Ligation-based detection of genetic variants
The present invention provides assays systems and methods for detection of genetic variants in a sample, including copy number variation and single nucleotide polymorphisms. The invention preferably employs the technique of tandem ligation—, e.g., the ligation of two or more fixed sequence oligonucleotides and one or more bridging oligonucleotides complementary to a region between the fixed sequence oligonucleotides—combined with detection of levels of particular genomic regions using array hybridization.
1. A method for detecting a fetal copy number variation (CNV) in a maternal plasma or serum sample comprising maternal and fetal cell-free DNA, comprising the steps of:
(a) introducing at least 48 first sets of first and second fixed sequence oligonucleotides to the cell-free DNA from the maternal plasma or serum sample under conditions that allow the first sets of fixed sequence oligonucleotides to specifically hybridize to non-polymorphic loci in a first genomic region of interest, wherein:
(i) at least one fixed sequence oligonucleotide in each first set comprises at first label and a binding region that binds selectively to an array feature, wherein the binding region in each first set is different from the binding regions in each other first set,
(ii) the first and second fixed sequence oligonucleotides in each first set hybridize directly adjacent to each other or are separated by a gap of one or more nucleotides, and
(iii) wherein melting temperatures (T m s) of first fixed sequence oligonucleotides of each first set of first and second fixed sequence oligonucleotides vary in a range of two degrees centigrade;
(b) introducing at least 48 second sets of first and second fixed sequence oligonucleotides to the cell-free DNA from the maternal plasma or serum sample under conditions that allow the second sets of fixed sequence oligonucleotides to specifically hybridize to non-polymorphic loci in a second genomic region of interest, wherein:
(i) at least one fixed sequence oligonucleotide in each second set comprises a second label and binding region that binds selectively to an array feature, wherein the binding region in each second set is different from the binding regions in each other second set, and wherein the binding region in each second set is the same as the binding region in one first set and binds to the same array feature,
(ii) the first and second fixed sequence oligonucleotides in each second set hybridize directly adjacent to each other or are separated by a gap of one or more nucleotides, and
(iii) wherein melting temperatures (T m s) of first fixed sequence oligonucleotides of each second set of first and second fixed sequence oligonucleotides vary in a range of two degrees centigrade;
(c) hybridizing the first and second sets of fixed sequence oligonucleotides to the cell-free DNA from the maternal plasma or serum sample;
(d) if the first and second fixed sequence oligonucleotides of each first and second set are separated by a gap of one or more nucleotides,
(i) hybridizing bridging oligonucleotides with the cell-free DNA from the maternal plasma or serum sample, wherein the bridging oligonucleotides are complementary to regions in the loci between the first and second fixed sequence oligonucleotides of each first and second set, to create adjacently hybridized oligonucleotides, or
(ii) extending the hybridized fixed sequence oligonucleotides with a polymerase to create adjacently hybridized oligonucleotides;
(e) ligating the hybridized fixed sequence oligonucleotides to create contiguous ligation products complementary to the non-polymorphic loci in the first and second genomic regions of interest;
(f) introducing the contiguous ligation products to an array comprising array features complementary to binding regions in the contiguous ligation products;
(g) detecting hybridization of the contiguous ligation products from the first and second sets of fixed sequence oligonucleotides to the array by detection of the first and second labels at each array feature;
(h) determining frequencies of the first and second genomic regions of interest based on the detected levels of the first and second labels; and
(i) detecting a fetal CNV based on the frequencies of the first and second genomic regions of interest.
2. The method of claim 1 , further comprising amplifying the contiguous ligation products prior to introduction to the array.
3. The method of claim 2 , wherein at least one of the first or second fixed sequence oligonucleotides of each first and second set comprise universal primer regions.
4. The method of claim 1 , wherein the hybridization products of the fixed sequence oligonucleotides and the non-polymorphic loci of interest to which they hybridize are isolated prior to introduction of the bridging oligonucleotides.
5. The method of claim 1 , wherein the one or more bridging oligonucleotides are introduced simultaneously with the fixed sequence oligonucleotides.
6. The method of claim 1 , wherein the first and second labels are fluorescent labels.
7. The method of claim 1 , wherein the method is carried out for at least 100 first sets of fixed sequence oligonucleotides complementary to non-polymorphic loci on the first chromosome and at least 100 second sets of fixed sequence oligonucleotides complementary non-polymorphic loci on the second chromosome.
8. The method of claim 1 , wherein the method is carried out for at least 200 first sets of fixed sequence oligonucleotides complementary to non-polymorphic loci on the first chromosome and at least 200 second sets of fixed sequence oligonucleotides complementary non-polymorphic loci on the second chromosome.
9. The method of claim 1 , wherein the method is carried out for at least 500 first sets of fixed sequence oligonucleotides complementary to non-polymorphic loci on the first chromosome and at least 500 second sets of fixed sequence oligonucleotides complementary non-polymorphic loci on the second chromosome.
10. The method of claim 1 , wherein unhybridized fixed sequence oligonucleotides are removed prior to introduction of the bridging oligonucleotides.
11. The method of claim 1 , wherein the first or second fixed sequence oligonucleotide of each first and second set comprises a sample index.
12. The method of claim 1 , wherein the first or second fixed sequence oligonucleotide of each first and second set comprises a locus index.
13. The method of claim 1 , wherein the first and second genomic regions of interest are chromosomes.
14. The method of claim 13 , wherein the chromosome is selected from the group consisting of: chromosome 13, chromosome 18, chromosome 21, the X chromosome, and the Y chromosome.
15. The method of claim 1 , wherein the copy number variation is selected from the group consisting of a deletion, a duplication, a rearrangement, and a translocation.
16. The method of claim 1 , further comprising determination of percent fetal DNA in the maternal plasma or serum sample.