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Method Article
Bisulfite mutagenesis is the gold standard for analyzing DNA methylation. Our modified protocol allows for DNA methylation analysis at the single-cell level and was specifically designed for individual oocytes. It can also be used for cleavage-stage embryos.
Epigenetics encompasses all heritable and reversible modifications to chromatin that alter gene accessibility, and thus are the primary mechanisms for regulating gene transcription1. DNA methylation is an epigenetic modification that acts predominantly as a repressive mark. Through the covalent addition of a methyl group onto cytosines in CpG dinucleotides, it can recruit additional repressive proteins and histone modifications to initiate processes involved in condensing chromatin and silencing genes2. DNA methylation is essential for normal development as it plays a critical role in developmental programming, cell differentiation, repression of retroviral elements, X-chromosome inactivation and genomic imprinting.
One of the most powerful methods for DNA methylation analysis is bisulfite mutagenesis. Sodium bisulfite is a DNA mutagen that deaminates cytosines into uracils. Following PCR amplification and sequencing, these conversion events are detected as thymines. Methylated cytosines are protected from deamination and thus remain as cytosines, enabling identification of DNA methylation at the individual nucleotide level3. Development of the bisulfite mutagenesis assay has advanced from those originally reported4-6 towards ones that are more sensitive and reproducible7. One key advancement was embedding smaller amounts of DNA in an agarose bead, thereby protecting DNA from the harsh bisulfite treatment8. This enabled methylation analysis to be performed on pools of oocytes and blastocyst-stage embryos9. The most sophisticated bisulfite mutagenesis protocol to date is for individual blastocyst-stage embryos10. However, since blastocysts have on average 64 cells (containing 120-720 pg of genomic DNA), this method is not efficacious for methylation studies on individual oocytes or cleavage-stage embryos.
Taking clues from agarose embedding of minute DNA amounts including oocytes11, here we present a method whereby oocytes are directly embedded in an agarose and lysis solution bead immediately following retrieval and removal of the zona pellucida from the oocyte. This enables us to bypass the two main challenges of single oocyte bisulfite mutagenesis: protecting a minute amount of DNA from degradation, and subsequent loss during the numerous protocol steps. Importantly, as data are obtained from single oocytes, the issue of PCR bias within pools is eliminated. Furthermore, inadvertent cumulus cell contamination is detectable by this method since any sample with more than one methylation pattern may be excluded from analysis12. This protocol provides an improved method for successful and reproducible analyses of DNA methylation at the single-cell level and is ideally suited for individual oocytes as well as cleavage-stage embryos.
DAY 1
Prepare the following solutions fresh on the day of oocyte collection with sterile, distilled water such as GIBCO water. To reduce the chance of DNA contamination, change gloves often and use filter tips. Keep tubes angled away when open, and recap all tubes when not in use. We recommend that solutions are made as n+1.
3% LMP Agarose
30 mg Low Melting Point (LMP) Agarose
up to 1 ml GIBCO H2O
dissolve @ 70 °C
Lysis Solution
8 μl lysis buffer
1 μl proteinase K
1 μl 10% IGEPAL
place on ice until ready for use
2:1 Agarose:Lysis Solution (10 μl per individual oocyte, amount is for 3 oocytes)
20 μl 3% LMP agarose
10 μl Lysis Solution
mix @ 70 °C
SDS Lysis Buffer (501 μl per individual oocyte)
1x TE pH 7.5 | 450 μl |
10% SDS | 50 μl |
Proteinase K | 1 μl |
501 μl |
1. Oocyte Collection
2. Agarose Embedding and Lysis
DAY 2
Prepare the following solutions fresh on the day of bisulfite mutagenesis. To reduce chance of DNA contamination, change gloves often and use filter tips. Keep tubes angled away when open, and recap all tubes when not in use. We recommend that solutions are made as n+1.
