Abstract 2424: Detection of prostate cancer in urine by fragment-level DNA methylation analysis
Authors: Bedekovics Tibor , Taylor William R. , Sun Zhifu , Baheti Saurabh , Cao Xiaoming , Burger Kelli N. , Doering Karen A. , Foote Patrick H. , Gochanour Benjamin , Mahoney Douglas, W. , Gettman Matthew T. , Kisiel John B. 
Published: March 2024

Introduction: Screening for prostate cancer (PCa) with molecular markers is hindered by low tumor allele fraction (TAF) in liquid biopsy samples such as blood or urine. Others have suggested that tumor allele enrichment can be achieved by evaluating fragment-level methylation patterns generated by next generation sequencing (NGS). Here, we test this concept using target capture NGS on cfDNA from prediagnostic collected urine samples to discriminate PCa from otherwise healthy men.

Methods: Buffered urines were collected from 23 patients with biopsy confirmed PCa (Gleason score 6-9) and 24 otherwise healthy age matched cancer-free controls. cfDNA was isolated and enzymatic methyl-seq libraries were prepared and enriched by hybrid capture using probes mapping to 197 differentially methylated regions (DMRs) from a previous PCa/normal tissue marker discovery study in our lab. These DMRs were subsequently vetted independently using a methylome-wide dataset from a cohort of 36 clean and disease control urines to ensure low non-cancer background signal (&lt;1%). Indexed libraries were sequenced at 560X deduplicated coverage and fragment-level CpG methylation information was extracted from individual sample .bam files. Mapped sequencing reads exhibiting concordant methylation were used to derive a quantitative α-score. Reads had to contain at minimum 5 CpGs and have an α-value (% 5mC per read) &gt; 60%. Positivity for the 197 markers required an α-score greater than 6 sigma (σ) above the mean of the control arm. Values were variance-adjusted to ensure equal weighting. Beta (β) analysis (mean % 5mC across all reads) of the data was also performed for comparison.

Results: Of the 197 markers, 187 were positive for one or more PCa patients. Tumor-derived fragments were detected in 22 of 23 cases, which included 8 G-6, 9 G-7, 2 G-8, and 3 G-9 patients. 1 G-7 was missed, however the sequencing coverage for this sample was significantly lower than the others due to poor library enrichment. 2 of the 24 healthy controls were positive, one with extremely high fragment counts. Upon re-review of the full control cohort, this patient had been diagnosed with bladder cancer 2 years after urine collection. The 2nd patient had was diagnosed with lung cancer in a similar timeframe. The other 22 confirmed cancer-free controls had α-scores below the 6σ threshold. Using a co-captured reference gene (B3GALT6) to mimic genomic equivalents, the TAFs ranged from 0.015% to 8%. β analysis of the data detected only 13 PCa patients (&gt;6σ) and marker positivity was lower than the fragment-level approach.

Conclusion: Target capture NGS coupled with fragment-level analysis is a rational approach for sensitive and specific detection of low tumor-derived DNA in liquid biopsy samples. For PCa assay in urine, we were able to uncover positivity in cancer patients approaching 1 in 10,000 ctDNA dilution levels. These results warrant further validation studies with this and other cancers.

Citation Format: Tibor Bedekovics, William R. Taylor, Zhifu Sun, Saurabh Baheti, Xiaoming Cao, Kelli N. Burger, Karen A. Doering, Patrick H. Foote, Benjamin Gochanour, Douglas, W. Mahoney, Matthew T. Gettman, John B. Kisiel. Detection of prostate cancer in urine by fragment-level DNA methylation analysis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2424.