We investigated the clinicopathologic features of 5 follicular lymphomas (FL) that transformed to morphologic diffuse large B-cell lymphomas (DLBCL) and had a primary mediastinal large B-cell lymphoma (PMBL)-like gene expression profile. None of the transformed FL (tFL) arose in the mediastinum, all cases tested had a germinal center B-cell immunophenotype, 20% were CD30+, 60% CD23+, 80% MAL+, 20% CD200+, and 0% CD273/PDL2+. Whole exome sequencing detected alterations in genes associated with both FL/DLBCL (CREBBP, KMT2C, KMT2D, ARID1A, HIST1 members, and TNFRSF14) and PMBL (JAK-STAT pathway genes, B2M, and CD58). Copy number (CN) analysis detected gains/amplification of REL in 60% and STAT6 in 40%, gains of chromosome 16, including IL4R, in 40%, and both deletions and gains of 11q in 20%. This cohort, although limited in size, supports a subset of tFL that has blended features between FL/DLBCL and PMBL. Despite having some features that are less common in DLBCL (MAL and CD23 expression, JAK-STAT activation), these cases lack the most characteristic CN alteration seen in PMBL ... (Show More)

We investigated the clinicopathologic features of 5 follicular lymphomas (FL) that transformed to morphologic diffuse large B-cell lymphomas (DLBCL) and had a primary mediastinal large B-cell lymphoma (PMBL)-like gene expression profile. None of the transformed FL (tFL) arose in the mediastinum, all cases tested had a germinal center B-cell immunophenotype, 20% were CD30+, 60% CD23+, 80% MAL+, 20% CD200+, and 0% CD273/PDL2+. Whole exome sequencing detected alterations in genes associated with both FL/DLBCL (CREBBP, KMT2C, KMT2D, ARID1A, HIST1 members, and TNFRSF14) and PMBL (JAK-STAT pathway genes, B2M, and CD58). Copy number (CN) analysis detected gains/amplification of REL in 60% and STAT6 in 40%, gains of chromosome 16, including IL4R, in 40%, and both deletions and gains of 11q in 20%. This cohort, although limited in size, supports a subset of tFL that has blended features between FL/DLBCL and PMBL. Despite having some features that are less common in DLBCL (MAL and CD23 expression, JAK-STAT activation), these cases lack the most characteristic CN alteration seen in PMBL (9p24.1 gain/amplification). This cohort expands the biologic heterogeneity of tFL, illustrating a subset with blended FL/DLBCL and PMBL features, with potential treatment implications that include the use of novel PMBL-targeted therapies.

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