The neighborhood relationships of chromosomes can be of great importance for basic cellular processes such as gene expression or translocation induction. In this study, the topological organization of chromosomes 9 and 22 was investigated in cell nuclei of G0-phase lymphocytes. We found that the territories of both chromosomes are predominantly located in the central region of cell nuclei. In addition to this, chromosomes 9 and 22 were frequently associated in pairs detected as false-positive ABL-BCR fusions. Both effects might substantially increase the probability of interaction between chromosomes. Because of this, exchange aberrations were studied in chromosomes 9 and 22 of human lymphocytes irradiated by neutrons. The rate of aberration induction between these chromosomes was 11 times higher than the expected frequency based on the fractional molecular weight of these chromosomes. We show that the increased rate of exchange between chromosomes 9 and 22 induced by neutrons corresponds to the neighborhood relationships of both chromosomes. Similar topological characteristics of ABL and BCR genes were found in several cell lines: T- and B-lymphocytes. HL60 cells and bone marrow cells. This finding suggests that the specific chromatin structure mentioned might be responsible for the high rate of induction of t(9;22)-positive leukemias in the human population.

Follicular lymphoma (FL) is a common indolent B-cell malignancy with a variable clinical course. An unfavorable event in its course is histological transformation to a high-grade lymphoma, typically diffuse large B-cell lymphoma. Recent studies show that genetic aberrations of MYC or its overexpression are associated with FL transformation (tFL). However, the precise molecular mechanisms underlying tFL are unclear. Here we performed the first profiling of expression of microRNAs (miRNAs) in paired samples of FL and tFL and identified 5 miRNAs as being differentially expressed. We focused on one of these miRNAs, namely miR-150, which was uniformly downmodulated in all examined tFLs (∼3.5-fold), and observed that high levels of MYC are responsible for repressing miR-150 in tFL by binding in its upstream region. This MYC-mediated repression of miR-150 in B cells is not dependent on LIN28A/B proteins, which influence the maturation of miR-150 precursor (pri-miR-150) in myeloid cells. We also demonstrated that low miR-150 levels in tFL lead to upregulation of its target, namely FOXP1 protein, which is a known positive regulator of cell survival, as well as B-cell receptor and NF-κB signaling in malignant B cells. We revealed that low levels of miR-150 and high levels of its target, FOXP1, are associated with shorter overall survival in FL and suggest that miR-150 could serve as a good biomarker measurable in formalin-fixed paraffin-embedded tissue. Overall, our study demonstrates the role of the MYC/miR-150/FOXP1 axis in malignant B cells as a determinant of FL aggressiveness and its high-grade transformation.

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