3D multicolor DNA FISH represents a tool to visualize multiple genomic loci within 3D preserved nuclei, unambiguously defining their reciprocal interactions and localization within the nuclear space at a single cell level. Here, a step by step protocol is described for a wide spectrum of human primary cells.
A major question in cell biology is genomic organization within the nuclear space and how chromatin architecture can influence processes such as gene expression, cell identity and differentiation. Many approaches developed to study the 3D architecture of the genome can be divided into two complementary categories: chromosome conformation capture based technologies (C-technologies) and imaging. While the former is based on capturing the chromosome conformation and proximal DNA interactions in a population of fixed cells, the latter, based on DNA fluorescence in situ hybridization (FISH) on 3D-preserved nuclei, allows contemporary visualization of multiple loci at a single cell level (multicolor), examining their interactions and distribution within the nucleus (3D multicolor DNA FISH). The technique of 3D multicolor DNA FISH has a limitation of visualizing only a few predetermined loci, not permitting a comprehensive analysis of the nuclear architecture. However, given the robustness of its results, 3D multicolor DNA FISH in combination with 3D-microscopy and image reconstruction is a possible method to validate C-technology based results and to unambiguously study the position and organization of specific loci at a single cell level. Here, we propose a step by step method of 3D multicolor DNA FISH suitable for a wide range of human primary cells and discuss all the practical actions, crucial steps, notions of 3D imaging and data analysis needed to obtained a successful and informative 3D multicolor DNA FISH within different biological contexts.
Higher eukaryotes need to systematically condense and compact a huge amount of genetic information in the minute 3D space of the nucleus1,2,3,4. Today, we know that the genome is spatially ordered in compartments and topologically associated domains5 and that the multiple levels of DNA folding generate contacts between different genomic regions that may involve chromatin loop formation6,7. The 3D dynamic looping of chromatin can influence many different biological processes such as transcription8,9, differentiation and development10,11, DNA repair12,13, while its perturbations are involved in various diseases14,15,16 and developmental defects15,17,18.
Many approaches have been developed to study the 3D genome organization. Chromosome conformation capture-based technologies (C-technologies, 3C, 4C, 5C, Hi-C and derivatives) have been developed to study genome organization in fixed cells3,4,19,20. Such approaches are based on the ability to capture the contact frequencies between genomic loci in physical proximity. C-technologies, depending on their complexity, catch the global 3D genome organization and nuclear topology of a cell population3,4,19,20. Nevertheless, 3D interactions are dynamic in time and space, highly variable between individual cells consisting of multiplex interactions, and are extensively heterogenous21,22.
3D multicolor DNA fluorescence in situ hybridization (FISH) is a technique that allows the visualization of specific genomic loci at a single cell level, enabling direct investigation of the 3D nuclear architecture in a complementary manner to C-technologies. It represents a technology currently used to unambiguously validate C-results. 3D multicolor DNA FISH uses fluorescently labeled probes complementary to the genomic loci of interests. The use of different fluorophores and suitable microscopy equipment allow contemporary visualization of multiple targets within the nuclear space23,24. In recent years, FISH has been combined with technological advances in microscopy to obtain the visualization of fine-scale structures at high resolution25,26 or with CRISPR-Cas approaches for the visualization of the nucleic acids in live imaging27,28. Despite wide adoption, the 3D multicolor DNA FISH approach is still considered difficult in many laboratories because the biological material used must be adapted.
Here, we provide a comprehensive protocol for 3D multicolor DNA FISH (from cell/probe preparation to data analysis) applicable to a wide range of human primary cells, enabling the visualization of multiple genomic loci and preserving the 3D structure of nuclei. In order to study nuclear architecture, the 3D structure of nuclei must be preserved. For this reason, contrasting from other existing protocols29,30,31, we avoid the use of an alcohol gradient and the storage of the coverslips in alcohol that can affect chromatin structure32. The method is adapted from preserved 3D DNA FISH protocols24,33 to be applied to a wide range of human primary cells, both isolated ex vivo or cultured in vitro. There are permeabilization and deproteinization parameters for different nuclear morphology and cytological characteristics (e.g., different degrees of nuclear compaction, cytoskeleton abundance)34. These parameters are often generally described in other protocols24,33, without providing a clear discrimination of the procedure within different cell types. Furthermore, we developed a specific tool named NuCLεD (nuclear contacts locator in 3D)16, providing principles for data analysis that will improve the 3D proximity between different loci and their nuclear topological distribution within the nuclear space in an automated way.
1. DNA probe preparation and labelling procedures with nick translation
2. Cell fixation, pre-treatment and permeabilization
NOTE: Permeabilization and deproteinization passages are crucial steps. The time of reaction and concentration of the reagents strongly depend on the cell type, the cytoplasm abundance, and the nuclear morphology.
