Genomic landscape of metastatic colorectal cancer.

Response to drug therapy in individual colorectal cancer (CRC) patients is associated with tumour biology. Here we describe the genomic landscape of tumour samples of a homogeneous well-annotated series of patients with metastatic CRC (mCRC) of two phase III clinical trials, CAIRO and CAIRO2. DNA copy number aberrations of 349 patients are determined. Within three treatment arms, 194 chromosomal subregions are associated with progression-free survival (PFS; uncorrected single-test P-values <0.005). These subregions are filtered for effect on messenger RNA expression, using an independent data set from The Cancer Genome Atlas which returned 171 genes. Three chromosomal regions are associated with a significant difference in PFS between treatment arms with or without irinotecan. One of these regions, 6q16.1-q21, correlates in vitro with sensitivity to SN-38, the active metabolite of irinotecan. This genomic landscape of mCRC reveals a number of DNA copy number aberrations associated with response to drug therapy.

Lack of genomic heterogeneity at high-resolution aCGH between primary breast cancers and their paired lymph node metastases.

Lymph-node metastasis (LNM) predict high recurrence rates in breast cancer patients. Systemic treatment aims to eliminate (micro)metastatic cells. However decisions regarding systemic treatment depend largely on clinical and molecular characteristics of primary tumours. It remains, however, unclear to what extent metastases resemble the cognate primary breast tumours, especially on a genomic level, and as such will be eradicated by the systemic therapy chosen. In this study we used high-resolution aCGH to investigate DNA copy number differences between primary breast cancers and their paired LNMs. To date, no recurrent LNM-specific genomic aberrations have been identified using array comparative genomic hybridization (aCGH) analysis. In our study we employ a high-resolution platform and we stratify on different breast cancer subtypes, both aspects that might have underpowered previously performed studies.To test the possibility that genomic instability in triple-negative breast cancers (TNBCs) might cause increased random and potentially also recurrent copy number aberrations (CNAs) in their LNMs, we studied 10 primary TNBC-LNM pairs and 10 ER-positive (ER+) pairs and verified our findings adding additionally 5 TNBC-LNM and 22 ER+-LNM pairs. We found that all LNMs clustered nearest to their matched tumour except for two cases, of which one was due to the presence of two distinct histological components in one tumour. We found no significantly altered CNAs between tumour and their LNMs in the entire group or in the subgroups. Within the TNBC subgroup, no absolute increase in CNAs was found in the LNMs compared to their primary tumours, suggesting that increased genomic instability does not lead to more CNAs in LNMs. Our findings suggest a high clonal relationship between primary breast tumours and its LNMs, at least prior to treatment, and support the use of primary tumour characteristics to guide adjuvant systemic chemotherapy in breast cancer patients.

Chromosomal copy number aberrations in colorectal metastases resemble their primary counterparts and differences are typically non-recurrent.

The metastatic process is complex and remains a major obstacle in the management of colorectal cancer. To gain a better insight into the pathology of metastasis, we investigated genomic aberrations in a large cohort of matched colorectal cancer primaries and distant metastases from various sites by high resolution array comparative genomic hybridization. In total, 62 primary colorectal cancers, and 68 matched metastases (22 liver, 11 lung, 12 ovary, 12 omentum, and 11 distant lymph nodes) were analyzed. Public datasets were used for validation purposes. Metastases resemble their matched primary tumors in the majority of the patients. This validates the significant overlap in chromosomal aberrations between primary tumors and corresponding metastases observed previously. We observed 15 statistically significant different regions between the primary tumors and their matched metastases, of which only one recurrent event in metastases was observed. We conclude, based on detailed analysis and large independent datasets, that chromosomal copy number aberrations in colorectal metastases resemble their primary counterparts, and differences are typically non-recurrent.

Detection limits of DNA copy number alterations in heterogeneous cell populations.

BACKGROUND: Array Comparative Genomic Hybridization (aCGH) is a widely used technique to assess chromosomal copy number alterations. Chromosomal content, however, is often not uniform throughout cell populations. Here we evaluated to what extent aCGH can detect DNA copy number alterations in heterogeneous cell populations. A systematic evaluation is currently lacking, despite its importance in diagnostics and research. The detection limits reported are a compound of analytical software and laboratory techniques and do not account for the number of probes in relation to sample homogeneity. METHODS: Detection limits were explored with DNA isolated from a patient with intellectual disability (ID) and from tumor cell line BT474. Both were diluted with increasing amounts of normal DNA to simulate different levels of cellularity. Samples were hybridized on microarrays containing 180,880 oligonucleotides evenly distributed over the genome (spacing ~17 kb). RESULTS: Single copy number alterations, represented by down to 249 probes (4 Mb) and present in 10 % of a cell population, could be detected. Alterations encompassing as few as 14 probes (~238 Kb) could also be detected, but for this a 35 % mosaic level was required. CONCLUSIONS: DNA copy number alterations can be detected in cell populations containing 10 % abnormal cells. Detection of sub-megabase alterations requires a higher percentage of abnormal cells or microarrays with a higher probe density.

