SUMMARYMammals compensate X chromosome gene dosage between the sexes by silencing of one of the two female X chromosomes. X inactivation is initiated in the early embryo and requires the non-coding Xist RNA, which encompasses the inactive X chromosome (Xi) and triggers its silencing. In differentiated cells, several factors including the histone variant macroH2A and the scaffold attachment factor SAF-A are recruited to the Xi and maintain its repression. Consequently, in female somatic cells the Xi remains stably silenced independently of Xist. Here, we identify the Trithorax group protein Ash2l as a novel component of the Xi. Ash2l is recruited by Xist concomitantly with Saf-A and macroH2A at the transition to Xi maintenance. Recruitment of these factors characterizes a developmental transition point for the chromatin composition of the Xi. Surprisingly, expression of a mutant Xist RNA that does not cause gene repression can trigger recruitment of Ash2l, Saf-A and macroH2A to the X chromosome, and can cause chromosome-wide histone H4 hypoacetylation. This suggests that a chromatin configuration is established on non-genic chromatin on the Xi by Xist to provide a repressive compartment that could be used for maintaining gene silencing. Gene silencing is mechanistically separable from the formation of this repressive compartment and, thus, requires additional pathways. This observation highlights a crucial role for spatial organization of chromatin changes in the maintenance of X inactivation.

Epitranscriptomic events such as adenosine‐to‐inosine (A‐to‐I) RNA editing by ADAR can recode mRNAs to translate novel proteins. Editing of the mRNA that encodes actin crosslinking protein Filamin A (FLNA) mediates a Q‐to‐R transition in the interactive C‐terminal region. While FLNA editing is conserved among vertebrates, its physiological function remains unclear. Here, we show that cardiovascular tissues in humans and mice show massive editing and that FLNA RNA is the most prominent substrate. Patient‐derived RNA‐Seq data demonstrate a significant drop in FLNA editing associated with cardiovascular diseases. Using mice with only impaired FLNA editing, we observed increased vascular contraction and diastolic hypertension accompanied by increased myosin light chain phosphorylation, arterial remodeling, and left ventricular wall thickening, which eventually causes cardiac remodeling and reduced systolic output. These results demonstrate a causal relationship between RNA editing and the development of cardiovascular disease indicating that a single epitranscriptomic RNA modification can maintain cardiovascular health.

X inactivation is the mechanism by which mammals adjust the genetic imbalance that arises from the different numbers of gene-rich X chromosomes between the sexes. The dosage difference between XX females and XY males is functionally equalized by silencing one of the two X chromosomes in females. This dosage-compensation mechanism seems to have arisen concurrently with early mammalian evolution and is based on the long functional Xist RNA, which is unique to placental mammals. It is likely that previously existing mechanisms for other cellular functions have been recruited and adapted for the evolution of X inactivation. Here, we critically review our understanding of dosage compensation in placental mammals and place these findings in the context of other cellular processes that intersect with mammalian dosage compensation.

BACKGROUND Recent studies show that vascular endothelial growth factor (VEGF) and its receptors Flt‐1 and KDR, and a series of other angiogenic molecules, are upregulated in advanced but not low stage human neuroblastoma. Neuropilin‐1 and 2 (NRP) are novel specific receptors of VEGF165, whose role is unknown in human neuroblastoma. METHODS Tissue biopsies of 37 children with Stage I‐IV neuroblastoma were obtained, as well as biopsies of 7 normal adrenals as controls. The mRNA expression of VEGF165 and its receptors Flt‐1, KDR, NRP1, and NRP2 was evaluated by real‐time reverse transcripton polymerase chain reaction. NRP protein expression was detected by immunocytochemistry and Western blotting. RESULTS VEGF165 mRNA was upregulated in Stage III and IV and Flt‐1 and KDR gene expression was increased in Stage III, while NRP1 and 2 mRNA and protein levels were higher in Stages I‐IV vs. controls (P < 0.05). NRP was expressed in vascular endothelial but not tumor cells. CONCLUSIONS These results show for the first time that human neuroblastoma expresses NRP, and that NRP co‐regulates VEGF angiogenic effect in human neuroblastoma. NRP might be a sensitive angiogenic measure of VEGF systems in neuroblastoma, particularly in its early stages. Cancer 2002;94:258–63. © 2002 American Cancer Society.