Hutchinson-Gilford syndrome (HGPS) is an extremely rare autosomal dominant genetic disorder in which symptoms resembling aspects of aging are manifested at a very early age. Its occurrence is usually the result of a sporadic germline mutation; although HGPS is genetically dominant, sufferers rarely live long enough to have children, preventing them from passing the disorder on in a hereditary manner. Ultrastructural analysis of the nuclear envelope in fibroblasts from a subject with HGPS. Low magnification transmission electron microscopic image of a passage 10 PT001 nucleus showed several herniations (a). Two higher-magnification images of the same nucleus at sites of blebs (b and c) showed a close apposition of the chromatin to the nuclear envelope. In a, b, and c, the nucleus is to the left. Scale bars correspond to 2 μm in panel a, and 500 nm in panels b and c. HPGS is caused by mutations that weaken the structure of the cell nucleus, making normal cell division difficult. The histone mark H4K20me3 is involved and caused by de novo mutations that occur in a gene that encodes lamin A. Lamin A is made but is not processed properly. This poor processing creates an abnormal nuclear morphology and disorganized heterochromatin. Patients also do not have appropriate DNA repair, and they also have increased genomic instability. In normal conditions, the LMNA gene codes for a structural protein called prelamin A, which undergoes a series of processing steps before attaining its final form, called lamin A. Prelamin A contains a “CAAX” where C is a cysteine, A an aliphatic amino acid, and X any amino acid. This motif at the carboxyl-termini of proteins triggers three sequential enzymatic modifications. First, protein farnesyltransferase catalyzes the addition of a farnesyl moiety to the cysteine. Second, an endoprotease that recognizes the farnesylated protein catalyzes the peptide bond's cleavage between the cysteine and -aaX. In the third step, isoprenylcysteine carboxyl methyltransferase catalyzes methylation of the carboxyl-terminal farnesyl cysteine. The farnesylated and methylated protein is transported through a nuclear pore to the interior of the nucleus. Once in the nucleus, the protein is cleaved by a protease called zinc metallopeptidase STE24 (ZMPSTE24), which removes the last 15 amino acids, which includes the farnesylated cysteine. After cleavage by the protease, prelamin A is referred to as lamin A. In most mammalian cells, lamin A, along with lamin B1, lamin B2, and lamin C, makes up the nuclear lamina, which provides shape and stability to the inner nuclear envelope. Before the late 20th century, research on<unk>  yielded very little information about the syndrome. In 2003, the cause of<unk>  was discovered to be a point mutation in position 1824 of the LMNA gene, which replaces a cytosine with thymine. This mutation creates a 5' cryptic splice site within exon 11, resulting in a shorter than normal mRNA transcript. When this shorter mRNA is translated into protein, it produces an abnormal variant of the prelamin A protein, referred to as<unk> .<unk> 's farnesyl group cannot be removed because the ZMPSTE24 cleavage site is lacking from<unk> , so the abnormal protein is permanently attached to the nuclear rim. One result is that the nuclear lamina does not provide the nuclear envelope with enough structural support, causing it to take on an abnormal shape. Since the support that the nuclear lamina normally provides is necessary for the organizing of chromatin during mitosis, weakening of the nuclear lamina limits the ability of the cell to divide. However, defective cell division is unlikely to be the main defect leading to<unk> , particularly because children develop normally without any signs of disease until about one year of age. Farnesylated prelamin A variants also lead to defective DNA repair, which may play a role in the development of<unk> .<unk>  expression also leads to defects in the establishment of fibroblast cell polarity, which is also seen in physiological aging. To date over 1,400 SNPs in the LMNA gene are known. They can manifest as changes in mRNA, splicing, or protein amino acid sequence (e.g. Arg471Cys, Arg482Gln, Arg527Leu, Arg527Cys, Ala529Val).<unk>  may also play a role in normal human aging, since its production is activated in typical senescent cells. Unlike other "accelerated aging diseases" (such as Werner syndrome, Cockayne syndrome or xeroderma pigmentosum),<unk>  may not be directly caused by defective DNA repair. These diseases each cause changes in a few specific aspects of aging, but never in every aspect at once, so they are often called "segmental<unk> s". A 2003 report in Nature said that<unk>  may be a de novo dominant trait. It develops during cell division in a newly conceived zygote or in the gametes of one of the parents. It is caused by mutations in the LMNA (lamin A protein) gene on chromosome 1; the mutated form of lamin A is commonly known as<unk> . One of the authors, Leslie Gordon, was a physician who did not know anything about<unk>  until her own son, Sam, was diagnosed at 22 months. Gordon and her husband, pediatrician Scott Berns, founded the<unk>  Research Foundation. Lamin A Lamin A is a major component of a protein scaffold on the inner edge of the nucleus called the nuclear lamina that helps organize nuclear processes such as RNA and DNA synthesis. Prelamin A contains a CAAX box at the C-terminus of the protein (where C is a cysteine and A is any aliphatic amino acids). This ensures that the cysteine is farnesylated and allows prelamin A to bind membranes, specifically the nuclear membrane. After prelamin A has been localized to the cell nuclear membrane, the C-terminal amino acids, including the farnesylated cysteine, are cleaved off by a specific protease. The resulting protein, now lamin A, is no longer membrane-bound and carries out functions inside the nucleus. In HGPS, the recognition site that the enzyme requires for cleavage of prelamin A to lamin A is mutated. Lamin A cannot be produced, and prelamin A builds up on the nuclear membrane, causing a characteristic nuclear blebbing. This results in the symptoms of<unk> , although the relationship between the misshapen nucleus and the symptoms is not known. A study that compared HGPS patient cells with the skin cells from young and elderly normal human subjects found similar defects in the HGPS and elderly cells, including down-regulation of certain nuclear proteins, increased DNA damage, and demethylation of histone, leading to reduced heterochromatin. Nematodes over their lifespan show progressive lamin changes comparable to HGPS in all cells but neurons and gametes. These studies suggest that lamin A defects are associated with normal aging.
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