3. 3D multicolor DNA FISH hybridization
4. 3D multicolor DNA FISH detection
NOTE: For directly labelled probes, skip steps 4.1 and 4.2.
5. 3D multicolor DNA FISH microscopy and analysis
The method of 3D multicolor DNA FISH described in this article allows contemporary visualization of different genomic loci within preserved 3D nuclei (Figure 1B). This protocol permits the measurement of distances between alleles, and different genomic loci in order to evaluate their spatial proximity, and to assess their location within the nuclear space (e.g., loci distance from the centroid or the periphery of the nuclei)16. However, there are many crucial steps that must be accurately and specifically set up for each cell type used; it is highly recommended to pay particular attention to the following steps for success of 3D multicolor DNA FISH.
For DNA probe preparation, check that probe size is <200 bp (Figure 1A). This size ensures a successful procedure of 3D multicolor DNA FISH (Figure 1B). Suboptimal DNA FISH probes produced by nick translation can be partially digested (Figure 2A) or over digested (Figure 2B). With partially digested probes, the procedure will have no signal in the cells, due to the inability of the probe to enter the nuclei and properly hybridize to the complementary genomic loci. Over digested probes will result in a nonspecific signal, due to a loss of specificity in the hybridization and a consequent increase of the background. A representative example of over digested probes is shown in Figure 3A in comparison to an optimal digested probe in Figure 3B.
For deproteinization and pepsinization, follow these steps according to the cell type. In particular, take into consideration nuclear size and cytoplasm abundance. For human primary ex vivo isolated T lymphocytes and cells with small, highly compacted nuclei and low abundant cytoplasm, HCl deproteinization is crucial. Treatment with 0.1 N HCl for 5 min is not sufficient for DNA FISH visualization. 0.1 N HCl treatment for 12 min is recommended to promote nuclei accessibility to DNA probes and preserve nuclear integrity (Figure 4A). Pepsin digestion of the cytoplasm is not needed to obtain a good signal of DNA FISH (Figure 4B).
For human primary myoblasts and cells that have large nuclei and abundant cytoplasm, the pepsinization step is fundamental. A short and suboptimal pepsinization of the cytoskeleton will hamper the entry of the probe in the nuclei (Figure 4C), ending in the absence of a DNA FISH signal. However, if the cells are over pepsinized, nuclei will not remain intact (Figure 4D), losing their 3D structure. An example of successful 3D multicolor DNA FISH is provided in Figure 4E.
During hybridization, seal the coverslip accurately; otherwise, the probe will disperse and dry. Denaturation and hybridization steps must be performed rapidly such that the probe and genomic DNA will not reanneal. The duration of the denaturation can be increased.
Figure 1: Representative DNA probes and 3D multicolor DNA FISH. (A) Nick translated DNA probes of optimal size run on an 2.2% agarose gel (lane 1, 2), 50 bp marker (M). (B) Representative 3D multicolor DNA FISH nucleus using probes mapping to 3q11.2 region (green), 10q26.3 region (red) and 8q24.13 region (magenta) in human primary myoblasts. Nuclei are counterstained with DAPI (blue). 63x magnification. Scale bar = 5 µm. Please click here to view a larger version of this figure.
Figure 2: Examples of not optimally digested DNA FISH probes. (A) Not digested (lane 1, 2) or partially digested (lane 3) nick translated DNA probes run on an 2.2% agarose gel, 2log marker (M). (B) Over digested nick translated DNA probes run on an 2.2% agarose gel, 2log marker (M). Please click here to view a larger version of this figure.
Figure 3: Comparison of 3D multicolor DNA FISH using suboptimal or optimal DNA FISH probes. (A) Representative 3D multicolor DNA FISH nuclei using over digested probe mapping to 8q24.13 region (magenta) in human primary myoblasts. Nuclei are counterstained with DAPI (blue). 63x magnification. Scale bar = 10 µm. (B) Representative 3D multicolor DNA FISH nuclei using optimally digested probe mapping to 8q24.13 region (magenta) in human primary myoblasts. Nuclei are counterstained with DAPI (blue). 63x magnification. Scale bar = 10 µm. Please click here to view a larger version of this figure.
