Calculating the bisulfite conversion rate

Final Cytosine Methylation Report ================================= Total number of C's analysed: 86084529 Total methylated C's in CpG context: 9308262 Total methylated C's in CHG context: 84996 Total methylated C's in CHH context: 131989 Total C to T conversions in CpG context: 5029566 Total C to T conversions in CHG context: 19274244 Total C to T conversions in CHH context: 52255472 C methylated in CpG context: 64.9% C methylated in CHG context: 0.4% C methylated in CHH context: 0.3%

3.8 years ago by

Jautis • 270

United States

Jautis • 270 wrote:

From this information, it is not possible to say and additional/untreated samples would not help. To estimate bisulfite conversion rate, you need to have DNA that you know is unmethylated added to your sample before bisulfite conversion (for example, our lab adds 1ng lambda phage DNA to ~200ng of our mammalian samples). In that sample, your bisulfite conversion rate is the proportion of Cs converted to Ts (since all Cs are unmethylated and should be converted). This can't be determined from your experimental sample because, even for CHG and CHH sites, the actual methylation status is unknown.

ADD COMMENT • link written 3.8 years ago by Jautis • 270

Moving on: Do you know if you have unmethylated DNA in the sample (and, if so, what type)? If you do, I can help you generate a bisfulfite conversion rate from that.

ADD REPLY • link written 3.8 years ago by Jautis • 270

Do you mean if I have a separate sample that did not undergo bisulfite conversion? I do not.

ADD REPLY • link written 3.8 years ago by igor ♦ 7.6k

No. I'm asking if you have DNA of known methylation in with your sample. Usually this is some bacterial or phage DNA.

The process should look like: (sample DNA + unmethylated DNA) --> bisulfite conversion --> sequencing --> mapping --> pull out reads mapping to your unmethylated DNA sample --> calculate conversion rate (converted/(converted + unconverted))

ADD REPLY • link written 3.8 years ago by Jautis • 270

Thanks for all your advice. Which Cs should be used for conversion rate? CpG, CHG, or CHH context? Or all three? Should I only be looking at the lambda phage reads to determine the rates?

ADD REPLY • link written 3.8 years ago by igor ♦ 7.6k

Only look at the lambda phage DNA to determine the rates (you don't know the methylation of your sample!). You can use all types; they should all be unmethylated regardless of the context.

ADD REPLY • link written 3.8 years ago by Jautis • 270

So let's say I add lambda phage to my sample, then do bisulfite conversion, then alignment to lambda phage genome, then get a certain CpG methylation rate.

For example, that rate is 3%. Does that mean bisulfite conversion rate is 97%? Is it that simple?

ADD REPLY • link written 3.8 years ago by igor ♦ 7.6k

Precisely! Just as a sanity check, in our lab it is usually over 99.5%

ADD REPLY • link written 3.8 years ago by Jautis • 270

Does lambda phage DNA affect mapping efficiency when the mixed reads ( phage DNA & sample DNA ) are aligned to reference genome ? Thank you.

ADD REPLY • link written 2.8 years ago by hxlei613 • 80

Hello, Jautis. Could you tell me how to extract the lambda DNA after mapping? Thanks.

ADD REPLY • link written 16 months ago by Kangli Wang • 0

Can you clarify how you are adding lamda phage? Would you have it as a separate low concentration library? When you align it, it would be to a different genome, so doesn't that also introduce another variable?

ADD REPLY • link written 3.8 years ago by igor ♦ 7.6k

We add it to the sample so you have <1% lambda phage DNA. As such, it is in the same library (same barcodes, same conversion tube, same PCR, etc.). Yes, it will align to a different genome, but you can map it separately or together. You just want to know what percentage of sites are converted (theoretically they all should be)

Mapping to another genome doesn't introduce new variables; you're not trying to measure actual methylation in the phage DNA; just determine how well the conversion worked.

ADD REPLY • link modified 3.8 years ago • written 3.8 years ago by Jautis • 270

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