Expression and localization of nuclear proteins in autosomal-dominant Emery-Dreifuss muscular dystrophy with LMNA R377H mutation

Ruth Klafke,Andrea Ewald,Clemens R Müller,Marie-Christine Dabauvalle,Christine Dreger,Isabell Motsch,Heinz Reichmann,Beate Reichart,Eleonora Krüger,Jochen Schäfer

doi:10.1186/1471-2121-5-12

Abstract

BackgroundThe autosomal dominant form of Emery-Dreifuss muscular dystrophy (AD-EDMD) is caused by mutations in the gene encoding for the lamins A and C (LMNA). Lamins are intermediate filament proteins which form the nuclear lamina underlying the inner nuclear membrane. We have studied the expression and the localization of nuclear envelope proteins in three different cell types and muscle tissue of an AD-EDMD patient carrying a point mutation R377H in the lamin A/C gene.ResultsLymphoblastoid cells, skin fibroblasts, primary myoblasts and muscle thin sections were studied by immunocytochemistry and electron microscopy. Cellular levels of A-type lamins were reduced compared to control cells. In contrast, the amount of emerin and lamin B appeared unaltered. Cell synchronization experiments showed that the reduction of the cellular level of A-type lamin was due to instability of lamin A. By electron microscopy, we identified a proportion of nuclei with morphological alterations in lymphoblastoid cells, fibroblasts and mature muscle fibres. Immunofluorescence microscopy showed that a major population of the lamin B receptor (LBR), an inner nuclear membrane protein, was recovered in the cytoplasm in association with the ER. In addition, the intranuclear organization of the active form of RNA polymerase II was markedly different in cells of this AD-EDMD patient. This aberrant intranuclear distribution was specifically observed in muscle cells where the pathology of EDMD predominates.ConclusionsFrom our results we conclude: Firstly, that structural alterations of the nuclei which are found only in a minor fraction of lymphoblastoid cells and mature muscle fibres are not sufficient to explain the clinical pathology of EDMD; Secondly, that wild type lamin A is required not only for the retention of LBR in the inner nuclear membrane but also for a correct localization of the transcriptionally active RNA pol II in muscle cells. We speculate that a rearrangement of the internal chromatin could lead to muscle-specific disease symptoms by interference with proper mRNA transcription.

Highlights

The autosomal dominant form of Emery-Dreifuss muscular dystrophy (AD-EDMD) is caused by mutations in the gene encoding for the lamins A and C (LMNA)
The cellular level of the A-type lamins is reduced in cells of AD-EDMD patient 99-3 In the course of the diagnostic work-up, a Western blot of proteins from lymphoblastoid cells of the patient 99-3 showed a normal expression of emerin
When lymphoblastoid cells with the mutation LMNA replaces arginine 377 by histidine (R377H) were labelled with the antibodies against emerin (Fig. 2a) or lamin B2 (Fig. 2c), all nuclei revealed the typical prominent staining of the nuclear periphery

Summary

Introduction

The autosomal dominant form of Emery-Dreifuss muscular dystrophy (AD-EDMD) is caused by mutations in the gene encoding for the lamins A and C (LMNA). Lamins are intermediate filament proteins which form the nuclear lamina underlying the inner nuclear membrane. We have studied the expression and the localization of nuclear envelope proteins in three different cell types and muscle tissue of an AD-EDMD patient carrying a point mutation R377H in the lamin A/C gene. The lamins are a group of intermediate filament proteins which form major components of the nuclear lamina in most differentiated eukaryotic cells. In order to build up the nuclear lamina, lamins form homo- and heteropolymers which associate with other proteins into a network that underlies and supports the nuclear membrane (for review see [3])

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