Genetic Insights into the Hutchinson-Gilford Progeria Syndrome
Gina Glass
4/7/18
Abstract
Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disorder characterized by clinical features that resemble premature aging. The majority of HGPS patients die before the age of 20 years due to cardiovascular problems resulting in heart failure. Classical HGPS usually caused by an autosomal dominant mutation; few atypical forms where progression is delayed follow an autosomal recessive pattern of inheritance. HGPS occurs as a result of a de novo point mutation in the DNA, specifically the LMNA gene. The LMNA gene codes for the intermediate filament protein Lamin A. Lamin A is one of the major components of the nuclear lamina, a scaffold structure of the nuclear envelope. Lamin A defines mechanical and chemical properties of the nucleus and is also involved in chromatin organization as well as epigenetic regulation. Although there is no known cure, treatments to delay the progression and alleviate symptoms for HGPS have been identified. The purpose of this review is to integrate and connect the mechanisms of Progeria to the normal aging process to better investigate possible treatments for age related disorders.
Introduction
Hutchinson-Gilford progeria syndrome (HGPS) is an extremely rare genetic disorder with about 114 diagnosed cases across 39 countries (Sinha et al., 2014). The disease is detected by phenotypes that resemble premature aging and diagnosed during the first year of the child’s life. At birth, HGPS patients appear normal. Profound failure to thrive occurs during the first year of the child’s development. The phenotypes observed during early development includes a receding mandible, narrow nasal bridge, and pointed nasal tip. During the first to the third year of the child’s life, features become more apparent: partial hair loss progressing to total hair loss, diminishing of subcutaneous fat, progressive joint contractures, bone changes, nail dystrophy, abnormal tightness and/or small soft outpouchings of the skin over the abdomen and upper thighs, and delayed tooth eruption (Gordon et al., 2015). It is estimated that these children biologically age about ten years in a single year (Sinha et al., 2014). Remarkably, these children have normal IQ and intelligence. The average survival of children diagnosed is 13.5 years. Death occurs due to stroke, myocardial infarction, heart failure and atherosclerosis.
HGPS is a genetic disorder, classical HGPS is most commonly caused by a sporadic autosomal dominant mutation. Atypical forms of progeria, also called non-classical progeria follows an autosomal recessive pattern of inheritance. Non-classical progeria can be differentiated also because the phenotypes of aging progress at a slower rate. HGPS occurs as a result of a de novo point mutation in the DNA (Sinha et al., 2014). This mutation occurs in the LMNA gene. The result is an abnormally formed Lamin A protein called Progerin. LMNA is present on chromosome 1, the point mutation results in deletion of 50 amino acids of preLamin A. This deletion destabilizes the nucleus further and is fatal for the cell.
The Cellular Role of Nuclear Lamins
Lamins are structural components of the nuclear lamina which determines the shape, integrity and size of the nucleus (Coutinho et al., 2009). Lamins A and C are important for nuclear stability, and they play an important role in transcription regulation in response to a chemical or mechanical stimulus (Coutinho et al., 2009). They are a part of the intermediate-type filament proteins which also form part of the nuclear matrix scaffold. Due to the interactions between chromatin and the nuclear matrix mutations, mutations in Lamins A and C are hypothesized to impair various nuclear functions such as chromatin and chromosome stability, telomere integrity, regulation of transcription, DNA replication, cell cycle control and cellular differentiation causing various disorders affecting striated muscle, adipocytes and peripheral nerves, the appearance of premature aging. Lamin A and C have been shown to play a significant role in the shape and structure of the nucleus of the cell, including the stiffness and elasticity of the nucleus. Higher levels of Lamin A and C are seen in cells of tissues often subjected to mechanical torsion, including the muscle and heart (Reddy et al., 2016). Maintaining the integrity of the nucleus is extremely important in our cells. Having a reduction in Lamin A/C especially in cells exposed to such high mechanical stress results in the integrity of the nucleus being damaged due to the depletion of these Lamins and the nucleus not being able to hold shape under stress. This results in the accelerated aging phenotypes that patients with Progeria experience such as myocardial infarction, heart failure, and stroke. HGPS is specifically characterized by definite defects in the nuclear shape due to the mutated LMNA gene. While patients with Progeria experience rapid acceleration of aging, HGPS and normal aging share many similar cellular phenotypes such as abnormal nuclear shape, loss of epigenetic marks and increased DNA damage, reduced bone density, cardiovascular disease, and overall dysfunctional tissues and organs.
