The boundary of macaque rDNA is constituted by low-copy sequences conserved during evolution

4q-D4Z4 chromatin architecture regulates the transcription of muscle atrophic genes in facioscapulohumeral muscular dystrophy

Despite increasing insights in genome structure organization, the role of DNA repetitive elements, accounting for more than two thirds of the human genome, remains elusive. Facioscapulohumeral muscular dystrophy (FSHD) is associated with deletion of D4Z4 repeat array below 11 units at 4q35.2. It is known that the deletion alters chromatin structure in cis, leading to gene up-regulation. Here we show a genome-wide role of 4q-D4Z4 array in modulating gene expression via 3D nuclear contacts. We have developed an integrated strategy of 4q-D4Z4-specific 4C-seq and chromatin segmentation analyses, showing that 4q-D4Z4 3D interactome and chromatin states of interacting genes are impaired in FSHD1 condition; in particular, genes that have lost the 4q-D4Z4 interaction and with a more active chromatin state are enriched for muscle atrophy transcriptional signature. Expression level of these genes is restored by the interaction with an ectopic 4q-D4Z4 array, suggesting that the repeat directly modulates the transcription of contacted targets. Of note, the up-regulation of atrophic genes is a common feature of several FSHD1 and FSHD2 patients, indicating that we have identified a core set of deregulated genes involved in FSHD pathophysiology.

Chromosomal dynamics of nucleolar organizer regions (NORs) in the house mouse: micro-evolutionary insights

Variation in the number and chromosomal location of nucleolar organizer regions (NORs) was studied in the house mouse, Mus musculus (2n=40). From an origin in Western Asia, this species colonized the Middle East, Europe and Asia. This expansion was accompanied by diversification into five subspecies. NOR diversity was revealed by fluorescence in situ hybridization using 18S and 28S probes on specimens spanning Asia to Western Europe. The results showed that the house mouse genome possessed a large number of NOR-bearing autosomes and a surprisingly high rate of polymorphism for the presence/absence of rRNA genes on all these chromosomes. All NOR sites were adjacent to the centromere except for two that were telomeric. Subspecific differentiation established from the NOR frequency data was concordant with the overall pattern of radiation proposed from molecular studies, but highlighted several discrepancies that need to be further addressed. NOR diversity in M. musculus consisted of a large number of polymorphic NORs that were common to at least two subspecies, and a smaller number of NORs that were unique to one subspecies. The most parsimonious scenario argues in favor of a subspecific differentiation by lineage sorting of ancestral NOR polymorphisms; only the unique NORs would have appeared by inter-chromosomal transposition, except for the two telomeric ones that may have originated by hybridization with another species. Such a scenario provides an alternative view from the one prevailing in most systematic and phylogenetic analyses that NORs have a high transposition rate due to concerted evolution of rRNA genes.

Remodeling of the chromatin structure of the facioscapulohumeral muscular dystrophy (FSHD) locus and upregulation of FSHD-related gene 1 (FRG1) expression during human myogenic differentiation

BACKGROUND

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant neuromuscular disorder associated with the partial deletion of integral numbers of 3.3 kb D4Z4 DNA repeats within the subtelomere of chromosome 4q. A number of candidate FSHD genes, adenine nucleotide translocator 1 gene (ANT1), FSHD-related gene 1 (FRG1), FRG2 and DUX4c, upstream of the D4Z4 array (FSHD locus), and double homeobox chromosome 4 (DUX4) within the repeat itself, are upregulated in some patients, thus suggesting an underlying perturbation of the chromatin structure. Furthermore, a mouse model overexpressing FRG1 has been generated, displaying skeletal muscle defects.

