[Molecular abnormalities in lymphomas]

Numerous molecular abnormalities have been described in lymphomas. They are of diagnostic and prognostic value and are taken into account for the WHO classification of these tumors. They also shed some light on the underlying molecular mechanisms involved in lymphomas. Overall, four types of molecular abnormalities are involved: mutations, translocations, amplifications and deletions of tumor suppressor genes. Several techniques are available to detect these molecular anomalies: conventional cytogenetic analysis, multicolor FISH, CGH array or gene expression profiling using DNA microarrays. In some lymphomas, genetic abnormalities are responsible for the expression of an abnormal protein (e.g. tyrosine-kinase, transcription factor) detectable by immunohistochemistry. In the present review, molecular abnormalities observed in the most frequent B, T or NK cell lymphomas are discussed. In the broad spectrum of diffuse large B-cell lymphomas microarray analysis shows mostly two subgroups of tumors, one with gene expression signature corresponding to germinal center B-cell-like (GCB: CD10+, BCL6 [B-Cell Lymphoma 6]+, centerine+, MUM1-) and a subgroup expressing an activated B-cell-like signature (ABC: CD10-, BCL6-, centerine-, MUM1+). Among other B-cell lymphomas with well characterized molecular abnormalies are follicular lymphoma (BCL2 deregulation), MALT lymphoma (Mucosa Associated Lymphoid Tissue) [API2-MALT1 (mucosa-associated-lymphoid-tissue-lymphoma-translocation-gene1) fusion protein or deregulation BCL10, MALT1, FOXP1. MALT1 transcription factors], mantle cell lymphoma (cycline D1 [CCND1] overexpression) and Burkitt lymphoma (c-Myc expression). Except for ALK (anaplastic lymphoma kinase)-positive anaplastic large cell lymphoma, well characterized molecular anomalies are rare in lymphomas developed from T or NK cells. Peripheral T cell lymphomas not otherwise specified are a heterogeneous group of tumors with frequent but not recurrent molecular abnormalities. Gene profiling analysis shows that the expression of several genes is deregulated including PDGFRA (platelet-derived growth factor receptor) gene, encoding a receptor with tyrosine kinase activity. In angio-immunoblastic T-cell lymphomas molecular abnormalities are found in follicular helper T-cell (TFH) that express some distinctive markers such as CD10, PD-1, CXCR5 and the CXCL13 chemokine. ALK-positive anaplastic large cell lymphoma is a paradigme of T-cell lymphoma since it is associated with an X-ALK oncogenic fusion protein due to a translocation involving ALK gene at 2p23. ALK tyrosine kinase activates downstream pathways (Stat3/5b, Src kinases, PLCγ, PI3 kinase) implicated in lymphomagenesis, proliferation and protection against apoptosis. Specific ALK inhibitors are currently in clinical evaluation. Lastly several lymphomas are associated with infectious agents that play a direct (EB virus, HTLV1) or indirect role (e.g. Helicobacter pylori in MALT lymphoma) in lymphomagenesis.