Mitochondrial DNA quality control

1. A method of identifying one or more contaminated biological samples, the method comprising:

performing a nucleic acid sequencing assay on each biological sample of a plurality of biological samples obtained from a single individual to obtain mitochondrial DNA (mtDNA) sequencing reads for each biological sample;

identifying a heteroplasmy and a homoplasmy in the mtDNA sequencing reads from the previous step for each of the biological samples;

assigning a primary mtDNA haplogroup to each biological sample;

determining the total heteroplasmy number for each biological sample;

assigning a biological sample having a high heteroplasmy number to a secondary mtDNA haplogroup;

detecting biological sample having an assigned secondary mtDNA haplogroup that is different than the primary mtDNA haplogroup assigned to a majority of the biological samples from the same individual;

determining that the biological sample having an assigned secondary mtDNA haplogroup that is different than the primary mtDNA haplogroup assigned to a majority of the biological samples from the same individual is a contaminated biological sample; and

discarding the one or more contaminated biological samples.

2. The method according to claim 1 , wherein identifying the heteroplasmy comprises determining the sequencing coverage, a minor allele frequency, and presence of the minor allele, wherein a heteroplasmy is present when:

i) the sequencing coverage is ≥50;

ii) the minor allele frequency is ≥1%; and

iii) for DNA data, the minor allele is observed at least twice from each strand, or for RNA data, the minor allele is observed at least three times.

3. The method according to claim 1 , wherein identifying the homoplasmy comprises determining the sequencing coverage and the presence of one or more alleles, wherein a homoplasmy is present when:

i) the sequencing coverage is ≥10; and

ii) only one allele is observed at a particular nucleic acid site and it is different than the corresponding reference allele, or multiple alleles are observed at a particular nucleic acid site and the major allele is different than the corresponding reference allele, and the particular nucleic acid site fails heteroplasmy criteria.

4. The method according to claim 1 , wherein assigning the primary mtDNA haplogroup to each biological sample comprises constructing a mtDNA sequence for each biological sample using the homoplasmy and major alleles of the heteroplasmy.

5. The method according to claim 1 , wherein assigning the secondary mtDNA haplogroup comprises constructing a secondary mtDNA sequence using the homoplasmy and minor alleles of the heteroplasmy, and identifying a biological sample having an assigned secondary mtDNA haplogroup that is different than the assigned primary mtDNA haplogroup as an unreliable biological sample that is contaminated.

6. The method according to claim 5 , the method further comprising determining the level of contamination of a biological sample by determining the median of the heteroplasmy frequencies of all heteroplasmies in the contaminated biological sample, wherein the greater the median of the heteroplasmy frequency, the greater the level of contamination.

7. The method according to claim 1 , the method further comprising, prior to identifying a heteroplasmy and a homoplasmy, processing the mtDNA sequencing reads obtained from the nucleic acid sequencing assay for quality control and adaptor sequence removal to produce quality controlled mtDNA sequencing reads.

8. The method according to claim 7 , the method further comprising:

mapping the quality controlled mtDNA sequencing reads to a mitochondrial reference genome to produce candidate mtDNA sequencing reads; and

mapping the candidate mtDNA sequencing reads to a human reference genome, and

retaining the candidate mtDNA sequencing reads when:

i) the candidate mtDNA sequencing read is uniquely mapped to the mitochondrial reference genome or has fewer mismatches to the mitochondrial reference genome than to the human reference genome; and

ii) the alignment mismatch count of the candidate mtDNA sequencing read is less than 5.

9. The method according to claim 8 , the method further comprising performing post-mapping processing of the retained candidate mtDNA sequencing read for sorting and duplicate removal.

10. The method according to claim 1 , the method further comprising down-sampling the mtDNA sequencing reads obtained from the nucleic acid sequencing assay to a desired depth prior to identifying the heteroplasmy and the homoplasmy and/or prior to processing the mtDNA sequencing reads for quality control and adaptor sequence removal.

11. The method according to claim 1 , the method further comprising obtaining the plurality of biological samples from the individual prior to performing the nucleic acid sequencing assay on the plurality of biological samples.

12. The method according to claim 1 , wherein the biological samples are blood or tissue.

13. The method according to claim 1 , the method further comprising amplifying nucleic acid molecules in the biological samples prior to performing the nucleic acid sequencing assay on the plurality of biological samples.

14. The method according to claim 1 , wherein the sequencing assay comprises next generation sequencing (NGS).

15. The method according to claim 14 , wherein the NGS comprises whole genome sequencing.

16. The method according to claim 14 , wherein the NGS comprises whole exome sequencing.

17. The method according to claim 14 , wherein the NGS comprises RNA sequencing.

18. The method according to claim 14 , wherein the NGS comprises bisulfite sequencing.

19. A method of identifying one or more contaminated biological samples, the method comprising:

performing a nucleic acid sequencing assay on each biological sample of a plurality of biological samples obtained from a single individual to obtain mitochondrial DNA (mtDNA) raw sequencing reads for each biological sample;

processing the mtDNA raw sequencing reads for quality control and adaptor sequence removal to produce quality controlled mtDNA sequencing reads;

mapping the quality controlled mtDNA sequencing reads to a mitochondrial reference genome to produce candidate mtDNA sequencing reads;

mapping the candidate mtDNA sequencing reads to a human reference genome and

retaining the candidate mtDNA sequencing reads when:

i) the candidate mtDNA sequencing read is uniquely mapped to the mitochondrial reference genome or has fewer mismatches to the mitochondrial reference genome than to the human reference genome; and

ii) the alignment mismatch count of the candidate mtDNA sequencing read is less than 5;

performing post-mapping processing of the retained candidate mtDNA sequencing reads for sorting and duplicate removal;

identifying heteroplasmy and homoplasmy in the retained candidate mtDNA sequencing reads for each of the biological samples;

assigning a primary mtDNA haplogroup to each biological sample;

determining the total heteroplasmy number for each biological sample;

assigning a biological sample having a high heteroplasmy number to a secondary mtDNA haplogroup;

detecting a biological sample having an assigned secondary mtDNA haplogroup that is different than the primary mtDNA haplogroup assigned to a majority of the biological samples from the same individual;

determining that the biological sample having an assigned secondary mtDNA haplogroup that is different than the assigned primary mtDNA haplogroup is a contaminated biological sample; and

discarding the one or more contaminated biological samples.