Proof-of-concept on Circulating Tumor DNA
It is a common topic now that multiple repeated tumor biopsies are often nonfeasible and more invasive. In this way, Murtaza et. al[4] in a recent study performed whole-exome sequencing of plasma DNA in six patients with advanced cancers. There were two cases with breast cancer, three cases with ovarian cancer and one case with non-small-cell lung cancer. Whole-exome sequencing was performed on multiple samples of plasma from each patient. It is reported that the followup was up to 665 days (range: 109–665 days, median: 433 days). The researchers used 4–20% of the DNA extracted from 2.0–2.2 ml of plasma, and an average of 169 million reads of sequencing per sample. They have analyzed the coding exons of all protein-coding genes at an average unique coverage depth ranging from 31- to 160-fold. For two cases, synchronous biopsies were also analyzed, confirming genome-wide representation of the tumor genome in plasma. To identify changes in the mutation profiles of the tumors, they have compared the abundance of somatic mutations found in plasma before and after each course of systemic therapy. In this way, they have shown that exome analysis of plasma ctDNA represents a novel noninvasive tool for characterization of tumor progress. The researchers involved in this innovative study strongly support that ctDNA are representative of the tumor genome and provide a valid alternative novel method of tumor sampling without the need of multiple invasive tumor biopsies.[4,14–15] What is more, new genes or important mutated genetic pathways maybe identified with this novel method.
Few months ago, Bettegowda et al.[16] published a very important and innovative study. Their aim was to compare many different types of cancer and to determine in which cancers ctDNA detection levels could represent a useful clinical tool. The researchers in this study have measured and evaluated an important number of tumor types. In this way, they have purified plasma and tumor DNA using specific protocols for all the tumor samples and they have used advanced technological methods to detect and evaluate ctDNA levels from each tumor. Notably, they were able to report the number of mutant templates per milliliter of plasma in every tumor type. They have enrolled in this exciting study 136 metastatic tumors from 14 different tissues, as well as 41 patients with primary brain tumors such as glioma and medulloblastoma. Additionally, they have included ten cases of stage III ovarian and hepatocellular carcinomas. The investigators showed that most patients with stage III ovarian and liver cancers and metastatic cancers of the pancreas, bladder, colon, stomach, breast, liver, esophagus, head and neck, as well as patients with neuroblastoma and melanoma, harbored detectable levels of ctDNA. On the other hand, less than 50% of patients with medulloblastomas, metastatic kidney, prostate or thyroid cancers and less than 10% of patients with gliomas, harbored detectable ctDNA levels. Moreover, we have to keep in mind that although ctDNA was detectable in most patients with metastatic cancers, the ctDNA's concentration varied among patients. Moreover, ctDNA's concentration varied also among patients with the same tumor type.
The most important aspect of this study was the evaluation of ctDNA levels in patients with early-localized disease. The researchers specified that with the term of localized disease it is intended a 'disease status' where there is no clinical or radiological sign of distant metastasis at the time of sample collection. We have to highlight the fact that among 223 patients with localized cancers of all types, the researchers showed that detectable levels of ctDNA were found in 55% of the patients (122 of 223 patients). Differences in the rate of patients with detectable levels of ctDNA were also correlated with the stage. In this way, 47% of patients with stage I cancers of any type had detectable ctDNA, whereas the fraction of patients with detectable ctDNA was 55, 69 and 82% for patients with stage II, III and IV cancers, respectively. Bettegowda et al.[16] concluded after this innovative study of 640 patients, that mutant DNA fragments are found at lower but detectable levels in patients with early-stage localized cancers and in high levels in the circulation of patients with advanced-stage cancers. We will expect that this research team or other scientific groups, in addition to ctDNA detection will also perform NGS analysis in this large number of samples. It is out of question that these amazing results may have several scientific implications and may suggest novel important pathways of future translational research.[15]
Taken all these data together, novel noninvasive biomarkers may provide many advantages as compared with invasive multiple tumor biopsies. The above-mentioned approach of ctDNA represents a novel exciting strategy for reaching personalized modern medicine.[4,16–17] Therefore, NGS-based analysis of ctDNA could be used as a valid biomarker for cancer monitoring. Recent improvements in NGS methods and ctDNA method could lead to great expectations in the translation of research into clinical personalized cancer medicine.[5,17] This ctDNA-based genomic diversity identified by NGS analyses also opens a new way for more accurate and effective therapeutic strategy with a combination of targeted drugs against the complete set of genome changes.[18–21] However, it should be emphasized that new clinical trials will be required to prove the robustness of these biomarkers and effectiveness of this therapeutic strategy.
Future Oncol. 2015;11(4):545-548. © 2015 Future Medicine Ltd.
