Correction: The SIRT1 Deacetylase Suppresses Intestinal Tumorigenesis and Colon Cancer Growth.

[This corrects the article DOI: 10.1371/journal.pone.0002020.].

X chromosome choice occurs independently of asynchronous replication timing.

In mammals, dosage compensation is achieved by X chromosome inactivation in female cells. Xist is required and sufficient for X inactivation, and Xist gene deletions result in completely skewed X inactivation. In this work, we analyzed skewing of X inactivation in mice with an Xist deletion encompassing sequence 5 KB upstream of the promoter through exon 3. We found that this mutation results in primary nonrandom X inactivation in which the wild-type X chromosome is always chosen for inactivation. To understand the molecular mechanisms that affect choice, we analyzed the role of replication timing in X inactivation choice. We found that the two Xist alleles and all regions tested on the X chromosome replicate asynchronously before the start of X inactivation. However, analysis of replication timing in cell lines with skewed X inactivation showed no preference for one of the two Xist alleles to replicate early in S-phase before the onset of X inactivation, indicating that asynchronous replication timing does not play a role in skewing of X inactivation.

The SIRT1 deacetylase suppresses intestinal tumorigenesis and colon cancer growth.

Numerous longevity genes have been discovered in model organisms and altering their function results in prolonged lifespan. In mammals, some have speculated that any health benefits derived from manipulating these same pathways might be offset by increased cancer risk on account of their propensity to boost cell survival. The Sir2/SIRT1 family of NAD(+)-dependent deacetylases is proposed to underlie the health benefits of calorie restriction (CR), a diet that broadly suppresses cancer in mammals. Here we show that CR induces a two-fold increase SIRT1 expression in the intestine of rodents and that ectopic induction of SIRT1 in a beta-catenin-driven mouse model of colon cancer significantly reduces tumor formation, proliferation, and animal morbidity in the absence of CR. We show that SIRT1 deacetylates beta-catenin and suppresses its ability to activate transcription and drive cell proliferation. Moreover, SIRT1 promotes cytoplasmic localization of the otherwise nuclear-localized oncogenic form of beta-catenin. Consistent with this, a significant inverse correlation was found between the presence of nuclear SIRT1 and the oncogenic form of beta-catenin in 81 human colon tumor specimens analyzed. Taken together, these observations show that SIRT1 suppresses intestinal tumor formation in vivo and raise the prospect that therapies targeting SIRT1 may be of clinical use in beta-catenin-driven malignancies.

Partial rescue of MeCP2 deficiency by postnatal activation of MeCP2.

In humans, mutations in the X-linked MECP2 gene, are the cause of Rett syndrome (RTT), a neurodevelopmental disorder that affects mainly girls. MeCP2 binds to methylated CpGs and is thought to act as a transcriptional repressor. In male mice, deletion or targeted mutation of Mecp2 leads to lethality and causes a neuronal phenotype. Selective mutation of Mecp2 in postnatal neurons results in a similar, although delayed, phenotype, suggesting that the symptoms are caused by MeCP2 deficiency in postmitotic neurons. In agreement with this idea, expression of a Mecp2 transgene in postmitotic neurons of Mecp2-null mutant mice resulted in the phenotypical rescue of the symptoms. To assess whether postnatal activation of MeCP2 in mutant animals could also affect the progression of the disorder, we constructed a conditionally active Mecp2 "rescue transgene" that was activated between P0 and P30. The Mecp2 transgene was under the control of the CAGGS promoter and was activated by using brain specific Cre-mediated recombination. Our results indicate that postnatal, neuron-specific activation of MeCP2 as late as 2-4 weeks of age significantly prolonged the lifespan of mutant animals and delayed the onset of neurologic symptoms.

Expression of MeCP2 in postmitotic neurons rescues Rett syndrome in mice.

Mutations in MECP2 are the cause of Rett syndrome (RTT) in humans, a neurodevelopmental disorder that affects mainly girls. MeCP2 is a protein that binds CpG dinucleotides and is thought to act as a global transcriptional repressor. It is highly expressed in neurons, but not in glia, of the postnatal brain. The timing of MeCP2 activation correlates with the maturation of the central nervous system, and recent reports suggest that MeCP2 may be involved in the formation of synaptic contacts and may function in activity-dependent neuronal gene expression. Deletion or targeted mutation of Mecp2 in mice leads to a Rett-like phenotype. Selective mutation of Mecp2 in postnatal neurons leads to a similar, although delayed, phenotype, suggesting that MeCP2 plays a role in postmitotic neurons. Here we test the hypothesis that the symptoms of RTT are exclusively caused by a neuronal MeCP2 deficiency by placing Mecp2 expression under the control of a neuron-specific promoter. Expression of the Mecp2 transgene in postmitotic neurons resulted in symptoms of severe motor dysfunction. Transgene expression in Mecp2 mutant mice, however, rescued the RTT phenotype.