A Non-random Mouse Model for Pharmacological Reactivation of Mecp2 on the Inactive X Chromosome

Summary

Here, we describe a protocol to generate a viable female murine model with non-random X chromosome inactivation, i.e., the maternally-inherited X chromosome is inactive in 100% of the cells. We also describe a protocol to test feasibility, tolerability, and safety of pharmacological reactivation of the inactive X chromosome in vivo.

Abstract

X chromosome inactivation (XCI) is the random silencing of one X chromosome in females to achieve gene dosage balance between the sexes. As a result, all females are heterozygous for X-linked gene expression. One of the key regulators of XCI is Xist, which is essential for the initiation and maintenance of XCI. Previous studies have identified 13 trans acting X chromosome inactivation factors (XCIFs) using a large-scale, loss-of-function genetic screen. Inhibition of XCIFs, such as ACVR1 and PDPK1, using short-hairpin RNA or small molecule inhibitors, reactivates X chromosome-linked genes in cultured cells. But the feasibility and tolerability of reactivating the inactive X chromosome in vivo remains to be determined. Towards this goal, a XistΔ:Mecp2/Xist:Mecp2-Gfp mouse model has been generated with non-random XCI due to deletion of Xist on one X chromosome. Using this model, the extent of inactive X reactivation was quantitated in the mouse brain following treatment with XCIF inhibitors. Recently published results show, for the first time, that pharmacological inhibition of XCIFs reactivates Mecp2 from the inactive X chromosome in cortical neurons of the living mouse brain.

Introduction

X chromosome inactivation (XCI) is a process of dosage compensation that balances X-linked gene expression by silencing one copy of the X chromosome in females¹. As a result, the inactive X chromosome (Xi) accumulates characteristic features of heterochromatin including DNA methylation and inhibitory histone modifications, such as histone H3-lysine 27 trimethylation (H3K27me3) and histone H2A ubiquitination (H2Aub)². The master regulator of X chromosome silencing is the X-inactivation center (Xic) region, around 100−500 kb, which controls the counting and pairing of the X chromosomes, the random choice of the X chr....

Protocol

Work involving mice was approved by the University of Virginia Institutional Animal Care and Use Committee (IACUC; #4112).

1. Generate a Non-random XCI Mouse Model with Genetically Labeled Mecp2 on Xi

NOTE: Mouse strains used in the study were as follows: Mecp2-Gfp/Mecp2-Gfp (Mecp2^tm3.1Bird, Table of Materials) and Xist/ΔXist (B6;129-Xist<tm5Sado>; provided by Antonio Bedalov, Fred Hutchinson Cancer Center, Seattle). Breeding strategies among the respective strains have been designed to expand the mouse colonies f....

Results

To demonstrate the feasibility of the XistΔ:Mecp2/Xist:Mecp2-Gfp mouse model for Xi reactivation studies, XCIF inhibitor-mediated reactivation of Xi-linked Mecp2-Gfp was tested in mouse embryonic fibroblasts (MEFs). Female MEFs were isolated from day 15.5 XistΔ:Mecp2/Xist:Mecp2-Gfp embryos as described in section 3 (Figure 1A). The genotypes of female XistΔ:Mecp2/Xist:Mecp2-Gfp MEFs were confirmed by genotyping-PCR, as described previou.......

Discussion

Previously, XCIFs that are selectively required for silencing of Xi-linked genes in mammalian female cells were identified¹². We further optimized potent small molecule inhibitors to target XCIFs, such as ACVR1 and downstream effectors of PDPK1, which efficiently reactivate Xi-linked Mecp2 in mouse fibroblast cell lines, mouse cortical neurons, and a human fibroblast cell line derived from a RTT patient. These results suggest that Xi reactivation is a plausible therapeutic approach to res.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
MICE
Mecp2tm3.1Bird	The Jackson Laboratory	#014610
B6;129-Xist (tm5Sado)	provided by Antonio Bedalov, Fred Hutchinson Cancer Center, Seattle
REAGENTS
22x22 mm coverslip	FISHERfinest (Fisher Scientific)	125488
32% Paraformaldehyde	Electron Microscopy Sciences	15714-S
50 ml syringe	Medline Industries	NPMJD50LZ
60mm culture dish	CellStar	628160
7-AAD	BioLegend	420403
ammonium chloride (NH4Cl)	Fisher Chemical	A661-3
anti-GFP-AlexaFluor647	Invitrogen	A-31852
anti-MAP2	Aves Labs	MAP
BSA	Promega	R396D
Buprenorphine SR	Zoopharm
citric acid	Sigma	C-1857
DMSO	Fisher Bioreagents	BP231-100
Dulbecco's Modified Eagle Medium (DMEM)	Corning Cellgro	10-013-CV
Ethanol	Decon Labs	2701
fetal bovine serum (FBS)	VWR Life Science	89510-198
gelatin	Sigma-Aldrich	G9391
glass slides	Fisherbrand	22-034-486
goat anti-chicken FITC-labeled secondary antibody	Aves Labs	F-1005
GSK650394	ApexBio	B1051
hamilton 10μl syringe	Hamilton Sigma-Aldrich	28615-U
Hank's Balanced Salt Solution (HBSS)	Gibco	14025-092
Ketamine	Ketaset	NDC 0856-2013-01
Large blunt/blunt curved scissors	Fine Science Tools	14519-14
LDN193189	Cayman Chemicals	11802
lodixanol	Sigma	1343517
magnesium chloride (MgCl2)	Fisher Chemical	M35-212
Methylcelulose	Sigma	M0262-100G
mounting medium with DAPI	Vectashield	H-1200
Needle tip, 26 GA x 1.25"	PrecisionGlide	305111
ophthalmic ointment	Refresh Lacri-Lube	93468
optimal cutting temperature (O.C.T.)	ThermoFisher
PCR mix
Penicillin/Streptomycin (Pen/Strep)	Corning	30-002-Cl
Phosphate buffered saline pH 7.4 (PBS)	Corning Cellgro	46-103-CM
Potassium chloride (KCl)	Fisher Scientific	P330-500
scalpel blades
Shallow glass or plastic tray
skin glue/tissue adhesive	3M Vetbond	1469SB
sodium azide	Fisher Scientific	CAS 26628-22-8
Sodium chloride (NaCl)	Fisher Chemical	S642-212
standard hemostat forceps	Fine Science Tools	13013-14
Standard tweezers	Fine Science Tools	11027-12
Straight iris scissors	Fine Science Tools	14058-11
sucrose	Fisher Scientific	BP220-1
Tris-base	Fisher Bioreagents	BP152-5
Triton X-100	Fisher Bioreagents	BP151-500
Trypsin-EDTA	Gibco	15400-054
Xylazine	Akorn	NDC: 59399-111-50
EQUIPMENT
Zeiss AxioObserver Live-Cell microscope	Zeiss	Zeiss AxioObserver
0.45mm burr	IDEAL MicroDrill	67-1000
BD FACScalibur
centrifuge
glass homogenizer
cell culture incubator	Thermo Scientific HERACELL VIOS 160i	13-998-213
Leica 3050S research cryostat
stereotactic platform
thermocycler
Timer
ultracentrifuge	Beckman Coulter Optima L-100 XP
Water bath (37 ºC)	Fisher Scientific Isotemp 2239