Hutchinson-Gilford progeria syndrome (HGPS) is the most dramatic human syndrome of premature aging. Children with this rare condition are normal at birth, but by age 2 they have stopped growing, lost their hair, and shown skin changes and loss of subcutaneous tissue that resemble the ravages of old age. Untreated, they rarely live past adolescence, dying almost always of advanced cardiovascular disease (heart attack and stroke). The classic syndrome has never been observed to recur in families. Our laboratory discovered that nearly all cases of HGPS harbor a de novo point mutation in codon 608 of the LMNA gene. This mutation causes disease by creating an abnormal splice donor, generating an mRNA with an internal deletion of 150 nt. This is translated into a mutant form of the lamin A protein (referred to as progerin) that lacks 50 amino acids near the C-terminus. Normally lamin A is post-translationally processed to add a farnesyl group at the C-terminus, and then the last 18 amino acids are cleaved off to produce mature lamin A. Progerin lacks the recognition site for this final cleavage, and so remains permanently farnesylated. We have shown that this abnormal protein acts as a dominant negative to disrupt the structure of the membrane scaffold. Data from our group has also demonstrated that progerin interferes with proper chromosome segregation during mitosis. A mouse model for HGPS has been developed. Animals carrying a human BAC transgene bearing the codon 608 mutation show progressive loss of smooth muscle cells in the media of large vessels. Thus, the mouse model nicely replicates the cardiovascular phenotype of HGPS. We have tested the use of farnesyl transferase inhibitors (FTIs), to see if these drugs could provide benefit in HGPS by reducing the amount of the toxic progerin protein. Treatment of HGPS fibroblasts growing in cell culture demonstrates that FTIs are capable of reversing the dramatic nuclear blebbing that is the hallmark of the disease. A trial of FTIs in the HGPS mouse model has demonstrated that this drug treatment is capable of preventing and even reversing the cardiovascular phenotype. An open label clinical trial of FTIs in 30 children with the disease was initiated in May 2007, and results have been submitted for publication. Homozygotes for the mouse BAC transgenic have also now been bred, and show a considerably more severe phenotype. Those animals are now being used to test the effect of the combination treatment of FTIs, statins, and bisphosphonates. A more recent line of research involves the use of rapamycin to increase turnover of progerin aggregates by activating autophagy. Based on cell culture results, rapamycin shows considerable promise, and we are now initiating a mouse model test of this therapeutic approach. While progerin has a dramatic effect on nuclear structure and mitosis, it also disrupts the connections between the nuclear scaffold and chromatin. The consequences include dysregulation of gene expression and epigenetic modification. To explore this in detail, we are studying passage-matched normal and HGPS fibroblasts, using gene expression microarray analysis and chromatin immunoprecipitation coupled with high throughput sequencing (ChIP-seq). Of considerable relevance to the study of normal human aging, we have also shown that progerin is made in small amounts in normal individuals, and appears to increase in quantity as cells approach senescence. Recent data points to an interesting connection between shortening of telomeres and activation of alternative splicing of dozens of genes, including production of progerin from a normal LMNA gene. In this way, senescence apparently proceeds by a positive feedback loop, once a cell has reached its maximum life span.
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