Somatic and germinal mosaicism in a Han Chinese family with laminopathies

"Laminopathies" refers to a wide spectrum of myopathies caused by mutations in the LMNA gene. These myopathies include limb girdle muscular dystrophy type 1B (LGMD1B) and dilated cardiomyopathy 1 A (DCM1A), which are both autosomal dominant neurogenetic diseases. There have been few studies on mosaicism in laminopathies. Herein, a Han Chinese family with laminopathies was enrolled in our study. Genetic analysis revealed that the proband carried a novel splice site mutation, c. 1158-3 C > T, in the LMNA gene due to her mother having de novo somatic and gonadal mosaicism. Reverse-transcription polymerase chain reaction (RT-PCR) analysis revealed reduced levels of LMNA mRNA in the proband, which were probably due to nonsense-mediated mRNA decay (NMD). Western blotting revealed reduced lamin A/C protein levels in the skeletal muscle tissue of the proband. In this family, the clinical phenotypes of the proband's mother were normal, and the c. 1158-3 C > T splicing mutation was identified in the blood sample of the proband's mother. Thus, the mutation could be easily considered to be nonpathogenic. Our study emphasizes the importance of mosaicism in the identification of pathogenic variants and genetic counseling.

Emerin modulates spatial organization of chromosome territories in cells on softer matrices

Cells perceive and relay external mechanical forces into the nucleus through the nuclear envelope. Here we examined the effect of lowering substrate stiffness as a paradigm to address the impact of altered mechanical forces on nuclear structure-function relationships. RNA sequencing of cells on softer matrices revealed significant transcriptional imbalances, predominantly in chromatin associated processes and transcriptional deregulation of human Chromosome 1. Furthermore, 3-Dimensional fluorescence in situ hybridization (3D-FISH) analyses showed a significant mislocalization of Chromosome 1 and 19 Territories (CT) into the nuclear interior, consistent with their transcriptional deregulation. However, CT18 with relatively lower transcriptional dysregulation, also mislocalized into the nuclear interior. Furthermore, nuclear Lamins that regulate chromosome positioning, were mislocalized into the nuclear interior in response to lowered matrix stiffness. Notably, Lamin B2 overexpression retained CT18 near the nuclear periphery in cells on softer matrices. While, cells on softer matrices also activated emerin phosphorylation at a novel Tyr99 residue, the inhibition of which in a phospho-deficient mutant (emerinY99F), selectively retained chromosome 18 and 19 but not chromosome 1 territories at their conserved nuclear locations. Taken together, emerin functions as a key mechanosensor, that modulates the spatial organization of chromosome territories in the interphase nucleus.

Nuclear margin irregularity and cancer: a review

Nuclear margin irregularity is an important diagnostic feature of malignant cells. The exact cause of nuclear margin irregularity is not fully understood. The distortion of the nuclear envelope is probably the major factor in nuclear margin irregularity. Multiple proteins on the nuclear envelope, particularly nuclear lamin, are responsible for the distortion of the nuclear envelope. The extracellular matrix may also indirectly affect the nuclear position and shape by the closely connected network of actin-nespirin-SUN-lamin links. The alteration of nuclear matrix protein and RET-oncogene expression may play a role in nuclear envelope distortion and in margin irregularity. In this review, the probable causes and impact of nuclear margin irregularities are discussed.

Proteomic analysis of the nuclear matrix in the early stages of rat liver carcinogenesis: identification of differentially expressed and MAR-binding proteins

Tumor progression is characterized by definite changes in the protein composition of the nuclear matrix (NM). The interactions of chromatin with the NM occur via specific DNA sequences called MARs (matrix attachment regions). In the present study, we applied a proteomic approach along with a Southwestern assay to detect both differentially expressed and MAR-binding NM proteins, in persistent hepatocyte nodules (PHN) in respect with normal hepatocytes (NH). In PHN, the NM undergoes changes both in morphology and in protein composition. We detected over 500 protein spots in each two dimensional map and 44 spots were identified. Twenty-three proteins were differentially expressed; among these, 15 spots were under-expressed and 8 spots were over-expressed in PHN compared to NH. These changes were synchronous with several modifications in both NM morphology and the ability of NM proteins to bind nuclear RNA and/or DNA containing MARs sequences. In PHN, we observed a general decrease in the expression of the basic proteins that bound nuclear RNA and the over-expression of two species of Mw 135 kDa and 81 kDa and pI 6.7-7.0 and 6.2-7.4, respectively, which exclusively bind to MARs. These results suggest that the deregulated expression of these species might be related to large-scale chromatin reorganization observed in the process of carcinogenesis by modulating the interaction between MARs and the scaffold structure.

