Abstract 2398: Colorectal primary tumors and metastases are highly homogeneous regarding driver mutations
Authors: Silva Miguel , Resende Carlos , Barranha Rui , Meireles Sara , Carneiro Fatima , Costa Jose L. , Machado Jose C. 
Published: July 2016

Genetic mutations play a key role in the phenotypic and functional heterogeneity that arise among cancer cells within a tumor. The distinct tumor subclones can be intermingled or spatially separated and recent studies suggest this subclonal architecture varies dynamically throughout the disease course. As such, there is a need to accommodate this level of genetic heterogeneity in the classical notion that the carcinogenic process evolves in a stepwise manner through the accumulation of a specific set of mutations along time.

To genetically compare independent lesions from the same patient in the progression of colorectal carcinoma (CRC), we selected 16 patients with metastatic CRC with available primary tumor (PT), lymph node (LN) and liver metastases (LM), and corresponding normal mucosa (NM), resulting in 65 samples. Clinical data were available for all patients. DNA was extracted from FFPE tissues after morphologic evaluation by a pathologist. Sequencing was performed with the Ion PGM system using Ampliseq primer panels consisting of a multiplex PCR covering approximately 2,800 COSMIC mutations from 50 oncogenes and tumor suppressor genes frequently mutated in human cancers. Genes include APC and CTNNB1, considered the initial genetic event in CRC development. NM from all patients was sequenced to identify germline genetic variants. Data was analyzed on the Ion Torrent Server v4.2 using the Variant Caller and Coverage Analysis plugins. Ion Reporter Server and IGV were used for variant annotation and reads visualization, respectively.

A total of 1212 somatic variants were detected among the 65 samples. C/G&gt;T transitions were the most prevalent alterations. In keeping with its initiating role, APC mutations showed the highest allelic fraction in the majority of samples. Seven samples exhibited a remarkably high number of variants (mean = 145). Excluding these, an average of 3,5 variants were observed per sample. For each patient, putative driver mutations (in APC, CTNNB1, KRAS or TP53) detected in PT were also detected in LN and LM. In LM of 6/16 patients (38%) an average of 4 private mutations were observed. Mutations in other genes, empirically classified as passenger mutations, were also frequently found in this series. However, these mutations were heterogeneously distributed in the different lesions of the same patient. In most patients treated with chemotherapy before liver metastasectomy, additional private mutations were detected in the LM.

Contrary to what was previously described, CRC presents a high level of homogeneity concerning well-established driver mutations, since all were present in every tissue of each patient. Conversely, a higher level of heterogeneity was detected between different lesions of the same patient for passenger mutations. Relapse after chemotherapy seems to associate with the emergence of new mutations, reflecting its influence in the dynamics of tumor cell populations.

Citation Format: Miguel Silva, Carlos Resende, Rui Barranha, Sara Meireles, Fatima Carneiro, Jose L. Costa, Jose C. Machado. Colorectal primary tumors and metastases are highly homogeneous regarding driver mutations. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2398.