Detection of cell-free fetal DNA in maternal plasma using two types of compound markers

Developmental Validation of a DIP-InDel/DIP-STR System for the Detection of Unbalanced DNA Mixtures

Extracting "masked" minor donor information from an unbalanced DNA mixture stain is important for the identification of the person of interest. In recent years, a series of compound markers genotyped based on allele-specific amplifications have been proposed to detect minor contributors of mixed stains. In this study, we selected out 18 DIP-InDel markers from previous literatures and established a multiplex system encompassing 18 DIP-InDel markers and 6 DIP-STR markers in two separate reactions. The allele frequencies were estimated based on 200 samples, and 23 of the 24 DIP-linked length polymorphic markers had a relatively high probability of informative markers (I value >0.267), which indicated their potential usefulness in the Chinese Han population. Moreover, the multiplex was highly sensitive (requiring >0.025 ng) and specific to human DNA. This system is capable of detecting the minor contributor comprising 1% in unbalanced mixtures of two individuals. The capacity of profiling cell-free DNA was also analyzed in few cases. At least one paternal allele could be detected in maternal plasma samples with gestation periods ranging from 27 to 40 weeks. To conclude, the DIP-linked length polymorphic markers may serve as a convenient method for detecting the presence of a minor donor in unbalanced mixtures and show promise for noninvasive prenatal diagnosis.

Early noninvasive prenatal paternity testing by targeted fetal DNA analysis

Today the challenge in paternity testing is to provide an accurate noninvasive assay that can be performed early during pregnancy. This requires the use of novel analytical methods capable of detecting the low fraction of circulating fetal DNA in maternal blood. We previously showed that forensic compound markers such as deletion/insertion polymorphisms-short tandem repeats (DIP-STR) can efficiently resolve complex mixed biological evidence including the target analysis of paternally inherited fetal alleles. In this study, we describe for the first time the validation of this type of markers in the first trimester of pregnancies, in addition to defining the statistical framework to evaluate paternity. To do so, we studied 47 DIP-STRs in 87 cases, with blood samples collected throughout gestation starting from the seven weeks of amenorrhea. Fetal DNA detection in the first trimester shows a false negative rate as low as 6%. The combined paternity index (CPI) results indicate that seven markers with fully informative genotypes are sufficient to determine the paternity. This study demonstrates that DIP-STR markers can be used from early pregnancy and that a small set of markers (about 40) is sufficient to address the question of paternity. The novel method offers substantial improvements over similar approaches in terms of reduced number of markers, lower costs and increased accuracy.

An MPS-Based 50plex Microhaplotype Assay for Forensic DNA Analysis

Microhaplotypes (MHs) are widely accepted as powerful markers in forensic studies. They have the advantage of both short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs), with no stutter and amplification bias, short fragments and amplicons, low mutation and recombination rates, and high polymorphisms. In this study, we constructed a panel of 50 MHs that are distributed on 21 chromosomes and analyzed them using the Multiseq multiple polymerase chain reaction (multi-PCR) targeted capture sequencing protocol based on the massively parallel sequencing (MPS) platform. The sizes of markers and amplicons ranged between 11–81 bp and 123–198 bp, respectively. The sensitivity was 0.25 ng, and the calling results were consistent with Sanger sequencing and the Integrative Genomics Viewer (IGV). It showed measurable polymorphism among sequenced 137 Southwest Chinese Han individuals. No significant deviations in the Hardy–Weinberg equilibrium (HWE) and linkage disequilibrium (LD) were found at all MHs after Bonferroni correction. Furthermore, the specificity was 1:40 for simulated two-person mixtures, and the detection rates of highly degraded single samples and mixtures were 100% and 93–100%, respectively. Moreover, animal DNA testing was incomplete and low depth. Overall, our MPS-based 50-plex MH panel is a powerful forensic tool that provides a strong supplement and enhancement for some existing panels.

Identification and characterization of novel DIP-STRs from whole-genome sequencing data

In an attempt to enhance forensic DNA mixture deconvolution several alternative DNA typing approaches have been developed. Among these, DIP-STR compound markers can resolve extremely unbalanced two-source DNA mixtures of same-or-opposite sex donors, up to a 1:1000 minor:major DNA ratio. A forensic set of 10 markers was validated for casework and a larger set of 23 DIP-STRs has proven suitable to biogeographic ancestry inference and for prenatal paternity testing. Yet, to promote the widespread use of this original approach, more markers and multiplex panels need to be developed. To this end, here we describe an extended set of forensic DIP-STRs identified using currently available whole-genome sequencing datasets. Complete lists of Indels and STRs were obtained from reported frequencies of genetic variants of 76,156 genomes. About 3000 identified DIP-STRs candidates were shorter than 200 bp and 500 showed high haplotype variability estimated using the genotypes of individuals homozygous for the DIP or the STR. Here, we present 23 additional DIP-STRs validated for sensitivity, specificity and Swiss population variability. Finally, a set of 30 markers comprising seven previously validated ones is proposed for the prospective development of a forensic DIP-STR multiplex panel.

Set of 15 SNP-SNP Markers for Detection of Unbalanced Degraded DNA Mixtures and Noninvasive Prenatal Paternity Testing

Unbalanced and degraded mixtures (UDM) are very common in forensic DNA analysis. For example, DNA signals from criminal suspects are masked by a large amount of DNA from victims, or cell-free fetal DNA (cffDNA) in maternal plasma is masked by a high background of maternal DNA. Currently, detecting minor DNA in these mixtures is complex and challenging. We developed a new set of SNP-SNP microhaplotypes with short amplicons, and we successfully genotyped them using the new method of amplification-refractory mutation system PCR (ARMS-PCR) combined with SNaPshot technology based on a capillary electrophoresis (CE) platform. This panel reflects a high polymorphism in the Southwest Chinese Han population and thus has excellent potential for mixture studies. We evaluated the feasibility of this panel for UDM detection and noninvasive prenatal paternity testing (NIPPT). Fifteen SNP-SNPs detected minor DNA of homemade DNA mixtures, with a sensitivity of 0.025-0.05 ng and a specificity of 1:1,000. In addition, the panel successfully genotyped degraded DNA from single and mixed samples. Finally, 15 SNP-SNPs were applied to 26 trios. All samples displayed positive results with at least one marker to detect cffDNA. Besides, all fetal alleles in maternal plasma were confirmed by genotyping fetal genomic DNA from amniocentesis and paternal genomic DNA from peripheral blood. The results indicated that the SNP-SNP strategy based on the CE platform was useful for UDM detection and NIPPT.