Methods for non-invasive prenatal ploidy calling
The present disclosure provides methods for determining the ploidy status of a chromosome in a gestating fetus from genotypic data measured from a sample of DNA from the mother of the fetus and from the fetus, and from genotypic data from the mother and optionally also from the father. The ploidy state is determined by using a joint distribution model to create a set of expected allele distributions for different possible fetal ploidy states given the parental genotypic data, and comparing the expected allelic distributions to the pattern of measured allelic distributions measured in the mixed sample, and choosing the ploidy state whose expected allelic distribution pattern most closely matches the observed allelic distribution pattern. In an embodiment, the mixed sample of DNA may be preferentially enriched at a plurality of polymorphic loci in a way that minimizes the allelic bias.
1. A method for preparing a preparation of amplified DNA derived from a cell-free sample comprising DNA of maternal origin and DNA of fetal origin useful for determining the ploidy state of a chromosome or chromosome region in a fetus, comprising:
(a) providing a polynucleotide composition comprising polymorphic loci derived from a cell-free sample comprising DNA of maternal origin and DNA of fetal origin, wherein the polymorphic loci comprise single nucleotide polymorphisms (SNPs),
(b) preparing a preparation of amplified DNA by: performing targeted multiplex nucleic acid amplification to enrich 1,000 to 10,000 of said polymorphic loci in one reaction mixture, wherein the targeted multiplex nucleic acid amplification comprises ligation of the polynucleotides with an adaptor comprising a universal sequence followed by a first PCR amplification using a universal primer and a second PCR amplification using 1,000 to 10,000 target-specific primers and a universal primer in a single reaction mixture, wherein the targeted multiplex nucleic acid amplification does not use molecular inversion probes (MIPs), and
(c) analyzing the amplified DNA by: (i) performing next-generation sequencing to obtain (a) quantitative measurements of at least two alleles present in each enriched polymorphic locus, and (b) an estimate of the amount of DNA of fetal origin from the number of sequenced polymorphic loci of fetal origin and the number of sequenced polymorphic loci of maternal origin, and
(ii) determining the copy number of the chromosome or chromosome region using the quantitative measurements of at least two alleles present in each enriched polymorphic locus and the estimate of the amount of DNA of fetal origin, wherein the copy number of the chromosome or chromosome region indicates the ploidy state of that chromosome or chromosome region.
2. The method of claim 1 , further comprising determining an allelic bias, wherein the allelic bias is the average difference between an allelic ratio at the 1,000 to 10,000 polymorphic loci in the cell-free sample comprising DNA of maternal origin and DNA of fetal origin after enrichment and the corresponding allelic ratio at the 1,000 to 10,000 polymorphic loci in the polynucleotide composition before enrichment, wherein the allelic ratio is the measured quantity of the second most frequent allele present at that locus divided by the total measured quantity of all the alleles for the locus, and using the allelic bias to statistically correct the quantitative measurements of at least two alleles present in each enriched polymorphic locus and the estimate of the amount of DNA of fetal origin.
3. The method of claim 2 , wherein the average difference between the allelic ratio at the 1,000 to 10,000 polymorphic loci in the cell-free sample comprising DNA of maternal origin and DNA of fetal origin after enrichment and the corresponding allelic ratio at the 1,000 to 10,000 polymorphic loci in the polynucleotide composition before enrichment is no more than a factor of 2.
4. The method of claim 1 , wherein the DNA of fetal origin comprises a trisomy.
5. The method of claim 1 , further comprising the step of adding sequencing-compatible adapters to the enriched loci.
6. A method for preparing a preparation of amplified DNA derived from a cell-free sample comprising DNA of maternal origin and DNA of fetal origin useful for determining the ploidy state of a chromosome or chromosome region in a fetus, comprising:
(a) providing a polynucleotide composition comprising polymorphic loci derived from a cell-free sample comprising DNA of maternal origin and DNA of fetal origin, wherein the polymorphic loci comprise single nucleotide polymorphisms (SNPs),
(b) preparing a preparation of amplified DNA by: performing targeted multiplex nucleic acid amplification to enrich 1,000 to 10,000 of said polymorphic loci in one reaction mixture, wherein the targeted multiplex nucleic acid amplification comprises ligation of the polynucleotides with an adaptor comprising a universal sequence followed by a first PCR amplification using a universal primer and a second PCR amplification using 1,000 to 10,000 target-specific primers and a universal primer in a single reaction mixture, wherein the targeted multiplex nucleic acid amplification does not use molecular inversion probes (MIPs), and
(c) analyzing the amplified DNA by: (i) performing next-generation sequencing to obtain (a) quantitative measurements of at least two alleles present in each enriched polymorphic locus, and (b) an estimate of the amount of DNA of fetal origin from the number of sequenced polymorphic loci of fetal origin and the number of sequenced polymorphic loci of maternal origin,
(ii) determining the copy number of the chromosome or chromosome region using the quantitative measurements of at least two alleles present in each enriched polymorphic locus, and
(iii) determining the confidence of the copy number determination using the estimate of the amount of DNA of fetal origin, wherein the copy number of the chromosome or chromosome region and the confidence of the copy number determination indicate the ploidy state of that chromosome or chromosome region.
7. The method of claim 1 , wherein the method comprises determining the copy number of one or more of chromosome 13, chromosome 18, chromosome 21, chromosome X, and chromosome Y.
8. The method of claim 6 , wherein the method comprises determining the copy number of one or more of chromosome 13, chromosome 18, chromosome 21, chromosome X, and chromosome Y.