BCL2 Negative Follicular Lymphoma with BCL6 Gene Rearrangement Show Mutations of STAT6 DNA Binding Domain

Abstract 1559

Follicular lymphoma (FL) is the most common indolent subtype of non-Hodgkin's lymphoma in the western world. The genetic hallmark of FL is the t(14;18)(q32;q21) translocation leading to the deregulation of BCL2 expression which occurs in up to 90% of the grade 1–2 FL. However, a minority of FL without BCL2 gene rearrangement harbour genetic abnormalities involving the BCL6 gene (5–15% of the cases) and show distinct pathological features. The activation of Signal Transducer and Activator of Transcription 6 (STAT-6) has been observed in Primary Mediastinal B-cell Lymphoma (PMBL) and Hodgkin Lymphoma but also in FL. Because missense mutations of STAT6 DNA binding domain have been described in PMBL, we searched for such mutations in FL. Using a PCR HRM (High Resolution Melting) assay as a screening tool and conventional Sanger sequencing in all cases with abnormal denaturation curves, we analyzed the frequency of STAT6 mutations in FL samples. We focused our screening on exon 12, which encodes part of the DNA binding domain and which had been shown to be a hotspot mutation in PMBL. A series of 40 FL lymphomas samples diagnosed at the University Hospital of Créteil (Henri Mondor) and Centre Henri Becquerel in Rouen were retrieved. These tumors showed characteristic histopathological features of FL according to the World Health Organization (WHO) classification. DNA analysis was performed on DNA extracted from fresh/frozen samples (Rouen) or FFPE (formalin-fixed and paraffin-embedded) tissues (Créteil) with standard procedures.

We detected 5 (12%) mutated tumors in this series of FL. These 5 mutations were single missense mutations targeting amino acids 419–421. All mutations were observed in histological grade 1or 2 lymphomas, and no mutations were found in the 9 cases classified as 3A or 3B. Only one classical FL out of 15 cases (6%) with BCL2 rearrangement showed STAT6 mutation. Strikingly, the 4 other mutated cases showed specific features. There were 3 female and 1 male, with a mean age of 52 years, and all presented with inguinal involvement and stage III or IV Ann Arbor disease. Morphologically, these cases displayed a follicular growth pattern. The immunophenotype was CD20+, CD5-, CD10+ (3/4), BCL6+, CD23+ (3/4) but BCL2 was negative (4/4). Cytogenetically, these 4 cases were characterized by BCL6 gene rearrangement without BCL2 gene rearrangement by interphase FISH on FFPE tissue sections. Thus, STAT6 mutations were observed in 4/11 (36%) grade 1–2, and 0/6 grade 3A/3B BCL2 negative FL with BCL6 rearrangement. No case was found to be mutated in the FL group without BCL2 and BCL6 rearrangement (8 cases).

In conclusion, this is the first time that mutations of STAT6 are found in FL and interestingly, they target a rare group of FL with distinct pathological and cytogenetical features. Further investigations are required to identify the mutational mechanisms involved and the oncogenic function associated with these mutations.