magnifications. Similar staining was observed for other two day15-iPSC-CM samples (subjects iPSCORE_2_3 and iPSCORE_2_9, not shown). (c) Hierarchical clustering and heatmap of RNA-seq expression data from 61 selected cell-type specific genes. In addition to the 56 samples from this study, Roadmap RNA-seq data from stem cell lines (H1, HUES64, iPS-20b and iPS-18) and human tissues (right ventricle, left ventricle, right atrium and fetal heart) are included as reference samples. Gene expression values are reported as vst-normalized read counts.
(a) Tag heatmap of sequence coverage at the TSSs of differentially expressed genes (n = 5,307 DEGs, minimum log2 fold change = 2, FDR < 0.05, DESeq2), listed in decreasing order of log2 ratio of expression in iPSCs versus iPSC-CMs. The density plot at the top shows the average normalized coverage (range 0-1) across iPSC-upregulated genes (2,444 genes, green) and iPSC-CM-upregulated genes (2,863 genes, purple). For a 2-kb window centered at the TSS, coverage values for 25-bp bins were obtained by combining all samples of each data type and normalized to a total of 107 reads. (b-f) Heatmap and hierarchical clustering of similarity based on overlap between enhancer annotations (from 25-state ChromHMM) for 127 samples from Roadmap Epigenomics (Ernst, J. and M. Kellis, Nat. Methods 9, 215-216, 2012; Kundaje, A. et al. Nature 518, 317-330, 2015) and ATAC-seq peaks or ChIP-Seq peaks from iPSCs (b, c) or iPSC-CMs (d-f) combined samples. Similarity was calculated using the Jaccard statistic (intersection/union of base pairs in each comparison), mean-centered across the 127 tissues and ordered by highest average enhancer similarity. A zoom in of the top 10 Roadmap tissues with highest average enhancer similarity is shown.
(a, c, e, g, i) Plots of the two first principal components (PC) calculated on all genes/peaks identified in each RNA-seq (a, iPSCs: 29 independent samples; e, iPSC-CMs: 26 independent samples and 1 technical replicate) or ChIP-seq dataset (c, H3K27ac in iPSCs: 17 independent samples and 4 technical replicates; g, H3K27ac in iPSC-CMs: 25 independent samples and 2 technical replicates; i, NKX2-5: 12 independent samples and 3 technical replicates). Samples are color-coded by subject and distinguished by a different symbol representing a different batch for iPSC (cultured, collected and sequenced at different times) or a different protocol (day 15 vs. day 25) for iPSC-CMs differentiation. Samples from the same individual are considered independent if iPSC lines were cultured -or iPSC-CMs were differentiated- at different times, and are considered technical replicates (indicated by A and B) if the assays were performed on cell material collected from the same sample. (b, d, f, h, j) Tables showing adjusted r-squared values from ANOVA tests are reported as a measure of association between PC1 to PC10 and different covariates in each dataset. Tables are color-coded according to –log10 of ANOVA P-values. (k-o) Plots of the average Spearman correlation coefficients between pairs of samples across the 1,000 most variable genes or peaks for the indicated molecular phenotypes. Each dot corresponds to the per-sample pairwise correlation coefficient averaged across samples from either the same subject or different subjects. Technical replicates were excluded for the comparisons between samples of the same subject. Plot lines indicate median, lower and upper quartiles. P-values from one-tailed Mann-Whitney test are shown.
Supplementary Figure 4 Comparison of number and effects of ASE-SNVs identified in ChIP-seq and ATAC-seq.
