Fluorescence In Situ Hybridization (FISH) Protocol: Principles, Steps, and Troubleshooting

Fluorescence In Situ Hybridization (FISH) is a powerful cytogenetic technique that uses fluorescent probes to detect and localize specific DNA or RNA sequences within fixed cells or tissues. Often described as the bridge between molecular biology and microscopy, FISH allows researchers to visualize the "where" and "when" of gene expression or chromosomal abnormalities with pinpoint accuracy.

You might also be interested in this PCR guide!

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Unlike solution-based techniques (like PCR) that require tissue destruction, FISH preserves the spatial context of the genetic material. Whether you are mapping a gene to a chromosome, diagnosing a genetic disease, or tracking mRNA in a developing embryo, FISH is the gold standard for spatial genomic analysis.

Core Principles: How FISH Works

At its molecular heart, FISH relies on the principle of complementarity—the base-pairing of the DNA/RNA double helix (Adenine-Thymine, Cytosine-Guanine).

1. The Probe: A short strand of DNA or RNA (typically 20–1000 nucleotides) is synthesized to match the target sequence. This probe is labeled with a fluorophore (direct labeling) or a hapten like biotin/digoxigenin (indirect labeling).

2. Denaturation: Both the target DNA in the cell and the probe are heated. This breaks the hydrogen bonds, separating the double strands into single strands.

3. Hybridization: The probe is applied to the sample. As they cool, the probe "hunts" for its complementary target sequence and binds (anneals) to it, reforming a double helix.

4. Detection: Under a fluorescence microscope, the probe emits a glowing signal (dot) at the specific location of the target gene.

Essential Materials & Reagents

Before starting, ensure you have the following grade-critical reagents. Impurities can lead to high background noise.

• Fixative: 4% Paraformaldehyde (PFA) in PBS or MOPS buffer.

• Permeabilization Agents: Proteinase K or Pepsin (for removing proteins that block DNA access).

• Hybridization Buffer:

  • Formamide: Lowers the melting temperature (Tm), allowing hybridization at lower, safer temperatures.

  • Dextran Sulfate: Acts as a volume excluder to artificially increase probe concentration.

  • SSC (Saline-Sodium Citrate): Provides necessary ionic strength.

• Probes: Nick-translated DNA probes or synthetic oligonucleotide probes.

• Counterstain: DAPI (4′,6-diamidino-2-phenylindole) to visualize nuclei.

• Mounting Media: Antifade reagent (essential to prevent signal photobleaching).

Fluorescence In Situ Hybridization Protocol

Note: This protocol synthesizes best practices from Sigma-Aldrich, Thermo Fisher, and recent academic standards. Always adjust incubation times based on your specific tissue type.

Phase 1: Sample Preparation & Fixation

The goal is to freeze cellular architecture while keeping DNA/RNA accessible.

1. Harvest Cells: For suspension cells, centrifuge at 2,000 x g for 5 mins. For adherent cells, grow directly on sterile glass coverslips.

2. Fixation: Resuspend or immerse sample in 4% Paraformaldehyde (PFA) for 10–30 minutes at room temperature.

   • Pro Tip: Freshly depolymerized PFA is crucial for RNA-FISH to prevent degradation.

3. Wash: Rinse 3 times with 1x PBS to remove fixative.

4. Dehydration: Dehydrate slides through an ethanol series (70%, 80%, 95%, 100%) for 2 minutes each. Air dry. This step helps in subsequent probe penetration and storage.

Phase 2: Permeabilization (The "Make or Break" Step)

DNA is wrapped in histone proteins; you must digest these to let the probe in.

1. RNase Treatment (DNA-FISH only): Incubate with RNase A (100 µg/mL) for 1 hour at 37°C to remove RNA that might cause non-specific binding.

2. Digestion: Immerse slides in Pepsin solution (0.005%) or Proteinase K (10–20 µg/mL) in 10 mM HCl at 37°C for 5–10 minutes.

   • Critical: Over-digestion destroys morphology (ghost cells); under-digestion yields no signal. Optimize this time!

