Progressive vascular smooth muscle cell defects in a mouse model of Hutchinson-Gilford progeria syndrome.

Children with Hutchinson-Gilford progeria syndrome (HGPS) suffer from dramatic acceleration of some symptoms associated with normal aging, most notably cardiovascular disease that eventually leads to death from myocardial infarction and/or stroke usually in their second decade of life. For the vast majority of cases, a de novo point mutation in the lamin A (LMNA) gene is the cause of HGPS. This missense mutation creates a cryptic splice donor site that produces a mutant lamin A protein, termed "progerin," which carries a 50-aa deletion near its C terminus. We have created a mouse model for progeria by generating transgenics carrying a human bacterial artificial chromosome that harbors the common HGPS mutation. These mice develop progressive loss of vascular smooth muscle cells in the medial layer of large arteries, in a pattern very similar to that seen in children with HGPS. This mouse model should prove valuable for testing experimental therapies for this devastating disorder and for exploring cardiovascular disease in general.

Inhibiting farnesylation of progerin prevents the characteristic nuclear blebbing of Hutchinson-Gilford progeria syndrome.

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disorder that is characterized by dramatic premature aging and accelerated cardiovascular disease. HGPS is almost always caused by a de novo point mutation in the lamin A gene (LMNA) that activates a cryptic splice donor site, producing a truncated mutant protein termed "progerin." WT prelamin A is anchored to the nuclear envelope by a farnesyl isoprenoid lipid. Cleavage of the terminal 15 aa and the farnesyl group releases mature lamin A from this tether. In contrast, this cleavage site is deleted in progerin. We hypothesized that retention of the farnesyl group causes progerin to become permanently anchored in the nuclear membrane, disrupting proper nuclear scaffolding and causing the characteristic nuclear blebbing seen in HGPS cells. Also, we hypothesized that blocking farnesylation would decrease progerin toxicity. To test this hypothesis, the terminal CSIM sequence in progerin was mutated to SSIM, a sequence that cannot be farnesylated. SSIM progerin relocalized from the nuclear periphery into nucleoplasmic aggregates and produced no nuclear blebbing. Also, blocking farnesylation of authentic progerin in transiently transfected HeLa, HEK 293, and NIH 3T3 cells with farnesyltransferase inhibitors (FTIs) restored normal nuclear architecture. Last, treatment of both early- and late-passage human HGPS fibroblasts with FTIs resulted in significant reductions in nuclear blebbing. Our results suggest that treatment with FTIs represents a potential therapy for patients with HGPS.

The calcium-binding aspartate/glutamate carriers, citrin and aralar1, are new substrates for the DDP1/TIMM8a-TIMM13 complex.

The biogenesis of the mitochondrial inner membrane is dependent on two distinct 70 kDa protein complexes. TIMM8a partners with TIMM13 in the mitochondrial intermembrane space to form a 70 kDa complex and facilitates the import of the inner membrane substrate TIMM23. We have identified a new class of substrates, citrin and aralar1, which are Ca2+-binding aspartate/glutamate carriers (AGCs) of the mitochondrial inner membrane, using cross-linking and immunoprecipitation assays in isolated mitochondria. The AGCs function in the aspartate-malate NADH shuttle that moves reducing equivalents from the cytosol to the mitochondrial matrix. Mohr-Tranebjaerg syndrome (MTS/DFN-1, deafness/dystonia syndrome) results from a mutation in deafness/dystonia protein 1/translocase of mitochondrial inner membrane 8a (DDP1/TIMM8a) and loss of the 70 kDa complex. A lymphoblast cell line derived from an MTS patient had decreased NADH levels and defects in mitochondrial protein import. Protein expression studies indicate that DDP1 and TIMM13 show non-uniform expression in mammals, and expression is prominent in the large neurons in the brain, which is in agreement with the expression pattern of aralar1. Thus, insufficient NADH shuttling, linked with changes in Ca2+ concentration, in sensitive cells of the central nervous system might contribute to the pathologic process associated with MTS.

Accumulation of mutant lamin A causes progressive changes in nuclear architecture in Hutchinson-Gilford progeria syndrome.

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disorder, commonly caused by a point mutation in the lamin A gene that results in a protein lacking 50 aa near the C terminus, denoted LADelta50. Here we show by light and electron microscopy that HGPS is associated with significant changes in nuclear shape, including lobulation of the nuclear envelope, thickening of the nuclear lamina, loss of peripheral heterochromatin, and clustering of nuclear pores. These structural defects worsen as HGPS cells age in culture, and their severity correlates with an apparent increase in LADelta50. Introduction of LADelta50 into normal cells by transfection or protein injection induces the same changes. We hypothesize that these alterations in nuclear structure are due to a concentration-dependent dominant-negative effect of LADelta50, leading to the disruption of lamin-related functions ranging from the maintenance of nuclear shape to regulation of gene expression and DNA replication.