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Chromosome 13q deletion [del(13q)], harboring the miR-15a/16-1cluster, is one of the most common genetic alterations in mature B-cell malignancies, which originate from germinal center (GC) and post-GC B cells. Moreover, miR-15a/16 expression is frequently reduced in lymphoma and multiple myeloma (MM) cells without del(13q), suggesting important tumor-suppressor activity. However, the role of miR-15a/16-1in B-cell activation and initiation of mature B-cell neoplasms remains to be determined. We show that conditional deletion of the miR-15a/16-1cluster in murine GC B cells induces moderate but widespread molecular and functional changes including an increased number of GC B cells, percentage of dark zone B cells, and maturation into plasma cells. With time, this leads to development of mature B-cell neoplasms resembling human extramedullary plasmacytoma (EP) as well as follicular and diffuse large B-cell lymphomas. The indolent nature and lack of bone marrow involvement of EP in our murine model resembles human primary EP rather than MM that has progressed to extramedullary disease. We corroborate human primary EP having low levels of miR-15a/16 expression, with del(13q) being the most common genetic loss. Additionally, we show that, although the mutational profile of human EP is similar to MM, there are some exceptions such as the low frequency of hyperdiploidy in EP, which could account for different disease presentation. Taken together, our studies highlight the significant role of the miR-15a/16-1cluster in the regulation of the GC reaction and its fundamental context-dependent tumor-suppression function in plasma cell and B-cell malignancies.

Defects involving α spectrin (Sp) are found in patients with hereditary elliptocytosis and a related disorder, hereditary pyropoikilocytosis (HPP). We have previously found that the severity of hemolysis was related to the total spectrin content of the cells and the percentage of unassembled dimeric Sp (SpD) in the membranes, which, in turn, reflected the amount of mutant Sp in the cell. However, no data are available comparing differences in the function of various α Sp mutations to clinical severity. We now report studies of nine homozygotes or double heterozygotes for four α Sp mutations: α1/74, α1/46, α1-65, and α1/61, whose red blood cells (RBCs) contained only the mutant Sp and no normal Sp. Sp α1/74, Sp α1/48, and Sp α1/65homozygotes differed strikingly in the severity of hemolysis that correlated with the severity of mutant Sp dysfunction, as reflected by the fraction of unassembled SpD in the membranes and the self-association of mutant Sp on inside-out vesicles. Homozygotes for Sp α1/74had a very severe hemolytic anemia and their SpD were virtually incapable of self-association, whereas SpD α1/46were not as severely affected. The Sp α1/65homozygotes had a relatively mild hemolytic anemia and their SpD showed the least impairment of function. Ultrastructural examination of membrane skeletons from subjects whose SpD self-association was severely impaired showed gross skeletal disruption and loss of hexagonal structure. In striking contrast, the homozygote for the mildly dysfunctional Sp α1/65had only a moderate disruption of the skeleton. Some of the homozygous or doubly heterozygous subjects also exhibited a partial deficiency of Sp that correlated with a RBC morphology characteristic of HPP, namely, marked microspherocytosis with virtual absence of elliptocytes. These data demonstrate striking differences in the function and structure of various α Sp mutants that underlie differences in clinical expression.

Oxidative injury to hemoglobin (Hb) leads to formation of methemoglobin (MetHb), reversible hemichromes (rHCRs), and irreversible hemichromes (iHCRs), iHCRs precipitate and form Heinz bodies that attach to the red cell membrane causing injury that leads to hemolysis. The molecular mechanisms of this membrane damage have not been fully elucidated. We have studied the effect of Hb oxidation products on the mechanical stability of red cell membrane skeletons and the associations of membrane skeletal proteins. Hb and MetHb stabilized the isolated membrane skeletons, whereas further oxidation to rHCRs abolished this stabilizing effect. Crude iHCRs prepared by phenylhydrazine oxidation of Hb destabilized membrane skeletons by decreasing formation of the spectrin–protein 4.1–actin complex, the effect similar to that of pure hemin. Whereas virtually no hemin was released from Hb and MetHb, high concentrations of hemin were released from crude iHCR preparations. After removal of this hemin fraction by Dowex resin, the iHCRs lost their destabilizing effect. We conclude that as the oxidation of Hb proceeds, the stabilizing effect of Hb on the membrane skeleton is gradually lost and the deleterious effect increases. The destabilization of the red cell membrane skeleton in the presence of crude iHCR is caused by release of hemin, which lowers the stability of membrane skeleton by weakening the spectrin–protein 4.1–actin interaction.

To elucidate the molecular basis of band 3 deficiency in a recently defined subset of patients with autosomal dominant hereditary spherocytosis (HS), we screened band 3 cDNA for single-strand conformation polymorphism (SSCP). In 5 of 17 (29%) unrelated HS subjects with band 3 deficiency, we detected substitutions R760W, R760Q, R808C, and R870W that were all coinherited with the HS phenotype. The involved arginines are highly conserved throughout evolution. To examine whether or not the product of the mutant allele is inserted into the membrane, we studied one HS subject who was doubly heterozygous for the R760Q mutation and the K56E (band 3MEMPHIS) polymorphism that results in altered electrophoretic mobility of the band 3 Memphis proteolytic fragments. We detected only the band 3MEMPHISin the erythrocyte membrane indicating that the protein product of the mutant, R760Q, band 3 allele is absent from the red blood cell membrane. These findings suggest that the R760Q substitution, and probably the other arginine substitutions, produce band 3 deficiency either by precluding incorporation of the mutant protein into the red blood cell membrane or by leading to loss of mutant protein from differentiating erythroid precursors.

Lymphoplasmacytic lymphoma (LPL)/Waldenström macroglobulinemia (WM) is a B-cell disorder resulting from the accumulation of cloncal lymphoplasmacytic cells in the bone marrow (BM) along with excessive secretion of immunoglobulin M paraprotein. Despite recent advances in treatment LPL/WM remains incurable. Generating a refined LPL/WM mouse model that would facilitate studying this disease as well as enable stringent in vivovalidation of potential therapeutic approaches has been a long-sought goal for the filed. Interestingly, among the most common mutations identified by recent high-throughput studies there was a single gain of function point mutation in the MYD88 gene, with a predicted non-synonymous amino acid (L265P) change, found in over 91% of patients. Therefore, we hypothesized that generation of transgenic mice overexpressing MYD88L256Pcould recapitulate the human setting of LPL/WM.