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Analysis of methylation using bisulphite sequencing

AnalysisofmethylationusingbisulphitesequencingContributedbyDr.A.Gratchev

Thismethodallowspreciseanalysisofmethylationinacertainregionbyconvertingallnonmethylatedcytosinesintotymines,whilemethylatedcytosinesremainunchanged.ThismethodrequiressmallamountofgenomicDNAandthereforeseemstobeveryusefulfortheanalysisofclinicalsamples,wherethematerialamountislimited.HoweverIsuggestoptimisingthemethodusinggenomicDNAfromacelllineandthenapplyittovaluablesamples.

BeforestartingtheexperimentyouhavetodevelopprimersforbisulphiteconvertedDNA.YoucangenerateamodelofbisulphitetreatedDNAbysubstitutingallcytosineswhicharenotinCGcontextintotymines.AndthendesignyourprimersinthewaythattheydonnotcontainanyCG.IfthisisimpossIBLe,youhavetouseC/TattheplaceofCinCGcontext.Usuallyprimerselectionisthemostcriticalinbisulphitebasedmethylationanalysis,sincethecomplexityofDNAisreduced.ThereforeIwouldsuggesttoselect2-3pairsofprimers,checkthemonbisulphitemodifiedDNA,andusethemostspecificones.

Protocol

  1. IsolategenomicDNAwiththequalitysufficientforrestrictionenzymedigestion.
  2. DigestDNA(50to200ng)withanyenzymewhichdoesnotcuttheregionofinterest,butresultinginasshortfragmentsaspossibleinsmallestpossiblevolume.Note:increasingtheamountofDNAwillmakedenaturingnotefficientenoughandthereforemakebisulphitereactionincomplete!
  3. StopthereactionbyboilingDNAfor5min.
  4. Add10NNaOHto0.3NfinalconcentrationanddenatureDNA37°Cfor15min.
  5. Prepare4-5Eppendorftubeswithcoldmineraloil.
  6. Add2xvolumeof2%lowmeltingagarosetotheDNAsolution,mixbypipettingupanddown.
  7. Formagarosebeadsbypipetting10µlaliquotsofDNA/agarosemixtureintocoldmineraloil.Note:Don"tpipettthesecondaliquotinthetubewhereyoualreadyhaveonebead!
  8. Transferbeadsinthetubecontaining1mlofmodifyingsolution(5Msodiumbisulphite(2.5MsodiummetABIsulphite),100mMHydroquinon).
  9. Incubatethetubes4hat50°Cinthedark.
  10. Washthebeads6timesfor15mininTEpH8.0.
  11. Completethemodificationbyincubatingthebeads2timesfor15minin0.2NNaOH.
  12. Washthebeads3timesfor15minindoubledistilledH2O.
  13. UseoneuloftheobtainedDNAforPCRwithselectedprimers.

ObtainedPCRproductcanbesequenceddirectly,inthiscaseyouwillobtainmoreorlessreliableresultsaboutthepercentageofmethylatedcytosineineveryposition.Anotherpossibilityistoclonetheproductandthensequence10ormoreindividualclones.Thismethodismuchmoretimeconsuming.ThirdmethodwhichcanbeusedfortheanalysisofbisulphitetreatedDNAisSingleNucleotidePrimerExtension(SNuPE).

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