In diffuse large B-cell lymphoma (DLBCL), patients typically are risk-stratified according to cell-of-origin, but recent genetics-based classifications led the World Health Organization to establish a new classification for lymphoid neoplasms: high-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangements, known as double-hit or triple-hit lymphoma (HGBL-DH/TH).
However, according to Brett Collinge, a graduate student at BC Cancer Research Institute in Vancouver, British Columbia, and co-author of an analysis published in Blood, “these classifications did not comprehensively incorporate structural variants (SVs) involving MYC – a key oncogene in aggressive B-cell lymphomas.” Copy number variations (CNVs), they added, “are another SV commonly associated with aberrant gene expression, offering a potential mechanism for [positive gene expression signature] in the absence of gene rearrangements.”
In their report, the researchers sought to elucidate the biological implication of MYC and BCL2 CNVs in a cohort of 802 tumors with DLBCL morphology. Samples underwent fluorescence in situ hybridization (FISH) testing, immunohistochemistry (IHC) staining, gene expression profiling, and molecular subtyping – as well as MYC mutation sequencing in tumors with unexplained MYC IHC negativity.
“Consistent with previous reports, MYC translocation (MYCTr) and BCL2 translocation (BCL2Tr) were both strongly associated with increased expression,” the authors reported. “In contrast, while BCL2 CNVs were strongly associated with increased expression, MYC CNVs were not.”
In addition, in the context of atypical double-hit lymphoma, MYC and BCL2 CNVs did not confer a similar gene expression profile as HGBL-DH/TH-BCL2.
The authors also evaluated tumor samples to determine mechanisms of IHC negativity in MYC-rearranged tumors, finding that approximately 30% of tumors with MYC rearrangement were negative for MYC protein expression by IHC (n=29/98). IHC positivity was associated with increased levels of MYC mRNA expression, while nine of the 11 tumors with low mRNA expression levels were negative for MYC IHC – providing a potential mechanism for false-negativity on IHC. The other major mechanism that uncoupled MYCTr from MYC protein expression was the presence of the MYC-N11S variant, or false-negative IHC staining mediated by a single nucleotide polymorphism resulting in an asparagine to serine substitution at the 11th amino acid residue of MYC.
“Taken together, these results support the current exclusion of MYC and BCL2 CNVs from HGBL-DH/TH and highlight the ability of a molecular-based classification system to identify tumors with shared biology that FISH and IHC fail to fully capture,” the investigators noted. They suggested moving from “a category defined simply by the presence of gene rearrangements to one defined by the strong molecular biology that these rearrangements typically produce.”
The implications of this study are limited to FISH testing due to its use in clinical practice, but the authors concluded that future investigations should use methodologies with greater resolution to fully elucidate the spectrum of MYC CNVs and their associated significance. “Such methods could additionally account for the confounding effects of cryptic translocations,” they wrote.
Study authors report no relevant conflicts of interest.
Collinge BJ, Ben-Neriah S, Chong LC, et al. Impact of MYC and BCL2 structural variants in tumors of DLBCL morphology and mechanisms of false-negative MYC IHC. Blood. 2020 October 29. [Epub ahead of print]