Mutations In MLL2 and MEF2B Genes In Follicular Lymphoma and Diffuse Large B-Cell Lymphoma

: Follicular lymphoma (FL) and diffuse large B cell lymphoma (DLBCL) are the most common types of B-cell derived non-Hodgkin lymphomas (NHL). A significant proportion of patients with FL develop transformed disease reminiscent of DLBCL, which arises from the cells of FL. A common feature of FL and a subset of DLBCL is the presence of a balanced cytogenetic translocation t(14;18)(q32;q21). In addition to chromosome rearrangements, inappropriate somatic hypermutation of proto-oncogenes, including MYC, PIM1, ARHH and PAX5, have been proposed to contribute to DLBCL, but not to FL. Evidence for other mutations contributing to the development of DLBCL and FL is lacking. Our research group had previously detected a residue of the SET domain of the methyltransferase EZH2 (Y641) that is frequently mutated both in FL and the GCB subtype of DLBCL (Morin, R et al. 2010 Nature Genetics 42 (2):181-5). It has also been recently reported that specific gene expression signatures can reveal unique diagnostic and prognostic information about FL and DLBCL. The presence of mutations in genes that contribute to chromatin modification (from transcriptionally active to silent, and vice versa) can have an effect on the regulation of gene expression, playing an important role in cancer.

We have deep-sequenced the whole transcriptome of DLBCL samples using Illumina second-generation sequencing to detect candidate mutations in different genes that may contribute to lymphoma development and progression. Based on our observations of these data, we subsequently PCR amplified and sequenced the entire MLL2 locus from genomic DNA isolated from FL and DLBCL tumor samples, and from matched germline DNA where available. Using an alternative approach, consisting of a targeted hybridization capture of MEF2B exons from genomic DNA, we re-sequenced the exons of MEF2B in an extended cohort of FL and DLBCL samples.

Among many other mutated genes we have characterized the mutations found in two chromatin modifying genes, MLL2 and MEF2B. MLL2, a gene that encodes a histone methyltransferase, is somatically mutated in 89% of FL (n=35) and 32% of DLBCL (n=37) samples. The majority of these mutations were truncations and frame-shift insertions and deletions, likely inactivating MLL2. MEF2B, which encodes a MADS/MEF2 DNA binding protein involved in the regulation of gene expression, was found mutated in 12% of FL (n=274) and 9% of DLBCL (n=321) tumor samples. In contrast with the mutational pattern of MLL2, we did not detect non-sense mutations in MEF2B. All mutations in MEF2B affected a small number of residues. Several studies in MEF2 family members have shown the importance of some of these mutated residues for the correct function of the protein.

After high-throughput sequencing of transcriptomes in a cohort of FL and DLBCL lymphoma samples, two targeted second generation re-sequencing approaches have enabled the screening of individual genes in a large cohort of samples. We chose two of the genes with strong evidence for recurrent somatic mutations and no previously known role in lymphoma, MLL2 and MEF2B, for detailed characterization. The high incidence of mutations in both of these genes suggests that these mutations might act as driver mutations of interest for further study.

No relevant conflicts of interest to declare.