MicroRNA-126-mediated control of cell fate in B-cell myeloid progenitors as a potential alternative to transcriptional factors.

Lineage specification is thought to be largely regulated at the level of transcription, where lineage-specific transcription factors drive specific cell fates. MicroRNAs (miR), vital to many cell functions, act posttranscriptionally to decrease the expression of target mRNAs. MLL-AF4 acute lymphocytic leukemia exhibits both myeloid and B-cell surface markers, suggesting that the transformed cells are B-cell myeloid progenitor cells. Through gain- and loss-of-function experiments, we demonstrated that microRNA 126 (miR-126) drives B-cell myeloid biphenotypic leukemia differentiation toward B cells without changing expression of E2A immunoglobulin enhancer-binding factor E12/E47 (E2A), early B-cell factor 1 (EBF1), or paired box protein 5, which are critical transcription factors in B-lymphopoiesis. Similar induction of B-cell differentiation by miR-126 was observed in normal hematopoietic cells in vitro and in vivo in uncommitted murine c-Kit(+)Sca1(+)Lineage(-) cells, with insulin regulatory subunit-1 acting as a target of miR-126. Importantly, in EBF1-deficient hematopoietic progenitor cells, which fail to differentiate into B cells, miR-126 significantly up-regulated B220, and induced the expression of B-cell genes, including recombination activating genes-1/2 and CD79a/b. These data suggest that miR-126 can at least partly rescue B-cell development independently of EBF1. These experiments show that miR-126 regulates myeloid vs. B-cell fate through an alternative machinery, establishing the critical role of miRNAs in the lineage specification of multipotent mammalian cells.

Mucocele-like lesions in the breast diagnosed with percutaneous biopsy: is surgical excision necessary?

OBJECTIVE: The purpose of this study was to determine the frequencies of atypia and cancer at excisional biopsy of lesions with a diagnosis of mucocele-like lesion (MLL) at percutaneous breast biopsy. MATERIALS AND METHODS: Retrospective review of 9286 lesions subjected to percutaneous imaging-guided biopsy identified MLLs in 35 (0.38%) patients. Medical records, imaging studies, and histologic results were reviewed. RESULTS: Of the 35 patients with core biopsy findings of MLL, 27 underwent stereotactic core needle biopsy (19 with microcalcifications, five with calcifications with an associated mass, and three with only a mass), and eight underwent ultrasound-guided core needle biopsy (four with a solid mass, three with a complex cystic mass). At core biopsy, 12 of 35 (34%) MLLs were associated with atypia (10 cases of atypical ductal hyperplasia, two of flat epithelial atypia), and 23 of 35 (66%) were benign MLL only. All 12 MLLs associated with atypia and 12 of 23 benign MLLs were surgically excised. Eleven patients did not undergo surgery, five of whom were lost to follow-up. One of the 12 (3% of the 35) MLLs associated with atypia was upgraded to DCIS. None were upgraded to invasive cancer. None of the benign MLLs were upgraded to malignancy, and findings at excision of four of the 23 (17%) benign MLLs led to a change in diagnosis to a high-risk lesion (three atypical ductal hyperplasia, one atypical lobular hyperplasia). CONCLUSION: MLL is a rare diagnosis but is encountered in large-volume breast practices. The findings are nonspecific with a range of imaging appearances. No imaging test is reliable for differentiating MLL from other suspicious lesions or lesions with associated atypia. Surgery is clearly warranted for MLL associated with atypia at core needle biopsy because it may be upgraded to malignancy upon excision. However, if the presence of atypia at excision of benign MLL will change clinical management, then benign MLL at core needle biopsy warrants surgical excision in some cases. In patients whose treatment will not change if atypia is found at excision, close surveillance with short-interval follow-up is a reasonable alternative.

miR-128b is a potent glucocorticoid sensitizer in MLL-AF4 acute lymphocytic leukemia cells and exerts cooperative effects with miR-221.

MLL-AF4 acute lymphocytic leukemia (ALL) has a poor prognosis. MicroRNAs (miRNA) are small noncoding RNAs that posttranscriptionally regulate expression of target mRNAs. Our analysis of previously published data showed that expression of miR-128b and miR-221 is down-regulated in MLL-rearranged ALL relative to other types of ALL. Reexpression of these miRNAs cooperatively sensitizes 2 cultured lines of MLL-AF4 ALL cells to glucocorticoids. Target genes down-regulated by miR-128b include MLL, AF4, and both MLL-AF4 and AF4-MLL fusion genes; miR-221 down-regulates CDKN1B. These results demonstrate that down-regulation of miR-128b and miR-221 is implicated in glucocorticoid resistance and that restoration of their levels is a potentially promising therapeutic in MLL-AF4 ALL.

Alteration of processing induced by a single nucleotide polymorphism in pri-miR-126.

MicroRNAs (miRNAs) are small non-coding RNAs that inhibit expression of specific target genes at the post-transcriptional level. Sequence variations in miRNA genes, including pri-miRNAs, pre-miRNAs and mature miRNAs, could potentially influence the processing and/or target selection of miRNAs. In this study, we have found the single nucleotide polymorphism (SNP) at the twenty-fourth nucleotide (+24) of the mature miR-126 in the genome of RS4;11 cells, derived from a MLL-AF4 ALL patient. Through a series of in vivo analyzes, we found that this miR-126 SNP significantly blocks the processing of pri-miRNA to mature miRNA, as well as reduces miRNA-mediated translational suppression. Moreover, its frequency is different among races. Thus, our study emphasizes the importance of identifying new miRNA SNP and its contribution to miRNA biogenesis which is possible link to human genetic disease.

A novel mutation in the miR-128b gene reduces miRNA processing and leads to glucocorticoid resistance of MLL-AF4 acute lymphocytic leukemia cells.

MLL-AF4 acute lymphocytic leukemia has a poor prognosis, and the mechanisms by which these leukemias develop are not understood despite intensive research based on well-known concepts and methods. MicroRNAs (miRNAs) are a new class of small noncoding RNAs that post-transcriptionally regulate expression of target mRNA transcripts. We recently reported that ectopic expression of miR-128b together with miR-221, two of the miRNAs downregulated in MLL-AF4 ALL, restores glucocorticoid resistance through downregulation of the MLL-AF4 chimeric fusion proteins MLL-AF4 and AF4-MLL that are generated by chromosomal translocation t(4;11). Here we report the identification of new mutations in miR-128b in RS4;11 cells, derived from MLL-AF4 ALL patient. One novel mutation significantly reduces the processing of miR-128b. Finally, this base change occurs in a primary MLL-AF4 ALL sample as an acquired mutation. These results demonstrate that the novel mutation in miR-128b in MLL-AF4 ALL alters the processing of miR-128b and that the resultant downregulation of mature miR-128b contributes to glucocorticoid resistance through the failure to downregulate the fusion oncogenes.