Genetics of Progeria
Lamin A in mammals is one element of the polypeptide family of lamins. Mutations in the gene encoding Lamin A can lead to numerous disorders, collectively these disorders are known as laminopathies (Pacheco et al., 2014). The main components are lamins A, B1, B2, and C. Lamins A and C are formed by the splicing of lamin A mRNA (Coutinho et al., 2009). Lamin A under normal circumstance contains a carboxyl-terminal CAAX box modified by farnesylation (does not occur in Lamin C). Farnesylation is the process when cysteine resides on the C-terminal region are post-translationally modified with a 15-carbon farnesyl group and the exposed carboxyl group is methylated (Tamanoi et al., 2001). This process increases the affinity of the protein for the membrane. Similarly, Lamin A is formed by farnesylation, cleavage and methylation of the carboxyl-terminal which generates the mature Lamin A.
The enzyme responsible for cleavage is zinc metalloproteinase ZMSPTE24, which gives rise to the mature Lamin A (McClintock et al., 2007). Normally, ZMSPTE24 cleaves 15 amino acids from the preLamin A to yield the mature Lamin A, in patients with HGPS the single point mutation in the LMNA gene which activates the cryptic splice site within exon 11 results in 50 amino acids being deleted from the preLamin A resulting in the termed protein Progerin (Ghosh et al., 2014). Progerin, the altered Lamin A in HGPS, is an incompletely processed Lamin A that remains farnesylated seen in Figure A where “FS” indicates the fanesylation site.
Figure A. Cleavage of pre-laminA into progerin and mature lamin A (Moiseeva et al., 2016).
Incomplete processing leads to apparent loss of mechanical properties of the nuclear envelope and nuclear matrix. The cells that express Progerin may experience delayed mitotic progression consistent with HGPS phenotype (Coutinho et al., 2009).
Progerin Expression in HGPS and Normal Aging Phenotypes
Lamin A is localized to the nuclear lamina on the inner-side of the nuclear envelope, contributing to structural stability and other nuclear functions. Lamin A regulates gene expression by directly binding to DNA, and sequesters heterochromatin and silenced or transcriptionally low genes in the periphery of the nucleus (Pacheco et al., 2014). DNA replication, DNA repair, chromatin and nuclear pore complex organization are also regulated by Lamin A. When the LMNA gene is mutated, or Progerin is expressed, all of these functions within the cell are affected. Figure B shows the path that accumulation of farnesylated intermediates and how they affect cellular processes.
Figure B. Process of aging due to accumulation of farnesylated intermediates (Reddy et al., 2014)
From the figure above, the effects of farnesylated Progerin intermediates accumulating in the cell has many detrimental outcomes. Changing chromatin remodeling factors, transcription factors, DNA repair, and nuclear transmembrane associated factors (Reddy et al., 2014). All of these processes lead to dysfunctional tissues and organs, leading to the phenotypes seen in aging such as heart failure and stroke.
Progerin is processed identically to Lamin A, but the protein is truncated and permanently farnesylated. Progerin expression delocalized nuclear envelope morphology, and increases DNA damage and repair (Pacheco et al., 2014). Progerin is expressed in the general population, and its expression significantly increases with age (Pacheco et al., 2014). Our genome contains large number of splice sites termed as cryptic splice sites (CSS). These sites are generally disadvantageous and are dormant or only at low levels unless activated by mutations. In the case of Lamin A, Progerin is expressed in the general population by cryptic splice sites in Lamin A being activated. Activation of these sites has been seen to increase with age. Progerin expression has been seen in atherosclerotic vascular tissue in aged, non-HGPS individuals caused by the increase in splice site activation, which could be due to exposure to mutagenic agents over time. Progerin is expressed in HGPS patients most commonly produced by a de novo point mutation in exon 11 (Pacheco et al., 2014). This silent mutation significantly increases the cryptic splice sites.
In an experiment lead by Laurin Pacheco, Progerin expression was observed in non-immortalized MSCs known as marrow-isolated adult multilineage inducible (MIAMI) stem cells. MIAMI cells are known to secrete repair-mediating cytokines, so they provide an excellent model for mechanisms which report decrease in vascular repair (Pacheco et al., 2014). To focus on the effects Progerin has on these cells the focus was on the following processes, self-renewal, proliferation, migration, membrane flexibility, and the more complex process of vascular repair (Pacheco et al., 2014).
MIAMI stem cells expression Progerin were seen to have significantly higher abnormal nuclei than Lamin A expression stem cells. Figure C below demonstrates results found that as the donor age increased, especially after age 65, expression of Progerin increased while the expression of Lamin A decreased.
Figure C. Progerin expression in MIAMI stem cells in relation to donor age (Pacheco et al., 2014).
Laurin Pacheco also found that Progerin expression decreased cell proliferation, as well as decreasing cell migration and increasing cell stiffness.