(a) Number of uniquely mapped reads for each data type and subject (n = 7). Each open circle corresponds to merged reads from different samples of the same subject. (b) Average FrIP across different samples of the same subject in each data type. (c) Median number of reads at all heterozygous SNVs tested for ASE in each individual. (d,e) Scatterplot showing increase in the median SNV coverage as a function of the number of mapped reads (d) and of FrIP (e) in each subject (n = 7). Continuous lines indicate that the relation is significant (linear regression, P < 0.05). (f) Scatterplot showing increase in the fraction of identified ASE-SNV in each subject (FDR < 0.05), as a function of the median SNV coverage. Continuous lines indicate that the relation is significant (linear regression, P < 0.05). (g) Distribution of the mean SNV coverage between subjects, across all SNVs analyzed in each data type (number of SNVs in each distribution from left to right: 30,463, 26,201, 116,898, 123,151 and 19,371). Median values are shown as white dots within the violin plot and are indicated. (h) Distribution of the number of ASE-SNVs (FDR < 10%) in 100 samplings of 100 SNVs with the same coverage in the different data types. (i) Distribution of the mean ASE effect sizes (allele frequencies) of the 100 SNVs samples shown in h. Median values are shown. Boxplot elements: median (thick line), lower and upper quartiles (box), maximum and minimum (wiskers). (j-l) Scatterplot showing correlation of ASE effects of the same SNV between peaks from different data types in iPSC-CMs: (j) NKX2-5 vs. ATAC-seq peaks; (k) NKX2-5 vs. H3K27ac peaks; (l) ATAC-seq vs. H3K27ac peaks. The union of significant ASE-SNVs in each pair of datasets is shown, with ASE effects expressed as the proportion of the reference allele at heterozygous SNVs. Gray dots denote ASE-SNVs significant (binomial test, FDR < 0.05) only in the peaks indicated in the x-axis, blue dots in the y-axis, and green dots in both. Pearson correlation coefficient (r), number of SNVs (n) and P-values are indicated.
Scatterplots showing relationship between the proportion of reads for the reference allele at ASE-SNVs in the NKX2-5 ChIP-seq data and the difference in motif strength between the reference and alternate allele. Spearman correlation statistics (r and P-value) and the number of motif-altering ASE-SNVs (n) are indicated. The 12 most enriched families of motifs in NKX2-5 peaks (Supplementary Table 4) were tested. TFBS motifs that were strengthened (red) or weakened (blue) by the preferred allele of ASE-SNVs are indicated.
(a) Heatmap of enrichment for GWAS SNPs in ChIP-seq and ATAC-seq peaks from iPSCs and iPSC-CMs combined samples from this study as well as in peaks from cardiac tissues from Roadmap (DHS of fetal heart, H3K27ac of right ventricle and right atrium). Heatmap is ordered by the most enriched GWAS traits on average in the cardiac datasets (iPSC-CMs and Roadmap tissues) and shows fold change values for significant enrichment at FDR corrected P-value < 0.05. A total of 125 GWAS traits were tested for enrichment, and the corresponding number of independent SNPs is given in parenthesis. The statistical enrichment test was performed using GREGOR software. (b-f) Volcano plots showing -log10 P-values (y-axis) and fold enrichment (x-axis) for GWAS loci showed in a, indicating the position of the 6 electrocardiographic traits. Red symbols indicate significant enrichment at FDR corrected P-value < 0.05. The iPSC-CMs NKX2-5, H3K27ac and ATAC-seq enrichment plots are shown in Fig. 5a–c.
Supplementary Figure 7 Enrichment of GWAS signals within iPSC-CM functional annotations using fgwas single state models.
Fgwas natural log fold enrichment of iPSC-CM genomic annotations (y-axis) in heart rate (den Hoed, M. et al. Nat. Genet. 45, 621-631, 2013), atrial fibrillation (Christophersen, I. E. et al. Nat. Genet. 49, 946-952, 2017), and PR interval (van Setten, J. et al. Nat. Commun. 9, 2904, 2018) GWAS signals. Solid circles indicate significant enrichment (defined as 95% CI above zero) and the bars indicate the 95% confidence intervals. The genomic annotations include NKX2-5 ASE-SNVs, NKX2-5 peaks, ATAC-seq peaks, H3K27ac peaks and H3K27ac ASE-SNVs. Peaks were called by combining all samples from each data type. The number of SNPs analyzed for each GWAS was: 2,516,407 SNPs for heart rate, 11,779,664 SNPs for atrial fibrillation, and 2,712,310 SNPs for PR interval.
Supplementary Figure 8 Functional characterization of candidate causal variants at four loci associated with heart rate.