Figure 4: Possible outcomes of suboptimal deproteinization and pepsinization steps on 3D multicolor DNA FISH results. (A) Representative 3D multicolor DNA FISH nuclei of human primary T lymphocytes treated for 5 min (left) or 12 min (right) with 0.1 N HCl, using probe mapping to 8q24.13 region (green). Nuclei are counterstained with DAPI (blue). 100x magnification. Scale bar = 10 µm. (B) Representative 3D multicolor DNA FISH nuclei of human primary T lymphocytes treated for 12 min with 0.1 N HCl (left) or coupled with 0.01 N HCl/0.0025% pepsin for 2 min (right), using probe mapping to 8q24.13 region (green). Nuclei are counterstained with DAPI (blue). 100x magnification. Scale bar = 10 µm. (C) Representative 3D multicolor DNA FISH nuclei of human primary myoblasts treated with short and suboptimal pepsinization, using probe mapping to 8q24.13 region (magenta). Nuclei are counterstained with DAPI (blue). 63x magnification. Scale bar = 10 µm. (D) Representative 3D multicolor DNA FISH nuclei of human primary myoblasts treated with prolonged pepsinization step, using probe mapping to 8q24.13 region (magenta). Nuclei are counterstained with DAPI (blue). 63x magnification. Scale bar = 5 µm. (E) Representative 3D multicolor DNA FISH nuclei of human primary myoblasts treated with optimal HCl/pepsin conditions using probe mapping to 8q24.13 region (magenta). Nuclei are counterstained with DAPI (blue). 63x magnification. Scale bar = 25 µm. Please click here to view a larger version of this figure.
Nick translation reagents | Initial concentration | Final concentration |
dNTPs (C-G-A) | 0.5 mM | 0.05 mM |
dTTP | 0.1 mM | 0.01 mM |
Biotin/Dig/Cy3 dUTP | 1 mM | 0.02 mM |
Tris HCl pH 7.8 | 1 M | 50 mM |
MgCl2 | 100 mM | 5 mM |
β-mercaptoethanol | 100 mM | 10 mM |
BSA | 100 ng/µL | 10 ng/µL |
DNA Pol I | 10 U/µL | 0.1 U/µL |
DNase I | 1 U/µL | 0.002 U/µL |
DNA 2 µg | x | x |
ddH2O | Up to 50 µL |
Table 1: Nick Translation. Table describing all the reagents, their concentration and suggested timing for nick translation reaction.
The current method describes a step by step protocol to perform 3D multicolor DNA FISH on a wide range of human primary cells. Although DNA FISH is a technology in wide use, 3D multicolor DNA FISH on preserved 3D interphase nuclei is still difficult to perform in many laboratories, mainly due to the characteristics of the samples used23,24.
Probe nick translation is a fundamental step for successful 3D multicolor DNA FISH; many different substrates (BAC, fosmid, plasmid, PCR products) can be used for this reaction, and the timing of the reaction and enzyme concentration can be accordingly adjusted with respect to the length of the substrate. A proper probe digestion is fundamental (Figure 1), as nonoptimal probes (Figure 2) will result in no signal or a nonspecific signal (Figure 3A). Permeabilization, deproteinization, and pepsinization steps are crucial passages that strongly depend on the cell type used. Cells with small nuclei and low cytoskeleton abundance, such as ex vivo isolated T lymphocytes, require deproteinization with a prolonged 0.1 N HCl treatment. Also, washes in PBS with higher percentages of Triton X-100 can help the probe entry in the nuclei of these cells. On the contrary, in vitro cultured human primary myoblasts that present larger nuclei, with a high content of cytoskeleton, need digestion of the cytosolic structures with pepsin. These general roles can be applied to a wide range of cells, eventually combining the different steps depending on the specific cellular characteristics.
The use of freshly prepared biological material, fresh solutions (in particular solutions with detergent), and fluorescent reagents are strongly suggested: filtered PFA at pH 7.0; autoclaved and filtered 20x SSC at pH 7.0; filtered formamide at pH 7.0; nuclease free water; and disposable aliquots of modified UTP. Prolonged incubation with 20% glycerol/PBS, or 50% formamide/2x SSC can facilitate the hybridization. HCl and/or pepsin treatment can be further increased. The timing of hybridization, the quantity of probes, the concentration and the timing of incubation of anti-digoxigenin and streptavidin can all be further adjusted to improve the signal to noise ratio.
3D multicolor DNA FISH represents a complementary tool to C-technologies, the standard method to validate C-based results. If combined with 3D microscopy and analysis, 3D multicolor DNA FISH can monitor the proximity between genomic loci and their topological distribution within the nuclear space at single cell level. 3D multicolor DNA FISH can be further integrated with other methodologies such as RNA FISH and immunofluorescence for a comprehensive overview of the dynamics and interactions between genomic loci, RNAs (messenger RNA or regulatory non coding RNA) and a wide range of proteins, providing a unique opportunity to visualize the nuclear structure and investigate the epigenetic mechanisms that subtend cellular identity.
