A quarter of tumour samples from patients with advanced oesophagogastric cancer showed MYC amplification by fluorescence in situ-hybridisation (FISH), according to findings from an analysis presented at the ESMO Asia 2017 Congress in Singapore. Both the number of cells showing MYC amplification and the range of amplification intensity seen across tumour samples were also seen to vary significantly. The data has been generated by Dr Michael Davidson and colleagues from The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, London, UK, as part of ongoing prospective screening for the iMYC study, a biomarker-driven trial of ibrutinib in patients with advanced oesophagogastric cancer displaying MYC amplification.
Dr Davidson explained that preclinical work has demonstrated a synthetically lethal interaction between BTK-inhibition with ibrutinib and MYC amplification in oesophagogastric cancer, leading to the development of the iMYC study. In order to screen patients for the trial a dual probe FISH assay to assess for MYC amplification was developed, with probes for the centrometric component of chromosome 8 and the coding region of the MYC gene used to distinguish between increased copies of chromosome 8 and extra copies of MYC.
Significant heterogeneity in MYC amplification seen
Findings from the initial screening component of the trial were presented. FISH analysis has so far successfully been performed on 109 archival tumour samples, with MYC amplification observed in 27/109 (25%) of samples. The percentage of cells demonstrating MYC amplification varied between samples, with a median 57.5% of cells (range 11 to 94%) showing amplification. The analysis also revealed a high degree of intra-tumour heterogeneity, with 20/27 (74%) amplified samples showing a range of amplification ratios within the tumour specimen. The investigators suggest that extra-chromosomal amplifications of the MYC gene may account for some of the genetic heterogeneity seen.
Example of a FISH result assessing for MYC amplification demonstrating heterogeneity of signalling patterns within the sample, with green arrow indicating a normal diploid pattern of 2 MYC (red) signals and 2 CEP8 (green) signals; yellow arrow indicating a polysomic pattern with multiple MYC and CEP8 signals; and red arrow indicating an amplified pattern with multiple extra MYC signals.
© Dr Michael Davidson.
Further exploratory work has involved the development of a novel digital droplet polymerase chain reaction (ddPCR) test to attempt to identify MYC amplifications from DNA isolated from both primary tumour and circulating tumour (ct)DNA. Positive detection of amplification by ddPCR was only possible in tumour samples that had a homogenous and high-level FISH amplification pattern.
Conclusions
The authors discussed that the iMYC trial represents the first attempt at screening for and targeting MYC in advanced oesophagogastric cancer and prospective data generated from the screening programme thus far illustrates the utility of FISH in assessing for MYC amplification, as well as demonstrating extensive heterogeneity of MYC amplification in this tumour type. They described the use of a novel ddPCR assay to detect MYC amplifications from both archival tumour and ctDNA, but commented that the ddPCR assay did not appear to be optimal for the detection of small clonal subpopulations of amplified cells.
Disclosure
The iMYC study received a research grant from Janssen Pharmaceuticals.
Reference
23O – Davidson M, et al. Clonal diversity of MYC amplification evaluated by FISH and digital droplet polymerase chain reaction (ddPCR) in oesophagogastric (OG) cancer.