Gene expression profiling more sensitive than FISH for identifying double-hit DLBCL

Diffuse large B-cell lymphoma (DLBCL), the most common subtype of NHL, presents with a wide range of clinical characteristics and prognostic factors, depending on the underlying biology. The two major biologically distinct subgroups of DLBCL are germinal center B-cell-like (GCB) and activated B-cell-like (ABC), with the ABC subtype conferring a poorer prognosis.¹ In addition to the ABC and GCB subtypes, the high-grade DLBCL can be divided by the presence or absence of rearrangements in genes encoding MYC and BCL2 and/or BCL6. Tumors with these genetic abnormalities are referred to as a double/triple hit (HGBL-DH/TH-BCL2) and are also associated with the worst outcomes.^2,3 Therefore, the profiling of DLBCL tumors is crucial for stratifying patients’ risk and planning treatment that targets the underlining biology of different molecular subgroups.

A recent report showed that nearly 30% of GCB-DLBCL tumors have a gene expression pattern that resembles the double-hit signature (DHITsig). However, both MYC and BCL2 rearrangements could only be identified by standard break-apart fluorescence in situ hybridization (FISH), in half of the cases.⁴ Laura Hilton from Simon Fraser University, Burnaby, CA, and colleagues, published in Blood the results of a study that aimed to identify a more sensitive way to detect the double-hit biology underlying the poorer outcomes within GCB-DLBCL.⁵

Methods

Whole genome sequencing (WGS) was performed on DNA from the 20 fresh-frozen GCB tumors showing double-hit signature (DHITsig+) with no MYC and/or BCL2 translocations detected by break-apart FISH.⁴ Copy number variants (CNVs) analysis was also completed.

Main findings

Six HGBL-DH/TH tumors were identified by WGS (three cases of MYC rearrangement and three of BCL2) that were not detected by break-apart FISH. The results were confirmed by immunohistochemistry. WGS structural variant calling detected all BCL2 and MYC rearrangements identified by FISH. All cryptic MYC translocations had non-IG partner loci, while BCL2 translocations had IG partner.

Alterations in copy number affecting MYC and associated downstream pathways were found in several DHITsig+ tumors, including copy number gains affecting MIR17HG (6/20 cases). SNP array copy number profiles revealed enrichment in DHITsig+ tumors for amplifications of MIR17HG and FCGR2B, as well as deletions of CDKN2A and 22q11.22 (IGL). Additionally, the authors did not find an association between the 11q aberration and the DHITsig in DLBCL.

Conclusion

The data demonstrate the limitations of FISH in identifying HGBL-DH/TH, a high-risk GCB-DLBCL. Gene expression profiling was more sensitive for identifying double-hit biology. Moreover, the DHITsig+ was associated with various genetic events affecting MYC, BCL2, and their downstream pathways.