$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
To illustrate the use of the presented protocol, we investigated the TBX5 gene in the human heart, exploring TBX5-associated enhancers using the comprehensive workflow involving H3K4me1, H3K27ac, and Hi-C data. TBX5 is a gene that contributes to limb and heart development, including the formation of the four chambers and septum separation24. Mutation in this gene is a main cause of Holt-Oram syndrome (HOS), which causes limb abnormalities and congenital heart disease (CHD), including septum defects24. The mutation of TBX5-associated cardiac enhancers may critically influence CHD24. A previous study discovered three known TBX5 enhancers in human cardiac-specific tissue - namely "Enhancer 2", "Enhancer 9", and "Enhancer 16" (Supplementary File 2), which were demonstrated to have comparative phenotypes in transgenic mice23.
We investigated H3K4me1- and H3K27ac-enriched regions between RBM19 and TBX3, which are two flanking genes downstream and upstream of TBX5 in human, to retrieve putative enhancers at the TBX5 locus (Figure 1 and Figure 2). To identify heart-specific enhancers, the cardiac muscle cells were chosen. The putative TBX5 cardiac enhancer regions were retrieved as coordinates (chr12: start-end), and 22 both H3K4me1- and H3K27ac-associated regions were identified (Figure 3 and Supplementary File 3). Putative TBX5 cardiac enhancers were retrieved from the EnsEMBL genomic database to cross-reference Hi-C data held in the 4DNucleome database (Figure 4). This was done to gauge possible interactions between potential enhancers and the TBX5 cardiac promoter. Following the protocol described here, 21 out of 22 genomic regions were confirmed to interact with the TBX5 promoter (chr12: 114400143-114410103) in the cardiac muscle cells (Supplementary File 4). There was one region that had no physical interaction with the promoter (Figure 4, step 4.8). Finally, we compared this protocol with these biologically validated enhancers and the current gold-standard database of heart enhancers, VISTA Cardiac Enhancers Browser, and revealed additional enhancers not currently captured by the database25.
We performed a cross-comparison of the 21 TBX5 enhancers retrieved by the protocol presented here with existing databases. We retrieved 4 TBX5 enhancers from VISTA Cardiac Enhancer Browser (Supplementary File 5)25. Of the 4 VISTA-identified cardiac enhancers, 3 enhancers, hs2329, mm1282, and m370 overlapped with the regions identified by this web-based enhancer detection protocol (Figure 5). Each of the predicted enhancers also shared the genomic regions with previously experimentally validated enhancers from Smemo et al.23, Enhancer 2 (chr12:114025907-114026275, GRCh38), and Enhancer 16 (chr12:114415466-114420433, GRCh38), while they did not show overlap with Enhancer 9 (chr12:114263402-114266886, GRCh38). One of the VISTA-identified enhancers, hs498 did not overlap with any predicted enhancers by this protocol or Smemo et al.'s experimentally validated enhancers23 (Figure 5), even though the region showed partial overlap with H34Kme1-marks (Figure 5). Similarly, Enhancer 9 did not overlap with the predicted enhancers by this pipeline but was associated with H3K4me1 marks (Figure 5).

Figure 1: Step-by-step guide to locate the GoI in the EnsEMBL Genome Browser. The user first opens the EnsEMBL homepage (1.1), selects the species (Human), and enters the gene into the search bar (1.2-1.3). From the list of results, the appropriate gene ID is selected (1.4), which opens the gene summary page. The user then clicks the Region in Detail hyperlink (1.5) to visualize the genomic region surrounding the GoI, including neighboring elements and regulatory features. Please click here to view a larger version of this figure.

Figure 2: Defining the enhancer detection region around the GoI using the EnsEMBL Genome Browser. To define the enhancer detection region, identify the two neighboring genes flanking the GoI using the Basic Gene Annotations from GENCODE track, where genes are shown as dark yellow blocks labelled with merged EnsEMBL/Havana annotations. The transcriptional direction of each gene is indicated by arrowheads (< or >) next to the gene name (2.1). To select the intergenic region between the neighboring genes, click and drag across the region of interest, then choose Jump to region in the pop-up box to zoom in (2.2). To add regulatory or enhancer-related annotations, click Add/remove tracks (2.3). Please click here to view a larger version of this figure.

