To define and highlight primary bone diffuse large B-cell lymphoma (PB-DLBCL)'s distinctive clinical presentation, morphology, phenotype, gene expression profile (GEP) and molecular genetics
Primary bone lymphoma (PBL) accounts for 1–2 % of all lymphomas. Patients present with one or more bone lesions, without significant lymph node or other extranodal involvement. It mainly comprises diffuse large B-cell lymphoma (DLBCL). Dismayingly, PBL has been left out of all current lymphoma classification systems precluding it from being recognised as a separate entity/subentity. Herein, in the narrow sense of primary bone DLBCL (PB-DLBCL), we define and highlight its distinctive clinical presentation, morphology, phenotype, gene expression profile (GEP) and molecular genetics.  We collected 27 cases (10 female, 17 male, age range 22–93, mean 63) and investigated the expression of CD10, CD20, BCL2, BCL6, MUM1, and MYC by immunohistochemistry (IHC), performed gDNA panel sequencing covering 172 genes, and carried out fluorescence in situ hybridisation (FISH) to evaluate MYC, BCL2 and BCL6 translocations. We attempted to genetically subclassify cases in terms of DLBCL subtype by using the Two-step classifier. Finally, we performed GEP for cell-of-origin subtyping and in silico comparison to uncover up- and down-regulated genes as opposed to other DLBCL.  By applying the Hans algorithm, 22 were GCB and only 5 – non-GCB. FISH showed 2 BCL2, 3 BCL6 and 1 MYC rearranged instances. One case had MYC and BCL2 rearrangements. DNA sequencing highlighted TP53 (n=7), B2M (n=5), EZH2 (n=5), KMT2D (n=5), TNFRSF14 (n=4) and SGK1 (n=4) as the most frequently mutated genes. The Two-step classifier categorised 8 of the cases as EZB, 3 as ST2 and 1 as MCD. All cases were classified as GCB by GEP, and one case was molecular high-grade (MHG). The top 10 up-regulated genes were: MAP2K7, HLA-DRA, UBE4A, MAF, LRMP, PAFAH1B2, NF2, GRB2, CFLAR, MAFB, whereas the top 10 down-regulated were: NREP, BAK1, CRIP1, CSTF1, PCSK7, TRPM2, TNFSF4, CKS1B, CD19, CDK16.  Herein, we prove that PB-DLBCL is a specific entity with a characteristic clinical and morphologic presentation. Almost all cases are GCB by IHC and all by GEP. Interestingly, its mutational profile is similar to the one of follicular lymphoma and nodal GCB-DLBCL, but its GEP and frequency of BCL2 rearrangements are unique.