Subtelomeres form the transition between chromosome specific sequences and terminal telomeric repeats. They might influence telomeric functions but underlying mechanisms are still unclear. Nevertheless, subtelomeres are associated with a number of human pathologies such as facioscapulohumeral muscular dystrophy (FSHD), an autosomal dominant disease secondary to the contraction of an array of D4Z4 macrosatellite repeats in the subtelomeric region 4q35. In order to study the biological function of the D4Z4 sequence, we created contructs that mimic the genomic organization of the 4q35 locus. We showed that D4Z4 is able to localize a telomere at the nuclear periphery. This perinuclear activity was dependant on interactions with CTCF and A type lamins and lied within a 80 bp proximal sequence that harbors an insulator activity. Moreover, the peripheral positionning of a telomere by D4Z4 is accompanied by a late replication timing of the telomere. We also searched for sequences able to counteract telomeric position effect (TPE) and identified a 30 bp element containing a CTCF binding site in the proximal region of D4Z4. In another construct, the introduction of a poly-adenylation signal between a reporter gene and telomeric repeats counteracted TPE. This effect is accompanied by the production of a hybrid transcript encompassing the reporter gene and telomeric repeats, suggesting a role for the TERRAs telomeric transcripts in TPE regulation. This work contibuted to characterize the role of subtelomeric sequences, especially the D4Z4 macrosatellite, in telomere regulation, their nuclear compartimentalization, their replication or the telomeric position effect. We will discuss the implications in the understanding of the pathophysiology of FSHD and other subtelomeric diseases.
