Localization of 4q35.2 to the nuclear periphery: is FSHD a nuclear envelope disease?

Abstract

Facioscapulohumeral muscular dystrophy (FSHD) may be a new member of the class of neuromuscular diseases (NMD) due to defects in the nuclear envelope. Unlike other NMDs with primary defects in nuclear envelope proteins, however, FSHD may result from inappropriate chromatin interactions at the envelope. 3D Immuno-FISH and a novel method of 3D by 2D analysis using NucProfile were developed to examine nuclear organization of the FSHD genomic region. In contrast to most other telomeres, the FSHD region at 4q35.2 localizes to the nuclear periphery. This localization is consistent in normal myoblasts, myotubes, fibroblasts and lymphoblasts, does not vary significantly throughout the cell cycle, and is independent of chromosome territory effects. The nuclear lamina protein lamin A/C is required for FSHD region chromatin localization to the nuclear envelope, as the association is lost in lamin A/C null fibroblasts. As both normal and affected alleles (deleted for the subtelomeric repeat D4Z4) localize to the nuclear periphery, FSHD likely arises instead from improper interactions with transcription factors or chromatin modifiers at the nuclear envelope. Interestingly, it is not D4Z4 itself that mediates interaction with the envelope, as sequences proximal to D4Z4 (i.e. D4S139) localize closer to the nuclear periphery, perhaps accounting for the chromosome 4 specificity of the disease.

D4Z4as a prototype of CTCF and lamins-dependent insulator in human cells

“…Our results reinforce the link between FSHD and the nuclear lamina 5,6 while limits heterochromatin formation at the DM1 locus respectively. Similar mechanisms might also be implicated in other triplet expansion diseases.…”

“…1B for methodology) reside within demonstrate that CTCF binding is necessary for D4Z4 insulator function. 1 The 4q35 locus was consistently visualized at the periphery of the nucleus in an A-type lamins dependent manner suggesting that the particular positioning of this chromosome end could explain FSHD pathogenesis 5,6 and we tested this hypothesis in our cellular models. A-type lamins dependence of D4Z4 boundary activity was demonstrated after siRNA knockdown of LMNA expression, which led to a reliable loss of the insulator function of the whole D4Z4 or subfragments harboring anti-silencing activity.…”

“…In agreement, dual-probes FISH analysis of the 4q35 in control (containing large arrays) and FSHD (containing shorter arrays) myoblasts using D4Z4 and D4S139 (215 kb centromeric to D4Z4) probes revealed that long D4Z4 arrays are usually less peripheral than the D4S139 marker. 5 The mechanism that regulates CTCF binding is still unknown but a role for DNA methylation can be hypothesized. DNA methylation impairs CTCFbinding at several loci 3,4,29 and is reduced in FSHD patients.…”

The radial arrangement of the human chromosome 7 in the lymphocyte cell nucleus is associated with chromosomal band gene density

“…We determined the radial nuclear location of each locus using a 2-dimensional (2-D) analysis, following previously described criteria (Croft et al 1999;Bridger et al 2000;Roix et al 2003;D'Antoni et al 2004;Masny et al 2004;Ono et al 2007). More precisely, a high number of hybridized nuclei were captured with MacProbe v4.2 software and the hybridization signals were localized in each cell nucleus using a dedicated computer software developed in our lab at the University of Catania (freely available on request).…”

Mécanismes et conséquences des mutations

“…Parmi les hypothèses avancées, la formation de boucles internes à la région D4Z4 pourrait prévenir l'interaction de D4Z4 avec des gènes en cis à de grandes distances ; la contraction observée dans la FSHD empêcherait la formation de ces boucles, entraînant une expression inappropriée de certains gènes [29]. Le modèle le plus récent, par ailleurs compatible avec le précédent, est celui d'une altération de l'organisation nucléaire, la région D4Z4 du chromosome 4 intervenant dans l'interaction avec la lamine nucléaire [30]. Cela pourrait notamment altérer le recrutement de facteurs de transcription et de facteurs modifiant la structure de la chromatine, et perturber ainsi l'expression d'un ou plusieurs gènes localisés ou non à proximité de la région D4Z4.…”