Sottoriva et al. profiled 349 colorectal tumour glands (comprising fewer than 10,000 cells) and bulk samples taken from 2 opposing sides of 15 colorectal tumours. Given the glandular structure of colorectal tumours, it can be considered that cells within a gland share a common ancestry and have not been 'mixed', allowing the detection of subclonal alterations. Using whole-genome single-nucleotide polymorphism (SNP) array-based profiling of copy number alterations (CNAs), the authors observed several patterns of spatial variation, including CNAs found in all samples, those found in one side and not the other, CNAs found in some glands on one side only and CNAs found in only one gland. Adenomas (n = 4), which were more genomically stable than the carcinomas (n = 11), had many side-specific CNAs. Conversely, most carcinomas had the same CNAs — to varying extents — on both sides of the tumour, indicating that CNAs occurring early in tumorigenesis become spread out to distant regions as the tumour grows (referred to as variegation). Thus, an important finding is that one region does not represent the entirety of the tumour.
Next, the authors assessed the mutational heterogeneity of the bulk tumour samples using whole-exome sequencing. On the basis of the patient-specific mutations identified, they carried out deep targeted sequencing in the glands and fragments of the bulk samples. Twelve of the tumours had nonsense mutations in adenomatous polyposis coli (APC), and they also found KRAS mutations in five of the tumours and TP53 mutations in three of the carcinomas. Like the CNA data, the mutation analysis showed clonal spatial segregation in the adenomas and variegation in the carcinomas.
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