Gene Expression Profiling Classifies Splenic Marginal Zone Lymphoma and Hairy Cell Leukemia-Variant as Related Diseases That Are Distinct From Typical Hairy Cell Leukemia.

Abstract 3467

Poster Board III-355

Hairy cell leukemia (HCL) is an uncommon B-cell malignancy that shares some features with a variant form, HCL-variant, and splenic marginal zone lymphoma (SMZL). The distinction between these disorders can be difficult, but correct diagnosis is important for patient management and clinical outcome. The recent WHO classification has recognized SMZL as a distinct entity and HCL-variant was included in a provisional category of unclassifiable splenic lymphomas involving the splenic red pulp that also includes splenic diffuse red pulp small B-cell lymphoma (SDSL). The molecular features of these disorders are still largely uncharacterized and genomic studies have been limited due to their rarity. We have used Affymetrix gene expression microarrays to compare the transcription profiles of 31 HCL, 24 HCL-variant, 44 SMZL and 5 cases of SDSL with the aim of elucidating the relationships between these disorders and to identify novel potential therapeutic and disease-specific diagnostic targets. Total RNA was extracted from PBMC samples enriched for tumor cells (median 89%, range 70-98%) and the expression of approximately 18,000 genes was interrogated using the Affymetrix Human Exon 1.0 ST Array. Data were normalized and analyzed using Partek Genomics Suite® and differentially expressed transcripts between the disorders were identified by ANOVA. Unsupervised hierarchical clustering revealed disease-related patterns within the transcription profiles. Strikingly, the HCL-variant, SMZL and SDSL profiles clustered together and separately from those of typical HCL. Using a false discovery rate threshold of <0.001 and an expression fold-change >2 we identified 366 transcripts that distinguished HCL from HCL-variant, SMZL and SDSL. Using these same criteria, and even those less stringent, we could not identify a gene expression signature that could distinguish between HCL-variant, SMZL and SDSL. Biological interpretation of the data revealed many of the differentially expressed transcripts linked to immune response and signal transduction processes, including key oncogenes and tumor suppressor genes. Transcripts with higher expression in HCL-variant, SMZL and SDSL relative to HCL were associated with lymphocyte activation and proliferation, NFκB signaling and integrin signaling. Conversely, transcripts with higher expression in HCL relative to HCL-variant, SMZL and SDSL, comprised amongst others, pathways of antigen processing, MAP kinase signaling, JAK-STAT signaling, and FLT3 signaling. Comparison of the transcription signatures of HCL, HCL-variant and SMZL provides insight into their relationships and for the first time provides strong evidence that HCL-variant and SDSL may be variant forms of SMZL and that HCL remains as a distinct disease entity characterized by different signaling pathways. These data further strengthen our previous observation that showed that the immunoglobulin heavy chain gene (IGH) repertoire of HCL-variant is more similar to SMZL than to HCL and that, in contrast to HCL, both HCL-variant and SMZL contain subsets of cases with unmutated IGHV. Interestingly, the expression profiles of both HCL-variant and SMZL could not be discriminated based on their IGHV mutation status. We are currently performing further analyses of these data, including comparison with the expression profiles of normal B cells, with the aim of gaining further insight into the clonal history and the normal B-cell counterpart of these disorders.