A progeria mutation reveals functions for lamin A in nuclear assembly, architecture, and chromosome organization

Abstract

Numerous mutations in the humancomprising the major structural components of the nuclear lamina, the fibrous meshwork underlying the inner nuclear membrane (1). They are major determinants of nuclear size and shape and are involved in essential functions such as DNA replication and transcription (1). Lamins A (LA) and C (LC) are alternatively spliced products of the LMNA gene, whereas lamins LB1 and LB2 are encoded by LMNB1 and LMNB2. Structurally, the lamins have a conserved central ␣-helical rod domain flanked by globular head and tail domains. The central rod is responsible for the formation of in-parallel and in-register coiled-coil dimers, the building blocks of lamin polymers. In vitro, lamin dimers form head-to-tail chains, which further interact laterally in an anti-parallel orientation to form highly ordered paracrystals (PCs) (2). However, little is known about the assembly of lamins into higher order structures in vivo.There are Ͼ250 mutations in human LMNA causing a wide range of diseases [for detail, see http//www.umd.be/LMNA/ and (3)]. Among these is the rare premature aging disease, Hutchinson-Gilford progeria syndrome (HGPS). HGPS children typically appear normal at birth, but show growth retardation before the age of 2 years. Further manifestations include loss of hair, lipodystrophy, sclerodermatous skin, osteolysis, and progressive atherosclerosis leading to death at an average age of 13 years due to myocardial infarcts and strokes (4). Most HGPS patients carry the 1824CϾT mutation (G608G), which activates a cryptic splice site resulting in the expression of LA with 50 amino acids deleted near its C terminus (LA⌬50/progerin) (5). As a result, LA⌬50/ progerin remains permanently farnesylated (6, 7), and its accumulation in patients' cells is correlated with the loss of heterochromatin and changes in histone methylation (1).In addition to 1824CϾT, there are 21 other LMNA mutations causing progeria (http://www.umd.be/LMNA/). Some of these are located in the central rod domain, where they could directly impact the assembly of lamins into dimers and higher order structures. Here, we have studied the alterations in nuclear architecture and chromatin organization in one of these mutations, 433GϾA (E145K), located in segment 1B of the central rod domain of LA/C (5). A patient bearing this mutation showed earlier onset cardiovascular defects, only partial loss of hair and ample s.c. fat (5). We show that fibroblasts derived from an E145K patient have severely misshapen nuclei and multiple defects in chromatin organization as reflected by centromere clustering and an abnormal distribution of telomeres throughout the cell cycle. These abnormalities are established during cell division as nuclei assemble in daughter cells. The results demonstrate that the nuclear changes in E145K progeria cells are significantly different from those seen in cells expressing LA⌬50/ progerin, and they also emphasize the essential role of lamins in establishing and maintaining nuclear architecture.

Oxydative stress alters nuclear shape through lamins dysregulation

“…For example, it was reported that the expression of lamin A (E145K progeria mutation) induced the formation of blebs in the nuclear membrane in which lamin B1 was excluded. 30 Another studies show that deletion of lamin B1 increases the size of the Lamin A/C meshwork and induces the formation of lamin A/C-rich and lamin B2-deficient NE blebs 32 23,[33][34][35][36] In contrast with HGP fibroblasts, the telomere length in hematopoietic cells, which typically do not express lamin A, was within the normal range in different HGPS patients' samples. These results suggest that expression of mutant lamin A is necessary for telomere loss in HGPS.…”

“…Interestingly, we, and other have shown that the overexpression of lamin B1 leads to NSA. 13,28,30 Particularly, A-T cells exhibit an accumulation of lamin B1 protein, alterations in nuclear architecture and premature senescence. Importantly, we show that the overexpression of lamin B1 in normal cells is sufficient to induce NSA and senescence.…”

Impaired mechanical response of an EDMD mutation leads to motility phenotypes that are repaired by loss of prenylation

“…For details, see references Bonne et al, 1999, Cowan et al, 2010, Speckman et al, 2000and Taimen et al, 2009. (n=18) and 1.59×10 −10 (n=38) in (C) and 0.31 (n=17), 0.75 (n=25), 0.03 (n=26), 9.44×10 −6 (n=21) and 0.08 (n=21) in (D) for EV, R460C, lmn-1 RNAi, Q159K and L535P, respectively.…”

Lamin B1 fluctuations have differential effects on cellular proliferation and senescence

“…LMNB1 is reduced in HGPS cells and declines in normal fibroblasts as they enter replicative senescence (Scaffidi and Misteli, 2005;Taimen et al, 2009;Shimi et al, 2011;Zhang et al, 2011;Freund et al, 2012). Shimi et al (2011) reported that LMNB1 reduction triggered senescence, whereas its overexpression delayed senescence.…”