Selective enrichment of CpG islands
View Patent ↗The present invention provides compositions and methods for selectively enriching genomic CpG island (CGI)- and other epigenetically informative CG-rich polynucleotide targets. The method involves co-incubation of denatured or partially denatured polynucleotide fragments containing the CGI- or CG-targeted region(s) of interest with an oligonucleotide capture pool collectively designed to selectively target CGIs. The oligonucleotide capture pool includes a plurality of different oligonucleotides, each oligonucleotide coupled to a capture tag, whereby the oligonucleotide includes a CpG target sequence restricted to 4 to 10 bases. Upon binding, capture oligonucleotides bound to the target fragments are enriched by separating the bound fragments from the unbound fragments. The enriched fragments may be subjected to further analyses, including bisulfite sequencing to generate a methylation profile at the single nucleotide level.
1. A method for selective enrichment of CpG island polynucleotide fragments from genomic DNA, the method comprising:
(a) providing genomic DNA;
(b) providing an oligonucleotide capture pool comprising a plurality of different capture oligonucleotides, each capture oligonucleotide in said plurality comprising a capture tag, and a CpG target sequence of between 4 and 10 bases in length, each CpG target sequence capable of binding to one of its complementary sequences in the genomic DNA, wherein the capture tag is not a nucleic acid;
(c) denaturing the genomic DNA to form plurality of denatured polynucleotide fragments;
(d) incubating the oligonucleotide capture pool with denatured polynucleotide fragments under conditions sufficient to promote targeted binding of the capture oligonucleotides to CG-rich regions of the polynucleotide fragments, wherein at least 60% of the collective bases in the capture oligonucleotides in the capture pool incubated with the denatured polynucleotide targets are cytosine or guanine; and
(e) separating and recovering CpG-rich polynucleotide fragments bound to the capture oligonucleotides from unbound polynucleotide fragments to enrich for target CpG island polynucleotide fragments, wherein CpG island polynucleotide fragments map to and contain at least part of a CpG island consisting of a continuous region of genomic DNA that satisfies the criteria of (1) having a frequency of CpG dinucleotides corresponding to an observed/expected ratio greater than 0.6, (2) having a GC content greater than 0.5, and (3) having a length of at least 0.2 kb, where the repeat regions matching these criteria are excluded or masked.
2. The method of claim 1 , wherein the CpG target sequence of each capture oligonucleotide in the capture pool is between 5 to 8 bases in length.
3. The method of claim 1 , wherein the collective percentage of cytosine and guanine bases in the capture oligonucleotide in the capture pool is selected from the group consisting of 70%, 80%, 90%, 95%, and 100%.
4. The method of claim 1 , wherein the collective percentage of cytosine and guanine bases in the capture pool is selected from the group consisting of 70%, 80%, 90%, 95%, and 100%.
5. The method of claim 1 , wherein the number (P) of different capture oligonucleotides in the capture pool is defined by the formula P= 2 L , wherein L is the number of bases of the CpG target sequence, wherein L=4, 5, 6, 7, 8, 9, or 10.
6. The method of claim 1 , wherein each capture oligonucleotide in the capture pool is cytosine or guanine, wherein the 3′-terminal dinucleotide in each capture oligonucleotide is CG, and wherein the CpG target sequence of each capture oligonucleotide is between 5 to 8 bases in length.
7. The method of claim 1 , wherein each capture oligonucleotide in the capture pool comprises a consecutive run of cytosine or guanine residues spanning the middle of the capture oligonucleotide sequence.
8. The method of claim 1 , wherein the capture oligonucleotides in the capture pool are designed in accordance with a progressive scanning analysis of CpG island DNA sequences in a given species genome, wherein the target sequences reflect their composition in the CpG island DNA sequences.
9. The method of claim 8 , wherein each capture oligonucleotide in the capture pool comprises at least one CG dinucleotide, wherein the cytosine nucleotide in the CG dinucleotide is positioned in the middle of the capture oligonucleotide or in a position immediately 5′ to the middle of the capture oligonucleotide.
10. The method of claim 8 , wherein each capture oligonucleotide in the capture pool contains a CG dinucleotide at the 3′-end.
11. The method of claim 8 , wherein each capture oligonucleotide in the capture pool comprises a cytosine or guanine residue, and wherein the scanning analysis selects GC-restricted oligonucleotide target sequences for the capture pool that represent the top 50%, the top 30%, or the top 10% of GC-restricted oligonucleotides L bases in length that are present in the CpG islands polynucleotide fragments.
12. The method of claim 1 , wherein each capture oligonucleotide in the capture pool includes at least one nucleotide comprising a member of the group consisting of DNA, RNA, LNA, and PNA.
13. The method of claim 1 , wherein the capture tag comprises a nanoparticle, biotin, or combination thereof.
14. The method of claim 1 , wherein each capture oligonucleotides in the capture pool comprises a spacer.
15. The method of claim 1 , wherein bound capture oligonucleotides are subjected to primer extension.
16. The method of claim 1 , wherein the capture oligonucleotides are suspended and freely diffusible in solution during hybridization to polynucleotide fragments.
17. The method of claim 1 , wherein the capture oligonucleotides in the capture pool are bound to a matrix in a column and wherein polynucleotide fragments bound to the oligonucleotides are eluted from the column using an aqueous medium.
18. The method of claim 1 , further comprising a renaturation step following after step (c) to selectively promote enhanced re-association of repetitive elements or comprising hybridization of capture oligonucleotides without capture tags to repetitive elements after step (c) so as to reduce recovery of polynucleotide fragments containing repetitive elements.
19. The method of claim 1 , wherein in the recovered CpG-rich polynucleotide fragments the nucleotide content of targeted CpG island polynucleotides or CG-rich polynucleotide fragments outnumbers the nucleotide content of non-targeted polynucleotide fragments by an enrichment factor of greater than 2:1.
20. The method of claim 1 , wherein the capture tags are bound to an immobilization substrate prior to recovery of the CpG island polynucleotide fragments.
21. The method of claim 20 , where the recovered CpG island polynucleotide fragments are incubated with one or more methylation-specific binding proteins or with one or more antibodies against 5-methyl cytidine to separate methylated polynucleotide targets from unmethylated polynucleotide targets.
22. The method of claim 20 , further comprising determining the methylation status in the target CpG island polynucleotide fragments.
23. The method of claim 22 , where the methylation status of the recovered target CpG island polynucleotide fragments is determined by DNA sequencing.
24. The method of claim 23 , wherein the recovered target CpG island polynucleotide fragments are subjected to bisulfite conversion and the methylation status of the bisulfate-converted polynucleotide fragments is determined by DNA sequencing.
25. The method of claim 22 , wherein the recovered CpG island polynucleotide fragments are subjected to bisulfite conversion and the methylation status of the bisulfate-converted polynucleotide fragments is determined by hybridization to a microarray.