Significant Differences in the IGVH and BCL6 Mutation Status in Aggressive B-Cell Lymphomas with and without MYC Breakpoints.

Somatically mutated IGVH regions are a hallmark of germinal center (GC) B-cells. Moreover, aberrant somatic hypermutation (SHM) of oncogenes, like changes in the 5′ non-coding region of the BCL6 gene occurring in 75% of DLBCL, are a mechanism of oncogene activation independently of chromosomal translocations.

Large scale mutational screening using DHPLC and direct sequencing was applied to determine the somatic hypermutation status of clonal IGVH as well as several oncogenes like BCL6 and MYC in a series of more than 250 aggressive B-cell lymphomas included in the Deutsche Krebshilfe funded network “Molecular Mechanisms in Malignant Lymphoma”. Mutation patterns were correlated with the results of molecular cytogenetic analyses. MYC breakpoints detected by FISH clearly differentiated two groups of aggressive B-NHL with significantly different VH mutation status. MYC-positive lymphomas by FISH carried VH genes with a mutation frequency significantly lower compared to aggressive lymphomas lacking these features (median 4.8% vs.11.,0%, p<0.0001). Similarly, the mutation frequency of BCL6 was significantly lower in MYC-positive than in MYC-negative lymphomas (median 0.13% vs 0.25% p=0,020). We observed a bias in VH gene usage in both groups with an overrepresentation of VH4 (40% in both groups) and VH3 gene (24% in MYC-positive, 40% in MYC-negative). The group of MYC-positive lymphomas was heterogeneous with regard to the pattern of chromosomal aberrations. (IGH, MYC, BCL2, BCL6, MALT1 and REL loci). Lymphomas with IG-MYC fusion lacking breakpoints in BCL2, BCL6, MALT1 or REL loci as well as non-MYC associated IGH translocations displayed a median IGVH mutation frequency of 4,8% vs. 12,2% in those cases with additional chromosomal translocations (p=0.0060).

Molecular classification of aggressive B-cell lymphomas according to MYC breakpoints distinguishes subgroups with significant differences in the VH and BCL6 mutation frequencies.