Atypical presentation of invasive myoepithelioma in a pediatric patient.

A 15-year-old girl presented with a mobile lesion with yellowish hue on the posterior lamella of the right lower eyelid adjacent to the punctum. Four years prior, a lesion thought to be a chalazion was excised from the same location. There was subsequent progressive painless enlargement. The patient ultimately underwent an internal excision of the mass. Histopathology demonstrated infiltrative nests and cords of epithelioid and plasmacytoid cells with abundant eosinophilic myxoid-collagenous stroma along with focal infiltration of skeletal muscle with immunohistochemical staining consistent with an invasive myoepithelioma of mixed-cell type. Given concern for invasive disease, the patient underwent subsequent Mohs resection resulting in a 25% full thickness eyelid defect, which was repaired with direct closure of the wedge defect. There has been no recurrence of the disease for 7 months since the Mohs resection. This case illustrates the atypical presentation of an invasive myoepithelioma of the eyelid in a pediatric patient.

Evidence of Xist RNA-independent initiation of mouse imprinted X-chromosome inactivation.

XX female mammals undergo transcriptional silencing of most genes on one of their two X chromosomes to equalize X-linked gene dosage with XY males in a process referred to as X-chromosome inactivation (XCI). XCI is an example of epigenetic regulation. Once enacted in individual cells of the early female embryo, XCI is stably transmitted such that most descendant cells maintain silencing of that X chromosome. In eutherian mammals, XCI is thought to be triggered by the expression of the non-coding Xist RNA from the future inactive X chromosome (Xi); Xist RNA in turn is proposed to recruit protein complexes that bring about heterochromatinization of the Xi. Here we test whether imprinted XCI, which results in preferential inactivation of the paternal X chromosome (Xp), occurs in mouse embryos inheriting an Xp lacking Xist. We find that silencing of Xp-linked genes can initiate in the absence of paternal Xist; Xist is, however, required to stabilize silencing along the Xp. Xp-linked gene silencing associated with mouse imprinted XCI, therefore, can initiate in the embryo independently of Xist RNA.

ABCG1-mediated generation of extracellular cholesterol microdomains.

Previous studies have demonstrated that the ATP-binding cassette transporters (ABC)A1 and ABCG1 function in many aspects of cholesterol efflux from macrophages. In this current study, we continued our investigation of extracellular cholesterol microdomains that form during enrichment of macrophages with cholesterol. Human monocyte-derived macrophages and mouse bone marrow-derived macrophages, differentiated with macrophage colony-stimulating factor (M-CSF) or granulocyte macrophage colony-stimulation factor (GM-CSF), were incubated with acetylated LDL (AcLDL) to allow for cholesterol enrichment and processing. We utilized an anti-cholesterol microdomain monoclonal antibody to reveal pools of unesterified cholesterol, which were found both in the extracellular matrix and associated with the cell surface, that we show function in reverse cholesterol transport. Coincubation of AcLDL with 50 µg/ml apoA-I eliminated all extracellular and cell surface-associated cholesterol microdomains, while coincubation with the same concentration of HDL only removed extracellular matrix-associated cholesterol microdomains. Only at an HDL concentration of 200 µg/ml did HDL eliminate the cholesterol microdomains that were cell-surface associated. The deposition of cholesterol microdomains was inhibited by probucol, but it was increased by the liver X receptor (LXR) agonist TO901317, which upregulates ABCA1 and ABCG1. Extracellular cholesterol microdomains did not develop when ABCG1-deficient mouse bone marrow-derived macrophages were enriched with cholesterol. Our findings show that generation of extracellular cholesterol microdomains is mediated by ABCG1 and that reverse cholesterol transport occurs not only at the cell surface but also within the extracellular space.

Differentiation-dependent requirement of Tsix long non-coding RNA in imprinted X-chromosome inactivation.

Imprinted X-inactivation is a paradigm of mammalian transgenerational epigenetic regulation resulting in silencing of genes on the paternally inherited X-chromosome. The preprogrammed fate of the X-chromosomes is thought to be controlled in cis by the parent-of-origin-specific expression of two opposing long non-coding RNAs, Tsix and Xist, in mice. Exclusive expression of Tsix from the maternal-X has implicated it as the instrument through which the maternal germline prevents inactivation of the maternal-X in the offspring. Here, we show that Tsix is dispensable for inhibiting Xist and X-inactivation in the early embryo and in cultured stem cells of extra-embryonic lineages. Tsix is instead required to prevent Xist expression as trophectodermal progenitor cells differentiate. Despite induction of wild-type Xist RNA and accumulation of histone H3-K27me3, many Tsix-mutant X-chromosomes fail to undergo ectopic X-inactivation. We propose a novel model of lncRNA function in imprinted X-inactivation that may also apply to other genomically imprinted loci.