3 M NaOH | 2.4 g NaOH in 20 ml autoclaved ddH2O |
0.1 M NaOH | 0.5 ml of 3M in 14.5 ml autoclaved ddH2O |
0.3 M NaOH | 1.5 ml of 3M in 13.5 ml autoclaved ddH2O |
2.5 M Bisulfite Solution
When fully dissolved, mix solution (a) and (b)
*Keep away from light*
3. Bisulfite Mutagenesis
4. 1st and 2nd Round PCR amplification
10 μM Primer Forward Outer | 0.5 μl |
10 μM Primer Reverse Outer | 0.5 μl |
240 ng/ml tRNA | 1 μl |
H2O | 13 μl |
Add to Illustra Ready-to-Go Hot Start PCR beads
Carefully slide the solid agarose bead into the PCR tube (~10 μl)
Heat to 70 °C and mix
Add 25 μl mineral oil
Total: 50 μl
10 μM Primer Forward Inner | 0.5 μl |
10 μM Primer Reverse Inner | 0.5 μl |
H2O | 19 μl |
Add to Illustra Ready-to-Go Hot Start PCR beads
Add 5 μl 1st Round product as a template. Heat the 1st round product to 70 °C for 1 minute to soften the agarose. Be sure to pipette below the layer of mineral oil.
Add 25 μl mineral oil
Total: 50 μl
Note: Nested primer sequences for Snrpn, H19, and Peg3 can be found in Market-Velker et al10,12.
2nd Round product | 4 μl |
Restriction Enzyme | 1 μl |
Buffer | 1 μl |
H2O | 4 μl |
5. TA Cloning and Colony PCR
2nd Round PCR | 1 μl |
pGEMT-EASY vector | 1 μl |
Ligase | 1 μl |
H2O | 2 μl |
2x Ligation Buffer | 5 μl |
Incubate overnight @ 4 °C in PCR machine.
20 μM M13 Forward Primer | 0.7 μl |
20 μM M13 Reverse Primer | 0.7 μl |
5X Green Go Taq Buffer | 7.0 μl |
10 mM dNTP | 0.7 μl |
Taq DNA polymerase | 0.28 μl |
H2O | 25.62 μl |
35 μl Total |
Add 35 μl Colony PCR master mix into a PCR tube. Pick a white bacterial colony from the plate with a pipette tip, and swirl it into the PCR reaction.
6. Representative Results
In our work, we assay imprinted methylation in individual oocytes and embryos (Figure 1). Following nested PCR amplification using bisulfite converted primers, it is possible to confirm a successful conversion by visualizing a correct fragment size on an agarose gel (Figure 2). An individual oocyte represents one parental allele, and in theory, has one imprinted methylation pattern. As such, second round PCR products can be tested for unintentional contamination. A restriction enzyme sensitive to DNA methylation (such as HinfI or DpnII) can be used to digest the second round PCR product to assess whether it contains a methylated or unmethylated allele (Figure 3). A methylated C within the enzyme recognition sequence is cleaved while an unmethylated C that is converted to T is no longer recognized by the enzyme and is uncut. Any MII oocyte sample containing both methylated and unmethylated alleles should be discarded, as it is indicative of cumulus cell contamination (Figure 3). Following ligation and transformation, successful colony PCR amplification can be visualized on an agarose gel to ensure samples with the correct product size are sent for sequencing (Figure 4). Finally, the sequence of five individual clones from an MII oocyte should produce five identical methylation patterns and identical nonCpG conversion rates (Figure 5a). Any samples that contain more than one pattern should be discarded (Figure 5b). Since ovulated MII oocytes have two chromosome copies or an attached polar body, there is a possibility for obtaining two similar sequence patterns (Figure 5c). We recommend discarding data from oocytes that have highly dissimilar methylation patterns since cumulus cell contamination cannot be ruled out.
Figure 1. Schematic of the Single Oocyte Bisulfite Mutagenesis assay.
Figure 2. Representative results from 2nd round amplification for Snrpn from a single MII oocyte on a 1.5% agarose gel. Lanes 1-4 are four single MII oocytes and lane 5 is a negative control (no oocyte). Expected amplicon size for Snrpn is 420 bp. L, ladder.
Figure 3. Representative results from 2nd round methylation-specific restriction digestion for Snrpn from a single MII oocyte on an 8% acrylamide gel. HinfI diagnostic restriction digestion shows unmethylated DNA which harbors a T that abolishes the restriction site (420bp, lane 1) or methylated DNA which contains a C within recognition site (cut, 262, 103, and 54 bp, lane 2). Digestion showing both methylated and unmethylated restriction enzyme sites (cut & uncut bands, lane 3) are indicative of cumulus cell contamination. L, ladder.
