Aneuploidy: the impact of chromosome imbalance on nuclear organization and overall genome expression

The organization and dynamics of chromatin within the interphase nucleus as chromosome territories (CTs) and the relationship with transcriptional regulation are not fully understood. We studied a natural example of chromosomal disorganization: aneuploidy due to trisomies 13, 18 and 21. We hypothesized that the presence of an extra copy of one chromosome alters the CT distribution, which perturbs transcriptional activity. We used 3D-FISH to study the position of the chromosomes of interest (18 and 21) in cultured amniocytes and chorionic villus cells from pregnancies with a normal or aneuploid karyotype. We studied the volumes of nuclei and CTs in both conditions and performed a compared transcriptome analysis. We did not observe any differences between euploid and aneuploid cells in terms of the radial and relative CT positions, suggesting that the same rules govern nuclear organization in cases of trisomy. We observed lower volumes for CTs 18 and 21. Overall genome expression profiles highlighted changes in the expression of a subset of genes in trisomic chromosomes, while the majority of transcriptional changes concerned genes located on euploid chromosomes. Our results suggest that a dosage imbalance of the genes on trisomic chromosomes is associated with a disturbance of overall genomic expression.

Viability and stress protection of chronic lymphoid leukemia cells involves overactivation of mitochondrial phosphoSTAT3Ser727

Chronic lymphoid leukemia (CLL) is characterized by the accumulation of functionally defective CD5-positive B lymphocytes. The clinical course of CLL is highly variable, ranging from a long-lasting indolent disease to an unpredictable and rapidly progressing leukemia requiring treatment. It is thus important to identify novel factors that reflect disease progression or contribute to its assessment. Here, we report on a novel STAT3-mediated pathway that characterizes CLL B cells-extended viability and oxidative stress control. We observed that leukemic but not normal B cells from CLL patients exhibit constitutive activation of an atypical form of the STAT3 signaling factor, phosphorylated on serine 727 (Ser₇₂₇) in the absence of detectable canonical tyrosine 705 (Tyr₇₀₅)-dependent activation in vivo. The Ser₇₂₇-phosphorylated STAT3 molecule (pSTAT3Ser₇₂₇) is localized to the mitochondria and associates with complex I of the respiratory chain. This pSer₇₂₇ modification is further controlled by glutathione-dependent antioxidant pathway(s) that mediate stromal protection of the leukemic B cells and regulate their viability. Importantly, pSTAT3Ser₇₂₇, but neither Tyr705-phosphorylated STAT3 nor total STAT3, levels correlate with prolonged in vivo CLL B cells survival. Furthermore, STAT3 activity contributes to the resistance to apoptosis of CLL, but not normal B cells, in vitro. These data reveal that mitochondrial (Mt) pSTAT3Ser₇₂₇ overactivity is part of the antioxidant defense pathway of CLL B cells that regulates their viability. Mt pSTAT3Ser₇₂₇ appears to be a newly identified cell-protective signal involved in CLL cells survival. Targeting pSTAT3Ser₇₂₇ could be a promising new therapeutic approach.

3D position of pericentromeric heterochromatin within the nucleus of a patient with ICF syndrome

ICF (immunodeficiency, centromeric region instability, facial anomalies) syndrome is a rare autosomal recessive disorder characterised by severe immunodeficiency, craniofacial anomalies and chromosome instability. Chromosome analyses from blood samples show a high frequency of decondensation of pericentromeric heterochromatin (PH) and rearrangements involving chromosomes 1 and 16. It is the first and, as far as we know, the only disease associated with a mutation in a DNA methyltransferase gene, DNMT3B, with significant hypomethylation of the classical satellite DNA, the major component of the juxtacentromeric heterochromatin. To better understand the complex links between the hypomethylation of the satellite DNA, the cytogenetic anomalies and the clinical features of ICF syndrome, we performed three-dimensional (3D) FISH on preserved cells from a patient with a suspected ICF phenotype. Analysis of DNMT3B did not reveal any mutation in our patient, making this case an ICF type 2. The results of 3D-FISH showed a statistically significant change in the intranuclear position of PH of chromosome 1 in cells of the patient as compared to normal cells. It is difficult to understand how a defect in the methylation pathway can be responsible for the various symptoms of this condition. From our observations we suggest a mechanistic link between the reorganisation of the nuclear architecture and the altered gene expression.