MyoD inhibits Fstl1 and Utrn expression by inducing transcription of miR-206.

Terminal differentiation of distinct cell types requires the transcriptional activation of differentiation-specific genes and the suppression of genes associated with the precursor cell. For example, the expression of utrophin (Utrn) is suppressed during skeletal muscle differentiation, and it is replaced at the sarcolemma by the related dystrophin protein. The MyoD transcription factor directly activates the expression of a large number of skeletal muscle genes, but also suppresses the expression of many genes. To characterize a mechanism of MyoD-mediated suppression of gene expression, we investigated two genes that are suppressed in fibroblasts converted to skeletal muscle by MyoD, follistatin-like 1 (Fstl1) and Utrn. MyoD directly activates the expression of a muscle-specific microRNA (miRNA), miR-206, which targets sequences in the Fstl1 and Utrn RNA, and these sequences are sufficient to suppress gene expression in the presence of miR-206. These findings demonstrate that MyoD, in addition to activating muscle-specific genes, induces miRNAs that repress gene expression during skeletal muscle differentiation.

Essential role for Dicer during skeletal muscle development.

microRNAs (miRNAs) regulate gene expression post-transcriptionally by targeting mRNAs for degradation or by inhibiting translation. Dicer is an RNase III endonuclease which processes miRNA precursors into functional 21-23 nucleotide RNAs that are subsequently incorporated into the RNA-induced silencing complex. miRNA-mediated gene regulation is important for organogenesis of a variety of tissues including limb, lung and skin. To gain insight into the roles of Dicer and miRNAs in mammalian skeletal muscle development, we eliminated Dicer activity specifically in the myogenic compartment during embryogenesis. Dicer activity is essential for normal muscle development during embryogenesis and Dicer muscle mutants have reduced muscle miRNAs, die perinatally and display decreased skeletal muscle mass accompanied by abnormal myofiber morphology. Dicer mutant muscles also show increased apoptosis and Cre-mediated loss of Dicer in Myod-converted myoblasts results in enhanced cell death. These observations demonstrate key roles for Dicer in skeletal muscle and implicate miRNAs as critical components required for embryonic myogenesis.

Tax recruitment of CBP/p300, via the KIX domain, reveals a potent requirement for acetyltransferase activity that is chromatin dependent and histone tail independent.

Robust transcription of human T-cell leukemia virus type 1 (HTLV-1) genome requires the viral transactivator Tax. Although Tax has been previously shown to interact with the KIX domain of CBP/p300 in vitro, the precise functional relevance of this interaction remains unclear. Using two distinct approaches to interrupt the physical interaction between Tax and KIX, we find that Tax transactivation from chromatin templates is strongly dependent on CBP/p300 recruitment via the KIX domain. Additionally, we find that the primary functional contribution of CBP/p300 to Tax transactivation resides in the intrinsic acetyltransferase activity of the coactivators. These studies unexpectedly uncover a specific requirement for CBP/p300 acetyltransferase activity on chromatin templates assembled with nucleosomes lacking their amino-terminal tails. Together, these data indicate that the KIX domain of CBP/p300 is essential for targeting the acetyltransferase activity of the coactivator to the Tax-CREB (Tax/CREB) complex. Significantly, these observations reveal the presence of one or more CBP/p300 acetyltransferase targets that function specifically on chromatin templates, are independent of the histone tails, and are critical to Tax transactivation.

p300-mediated tax transactivation from recombinant chromatin: histone tail deletion mimics coactivator function.

Efficient transcription of the human T-cell leukemia virus type 1 (HTLV-1) genome requires Tax, a virally encoded oncogenic transcription factor, in complex with the cellular transcription factor CREB and the coactivators p300/CBP. To examine Tax transactivation in vitro, we used a chromatin assembly system that included recombinant core histones. The addition of Tax, CREB, and p300 to the HTLV-1 promoter assembled into chromatin activated transcription several hundredfold. Chromatin templates selectively lacking amino-terminal histone tails demonstrated enhanced transcriptional activation by Tax and CREB, with significantly reduced dependence on p300 and acetyl coenzyme A (acetyl-CoA). Interestingly, Tax/CREB activation from the tailless chromatin templates retained a substantial requirement for acetyl-CoA, indicating a role for acetyl-CoA beyond histone acetylation. These data indicate that during Tax transcriptional activation, the amino-terminal histone tails are the major targets of p300 and that tail deletion and acetylation are functionally equivalent.

Coordinated regulation of cell cycle transcripts by p53-Inducible microRNAs, miR-192 and miR-215.

Cell cycle arrest in response to DNA damage is an important antitumorigenic mechanism. MicroRNAs (miRNAs) were recently shown to play key regulatory roles in cell cycle progression. For example, miR-34a is induced in response to p53 activation and mediates G(1) arrest by down-regulating multiple cell cycle-related transcripts. Here we show that genotoxic stress promotes the p53-dependent up-regulation of the homologous miRNAs miR-192 and miR-215. Like miR-34a, activation of miR-192/215 induces cell cycle arrest, suggesting that multiple miRNA families operate in the p53 network. Furthermore, we define a downstream gene expression signature for miR-192/215 expression, which includes a number of transcripts that regulate G(1) and G(2) checkpoints. Of these transcripts, 18 transcripts are direct targets of miR-192/215, and the observed cell cycle arrest likely results from a cooperative effect among the modulations of these genes by the miRNAs. Our results showing a role for miR-192/215 in cell proliferation combined with recent observations that these miRNAs are underexpressed in primary cancers support the idea that miR-192 and miR-215 function as tumor suppressors.

An antisense transcript spanning the CGG repeat region of FMR1 is upregulated in premutation carriers but silenced in full mutation individuals.

Expansion of the polymorphic CGG repeats within the 5'-UTR of the FMR1 gene is associated with variable transcriptional regulation of FMR1. Here we report a novel gene, ASFMR1, overlapping the CGG repeat region of FMR1 and transcribed in the antisense orientation. The ASFMR1 transcript is spliced, polyadenylated and exported to the cytoplasm. Similar to FMR1, ASFMR1 is upregulated in individuals with premutation alleles and is not expressed from full mutation alleles. Moreover, it exhibits premutation-specific alternative splicing. Taken together, these observations suggest that in addition to FMR1, ASFMR1 may contribute to the variable phenotypes associated with the CGG repeat expansion.