Compositions and methods for analyzing modified nucleotides
A method for identifying any of the presence, location and phasing of modified cytosines (C) in long stretches of nucleic acids is provided. In some embodiments, the method may comprise (a) reacting a first portion of a nucleic acid sample containing at least one C and/or at least one modified C with a DNA glucosyltransferase and a cytidine deaminase to produce a first product and/or reacting a second portion of the sample with a dioxygenase, optionally a DNA glucosyltransferase and a cytidine deaminase to produce a second product and; (b) comparing the sequences from the first and optionally the second product obtained in (a), or amplification products thereof, with each other and/or an untreated reference sequence to determine which Cs in the initial nucleic acid fragment are modified. A modified TET methylcytosine dioxygenase with improved efficiency compared to unmodified TET2 at converting methylcytosine to carboxymethylcytosine is also provided.
1. A method for determining the location of modified cytosines in a nucleic acid fragment, comprising:
(a) reacting a nucleic acid sample containing at least one Cytosine (C) and/or at least one modified C with a methylcytosine dioxygenase and a DNA glucosyltransferase in a single buffer either together or sequentially, wherein the methylcytosine dioxygenase comprises an amino acid sequence that is at least 90% identical to SEQ ID NO:1 and comprises the amino acid sequence of SEQ ID NO:2;
(b) reacting the product of (a) with a cytidine deaminase;
(c) sequencing the product of (b) or an amplification product thereof; and
(d) comparing the sequences obtained in (c), with an reference sequence to determine which Cs in the nucleic acid fragment are modified.
2. The method according to claim 1 , further comprising: treating an aliquot of the nucleic acid sample with glucosyltransferase in the absence of a dioxygenase and subsequently with cytidine deaminase to produce a reaction product in which substantially all the 5-hydroxymethylcytosines ( 5hm Cs) in the aliquot are glucosylated, substantially all the unmodified Cs are converted to uracil (U) and substantially all the methylcytosines ( 5m Cs) are converted to thymine (Ts) and sequencing the aliquot.
3. The method according to claim 2 , further comprising sequencing the reaction product or an amplification product thereof and determining which cytosines are hydroxymethylated.
4. The method according to claim 1 , wherein the nucleic acid sample comprises a library of DNA fragments wherein the DNA fragments are ligated to nucleic acid adaptors in which cytosine in the adaptors is replaced by pyrollo-dC.
5. The method according to claim 1 , wherein the sequencing of (c) is done by a single molecule sequencing method.
6. The method according to claim 1 , wherein the nucleic acids in the sample are greater than 2 kb in length.
7. The method according to claim 1 , wherein the sample comprises genomic DNA.
8. The method according to claim 1 , wherein the nucleic acid sample contains a nucleic acid fragment linked to a transcribed gene.
9. The method according to claim 1 , further comprising obtaining the reference sequence by sequencing a third aliquot of the nucleic acid sample not reacted with the dioxygenase, glucosyltransferase or deaminase, or obtaining a reference sequence from a database of sequences.
10. The method according to claim 9 , further comprising: comparing the sequences obtained in (c) with the reference sequence and optionally another reaction product to determine which cytosines (Cs) in the nucleic acid sample are unmodified and optionally which of the modified cytosine are methylated or hydroxymethylated.
11. The method according to claim 1 , wherein the nucleic acid sample contains fragments that are linked, in cis, to a gene in a transcriptionally active or inactive state whereas the reference sequence is the same fragment that is linked, in cis, to the same gene in an opposite transcriptionally active or inactive state; and determining an altered pattern of cytosine modification associated with transcriptional activity.
12. The method according to claim 11 , wherein the altered pattern of cytosine modification associated with transcriptional activity of the gene in an ex vivo cell or tissue is correlated with a disease or condition.
13. The method according to claim 1 , wherein the method comprises comparing (i) the pattern of cytosine modification for a nucleic acid fragment from a mammalian subject that has a disease or condition with (ii) the pattern of cytosine modification in the same nucleic acid fragment from a mammalian subject that does not have the disease or condition.
14. The method of according to claim 1 , wherein the method comprises comparing (i) the pattern of cytosine modification for a nucleic acid fragment from a mammalian subject undergoing a treatment with (ii) the pattern of cytosine modification in the same intact nucleic acid fragment from a mammalian subject that has not been treated with the agent for detecting differences.
15. The method of according to claim 1 , wherein the methylcytosine dioxygenase and glucosyltransferase are combined together prior to addition to the reaction.
16. The method according to claim 1 , further comprises treating an aliquot of the nucleic acid sample with cytidine deaminase in the absence of dioxygenase and glucosyltransferase to produce a reaction product in which substantially all the cytosines are converted to uracil (U) and substantially all the methylcytosines ( 5m Cs) and 5-hydroxymethylcytosines ( 5hm Cs) are converted to thymine (T) and sequencing the aliquot.
17. The method according to claim 1 , further comprising: treating an aliquot of the nucleic acid sample with dioxygenase in the absence of a glucosyltransferase and subsequently with cytidine deaminase and sequencing the product.
18. The method of according to claim 1 , wherein the cytidine deaminase is APOBEC3A.