Methods for determining fraction of fetal nucleic acids in maternal samples
The invention provides compositions and methods for determining the fraction of fetal nucleic acids in a maternal sample comprising a mixture of fetal and maternal nucleic acids. The fraction of fetal nucleic acids can be used in determining the presence or absence of fetal aneuploidy.
1. A method for determining the fraction of fetal cell-free DNA (cfDNA) in a maternal sample comprising a mixture of fetal and maternal cfDNA, said method comprising:
(a) isolating said mixture of cfDNA from said sample;
(b) amplifying a plurality of predetermined polymorphic target nucleic acids in said mixture, wherein each of said predetermined polymorphic target nucleic acids comprises at least one single nucleotide polymorphism (SNP);
(c) preparing a sequencing library using at least a portion of the amplified product obtained in step (b);
(d) performing massively parallel sequencing of at least a portion of said library obtained in step (c) to provide sequence information for a plurality of sequence tags;
(e) using said sequence information to provide a number of sequence tags aligning to a reference sequence, wherein said reference sequence comprises allelic sequences for said at least one SNP in each of said plurality of predetermined target nucleic acids;
(f) counting the number of sequence tags aligned to said allelic sequences;
(g) identifying a plurality of informative SNPs from the number of sequence tags obtained in step (f), wherein said informative SNPs are identified by the difference in allelic sequences and the number of sequence tags aligned to each of the possible allelelic sequences for each SNP; and
(h) for each of said informative SNPs, calculating said fraction of fetal cfDNA from the number of sequence tags aligned to said possible allelic sequences for said informative SNPs.
2. The method of claim 1 , wherein amplifying said plurality of predetermined polymorphic target nucleic acids in step (b) comprises performing PCR.
3. The method of claim 1 , wherein said massively parallel sequencing is sequencing-by-synthesis with reversible dye terminators.
4. The method of claim 1 , wherein said massively parallel sequencing is sequencing-by-ligation.
5. The method of claim 1 , wherein said sequencing is single molecule sequencing.
6. The method of claim 1 , wherein said sequencing comprises an amplification.
7. The method of claim 1 , wherein said maternal sample is selected from blood, plasma, serum, urine and saliva.
8. The method of claim 1 , wherein said method is a fetal gender-independent method.
9. The method of claim 1 , wherein said plurality of polymorphic nucleic acids are located on a plurality of different chromosomes.
10. The method of claim 1 , wherein said plurality of polymorphic sites are located on a chromosome other than chromosome 13, 18, 21, X or Y.
11. The method of claim 10 , wherein said plurality of different chromosomes are selected from chromosomes 1-22.
12. The method of claim 1 , wherein said plurality of polymorphic nucleic acids comprises at least 3 informative polymorphic sites.
13. The method of claim 1 , wherein said plurality of polymorphic nucleic acids comprises at least 10 informative polymorphic sites.
14. The method of claim 1 , wherein said at least one SNP is a single SNP selected from rs560681, rs1109037, rs9866013, rs13182883, rs13218440, rs7041158, rs740598, rs10773760, rs4530059, rs7205345, rs8078417, rs576261, rs2567608, rs430046, rs9951171, rs338882, rs10776839, rs9905977, rs1277284, rs258684, rs1347696, rs508485, rs9788670, rs8137254, rs3143, rs2182957, rs3739005, and rs530022.
15. The method of claim 1 , further comprising using the calculated fraction of fetal cfDNA obtained in step (h) to set thresholds for calling an aneuploidy, a normal or a no call state.
16. The method of claim 1 , further comprising using the calculated fraction of fetal cfDNA obtained in step (h) to estimate the probability of correctly identifying an aneuploidy.