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.

A newly identified splice site mutation in ZMPSTE24 causes restrictive dermopathy in the Middle East.

Restrictive dermopathy (RD) is a severe neonatal inherited skin syndrome of which children die shortly after birth. Clinical features include intrauterine growth retardation, taut translucent and easily eroded skin, multiple joint ankylosis and distinct facial features. RD is usually caused by homozygous or compound heterozygous mutations in ZMPSTE24, predicted to cause loss of function of the encoded zinc metalloproteinase STE24. ZMPSTE24 is essential for the processing of the nuclear intermediate filament protein prelamin A. We report two distantly related children from the United Arab Emirates with RD. Remarkably, they lived up to 2 months, suggesting some residual function of the mutant protein. We sought to confirm the diagnosis by thorough microscopic analysis of patient skin, to identify the causative mutation and to study its functional consequences. A skin biopsy was obtained and processed for light and electron microscopy. Peripheral blood leucocytes were used for DNA and RNA isolation, and detection of prelamin A by immunofluorescence. Analysis of the skin confirmed the earlier reported densely packed collagen bundles and lack of elastin fibres. In both patients a homozygous splice site mutation c.627+1G>C in ZMPSTE24 was identified. Analysis of the ZMPSTE24 mRNA revealed an in-frame exon 5 skipping. Accumulation of prelamin A could be detected at the nuclear envelope of patient blood lymphocytes. We thus report the first splice site mutation in ZMPSTE24, which is likely to be a founder mutation in the United Arab Emirates. The accumulation of prelamin A at the nuclear periphery is consistent with defective ZMPSTE24 function. Interestingly, a regular blood sample can be used to investigate prelamin A accumulation.

The nuclear envelope, a key structure in cellular integrity and gene expression.

The envelope that encapsulates the cell nucleus has recently gained considerable interest, as several clinical syndromes are linked to mutations in its molecular components. Most disorders recognized so far are caused by defects in the nuclear lamins, building blocks of a filamentous network lining the nucleoplasmic side of the inner nuclear membrane. Nuclear lamins are the evolutionary precursors of cytoskeletal intermediate filaments and associate in a head-to-tail manner into a stable lamina at the nuclear periphery and into a more dispersed structure in the nucleoplasm. Lamins have a scaffolding function for several nuclear processes such as transcription, chromatin organization and DNA replication, and maintain nuclear and cellular integrity. Mutations in the LMNA gene, encoding A-type lamins, can cause cardiac and skeletal muscle disease, lipodystrophy and premature ageing phenotypes. Hence, the integrity of the nuclear envelope seems essential for longevity. Furthermore, the laminopathies provide evidence that metabolism and ageing are as tightly linked in humans as they are in model organisms such as C. elegans. In this review, we elaborate on the structure and functions of nuclear lamins, the spectrum of syndromes related to mutations in nuclear envelope components and pathogenic concepts unifying these disorders.