The vascular system is constantly under mechanical and inflammatory stress combined by fluid pressure and shear stress, which can result in injury and death of endothelial, vascular smooth muscle cells, and cells or the arterial and arteriole walls in the heart (Pacheco et al., 2014). Cell proliferation, migration, and flexibility are all important aspects in vascular repair. The expression of Progerin in both HGPS patients and the elderly decreases the cells ability to repair itself when exposed to these stressors. Inability to repair results in cell death. The death of these cells results in the phenotypes observed in both the normal aging process and Progeria.
Telomere shortening and dysfunction have been proved to contribute to DNA lesions, DNA damage, and defects to DNA repair mechanisms. Telomere attrition is a hallmark to aging and cell senescence. Telomeres shorten each time after a replication cycle which leads to the normal aging process of our cells. Recent studies have shown that accumulation of Progerin is associated with telomere dysfunction (Ghosh et al., 2014). In an experiment lead by Kan Cao, effects of cellular passage and donor age on the activation of Progerin production were investigated. They found an inverse correlation between cell immortalization and Progerin transcription, and the expression of telomerase in normal fibroblast cells resulted in a significant decrease in Progerin production (Kan et al., 2011).
The potential causal relationship between telomere-induced cell senescence and Progerin production found by Kan’s experiment is another explanation why the expression of Progerin in both HGPS patients and the normal aging process yields the aging phenotypes. Extending telomeres and activating telomerase could also be a potential investigation into treatment options for HGPS patients and other age-related disorders.
Diagnostic Methods, Treatments, and Future Directions
The diagnostic methods for HGPS are clinical, histological, radiological and by screening for mutations in the LMNA gene. Clinical tests are offered for confirmatory diagnosis are sequential analysis of the gene LMNA, which reveals point mutations in around 90% of patients with HGPS. Another test looks for uniparental disomy of chromosome 1 and deletions associates with HGPS. Radiology tests are used to detect manifestations that usually occur in the skull, thorax, long bones, and phalanges. Light microscopy, histological tests using skin biopsies from HGPS patients exhibit the irregular nuclear envelope outlines indicating the massive alterations of gene expression (Coutinho et al., 2009).
Presently, there is no known cure for progeria. However, there are treatments to improve clinical conditions. Farnesyltransferase inhibitors (FTI) have been tested and shown to reverse abnormalities in nuclear morphology in cells expressing Progerin. Farnesyltransferase inhibitors prevent the accumulation of farnesylated Progerin, producing less toxic proteins (Mehta et al., 2011). In an experiment lead by Ishita Mehta, the impact of FTI on the survival LMNA HGPS positive mice was investigated. The treatment of FTI significantly improved the survival of both male and female LMNA HGPS positive mice (Mehta et al., 2011). Treatment improved body weight curves and reduced the number of spontaneous rib fractures. A research study treated HGPS fibroblasts with FTI and analyzed the nuclear location of individual chromosome territories (Mehta et al., 2011). Results indicate that the HGPS fibroblasts treated with the FTI were able to restore normal interphase chromosome positioning.
Oxidative stress has been closely linked to the control of aging and age related diseases (Kang et al., 2017). Chemical screenings have identified that reactive oxygen species (ROS) play important roles in HGPS phenotype progression. ROS levels have been identified as five times higher in HGPS fibroblasts compared to normal fibroblasts. Recently, reduction of ROS levels has been proposed as a potentially effective treatment for patients with HGPS. In an experiment lead by Hyun Kang, compounds were screened to look for the reduction of ROS levels in HGPS fibroblasts. The compound identified was rho-associated protein kinase (ROCK) inhibitor as an effective agent to reduce ROS levels. ROCK regulates mitochondrial ROS generation (Kang et al., 2017). ROCK inactivation along with Y-27632 (ROCK specific inhibitor) abolished the interactions necessary to generate ROS species.
Conclusion
Since many of the mechanisms behind the expression of the mutant Lamin A protein are still unclear, identifying treatments and targets over the years has been difficult. Research is looking into each pathway and focusing on which ones are most relevant and causally involved in the pathologies of HGPS. Treatments involved to alleviate symptoms and the research to treat age-related disorders not only benefits patients diagnosed with Progeria but the general population as well. Efforts focusing on FTI treatments to alleviate symptoms that HGPS patients experience could also be used to treat symptoms of normal age related disorders where Progerin is expressed such as atherosclerosis. Further research should be conducted to evaluate the effectiveness of FTI treatments for HGPS patients and all other age related disorders involving nuclear Lamins.
Future directions looking into the expression of Progerin in HGPS patients and during the normal aging process in relation to telomere length to prevent cell senescence is a possible direction for treatment research for these age-related disorders. The effects of Progerin expression in HGPS patients can help us better understand the process of normal aging. Treatments and research to alleviate symptoms of this process in Progeria patients can be used to help treat both normal aging and age-related disorders.