(a, d, f, h) For each of the four loci, the top panel shows the regional plot of association P-values with heart rate (den Hoed, M. et al. Nat. Genet. 45, 621-631, 2013); SNPs are color coded based on r2 values from the 1000 Genome Project CEU population (Johnson, A. D. et al. Bioinformatics, 24, 2938-2939, 2008); lead GWAS variants in the locus are indicated by a diamond. The second panel shows the posterior probability of causality (PPA) of the variants in the locus calculated using fgwas, and panels three through five show epigenetic tracks from iPSC-CM combined samples (NKX2-5, ATAC-seq and H3K27a). The bottom panel shows the Roadmap fetal heart ChromHMM and genes from UCSC genome browser (conventional ChromHMM color code). For d and h, the bottom panel shows the locus at lower scale. For d, f and h, the locations of Hi-C loops from iPSC-CM are shown in red. For the candidate causal variants (turquoise lines), the allelic imbalance (pie chart) of NKX2-5 ASE and FRD-corrected P-values are shown; for a, the altered TF motif is shown. Significant associations (P < 0.05, linear regression) between putative variants genotypes and normalized gene expression of candidate genes in iPSC-CMs from 128 different individuals from iPSCORE are shown (c, e, g, i). Boxplot elements: median (thick line), lower and upper quartiles (box), maximum and minimum (whiskers). (b) EMSA with iPSC-CM nuclear extract using probes containing both allelic variants of rs7612445. An independent replicate is shown in Supplementary Figure 9.
(a-c) Second independent replicate of EMSA with iPSC-CMs nuclear extract using probes containing two allelic variants of rs590041 (a), rs3807989 (b), and rs7612445 (c). (d) Original (not cropped) blots of all presented EMSAs. The figures and supplementary figures where we showed the corresponding cropped versions are indicated.
(a) Representative fluorescence microscopy image showing that in the luciferase assays we achieved approximately 70% transfection efficiency of a GFP-over-expressing plasmid in iPSC-CMs. Efficiency was measured once. (b) Test of gRNA efficiency for CRISPR system in HEK293T cells. HEK293T were transfected with two vectors: one containing two gRNAs targeting the indicated SNP (2sgRNA-ccdB-EF1a-Puromycin) and the other expressing Cas9 (Lenti-Cas9-Blast, Addgene #52962). The gRNAs for SNPs rs590041 (SSBP3 intron) and rs3807989 (CAV1 intron) showed additional bands corresponding to targeted deletions (arrows) and were used for CRISPRi experiments in iPSC-CMs. The gRNAs for SNPs rs7612445 (GNB4) and rs8044595 (MYH11) did not show additional bands and were not used for CRISPRi. Efficiency was tested once. (c,d) qPCR expression of SSBP3 (c) or CAV1 and CAV2 (d) in iPSC-CMs (id: iPSCORE_1_57) stably expressing dCas9-KRAB (CRISPRi) and either a control guide RNA (gCTL) or two guide RNAs targeting the region encompassing rs590041 (c) or rs3807989 (d). Bars and error bars represent the mean and the standard deviation from three qPCR measurements, respectively; two-tailed t-test P-values are shown. Similar results were obtained in an independent cell line presented in the main manuscript (Fig. 5i and Fig. 7h).
Supplementary Figure 11 UCSC genome browser screenshots showing quality of ChIP-seq and ATAC-seq data.
Bedgraph tracks are shown for one representative sample from each of the 7 individuals for H3K27ac ChIP-seq: (a) iPSCs and (b) iPSC-CMs; (c) iPSCs and iPSC-CMs samples from 3 individuals are shown for ATAC-seq; and (d) iPSC-CMs for 7 individuals for NKX2-5.
Supplementary Figs. 1–11, Tables 1, 3 and 7, and Note
Metadata and per-sample sequence data metrics
Motif enrichment analysis of ATAC-Seq and ChIP-Seq peaks
Annotation of SNVs showing ASEs in ChIP-Seq and ATAC-Seq datasets
Results from fgwas fine-mapping analysis of heart rate, atrial fibrillation and PR interval GWAS studies using iPSC-CM functional genomics data
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Benaglio, P., D’Antonio-Chronowska, A., Ma, W. et al. Allele-specific NKX2-5 binding underlies multiple genetic associations with human electrocardiographic traits. Nat Genet 51, 1506–1517 (2019). https://doi.org/10.1038/s41588-019-0499-3