Methods and systems for dynamic variant thresholding in a liquid biopsy assay

Patent metadata

Field	Value
Publication number	US-11475981-B2
Application number	US-202117179086-A
Country	US
Kind code	B2
Filing date	Feb 18, 2021
Priority date	Feb 18, 2020
Publication date	Oct 18, 2022
Grant date	Oct 18, 2022

Abstract

Methods, systems, and software are provided for validating a somatic sequence variant in a subject having a cancer condition. Sequence reads are obtained from sequencing cell-free DNA fragments in a liquid biopsy sample of the subject. Sequence reads are aligned to a reference sequence. A variant allele fragment count and locus fragment count are identified for a candidate variant that maps to a locus in the reference sequence. The variant allele fragment count is compared against a dynamic variant count threshold for the locus. The threshold is based on a pre-test odds of a positive variant call for the locus, based on the prevalence of variants in a genomic region including the locus in a cohort of subjects having the cancer condition. The somatic sequence variant in the subject is validated, or rejected, when the variant allele fragment count for the candidate variant satisfies, or does not satisfy, the threshold.

First claim

What is claimed is:

1. A method of validating a somatic sequence variant in a cancerous tissue of a test subject having a cancer condition, the method comprising:
at a computer system having one or more processors, and memory storing one or more programs for execution by the one or more processors:
(A) obtaining, from a first sequencing reaction, a corresponding sequence of each cell-free DNA fragment in a plurality of cell-free DNA fragments in a liquid biopsy sample of the test subject, thereby obtaining a plurality of sequence reads, wherein:
the plurality of sequence reads comprises at least 100,000 sequence reads,
the plurality of sequence reads represents at least 100 genomic loci, and
the average length of respective sequence reads in the plurality of sequence reads is at least 50 nucleotides;

(B) aligning each respective sequence read in the plurality of sequence reads to a reference construct, wherein the reference construct represents at least 1 Mb of the genome for the species of the subject, thereby identifying a candidate somatic sequence variant mapping to a respective locus in the reference construct;
(C) determining for the candidate somatic sequence variant, (i) a respective variant allele fragment count for the first sequencing reaction, and (ii) a respective locus fragment count for the first sequencing reaction; and
(D) comparing the respective variant allele fragment count for the candidate somatic sequence variant against a threshold for the respective locus in the reference construct, wherein the threshold is determined by comparing (i) a pre-test odds of a positive variant call for the respective locus based upon a prevalence of a plurality of variants in a genomic region that includes the respective locus in a cohort of training subjects having the cancer condition and (ii) a desired post-test odds of a positive variant call for the respective locus, accounting for an observed variant fraction for the candidate somatic sequence variant in the liquid biopsy sample, and:
when the respective variant allele fragment count for the candidate somatic sequence variant satisfies the threshold for the respective locus, not rejecting the presence of the candidate somatic sequence variant in the test subject, or
when the variant allele fragment count for the candidate somatic sequence variant does not satisfy the threshold for the respective locus, rejecting the presence of the candidate somatic sequence variant in the test subject.

2. The method of claim 1 , wherein the comparing further uses a sequencing error rate of the sequencing reaction.

3. The method of claim 2 , wherein the sequencing error rate for the sequencing reaction is a trinucleotide sequencing error rate.

4. The method of claim 1 , wherein the threshold is also based upon comparing further uses a background sequencing error rate determined for the locus.

5. The method of claim 1 , wherein the comparing applies a variant detection sensitivity to the pre-test odds and desired post-test odds, wherein the variant detection sensitivity is selected from a distribution of variant detection sensitivities, wherein the distribution of variant detection sensitivities is based on a correlation between (i) a detection rate of a reference variant allele, in one or more sequencing reactions that are process-matched with the first sequencing reaction, for a plurality of cancer samples, and (ii) a plurality of variant allele fractions for the reference variant allele in the plurality of cancer samples.

6. The method of claim 5 , wherein the correlation is established by determining, for each respective bin in a plurality of bins collectively representing a span of variant allele fractions represented in the plurality of cancer samples, wherein each respective bin corresponds to a respective contiguous span of variant allele fractions that does not overlap with any other respective bin in the plurality of bins, a corresponding sensitivity for detection of the reference variant alleles for a corresponding subset of cancer samples in the plurality of cancer sample having a variant allele fraction in the corresponding contiguous span of variant allele fractions of the respective bin.

7. The method of claim 5 , wherein the applying the variant detection sensitivity to the pre-test odds and desired post-test odds is used to select a quantile of a beta-binomial distribution of the minimal variant allele fragment count required to support a positive variant call for the respective locus, thereby defining the threshold for the respective locus, wherein the beta-binomial distribution is defined by a sequencing error rate for the sequencing reaction and a background sequencing error rate determined for the locus.

8. The method of claim 1 , wherein the pre-test odds of the positive variant call for the respective locus is based on the prevalence of the plurality of variants in the genomic region that includes the respective locus from a set of nucleic acids obtained from the cohort of training subjects having the cancer condition.

9. The method of claim 8 , wherein, when the genomic region is associated with a mutation known to confer resistance against a therapy used to treat the cancer condition, the pre-test odds are boosted based on a pre-test-odds multiplier specific for the genomic region.

10. The method of claim 8 , wherein the pre-test odds of a positive variant call for the respective locus is further based on a known or inferred effect of the plurality of variants, wherein:
when the known or inferred effect of a variant in the plurality of variants is loss-of-function of a gene that includes the locus, the genomic region used to compute the pre-test odds is the entire gene, and
when the known or inferred effect of a variant in the plurality of variants is gain-of-function of the gene that includes the locus, the genomic region used to compute the pre-test odds is the exon, of the gene, that includes the locus.

11. The method of claim 10 , wherein the effect of the plurality of variants is inferred by:
binning each respective variant of the plurality of variants in the genomic region that includes the locus from the set of nucleic acids obtained from the cohort of training subjects having the cancer condition into a respective bin, in a plurality of bins for the gene that include the locus, corresponding to the exon encompassing the respective variant in the gene, wherein each bin in the plurality of bins corresponds to a different exon of the respective gene; and
determining whether any bin in the plurality of bins contains significantly more variants than the other bins in the plurality of bins, wherein:
when a bin contains significantly more variants than the other bins in the plurality of bins, the effect of the sequence variant is inferred to be a gain-of-function of the gene, and
when no bin in the plurality of bins contains significantly more sequence variants than the other bins in the plurality of bins, the effect of the sequence variant is inferred to be a loss-of-function of the gene.

12. The method of claim 11 , wherein determining whether any bin in the plurality of bins contains significantly more variants than the other bins in the plurality of bins comprises applying a rolling Poisson test of difference between bin counts corresponding to adjacent exons in the gene.

13. The method of claim 1 , wherein the liquid biopsy sample is blood.

14. The method of claim 1 , wherein the liquid biopsy sample comprises blood, whole blood, peripheral blood, plasma, serum, or lymph of the test subject.

15. The method of claim 1 , wherein:
the first

Assignees

• Tempus Labs Inc

Inventors

• Tell Robert
• Zhu Wei
• Finkle Justin David
• Lo Christine
• Driessen Terri M

Classifications

• G16B30/10 Primary

  Sequence alignment; Homology search · CPC title

• G16B20/20

  Allele or variant detection, e.g. single nucleotide polymorphism [SNP] detection · CPC title

• G16B20/40

  Population genetics; Linkage disequilibrium · CPC title