Regulation of mir-196b by MLL and its overexpression by MLL fusions contributes to immortalization.

Chromosomal translocations involving the Mixed Lineage Leukemia (MLL) gene produce chimeric proteins that cause abnormal expression of a subset of HOX genes and leukemia development. Here, we show that MLL normally regulates expression of mir-196b, a hematopoietic microRNA located within the HoxA cluster, in a pattern similar to that of the surrounding 5' Hox genes, Hoxa9 and Hoxa10, during embryonic stem (ES) cell differentiation. Within the hematopoietic lineage, mir-196b is most abundant in short-term hematopoietic stem cells and is down-regulated in more differentiated hematopoietic cells. Leukemogenic MLL fusion proteins cause overexpression of mir-196b, while treatment of MLL-AF9 transformed bone marrow cells with mir-196-specific antagomir abrogates their replating potential in methylcellulose. This demonstrates that mir-196b function is necessary for MLL fusion-mediated immortalization. Furthermore, overexpression of mir-196b was found specifically in patients with MLL associated leukemias as determined from analysis of 55 primary leukemia samples. Overexpression of mir-196b in bone marrow progenitor cells leads to increased proliferative capacity and survival, as well as a partial block in differentiation. Our results suggest a mechanism whereby increased expression of mir-196b by MLL fusion proteins significantly contributes to leukemia development.

Behavioral screening for nightblindness mutants in zebrafish reveals three new loci that cause dominant photoreceptor cell degeneration.

Here we report three dominant nightblindness mutations in zebrafish: nightblindness e (nbe), nightblindness f (nbf) and nightblindness g (nbg). The mutants were isolated in the F1 generation of N-ethyl-N-nitrosourea (ENU) mutagenized zebrafish using a behavioral assay based on visually mediated escape responses. Subsequently, electroretinographic (ERG) recordings were made, and histological sections were screened for degenerative processes. For each mutant line, correlation analysis between behavioral, ERG and histological parameters was performed, and their relationships were determined by either calculating the Pearson correlation coefficient or by ANOVA. nbe is characterized by severe rod outer segments (ROS) degeneration. The degeneration correlates weakly with behavioral threshold and ERG b-wave amplitude, however, behavioral threshold correlates strongly with ERG b-wave. nbf is characterized by a dual histological pathology: patchy ROS-degeneration and 'gaps' homogeneously distributed over the outer nuclei layer (ONL) and between cone outer segments (COS). The correlations between histological pathology and behavioral threshold, and between behavioral threshold and ERG b-wave amplitude are obvious, but the correlation between histology and b-wave amplitude is less prominent. nbg is characterized by moderate ROS degeneration and moderate correlation between histology and behavioral threshold. Interestingly, behavioral threshold correlated inversely with ERG b-wave amplitude and threshold. Thus, contrary to what is normally seen in other nightblindness mutants, in nbg, the fish with the lowest behavioral threshold had the smallest b-waves amplitudes and the highest b-wave threshold. In our interpretation, the major impairment in nbe is photoreceptor-specific. In nbf, both photoreceptor degeneration and altered post-photoreceptor signaling are responsible for the behavioral deficit. In nbg, we find hypersensitivity at a post-photoreceptoral level concurrently with behavioral impairment.