Abstract 2846: Ultrasensitive detection of actionable subclones in chronic myeloid leukemia

Abstract

Abstract Targeted Abl inhibitors such as imatinib have vastly improved survival in chronic myeloid leukemia (CML). However resistance occurs in many patients, most frequently due to point mutations in the Abl kinase. At the time of diagnosis, resistance mutations may already exist in a small fraction of the malignant cells. Second-generation inhibitors have been developed which can overcome specific resistance mutations. Identification of pre-existing resistance mutations would thus have important therapeutic implications, as it could inform which therapy would result in the most durable response in a given patient. Prior searches for pre-existing Abl mutations in CML have used methods such as allele-specific PCR or next-generation DNA sequencing; these methods are prone to background errors resulting in frequent false positive mutations. For example, next-generation sequencing has an error rate of 0.1%-1%, which results in a large number of erroneous mutations that obscure true variants. Enrichment of the Abl gene is also a major limitation. Nearly all prior reports have used PCR amplification for dozens of cycles to enrich the Abl gene prior to mutation detection; this process introduces thousands of PCR errors which make differentiation of true mutations from artifactual mutations challenging. We have developed a protocol for DNA sequencing, Duplex Sequencing, that separately tags and sequences each of the two strands of single DNA molecules. True mutations are present at the same position in both strands and are complementary, while PCR or sequencing errors occur in only one of the strands and are not scored. Duplex Sequencing has a calculated error rate of less than one artifactual mutation per billion nucleotides sequenced. To apply Duplex Sequencing to the Abl oncogene, we established a new, extremely efficient targeted capture approach that utilizes sequential rounds of capture with biotinylated DNA probes. This protocol results in >1,000,000 fold enrichment, with up to 99.8% of reads corresponding to the Abl gene. Duplex Sequencing was carried out at high depth (>100,000 fold), resulting in an average of 2,000 duplex reads per nucleotide position. The estimated accuracy for mutation identification was 99.9999%. We are in the process of sequencing samples from CML at the time of diagnosis and relapse to assay for drug resistance mutations below the limit of detection of conventional assays. Our approach could be extended to any targeted molecular agent for which mutation of the target confers resistance to the drug. Citation Format: Michael W. Schmitt, Bella Aminov, Jerald P. Radich, Lawrence A. Loeb. Ultrasensitive detection of actionable subclones in chronic myeloid leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2846. doi:10.1158/1538-7445.AM2015-2846