Requirement for dicer in survival of proliferating thymocytes experiencing DNA double-strand breaks

DICER-dependent biogenesis of let-7 miRNAs affects human cell response to DNA damage via targeting p21/p27

Recently, it was reported that knockdown of DICER reduced the ATM-dependent DNA damage response and homologous recombination repair (HRR) via decreasing DICER-generated small RNAs at the damage sites. However, we found that knockdown of DICER dramatically increased cell resistance to camptothecin that induced damage required ATM to facilitate HRR. This phenotype is due to a prolonged G1/S transition via decreasing DICER-dependent biogenesis of miRNA let-7, which increased the p21(Waf1/Cip1)/p27(Kip1) levels and resulted in decreasing the HRR efficiency. These results uncover a novel function of DICER in regulating the cell cycle through miRNA biogenesis, thus affecting cell response to DNA damage.

The microRNA biogenesis machinery modulates lineage commitment during αβ T cell development

Differentiation of CD4(+) helper and CD8(+) cytotoxic αβ T cells from CD4(+)CD8(+) thymocytes involves upregulation of lineage-specifying transcription factors and transcriptional silencing of CD8 or CD4 coreceptors, respectively, in MHC class II or I (MHCII or I)-restricted thymocytes. In this study, we demonstrate that inactivation of the Dicer RNA endonuclease in murine thymocytes impairs initiation of Cd4 and Cd8 silencing, leading to development of positively selected MHCI- and MHCII-restricted mature CD4(+)CD8(+) thymocytes. Expression of the antiapoptotic BCL2 protein or inactivation of the p53 proapoptotic protein rescues these thymocytes from apoptosis, increasing their frequency and permitting accumulation of CD4(+)CD8(+) αβ T cells in the periphery. Dicer-deficient MHCI-restricted αβ T cells fail to normally silence Cd4 and display impaired induction of the CD8 lineage-specifying transcription factor Runx3, whereas Dicer-deficient MHCII-restricted αβ T cells show impaired Cd8 silencing and impaired induction of the CD4 lineage-specifying transcription factor Thpok. Finally, we show that the Drosha RNA endonuclease, which functions upstream of Dicer in microRNA biogenesis, also regulates Cd4 and Cd8 silencing. Our data demonstrate a previously dismissed function for the microRNA biogenesis machinery in regulating expression of lineage-specifying transcription factors and silencing of Cd4 and Cd8 during αβ T cell differentiation.

MicroRNA Machinery Genes as Novel Biomarkers for Cancer

MicroRNAs (miRNAs) directly and indirectly affect tumorigenesis. To be able to perform their myriad roles, miRNA machinery genes, such as Drosha, DGCR8, Dicer1, XPO5, TRBP, and AGO2, must generate precise miRNAs. These genes have specific expression patterns, protein-binding partners, and biochemical capabilities in different cancers. Our preliminary analysis of data from The Cancer Genome Atlas consortium on multiple types of cancer revealed significant alterations in these miRNA machinery genes. Here, we review their biological structures and functions with an eye toward understanding how they could serve as cancer biomarkers.

Noncore RAG1 regions promote Vβ rearrangements and αβ T cell development by overcoming inherent inefficiency of Vβ recombination signal sequences

The RAG proteins are comprised of core endonuclease domains and noncore regions that modulate endonuclease activity. Mutation or deletion of noncore RAG regions in humans causes immunodeficiency and altered TCR repertoire, and mice expressing core but not full-length Rag1 (Rag1(C/C)) or Rag2 (Rag2(C/C)) exhibit lymphopenia, reflecting impaired V(D)J recombination and lymphocyte development. Rag1(C/C) mice display reduced D-to-J and V-to-DJ rearrangements of TCRβ and IgH loci, whereas Rag2(C/C) mice show decreased V-to-DJ rearrangements and altered Vβ/VH repertoire. Because Vβs/VHs only recombine to DJ complexes, the Rag1(C/C) phenotype could reflect roles for noncore RAG1 regions in promoting recombination during only the D-to-J step or during both steps. In this study, we demonstrate that a preassembled TCRβ gene, but not a preassembled DβJβ complex or the prosurvival BCL2 protein, completely rescues αβ T cell development in Rag1(C/C) mice. We find that Rag1(C/C) mice exhibit altered Vβ utilization in Vβ-to-DJβ rearrangements, increased usage of 3'Jα gene segments in Vα-to-Jα rearrangements, and abnormal changes in Vβ repertoire during αβ TCR selection. Inefficient Vβ/VH recombination signal sequences (RSSs) have been hypothesized to cause impaired V-to-DJ recombination on the background of a defective recombinase as in core-Rag mice. We show that replacement of the Vβ14 RSS with a more efficient RSS increases Vβ14 recombination and rescues αβ T cell development in Rag1(C/C) mice. Our data indicate that noncore RAG1 regions establish a diverse TCR repertoire by overcoming Vβ RSS inefficiency to promote Vβ recombination and αβ T cell development, and by modulating TCRβ and TCRα gene segment utilization.