Micro-RNA Gene Deregulation by Chromosome Translocations with Fragile Genomic Regions in B-Cell Lymphoma Cells
Authors: Schneider Bjoern , Nagel Stefan , Kaufmann Maren , Drexler Hans G. , MacLeod Roderick A.F. 
Published: October 2019

Micro-RNA (miR) genes posttranscriptionally modulate target gene expression via imperfect 3′-UTR matching sequences and play key roles in development, homeostasis and cancer. Little is known how miR genes are themselves regulated, or deregulated in cancer. Chief paradigm for neoplastic miR deregulation concerns miR-17/92 cluster members subject to genomic amplification in B-cell lymphoma. While the repeated occurrence of oncogenic miR genes at or near chromosomal breakpoints in cancer links chromosome fragility to oncogenic miR deregulation, direct evidence of a causal connection remains tenuous. We found that t(3;7)(q27;q32) in a B-cell lymphoma cell line joins 5′-BCL6 to a noncoding region of chromosome 7 inside a common chromosomal fragile site (FRA7H). In these cells hybrid mRNA was absent, unlike canonical BCL6 translocations which involve promoter exchange yielding hybrid mRNA. Affected cells instead showed downregulation of miR-29b-1, the only gene located within FRA7H - a recurrent transcriptional feature of B-cell lymphoma subsets. In another BCL6 translocation, t(3;13)(q27;q31)t(13;12)(q31;p11), which 5′-RACE also showed to be non-fusogenic, long distance inverse (LDI)-PCR revealed junction of 5′-BCL6 to chromosome 13 sequences inside the miR-17/92 host gene MIRH1 (alias c13orf25). FISH using a sensitive tyramide amplification protocol with c13orf25 clones confirmed the presence of a cryptic BCL6-MIRH1 rearrangement. Surprisingly, reverse transcriptase quantitative (q) PCR assay revealed weak MIRH1 expression using 3′-primers. In contrast, repeating the assay using more central primers covering the miR-17/92 coding region showed massive upregulation. 3′-RACE confirmed a novel high level MIRH1 transcript truncated by 3.1 kbp. Quantitative genomic PCR and FISH excluded miR-17/92 genomic copy number alteration, while LDI-PCR analysis showed that formation of truncated MIRH1 involved multiple DNA cuts at 3q27 (x1), 12p11 (x1), and 13q31 (x5) – the last including a complex excision/inversion/insertion rearrangement. Stress induced DNA duplex destabilization (SIDD) analysis revealed that 6 of 7 breaks precisely coincided with fragility peaks. Taken together, these data suggest a novel role for BCL6 translocations in the deregulation of miR genes near sites of chromosome or DNA instability. BCL6 has been shown to suppress p53 in germinal center B-cells thus protecting B-cells from apoptosis induced by DNA damage, offering a possible explanation for chromosome rearrangements associated with genomic fragility therein. Chromosomal MIRH1 dysregulation is not limited to BCL6 expressing lymphomas, however: cytogenetic investigations performed on diverse leukemia-lymphoma cell lines, including those derived from multiple myeloma and plasma cell leukemia, showed 11/50 with cytogenetic rearrangements at or near MIRH1. In sister cell lines sequentially established at diagnosis and relapse of multiple myeloma, only the latter showed miR-17/92 chromosomal rearrangement and upregulation. Interestingly miR overexpression was limited to miR-92, while miR-17/18 were barely expressed. FISH analysis and qPCR showed that discrepant expression was associated with rearrangement upstream of MIRH1. In brief, our data show that like other cancer genes, oncogenic miRs are subject to dysregulation mediated by structural chromosome rearrangements.