Dynamic changes in clonal architecture during disease progression in follicular lymphoma

Follicular lymphoma (FL) is typically a slow growing cancer that can be effectively treated. Some patients undergo transformation to diffuse large B cell lymphoma (DLBCL), which is frequently resistant to chemotherapy and is generally fatal. Targeted sequencing of DNA and RNA was applied to identify mutations and transcriptional changes that accompanied transformation in a cohort of 16 patients, including 14 with paired samples. In most cases we found mutations that were specific to the FL clone dominant at diagnosis, supporting the view that DLBCL does not develop directly from FL, but from an ancestral progenitor. We identified frequent mutations in TP53, cell cycle regulators (cyclins and cyclin-dependent kinases) and the PI3K pathway, as well as recurrent somatic copy number variants (SCNVs) on chromosome 3, 7 and 17p associated with transformation. An integrated analysis of RNA and DNA identified allele specific expression changes in oncogenes, including MYC, that could be attributed to structural rearrangements. By focusing on serial samples taken from two patients, we identified evidence of convergent tumour evolution, where clonal expansion was repeatedly associated with mutations targeting the same genes or pathways. Analysis of serial samples is a powerful way to identify core dependencies that support lymphoma growth.