Cell-free DNA Size Selection for ctDNA Variant Detection
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Solution Overview
Problem
Current methods for detecting low-frequency variants in circulating tumor DNA (ctDNA) face challenges due to high sample complexity and sequencing errors, leading to difficulties in accurately identifying low-abundance ctDNA fragments amidst abundant normal cell-free DNA, which limits the effectiveness of ctDNA diagnostics in cancer detection.
Innovation Solution
The method involves size-based selection of ccfDNA fragments, isolating shorter fragments (≤160 bp) prior to sequencing, which reduces sample complexity and increases the variant allele frequency, thereby enhancing sensitivity and specificity through the use of unique molecular identifiers and subsequent PCR amplification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If size-based selection of ccfDNA fragments is performed to enrich shorter fragments, then variant allele frequency increases and detection sensitivity improves, but sample complexity increases and sequencing cost increases
Solution Approach 1:
The method segments the ccfDNA population by size, separating shorter fragments (enriched for tumor variants) from longer fragments (predominantly normal DNA). This segmentation allows focused sequencing on the most informative fraction, improving detection sensitivity while managing sample complexity through targeted analysis
Solution Approach 2:
The method extracts and isolates the shorter ccfDNA fragment fraction from the total DNA population using size-selection techniques. By taking out only the relevant shorter fragments for sequencing, the method improves variant detection while reducing the burden of analyzing the complete, more complex DNA mixture
2Reliability
If family size is increased to reduce false positives from sequencing errors, then accuracy improves, but the ability to detect low-frequency variants decreases
Solution Approach 1:
The method performs preliminary enrichment of tumor-derived short fragments before sequencing and family size expansion. By pre-concentrating the rare variant alleles in the short fraction, the method ensures that even after PCR amplification creates large family sizes, sufficient true variant signals remain detectable above the background noise
Solution Approach 2:
The method changes the size parameter of the DNA fragments under analysis, focusing specifically on shorter fragments (≤160 bp) that are enriched for tumor variants. This parameter change shifts the composition of the sequenced population, allowing larger family sizes to be formed from a pool richer in true variants, thereby maintaining detection sensitivity while improving accuracy
Data Source
AI summary
A method of increasing detection of low-abundant fragments of cell-free DNA (ccfDNA) in a biological sample is disclosed and discussed. Such a method can include isolating an initial fraction of ccfDNA fragments from a biological sample, ligating a unique molecular identifier (UMI) to each of the ccfDNA fragments in the initial fraction, amplifying the plurality of ccfDNA fragments to generate a ccfDNA library, isolating a short fraction of ccfDNA fragments from the ccfDNA library, where the ccfDNA fragments in the short fraction are limited to a size of less than or equal to 160 base pairs (bp), amplifying the ccfDNA fragments in the short fraction, and sequencing the ccfDNA fragments in the short fraction to generate sequenced ccfDNA fragments.