Whole Exome Sequencing Defines the Genetic Differences Between Pediatric and Adult Follicular Lymphoma

Follicular lymphoma (FL) is the most common low grade lymphoma in adults, but uncommon in children. While the two diseases are similar in morphology and immunophenotype, pediatric-type FLs manifest higher grade histology with most cases being localized and curable with standard therapy. Pediatric-type FLs are thought arise from germinal center B cells and typically demonstrate higher expression of the germinal center B cell markers BCL6 and CD10.

While the genetic basis of adult FL has been studied previously, the genetic basis of pediatric-type FL remains largely unknown. In this study, we sought to define the gene coding mutations occurring in pediatric FL and compared them to those of 48 adult FLs, as well as other adult lymphomas derived from germinal center B cells including 88 GCB DLBCLs and 67 cases of BL that we, and others, have published previously.

We obtained a total of 32 pediatric-type FLs along with paired normal tissue. The median age wa 8 years with 62% male patients. We performed whole-exome sequencing for these cases using the Agilent solution-based system of exon capture, which uses RNA baits to target all protein coding genes (CCDS database), as well as ~700 human miRNAs from miRBase (v13).  In all, we generated over 6 GB of sequencing data using high throughput sequencing on the Illumina platform.

Using bioinformatics approaches that we have described previously, we identified 26 candidate cancer genes that were recurrently somatically mutated in pediatric FL. We found that the genes mutated in pediatric FLs were from three predominant ontologies/pathways. These included DNA damage repair (40%, ATM, RB1), chromatin modification (34%, SETD2, SMARCA4 and TET2) and alternative splicing (12%, SF3B1). We also noted IRF4 translocations and TNFRSF14 mutations in 10% of the cases as has been described previously.

We further compared pediatric FLs to other germinal center B cell derived lymphomas and, in particular, FLs occurring in adults. We found that the mutations that characterize adult FLs including MLL2, CREBBP, and GNA13 were absent in pediatric FLs. There was only one pediatric FL case with an EZH2 mutation, which otherwise occur commonly in germinal center B cell derived lymphomas.

Thus, pediatric FLs are genetically unlike both adult FLs and other germinal center B cell derived lymphomas. Our data provide new clues to the distinctive genetic basis of these tumors and their clinical behavior.