3. Post-Fixation: Wash in PBS, then refix in 1% PFA for 5 minutes to stabilize the digested structures.

4. Wash: Rinse thoroughly in 2x SSC.

Phase 3: Denaturation & Hybridization

1. Prepare Probe Mix: Mix probe (5–10 ng/µL) with Hybridization Buffer (50% formamide, 10% dextran sulfate, 2x SSC).

2. Denature: Apply probe mix to the slide, cover with a coverslip, and seal edges with rubber cement (optional). Place on a heating block at 75°C for 5 minutes.

   • Alternative: If using a specialized hybridizer, program it for 75°C denaturation followed by the hybridization temp.

3. Hybridize: Incubate overnight in a humidified dark chamber at 37°C–42°C.

   • Note: The temperature depends on the probe's Tm. A general rule is Tm - 20°C.

Phase 4: Washing & Detection

Remove unbound probe to reveal the signal ("Stringency Washes").

1. Remove Coverslips: Soak in 2x SSC until coverslips slide off.

2. High Stringency Wash: Wash in 0.4x SSC with 0.3% NP-40 at 72°C for 2 minutes. (Temperature and salt concentration determine stringency—higher temp/lower salt = cleaner signal).

3. Low Stringency Wash: Wash in 2x SSC with 0.1% NP-40 at room temperature for 1 minute.

4. Counterstain: Apply DAPI (0.5–1 µg/mL) for 5–10 minutes.

5. Mount: Apply Antifade Mounting Medium and seal with nail polish.

6. Visualization: Store in the dark at -20°C or image immediately using a fluorescence microscope with appropriate filters (e.g., FITC, TRITC, Cy5).

Advanced Variations

1. ECHO-FISH (Fast Protocol)

Recent advances (PMC3261739) introduced ECHO-FISH, using Exciton-Controlled Hybridization-Sensitive fluorescent probes. These probes are "silent" until they bind to their target, meaning:

• No Washing Required: Unbound probes don't fluoresce.

• Speed: The entire protocol (Fixation to Mounting) takes ~25 minutes.

• Workflow: Fix → Ethanol Dehydrate → Apply ECHO Probe → Mount.

2. Tyramide Signal Amplification (TSA)

For low-abundance targets, standard FISH may be too dim. TSA (Thermo Fisher) adds an enzyme-linked step:

• The probe is labeled with Peroxidase (HRP).

• Tyramide-fluorophores are added. The HRP activates tyramide, causing it to deposit continuously near the probe.

• Result: 10–200x brighter signal, ideal for detecting rare mRNA transcripts.

Troubleshooting: The "Why My Fluorescence In Situ Hybridization Failed?" Guide

Issue	Possible Cause	Solution
No Signal / Weak Signal	Probe denaturation failed	Ensure temperature reached 75°C. Check thermometer accuracy.
	Over-digestion	Reduce Proteinase K/Pepsin concentration or time.
	Photobleaching	Use Antifade media. Keep slides in the dark.
High Background	Non-specific binding	Increase wash temperature or decrease salt (SSC) concentration (Higher Stringency).
	Drying out	Ensure slides remain moist during hybridization. Use a humidity chamber.
	Inadequate blocking	Use proper blocking agents (BSA or Salmon Sperm DNA) in the hybridization buffer.
Autofluorescence	Tissue artifacts	Avoid Glutaraldehyde fixation; use PFA. Treat with Sodium Borohydride if necessary.
"Ghost" Nuclei	Over-permeabilization	Reduce digestion time. Ensure post-fixation step is included.

Did you know we have many more SOPs and guides like this?

References

• Wang, D. O., et al. (2012). A quick and simple FISH protocol with hybridization-sensitive fluorescent linear oligodeoxynucleotide probes. RNA, 18(1), 166–175. NCBI PMC

• Sigma-Aldrich. Fluorescent in situ Hybridization (FISH) Procedure and Reagents. Link

• Creative BioMart. Principle & Protocol: Fluorescence In Situ Hybridization (FISH). Link

• Thermo Fisher Scientific. Fluorescence In Situ Hybridization (FISH) Overview & Kits. Link

• Paganos, P., et al. (2022). FISH for All: A Fast and Efficient Fluorescent In situ Hybridization (FISH) Protocol for Marine Embryos and Larvae. Frontiers in Physiology. Link