Differential expression of nuclear lamins in normal and cancerous prostate tissues

The process of carcinogenesis is characterized by definite changes in the protein composition of the nuclear matrix. We have recently found that lamins form, in addition to the nuclear lamina, an intranuclear web of thin fibrils. This finding prompted us to address the question of whether changes in the expression of lamins occur in the course of tumor development. In prostate cancer, lamin B undergoes a significant increase; interestingly, its nuclear content strongly correlates with tumor differentiation. Moreover, all the lamins show reproducible alterations in the distribution of the isoelectric variants, suggesting that dephosphorylation events could trigger changes in the pattern of gene expression by inducing structural rearrangements of the nuclear scaffold.

HIM-8 binds to the X chromosome pairing center and mediates chromosome-specific meiotic synapsis

The him-8 gene is essential for proper meiotic segregation of the X chromosomes in C. elegans. Here we show that loss of him-8 function causes profound X chromosome-specific defects in homolog pairing and synapsis. him-8 encodes a C2H2 zinc-finger protein that is expressed during meiosis and concentrates at a site on the X chromosome known as the meiotic pairing center (PC). A role for HIM-8 in PC function is supported by genetic interactions between PC lesions and him-8 mutations. HIM-8 bound chromosome sites associate with the nuclear envelope (NE) throughout meiotic prophase. Surprisingly, a point mutation in him-8 that retains both chromosome binding and NE localization fails to stabilize pairing or promote synapsis. These observations indicate that stabilization of homolog pairing is an active process in which the tethering of chromosome sites to the NE may be necessary but is not sufficient.

Dermal fibroblasts in Hutchinson-Gilford progeria syndrome with the lamin A G608G mutation have dysmorphic nuclei and are hypersensitive to heat stress

Background

Hutchinson-Gilford progeria syndrome (HGPS, OMIM 176670) is a rare sporadic disorder with an incidence of approximately 1 per 8 million live births. The phenotypic appearance consists of short stature, sculptured nose, alopecia, prominent scalp veins, small face, loss of subcutaneous fat, faint mid-facial cyanosis, and dystrophic nails. HGPS is caused by mutations in LMNA, the gene that encodes nuclear lamins A and C. The most common mutation in subjects with HGPS is a de novo single-base pair substitution, G608G (GGC>GGT), within exon 11 of LMNA. This creates an abnormal splice donor site, leading to expression of a truncated protein.

Results

We studied a new case of a 5 year-old girl with HGPS and found a heterozygous point mutation, G608G, in LMNA. Complementary DNA sequencing of RNA showed that this mutation resulted in the deletion of 50 amino acids in the carboxyl-terminal tail domain of prelamin A. We characterized a primary dermal fibroblast cell line derived from the subject's skin. These cells expressed the mutant protein and exhibited a normal growth rate at early passage in primary culture but showed alterations in nuclear morphology. Expression levels and overall distributions of nuclear lamins and emerin, an integral protein of the inner nuclear membrane, were not dramatically altered. Ultrastructural analysis of the nuclear envelope using electron microscopy showed that chromatin is in close association to the nuclear lamina, even in areas with abnormal nuclear envelope morphology. The fibroblasts were hypersensitive to heat shock, and demonstrated a delayed response to heat stress.

Conclusion

Dermal fibroblasts from a subject with HGPS expressing a mutant truncated lamin A have dysmorphic nuclei, hypersensitivity to heat shock, and delayed response to heat stress. This suggests that the mutant protein, even when expressed at low levels, causes defective cell stability, which may be responsible for phenotypic abnormalities in the disease.