Figure 4. Representative results for colony PCR amplification for Snrpn from a single MII oocyte on a 1.5% agarose gel. Expected amplicon size following ligation of Snrpn into the pGEM-T Easy vector and using M13 forward and reverse primers is 656 bp. Lane 1-8, amplicons from clones 1-8. Clone 5 has an incorrect amplicon size and should not be sent for sequencing.
Figure 5. Representative sequencing results for Snrpn from a single MII oocyte. Snrpn is methylated in oocytes. Black circles indicate methylated CpGs. White circles indicate unmethylated CpGs. CpG number and placement is representative for a B6 strain female mouse. a) Expected sequencing results for Snrpn from a single MII oocyte. Only a single strand of DNA should amplify in all five clones. Oocytes with a single methylation pattern and identical non-CpG conversion pattern should be included in analyses (percent conversion of non-CpGs indicated to the right was calculated as the number of non-CpG cytosines converted to thymine as a percentage of total non-CpG cytosines). b) Sequencing results for Snrpn from a single MII oocyte with cumulus cell contamination. Note the dissimilarity between methylation states and conversion patterns indicating multiple strand amplification. c) Sequencing results for Snrpn from a single MII oocyte with both chromosome copies or polar body inclusion.
This single oocyte assay contains many steps with a number that are critical and require special care. The first is oocyte washing. It is particularly important to wash each oocyte multiple times in fresh medium drops following hyaluronidase treatment to remove as many cumulus cells as possible. Moreover, when transferring oocytes to acidic tyrode's solution for zona pellucida removal make sure surrounding medium is clear of cumulus cells. The oocyte is very sticky following zona removal, and any surrounding cumulus cell...
The authors have nothing to disclose.
This work was supported by the University of Western Ontario, the Department of Obstetrics and Gynaecology; and a grant ER06-02-188 from the Ministryof Research and Innovation, Early Researcher Award. MMD was supported by a CIHR Training Program in Reproduction, Early Development and the Impact on Health (REDIH) Graduate Scholarship.
Table of specific reagents and equipment.
Name | Company | Catalog Number | Comments |
Name of the reagent | Company | Catalogue number | Comments |
Oocyte Collection | |||
Hyaluronidase | Sigma | H4272 | |
Acidic Tyrode | Sigma | T1788 | |
Proteinase K | Sigma | P5568 | |
10% IGEPAL | Bioshop | NON999.500 | |
Lysis Solution | |||
Tris pH 7.5 | Bioshop | TRS001.5 | |
LiCl | Sigma | L9650 | |
EDTA pH 8.0 | Sigma | E5134 | |
LiDS | Bioshop | LDS701.10 | |
DTT | Invitrogen | P2325 | |
SDS Lysis Buffer | |||
TE pH 7.5 |
Bioshop(Tris) Sigma (EDTA) |
TRS001.5 E5134 | |
10% SDS | Bioshop | SDS001.500 | |
Bisulfite Conversion | |||
Sodium Hydroxide | Sigma | S8045 | |
Sodium Hydrogensulfite (Sodium Bisulfite) | Sigma | 243973 | |
Hydroquinone | Sigma | H9003 | |
Low Melting Point (LMP) Agarose | Sigma | A9414 | |
Mineral Oil | Sigma | M8410 | |
M2 Medium | Sigma | M7167 | |
GIBCO Distilled water | Invitrogen | 15230-196 | |
Autoclaved double distilled (dd) water | |||
PCR | |||
Illustra Hot Start Mix RTG | GE Healthcare | 28-9006-54 | |
240 ng/ml yeast tRNA | Invitrogen | 15401-011 | |
5x Green GoTaq Reaction Buffer | Promega | M7911 | |
Inner and outer nested primers | Sigma | ||
Ligation | |||
Promega pGEM-T Easy Vector | Fisher Scientific | A1360 | |
TA Cloning | |||
Competent E.coli cells | Zymo Research Corp. | T3009 | |
Equipment | |||
Dissecting Microscope | |||
70°C and 90°C Heat Blocks | |||
37°C and 50°C Waterbaths (42°C for transformations) | |||
Rocker | |||
PCR machine |
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