Cancers contain ‘mixed’ populations of malignant cells that arise from malignant conversion of a “cell of origin” via a process referred to as clonal evolution. This gives rise to ‘intratumoural heterogeneity’, and explains why cancers comprise morphologically, genetically and phenotypically distinct populations of cells. Most recently, clonal evolution has been shown to explain the emergence of drug resistance (Saunders et al, EMBO Mol Med). Moreover, the novel sampling, deep-sequencing and computational approaches employed in clonal evolution studies enables the profiling of behavior of clonal populations with a precision not currently achievable with single whole-tumour biopsies.
The skin represents a complex tissue to follow malignant conversion since tumours (eg SCC) arise within a field of nonmalignant epithelial cells which have been exposed to UV radiation. Thus, discriminating “driver” events from bystander events within a highly mutagenised field represents a considerable challenge. To address this we have employed a novel sampling and exome sequencing approach. We employed a micro-biopsy device and sampled 8-12 micro-biopsies of approximately 100-1000 cells from each of three SCC lesions and adjacent photodamaged skin. All samples were taken from sun-exposed extremities from patients at the Princess Alexandra Hospital, Brisbane. DNA was amplified by multiple displacement amplification prior to exome sequencing. Analysis of exome sequencing data revealed the majority of mutations detected were ‘UV-Signature’, C>T nucleotide mutations. Mutations were detected in genes which have previously been described in SCC, such as FAT1, p53 and NOTCH2. We detected a high burden of mutations in the photodamaged skin adjacent to SCC, in addition to significant mutational heterogeneity within individual SCC lesions. Of note, photodamaged skin harbored UV-signature mutations in genes commonly thought to be malignant “drivers” (eg p53). Further analysis of the exome sequencing data will allow for construction of a phylogenetic tree to provide insights into evolution of cutaneous SCC.