[1355] EZH2 Codon 641 Mutations Are Common in BCL2-Rearranged B Cell Lymphomas.

Russell JH Ryan, Mai Nitta, Darrell Borger, A John Iafrate, Lawrence Zukerberg, Aliyah Sohani, Long Le. Mass General Hospital, Boston

Background: EZH2 encodes the enzymatic subunit of the polycomb repressive complex 2 that mediates gene repression through trimethylation of histone H3 at lysine 27. Point mutations affecting the first 2 nucleotides of codon 641 of EZH2 were recently described in a subset of germinal center B-cell type (GCB) diffuse large B-cell lymphomas (DLBCL) and <10% of follicular lymphomas (FL). The first EZH2 mutant FL case lacked a BCL2 rearrangement (BCL2-R) and the status of other cases was not reported. We sought to compare the frequency of EZH2 mutations in B-cell lymphomas with BCL2-R to germinal center-derived lymphomas lacking the BCL2-R.
Design: Cases of DLBCL, FL, MYC and BCL2 rearranged “double hit” lymphomas (DHL), and Burkitt lymphoma (BL) were identified from our pathology archives. DHL cases were classified as DLBCL or intermediate between DLBCL and BL by WHO 2008 criteria, but were analyzed as one unique group for this study. All cases of DLBCL expressed a GCB immunophenotype by the Hans classifier or contained a known BCL2-R. DNA was extracted from paraffin-embedded tissue and genomic DNA containing EZH2 codon 641 was PCR amplified. We performed SNaPshot genotyping (Applied Biosystems) with 4 extension primers designed to interrogate the 1st 2 nucleotides of EZH2 codon 641 on both the coding and noncoding strand. Extension products were analyzed by capillary electrophoresis. All cases of unknown BCL2 status were assessed for the presence of BCL2-R by FISH. Low grade FL cases were assessed by Ki67 staining for proliferation index ≥30% (high PI).
Results: 18 EZH2 codon 641 mutations were detected as listed in table 1. Mutations were seen in 12/54 FL (22%) (11/41 FL grade 1-2 (27%) and 1/13 FL grade 3 (8%)). Mutations were seen in 4/38 DLBCL overall (11%). 15 genotyped DLBCL cases had either prior or concurrent diagnoses of FL, and all 4 DLBCL EZH2 mutations were found in this group. There was no association between EZH2 mutation status and PI in grade 1-2 FL (low PI 6/23 (26%), high PI 5/18 (28%)).

Table 1
 EZH2 Y641 WTEZH2 Y641 mutant% mutant
FL with BCL2-R (n=42)301229%
FL w/o BCL2-R (n=12)1200%
DLBCL with BCL2-R (n=12)9325%
GCB-DLBCL w/o BCL2-R (n=26)2514%
DHL (n=11)9218%
BL (n=23)2300%

Conclusions: EZH2 mutations may be more common in FL than previously reported, and often co-occur with BCL2-R in both FL and high-grade B cell lymphomas. Thus EZH2 mutations do not substitute for BCL2-R, but may represent a functionally distinct oncogenic 'hit'. EZH2 mutations are common in DLBCL with concurrent or prior FL, but are rare or absent in BL despite its shared GC origin.
Category: Hematopathology

Monday, February 28, 2011 11:15 AM

Platform Session: Section B, Monday Morning


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