Chromosome 20p11 gains are associated with liver-specific metastasis in patients with colorectal cancer.

OBJECTIVE: Metastatic colorectal cancer (CRC) cells have a selective preference for certain target organs that cannot be explained by circulatory patterns alone. This study aimed to identify clinicopathological features and chromosomal aberrations of primary tumours associated with organ-specific CRC metastasis. DESIGN: Clinicopathological features were investigated in patients with CRC who had exclusively hepatic (n=182) versus exclusively extrahepatic (n=139) metastases. A total of 139 primary tumours of patients with hepatic (n=85) and extrahepatic metastases (n=54) were screened for chromosomal aberrations by microarray-based comparative genomic hybridisation, and the findings were validated in an independent set of 80 primary tumours. A publicly available database was used to correlate chromosomal aberrations with gene expression. Protein expression was evaluated by immunohistochemistry on tissue microarrays. RESULTS: Patients with hepatic metastases were significantly more often male (71% vs 53% p=0.002), more often had abnormal lactate dehydrogenase activity (37% vs 14% p<0.0001), exhibited primary tumour localisation in the colon (52% vs 40% p=0.03) and had synchronous onset of metastases (70% vs 19% p<0.0001). Primary tumours of patients with hepatic metastases were more commonly T3 tumours (79% vs 63% p=0.006) and less commonly of mucinous histology (5% vs 16% p=0.02). Gain of 20p11 was more often observed in patients with hepatic metastases (p<0.05), which was confirmed in an independent dataset (p<0.05; false discovery rate <0.05). Twelve genes mapping at 20p11 were significantly overexpressed as a consequence of 20p11 copy number gain. C20orf3 showed the strongest correlation between RNA expression and DNA copy number. This was reflected in significantly higher protein expression in patients with hepatic metastases (59%; n=325) than in those with extrahepatic metastases (41%; n=256) (p=0.01). CONCLUSION: C20orf3 mapping at 20p11 is associated with hepatic-specific metastasis in patients with CRC. This gene is a candidate biomarker for liver metastases and may be of clinical value in early-stage CRC.

CGH arrays compared for DNA isolated from formalin-fixed, paraffin-embedded material.

Formalin-fixed, paraffin-embedded (FFPE) archival tissue is an important source of DNA material. The most commonly used technique to identify copy number aberrations from chromosomal DNA in tumorigenesis is array comparative genomic hybridization (aCGH). Although copy number analysis using DNA from FFPE archival tissue is challenging, several research groups have reported high quality and reproducible DNA copy number results using aCGH. Aim of this study is to compare the commercially available aCGH platforms suitable for high-resolution copy number analysis using FFPE-derived DNA. Two dual channel aCGH platforms (Agilent and NimbleGen) and a single channel SNP-based platform (Affymetrix) were evaluated using seven FFPE colon cancer samples, and median absolute deviation (MAD), deflection, signal-to-noise ratio, and DNA input requirements were used as quality criteria. Large differences were observed between platforms; Agilent and NimbleGen showed better MAD values (0.13 for both) compared with Affymetrix (0.22). On the contrary, Affymetrix showed a better deflection of 0.94, followed by 0.71 for Agilent and 0.51 for NimbleGen. This resulted in signal-to-nose ratios that were comparable between the three commercially available platforms. Interestingly, DNA input amounts from FFPE material lower than recommended still yielded high quality profiles on all platforms. Copy number analysis using DNA derived from FFPE archival material is feasible using all three high-resolution copy number platforms and shows reproducible results, also with DNA input amounts lower than recommended.

Across array comparative genomic hybridization: a strategy to reduce reference channel hybridizations.

Array comparative genomic hybridization (array CGH) is widely used for studying chromosomal copy number aberrations (CNAs) on a genome-wide and high-resolution scale in heritable disorders and cancers. The aim of this study was to test if the separate channels of dual channel arrays can be interchanged (across array) to either make array CGH more sensitive and cost effective and/or to generate profiles of CNAs and copy number variations (CNVs). Therefore the BT474 breast cancer cell line was compared with a mix of normal reference DNAs hybridized on different arrays and days and DNA copy number profiles were evaluated. Quality was assessed, using regular dual channel array CGH as a standard, using four quality measures, i.e., the median absolute deviation value of chromosome 2, the amplitude of the ERBB2 gene amplification, a deletion on chromosome 9, and the deflection on chromosome 8. The quality of the across array CGH profiles matched or even surpassed the quality of regular dual channel array CGH. In addition, this across array approach was tested for genomic DNA derived from formalin-fixed paraffin-embedded tumors tissue samples, resulting in high-quality copy number profiles, comparable to regular dual channel arrays. Finally, we demonstrated this approach to obtain both CNA and CNV profiles. In summary, across array CGH avoids redundant hybridizations of the same reference material in every experiment either allowing hybridization of two test samples on one array or producing both CNA and CNV profiles simultaneously.