U-2932: two clones in one cell line, a tool for the study of clonal evolution

Genetic heterogeneity is common in tumors, explicable by the development of subclones with distinct genetic and epigenetic alterations. We describe an in vitro model for cancer heterogeneity, comprising the diffuse large B-cell lymphoma cell line U-2932 which expresses two sets of cell surface markers representing twin populations flow-sorted by CD20 vs CD38 expression. U-2932 populations were traced to subclones of the original tumor with clone-specific immunoglobulin IgVH4-39 hypermutation patterns. BCL6 was overexpressed in one subpopulation (R1), MYC in the other (R2), both clones overexpressed BCL2. According to the combined results of immunoglobulin hypermutation and cytogenetic analysis, R1 and R2 derive from a mother clone with genomic BCL2 amplification, which acquired secondary rearrangements leading to the overexpression of BCL6 (R1) or MYC (R2). Some 200 genes were differentially expressed in R1/R2 microarrays including transcriptional targets of the aberrantly expressed oncogenes. Other genes were regulated by epigenetic means as shown by DNA methylation analysis. Ectopic expression of BCL6 in R2 variously modulated new candidate target genes, confirming dual silencing and activating functions. In summary, stable retention of genetically distinct subclones in U-2932 models tumor heterogeneity in vitro permitting functional analysis of oncogenes against a syngenic background.