Despite the huge improvement of FISH technologies with super resolution25,26, live cell imaging27,28,36, single molecule detection37, and contemporary visualization of multiple targets with oligonucleotide arrays such as Oligopaint37,38 with 3D high-throughput approaches39, a limitation of the technology remains the discrete number of predetermined genomic loci that can be visualized. This prevents a wide-ranging analysis of nuclear architecture. Several studies have recently described sequential methods of hybridization to address genome organization in single cells such as barcode DNA FISH40,41,42,43. Further efforts will be needed to couple the single cell nature of 3D multicolor DNA FISH to genome wide features to broadly visualize nuclear architecture heterogeneity with imaging technologies, as the number of loci that can be tested at a time will increase.
The authors have nothing to disclose.
The authors acknowledge the technical assistance of the INGM Imaging Facility (Istituto Nazionale di Genetica Molecolare "Romeo ed Enrica Invernizzi" (INGM), Milan, Italy), in particular C. Cordiglieri, for assistance during 3D multicolor DNA FISH images acquisition. This work has been supported by the following grants to B.B.: EPIGEN Italian flagship program, Association Française contre les Myopathies (AFM-Telethon, grant nr 18754) and Giovani Ricercatori, Italian Ministry of Health (GR-2011-02349383). This work has been supported by the following grant to F.M.: Fondazione Cariplo (Bando Giovani, grant nr 2018-0321).
24-well plates | Thermo Fisher Scientific | 142475 | |
6-well plates | Thermo Fisher Scientific | 140675 | |
Anti-Digoxigenin 488 | DBA | DI7488 | |
b-Mercaptoethanol | Sigma | M3148 | |
bFGF | PeproTech | 100-18B | |
Biotin 11 d-UTP | Thermo Fisher Scientific | R0081 | |
BSA (bovine serum albumine) | Sigma | A7030 | |
Coverlsips | Marienfeld | 117500 | |
CY3 d-UTP | GE Healthcare | PA53022 | |
DAPI (4,6-diamidino-2-phenylindole) | Thermo Fisher Scientific | D21490 | |
Deoxyribonucleic acids single strand from salmon testes | Sigma | D7656 | |
Dextran sulfate (powder) | Santa Cruz | sc-203917A | |
Digoxigenin 11 d-UTP | Roche | 11093088910 | |
DMEM | Thermo Fisher Scientific | 21969-035 500mL | |
DNA polymerase I | Thermo Fisher Scientific | 18010-017 | |
DNase I | Sigma | AMPD1 | |
dNTPs (C-G-A-T) | Euroclone | BL0423A/C/G | |
EGF | Sigma | E9644.2MG | |
Ethanol | Sigma | 02860-1L | |
FBS Hyclone | Thermo Fisher Scientific | SH30109 | |
Formaldehyde solution | Sigma | F8775-25mL | |
Formamide | Sigma | F9037 | |
Glutammine | Thermo Fisher Scientific | 25030-024 100mL | |
Glycerol | Sigma | G5516-100mL | |
Glycogen | Thermo Fisher Scientific | AM9510 | |
HCl | Sigma | 30721 | |
Human Cot-1 DNA | Thermo Fisher Scientific | 15279-001 | |
Insulin Human | Sigma | I9278-5 mL | |
MgCl2 | Sigma | 63069 | |
NaAc (Sodium Acetate, pH 5.2, 3 M) | Sigma | S2889 | |
NaCl | Sigma | S9888 | |
Paraformaldehyde | Sigma | 158127-25G | |
PBS (phosphate-buffered saline) | Sigma | P4417 | |
Pennycillin/Streptavidin | Thermo Fisher Scientific | 15070-063 100mL | |
Pepsin | Biorad | P6887 | |
PhasePrep BAC DNA Kit | Sigma | NA0100-1KT | |
Poly-L-lysine solution | Sigma | P8920 | |
ProLong Diamond Antifade Mountant | Thermo Fisher Scientific | P36970 | |
PureLink Quick Gel Extraction & PCR Purification Combo Kit | Thermo Fisher Scientific | K220001 | |
PureLink Quick Plasmid Miniprep Kit | Thermo Fisher Scientific | K210010 | |
RNAse cocktail | Thermo Fisher Scientific | AM2288 | |
Rubbercement | Bostik | ||
Slides | VWR | 631-0114 | |
Streptavidina Alexa fluor 647 | Thermo Fisher Scientific | S21374 | |
Tri-Sodium Citrate | Sigma | 1110379026 | |
Tris-HCl | Sigma | T3253-500g | |
Triton X-100 | Sigma | T8787-250mL | |
TWEEN 20 | Sigma | P9416-100mL |