Glioblastoma multiforme (GBM) is a malignant tumor of the central nervous system (CNS). The poor prognosis of GBM due to resistance to therapy has been associated with high chromosomal instability (CIN). Replication stress is a major cause of CIN that manifests as chromosome rearrangements, fragility, and breaks, including those cytologically expressed within specific chromosome regions named common fragile sites (CFSs). In this work, we characterized the expression of human CFSs in the glioblastoma U-251 MG cell line upon treatment with the inhibitor of DNA polymerase alpha aphidicolin (APH). We observed 52 gaps/breaks located within previously characterized CFSs. We found 17 to be CFSs in GBM cells upon treatment with APH, showing a frequency equal to at least 1% of the total gaps/breaks. We report that two CFSs localized to regions FRA2E (2p13/p12) and FRA2F (2q22) were only found in U-251 MG cells, but not lymphocytes or fibroblasts, after APH treatment. Notably, these glioblastoma-specific CFSs had a relatively high expression compared to the other CFSs with breakage frequency between ∼7 and 9%. Presence of long genes, incomplete replication, and delayed DNA synthesis during mitosis (MiDAS) after APH treatment suggest that an impaired replication process may contribute to this loci-specific fragility in U-251 MG cells. Altogether, our work offers a characterization of common fragile site expression in glioblastoma U-251 MG cells that may be further exploited for cytogenetic and clinical studies to advance our understanding of this incurable cancer.
The human centromere comprises large arrays of repetitive alpha-satellite DNA at the primary constriction of mitotic chromosomes. In addition, centromeres are epigenetically specified by the centromere-specific histone H3 variant CENP-A that supports kinetochore assembly to enable chromosome segregation. Since CENP-A is bound to only a fraction of the alpha-satellite elements within the megabase-sized centromere DNA, correlating the three-dimensional (3D) organization of alpha-satellite DNA and CENP-A remains elusive. To visualize centromere organization within a single chromatid, we used a combination of the Centromere Chromosome Orientation Fluorescent In Situ Hybridization (Cen-CO-FISH) technique together with Structured Illumination Microscopy (SIM). Cen-CO-FISH allows the differential labeling of the sister chromatids without the denaturation step used in conventional FISH that may affect DNA structure. Our data indicate that alpha-satellite DNA is arranged in a ring-like organization within prometaphase chromosomes, in presence or absence of spindle's microtubules. Using expansion microscopy (ExM), we found that CENP-A organization within mitotic chromosomes follows a rounded pattern similar to that of alpha-satellite DNA, often visible as a ring thicker at the outer surface oriented towards the kinetochore-microtubules interface. Collectively, our data provide a 3D reconstruction of alpha-satellite DNA along with CENP-A clusters that outline the overall architecture of the mitotic centromere. [Media: see text] [Media: see text] [Media: see text] [Media: see text]
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