cfDNA Methylation Enrichment Using CpG-Selective cfRRBS
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Solution Overview
Problem
Existing methods for DNA methylation profiling using Reduced Representation Bisulfite Sequencing (RRBS) are ineffective for analyzing cell-free DNA (cfDNA) due to its fragmented nature and lack of CpG enrichment, which is crucial for clinical diagnostic applications.
Innovation Solution
Adaptation of RRBS for cfDNA, known as cfRRBS, involves modifying cfDNA ends to prevent adapter coupling, digesting at CpG sites, and coupling adapters to fragments, ensuring each fragment contains at least one CpG site, followed by size selection and bisulfite conversion to distinguish methylated from unmethylated bases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If typical RRBS procedure is followed to select fragments between 40-220 bp, then the procedure is simple and cost-effective, but it lacks CpG enrichment for cfDNA
Solution Approach 1:
The patent applies preliminary action by modifying the ends of cfDNA fragments before size selection to prevent adapter ligation. This is achieved by treating cfDNA with phosphatase to remove 5' phosphates or using terminal transferase to add non-ligatable nucleotides to 3' ends. By performing this modification before size selection and adapter ligation, the method ensures that only fragments with intact ends (indicating they contain CpG sites) will successfully ligate adapters, thereby achieving CpG enrichment without complicating the overall workflow
Solution Approach 2:
The patent applies local quality by making specific modifications only at the ends of cfDNA fragments rather than treating the entire DNA population uniformly. By selectively modifying terminal ends to prevent adapter ligation, the method creates a local distinction between fragments that contain CpG sites (which retain ligatable ends) and those that don't (which have modified ends). This localized modification strategy enables CpG enrichment while maintaining the simplicity of the RRBS approach
2Measurement precision
If whole-genome bisulfite sequencing is performed, then comprehensive DNA methylome view is obtained, but it is expensive to deep sequence the entire genome
Solution Approach 1:
The patent applies the extraction principle by isolating and analyzing only the CpG-rich regions of the genome from cfDNA samples. Through the end-modification-based enrichment approach, the method extracts information from approximately 3% of the genome (CpG-dense regions) that provides comprehensive methylation profiling capability. This selective extraction of relevant genomic regions maintains measurement precision for cancer detection while dramatically reducing sequencing costs and data complexity compared to whole-genome approaches
3Ease of operation
If cfDNA fragments are size-selected between 40-220 bp, then the selection process is straightforward, but almost every fragment is selected similarly to the whole population without CpG enrichment
Solution Approach 1:
The patent applies preliminary action by performing end modifications on cfDNA fragments before size selection and adapter ligation. The end modifications (phosphatase treatment or terminal transferase addition) are conducted prior to the size selection step, creating a state where only fragments with specific characteristics (those containing CpG sites) will subsequently be able to ligate adapters. This preliminary modification step maintains the simplicity of size selection while enabling downstream CpG enrichment
Solution Approach 2:
The patent introduces an intermediary mechanism through the use of end modifications as a mediator between size selection and adapter ligation. The end modifications act as an intermediate step that selectively prevents adapter ligation on fragments without CpG sites, while allowing ligation on fragments with CpG sites. This intermediary approach maintains the straightforward nature of size selection while achieving CpG enrichment through the selective ligation behavior of modified versus unmodified fragment ends
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enriches CpG-rich regions in cfDNA for effective methylation profiling, providing cost-effective sequencing suitable for clinical diagnostics and non-invasive cancer screening.
Implementation Method 1
genomic DNA is first digested with a restriction endonuclease (usually MspI)
Implementation Method 2
these regions compose only ~ 3% of the genome but provides comprehensive DNA methylation information about the genome
Data Source
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AI summary
The present disclosure concerns embodiments related to methods of enriching particular DNA for analysis of methylation status and/or profiles, for example in the process of diagnosis of cancer. In particular embodiments, the methods utilize cell-free DNA as a source of DNA instead of genomic DNA and allow for focused enrichment of fragments having two or more enzyme digestion sites and containing at least one CpG site.