Figure 3: Configuration of histone modification tracks in the enhancer detection region using the EnsEMBL Genome Browser. In the left tool bar, click Configure this page (3.1) to access the track configuration panel and navigate to "Activity by Cell/Tissue" under the Regulation section (3.2). In the opened tab, select the "Experiments" section (3.3), and use the Cell/Tissue search bar to locate and select your tissue of interest (cardiac muscle cell) (3.4). In the histone mark panel (3.5), enable H3K4me1 and H3K27ac as active enhancer marks and H3K4me3 as a promoter mark then click "Configure track display" (3.6). After confirming the track selections, click "View tracks" (3.7) to return to the genome viewer. Histone mark peaks are now shown in the detection region (3.8) as coloured blocks under the corresponding tissue label (yellow: H3K4me1, blue: H3K27ac, and orange: H3K4me3). "Hists & Pols" pop-up containing the genomic coordinates of the region in base pairs (chr:start-end), which can be copied and saved for downstream analysis. A "Hists & Pols" pop-up appears after clicking on the coloured elements in the track. The pop-up contains the genomic coordinates of the region in base pairs (e.g., chr12:11443450-114451611 for the promoter region), which can be copied and saved for downstream analysis (3.8). Likewise, to extract candidate enhancers, prioritize regions where H3K4me1 and H3K27ac peaks overlap, as shown by vertical alignment of peaks and boxes across tracks (3.9). Overlapping regions can be selected directly by clicking their boxes or by manually click-dragging across the aligned peaks to define a region (e.g., chr12:114400143-114410103 for an active candidate region). Coordinates shown in the pop-up should be saved in BED format for downstream validation or visualization. Please click here to view a larger version of this figure.

Figure 4: Visualisation of promoter-enhancer chromatin interactions using Hi-C heatmaps from the 4D Nucleome Data Portal. The 4D Nucleome Data Portal homepage displays a stacked bar chart summarizing available experiment types by organism. The "in situ Hi-C" dataset for human samples is selected by clicking the corresponding section of the bar (4.1). A filtered list of relevant datasets is displayed; a Hi-C dataset derived from H9 cells differentiated into cardiac myoblasts is selected (4.2). The selected dataset (4.3) is opened in the HiGlass browser via the Explore Data button (4.4). The genomic region of interest is entered in the coordinate box (4.5), and the contact matrix is rendered as a colour-scaled heatmap. Darker colours (deep red to black) indicate stronger chromatin contact frequency, while lighter colours (white to orange) represent weaker interactions. A horizontal rule is placed at the promoter coordinate, and vertical rules are drawn at the positions of three experimentally validated control enhancers (4.6). These intersections are used to define a strict interaction threshold, set by the strongest visible signal (darkest colour) among the promoter-enhancer contacts (4.7). Additional vertical rules are drawn at the locations of candidate H3K27ac and H3K4me1-marked enhancers (from step 3.8). Candidates whose promoter-enhancer intersections are equal to or darker than the threshold are retained, while those with weaker signals (lighter color squares) are excluded (4.8). Retained coordinates are extracted manually and saved in BED format for downstream analyses. (a. Enhancer 2, b. Enhancer 9, and c. Enhancer 16) Please click here to view a larger version of this figure.

Figure 5: Genomic browser view of predicted TBX5 enhancers compared with VISTA-validated cardiac enhancers and control enhancers. Genome browser snapshots display the enhancer searching range (STEP 2) comparing predicted enhancers retrieved by the web-based protocol (bottom) with VISTA-validated enhancers (top) and experimentally validated control enhancers (center). The main panel shows the full genomic locus with annotated regulatory elements, including cardiac muscle cell-specific H3K4me1 (yellow), H3K27ac (blue), and H3K4me3 peaks (orange). Three zoomed-in figures capture the alignment between the retrieved by the protocol, VISTA, and control enhancers. The overlap with control enhancers is outlined with red boxes. Coordinates for each sub-region are displayed in the lower browser panels. Please click here to view a larger version of this figure.
Table 1: Data used in the study. Please click here to download of this Table.
Table 2: Web-based tools used in the study. Please click here to download of this Table.
Supplementary File 1: Troubleshooting instructions for EnsEMBL Genome Browser. Please click here to download of this File.
Supplementary File 2: A BED file in GRCh38 format, the experimentally validated TBX5 cardiac control enhancers23. Please click here to download of this File.
Supplementary File 3: A BED file in GRCh38 format, TBX5 cardiac enhancers retrieved by STEP3 from EnsEMBL. Please click here to download of this File.
Supplementary File 4: A BED file in GRCh38 format, TBX5 cardiac enhancers retrieved by STEP4 from EnsEMBL. Please click here to download of this File.
Supplementary File 5: A BED file in GRCh38 format, TBX5 cardiac enhancers retrieved from the VISTA cardiac enhancer browser25. Please click here to download of this File.