Valerie L.R.M. Verstraeten

Repetitive disruptions of the nuclear envelope invoke temporary loss of cellular compartmentalization in laminopathies.

The nuclear lamina provides structural support to the nucleus and has a central role in nuclear organization and gene regulation. Defects in its constituents, the lamins, lead to a class of genetic diseases collectively referred to as laminopathies. Using live cell imaging, we observed the occurrence of intermittent, non-lethal ruptures of the nuclear envelope in dermal fibroblast cultures of patients with different mutations of lamin A/C. These ruptures, which were absent in normal fibroblasts, could be mimicked by selective knockdown as well as knockout of LMNA and were accompanied by the loss of cellular compartmentalization. This was demonstrated by the influx of cytoplasmic transcription factor RelA and regulatory protein Cyclin B1 into the nucleus, and efflux of nuclear transcription factor OCT1 and nuclear structures containing the promyelocytic leukemia (PML) tumour suppressor protein to the cytoplasm. While recovery of enhanced yellow fluorescent protein-tagged nuclear localization signal in the nucleus demonstrated restoration of nuclear membrane integrity, part of the mobile PML structures became permanently translocated to the cytoplasm. These satellite PML structures were devoid of the typical PML body components, such as DAXX, SP100 or SUMO1. Our data suggest that nuclear rupture and loss of compartmentalization may add to cellular dysfunction and disease development in various laminopathies.

The R439C mutation in LMNA causes lamin oligomerization and susceptibility to oxidative stress

Dunnigan‐type familial partial lipodystrophy (FPLD) is a laminopathy characterized by an aberrant fat distribution and a metabolic syndrome for which oxidative stress has recently been suggested as one of the disease‐causing mechanisms. In a family affected with FPLD, we identified a heterozygous missense mutation c.1315C>T in the LMNA gene leading to the p.R439C substitution. Cultured patient fibroblasts do not show any prelamin A accumulation and reveal honeycomb‐like lamin A/C formations in a significant percentage of nuclei. The mutation affects a region in the C‐terminal globular domain of lamins A and C, different from the FPLD‐related hot spot. Here, the introduction of an extra cysteine allows for the formation of disulphide‐mediated lamin A/C oligomers. This oligomerization affects the interaction properties of the C‐terminal domain with DNA as shown by gel retardation assays and causes a DNA‐interaction pattern that is distinct from the classical R482W FPLD mutant. Particularly, whereas the R482W mutation decreases the binding efficiency of the C‐terminal domain to DNA, the R439C mutation increases it. Electron spin resonance spectroscopy studies show significantly higher levels of reactive oxygen species (ROS) upon induction of oxidative stress in R439C patient fibroblasts compared to healthy controls. This increased sensitivity to oxidative stress seems independent of the oligomerization and enhanced DNA binding typical for R439C, as both the R439C and R482W mutants show a similar and significant increase in ROS upon induction of oxidative stress by H2O2.

Soft substrates normalize nuclear morphology and prevent nuclear rupture in fibroblasts from a laminopathy patient with compound heterozygous LMNA mutations

Laminopathies, mainly caused by mutations in the LMNA gene, are a group of inherited diseases with a highly variable penetrance; i.e., the disease spectrum in persons with identical LMNA mutations range from symptom-free conditions to severe cardiomyopathy and progeria, leading to early death. LMNA mutations cause nuclear abnormalities and cellular fragility in response to cellular mechanical stress, but the genotype/phenotype correlations in these diseases remain unclear. Consequently, tools such as mutation analysis are not adequate for predicting the course of the disease. Here, we employ growth substrate stiffness to probe nuclear fragility in cultured dermal fibroblasts from a laminopathy patient with compound progeroid syndrome. We show that culturing of these cells on substrates with stiffness higher than 10 kPa results in malformations and even rupture of the nuclei, while culture on a soft substrate (3 kPa) protects the nuclei from morphological alterations and ruptures. No malformations were seen in healthy control cells at any substrate stiffness. In addition, analysis of the actin cytoskeleton organization in this laminopathy cells demonstrates that the onset of nuclear abnormalities correlates to an increase in cytoskeletal tension. Together, these data indicate that culturing of these LMNA mutated cells on substrates with a range of different stiffnesses can be used to probe the degree of nuclear fragility. This assay may be useful in predicting patient-specific phenotypic development and in investigations on the underlying mechanisms of nuclear and cellular fragility in laminopathies.

Cytoplasmic localization of PML particles in laminopathies

There is growing evidence that laminopathies, diseases associated with mutations in the LMNA gene, are caused by a combination of mechanical and gene regulatory distortions. Strikingly, there is a large variability in disease symptoms between individual patients carrying an identical LMNA mutation. This is why classical genetic screens for mutations appear to have limited predictive value for disease development. Recently, the widespread occurrence of repetitive nuclear ruptures has been described in fibroblast cultures from various laminopathy patients. Since this phenomenon was strongly correlated with disease severity, the identification of biomarkers that report on these rupture events could have diagnostic relevance. One such candidate marker is the PML nuclear body, a structure that is normally confined to the nuclear interior, but leaks out of the nucleus upon nuclear rupture. Here, we show that a variety of laminopathies shows the presence of these cytoplasmic PML particles (PML CPs), and that the amount of these protein aggregates increases with severity of the disease. In addition, between clinically healthy individuals, carrying LMNA mutations, significant differences can be found. Therefore, we postulate that detection of PML CPs in patient fibroblasts could become a valuable marker for diagnosis of disease development.

Jejunal arteriovenous malformation, diagnosed by angiography and treated by embolization and catheter-guided surgery: case report and review of literature

Abstract. A case of chronic gastrointestinal hemorrhage caused by a small jejunal arteriovenous malformation (AVM) is reported. Treatment by endovascular embolization was temporarily successful. Subsequently, the patient underwent laparoscopic resection, guided by intraoperative catheter localization with methylene blue. Histopathology confirmed a true AVM. Eighteen months after treatment, the patient is free of symptoms. Literature of jejunal AVMs is reviewed.

Buschke-Ollendorf Syndrome: Report of a Case and a Brief Molecular Overview

Buschke-Ollendorf syndrome (BOS) is a rare autosomal dominant disorder characterized by localized increases in bone density manifesting as osteopoikilosis or melorheostosis and connective tissue nevi, collagenomas. Manifestations are highly variable. It is caused by loss-of-function mutations in the LEMD3 gene, which codes for an inner nuclear membrane protein that is also known as MAN1. Six different mutations have been described to date without a clear genotype-phenotype correlation. Buschke-Ollendorf syndrome exemplifies the importance of TGFß signaling for bone and connective tissue homeostasis. Here, we report on a father and his daughter with typical BOS syndrome caused by a known nonsense mutation and provide an overview of what is now known of this rare disorder.