Interaction of the miR-106a~363 microRNA cluster and the p27Kip1 CDK inhibitor in T cell development and lymphomagenesis
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MicroRNAs are 21-22 nucleotide RNAs that regulate gene expression by binding mRNA 3' UTRs and inhibiting translation or targeting them for degradation. The Xpcl1 locus, encoding the miR-106a~363 miRNA cluster, was first identified as a common integration site in a Moloney murine leukemia virus (M-MuLV) screen for oncogenes cooperating with p27Kip1 loss. The miR-106a~363 cluster is one of three paralogous clusters that have been implicated in oncogenesis and regulate development. In this dissertation I have examined the gene expression profiles associated with the M-MuLV tumors and validated that the viral integrations at Xpcl1 induced expression of the miR-106a~363 miRNAs. The gene expression profile associated with Xpcl1 tumors involved functional classes similar to p27-/- tumors, but with an opposite pattern of expression. This suggests that p27Kip1 loss may overcome an anti-oncogenic effect of the miRNAs. I also determined that miR-106a~363 is primarily expressed in T cells and is differentially expressed during T cell development. To validate miR-106a~363 as an oncogene, I generated transgenic mice overexpressing miR-106a~363 in T cells using the Lck promoter (Lx+). These mice developed a T cell developmental phenotype characterized by increased DP thymocytes and a deficiency in SP thymocytes, but have normal numbers of mature T cells. I determined that the miR-106a~363 cluster inhibits expression of CD69, a negative regulator of lymphocyte egress from lymphoid organs. This suggests that the developmental phenotype is caused by an altered rate of thymocyte egress due to inhibition of CD69. The Lx+ mice also spontaneously develop T cell lymphomas with a 46% penetrance by one year. There is a synergistic effect of p27Kip1 loss and miR-106a~363 overexpression, and double mutant mice develop lymphomas with a 94% penetrance by 28 weeks. The data suggests that the synergy is due to p27Kip1 loss overcoming a miRNA induced anti-oncogenic increase in p27Kip1 expression. The increase in p27Kip1 occurs at the transcriptional level through the FoxO3a and FoxO4 transcription factors. The work in this dissertation validates miR-106a~363 as an oncogene, identifies its potential involvement in regulating T cell development, and determines the mechanism of cooperation between the miRNAs and p27Kip1 loss in lymphomagenesis.