Major finding: One tumor can contain unique mutations in distinct regions.
Concept: A single biopsy may not be representative of the entire tumor.
Impact: Intratumoral variation may confound prognostics and personalized medicine.
Single tumor biopsy samples are used to characterize the mutational landscape, identify biomarkers of treatment response, stratify patients, and inspire targeted therapies, but emerging evidence suggests that genomic instability and evolutionary forces may drive heterogeneity within tumors and thus render the information derived from single biopsy samples incomplete. Gerlinger and colleagues performed whole-exome sequencing and single-nucleotide polymorphism array analyses on DNA extracted from multiple regions within primary tumors and metastases of 4 patients with renal carcinoma who were participating in a phase II preoperative trial of everolimus. Phylogenetic analyses based on shared and unique mutations revealed that tumor evolution occurred in a branched, nonlinear manner, although the acquisition of different mutations within the same gene in separate regions indicated that convergent evolution also occurs. Strikingly, more than half of all somatic mutations or chromosomal aberrations identified were present only in one or several regions, suggesting that a single biopsy would not be representative of the entire tumor. Indeed, gene expression analysis revealed that regions within the same tumor were enriched for signatures associated with both a poor and a good prognosis, indicating that multiregion sampling may improve biomarker accuracy. Additionally, analysis of the mTOR pathway revealed that downstream targets were only activated in the regions harboring an MTOR mutation, suggesting that divergent signal transduction pathway activity within individual tumors may further confound biomarker analysis and underlie drug resistance. Although future studies will need to confirm whether this high degree of intratumoral variation exists in other tumor types, these findings imply that focusing on ubiquitous alterations may lead to more robust prognostic indicators and improved responses to targeted therapy. Understanding the mechanisms that contribute to such diversity may also enable focused strategies to limit tumor adaptation.
Note: Research Watch is written by Cancer Discovery Science Writers. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at http://CDnews.aacrjournals.org.