RESULTS

In the context of myogenic differentiation, we compared the chromatin structure and tridimensional interaction of the D4Z4 array and FRG1 gene promoter, and FRG1 expression, in control and FSHD cells. The FRG1 gene was prematurely expressed during FSHD myoblast differentiation, thus suggesting that the number of D4Z4 repeats in the array may affect the correct timing of FRG1 expression. Using chromosome conformation capture (3C) technology, we revealed that the FRG1 promoter and D4Z4 array physically interacted. Furthermore, this chromatin structure underwent dynamic changes during myogenic differentiation that led to the loosening of the FRG1/4q-D4Z4 array loop in myotubes. The FRG1 promoter in both normal and FSHD myoblasts was characterized by H3K27 trimethylation and Polycomb repressor complex binding, but these repression signs were replaced by H3K4 trimethylation during differentiation. The D4Z4 sequences behaved similarly, with H3K27 trimethylation and Polycomb binding being lost upon myogenic differentiation.

CONCLUSION

We propose a model in which the D4Z4 array may play a critical chromatin function as an orchestrator of in cis chromatin loops, thus suggesting that this repeat may play a role in coordinating gene expression.

Segmental duplications and evolutionary plasticity at tumor chromosome break-prone regions

We have previously found that the borders of evolutionarily conserved chromosomal regions often coincide with tumor-associated deletion breakpoints within human 3p12-p22. Moreover, a detailed analysis of a frequently deleted region at 3p21.3 (CER1) showed associations between tumor breaks and gene duplications. We now report on the analysis of 54 chromosome 3 breaks by multipoint FISH (mpFISH) in 10 carcinoma-derived cell lines. The centromeric region was broken in five lines. In lines with highly complex karyotypes, breaks were clustered near known fragile sites, FRA3B, FRA3C, and FRA3D (three lines), and in two other regions: 3p12.3-p13 ( approximately 75 Mb position) and 3q21.3-q22.1 ( approximately 130 Mb position) (six lines). All locations are shown based on NCBI Build 36.1 human genome sequence. The last two regions participated in three of four chromosome 3 inversions during primate evolution. Regions at 75, 127, and 131 Mb positions carry a large ( approximately 250 kb) segmental duplication (tumor break-prone segmental duplication [TBSD]). TBSD homologous sequences were found at 15 sites on different chromosomes. They were located within bands frequently involved in carcinoma-associated breaks. Thirteen of them have been involved in inversions during primate evolution; 10 were reused by breaks during mammalian evolution; 14 showed copy number polymorphism in man. TBSD sites showed an increase in satellite repeats, retrotransposed sequences, and other segmental duplications. We propose that the instability of these sites stems from specific organization of the chromosomal region, associated with location at a boundary between different CG-content isochores and with the presence of TBSDs and "instability elements," including satellite repeats and retroviral sequences.

Evolutionary genomic remodelling of the human 4q subtelomere (4q35.2)

BACKGROUND

In order to obtain insights into the functionality of the human 4q35.2 domain harbouring the facioscapulohumeral muscular dystrophy (FSHD) locus, we investigated in African apes genomic and chromatin organisations, and the nuclear topology of orthologous regions.

RESULTS

A basic block consisting of short D4Z4 arrays (10-15 repeats), 4q35.2 specific sequences, and approximately 35 kb of interspersed repeats from different LINE subfamilies was repeated at least twice in the gorilla 4qter. This genomic organisation has undergone evolutionary remodelling, leading to the single representation of both the D4Z4 array and LINE block in chimpanzee, and the loss of the LINE block in humans. The genomic remodelling has had an impact on 4qter chromatin organisation, but not its interphase nuclear topology. In comparison with humans, African apes show very low or undetectable levels of FRG1 and FRG2 histone 4 acetylation and gene transcription, although histone deacetylase inhibition restores gene transcription to levels comparable with those of human cells, thus indicating that the 4qter region is capable of acquiring a more open chromatin structure. Conversely, as in humans, the 4qter region in African apes has a very peripheral nuclear localisation.

CONCLUSION

The 4q subtelomere has undergone substantial genomic changes during evolution that have had an impact on chromatin condensation and the region's transcriptional regulation. Consequently, the 4qter genes in African apes and humans seem to be subjected to a different strategy of regulation in which LINE and D4Z4 sequences may play a pivotal role. However, the effect of peripheral nuclear anchoring of 4qter on these regulation mechanisms is still unclear. The observed differences in the regulation of 4qter gene expression between African apes and humans suggest that the human 4q35.2 locus has acquired a novel functional relevance.