Modulation of Farnesylation
Just to do a quick recap, Hutchinson-Gilford progeria syndrome is caused by a single-base substitution in exon 11 of the LMNA gene, which results in the production of the toxic form of prelamin A, also known as progerin. Progerin is unable to be properly processed due to is lack of a cleavage site for the processing enzyme, Zmpste24, which is unable to remove a farnesylated cysteine. The accumulation of the progerin in the nuclear membrane can disrupt the shape of the nucleus and produces various cellular dysfunctions, such as premature cell death, defects in DNA repair, cell proliferation, differentiation, and so on (Blondel et al. 2016).
Lamin A and lamin AΔ150 contain CAAX boxes at their C-terminal ends, which undergo farnesylation and other post-translational modification to become mature proteins (Rodriguez et al. 2009). Farnesylation, a type of post-translational modification involving the addition of an isoprenoid lipid and conducting a methylation of the exposed carboxyl group to a cysteine residue is believed to control the progerin’s toxicity. If farnesylation is inhibited, then, theoretically, the processing enzyme, Zmpste24, does not have the obligation to remove the 15 amino acid section (Blondel et al. 2016).
Prenylation inhibitors have been correlated with improvement of the altered nuclear ship and other defects caused by HGPS. Some pyenylation inhibitors exist, including the HMG-CoA reductase inhibitor, pravastatin, combined with aminobisphosphonate zolefronate which inhibits farnesyl pyrophosphate synthetase and the farnesyl transferase inhibitor, lonafarnib.
Studies were conducted via high-throughput screening to identify new inhibitors of farnesylation. It was found that aminopyrimidines and monoaminopyrimidines target two farnesylation process enzymes called farnesyl pyrophosphate synthase and farnesyl transferase, and therefore have the ability to revert in vitro HGPS phenotypes, such as shape disruption and premature differentiation (Blondel et al. 2016). Not only were molecules found that inhibited farnesylation, but they also found molecules that promoted cleavage of the farnesylated cysteine residue by Zmptse24. There is much promise that these compounds can be used as a therapeutic treatment of HGPS and similar disorders, or even to work in the prevention of farnesylation, such as in cancers (Blondel et al. 2016).
Figure 1. Molecular Docking of Monoaminopyrimidines (Blondel et al. 2016).
Modulation of Splicing
As already discussed, Hutchinson-Gilford progeria syndrome is caused by a single point mutation in lamin A which is encoded by the LMNA gene. This mutation activates a splice site which leads to a truncated form of lamin A protein (prolamin A or progerin) and reduction in the concentration of the wild type form. This leads to the fibroblasts of HGPS patients then having abnormalities in the structure of their nuclear envelopes.
While the addition of wild type lamin A does not rescue the symptoms of HGPS patients, mutant LMNA mRNA and mutant lamin A can be eliminated by correcting the problematic splicing event. This correction can be corrected by using a modified oligonucleotide which is targeted to the splice site and corrects its action. The HGPS fibroblasts then can go back to their normal morphology and function. This includes the aberrant nuclear distribution and cellular levels of lamina proteins being rescued, the defects in heterochromatin-specific histone modifications being corrected, and the proper expression of misregulated genes being reestablished (Scaffidi and Misteli 2005).
Other researches tried to redirect the alternative splicing by promoting heathy production of the lamin C protein (an A-type lamin), which would then result in a decrease in the production of lamin A and ideally a decrease in progerin. A RNA-binding protein and a splicing factor, SRSF1, was discovered to affect the splicing of the LMNA gene in HGPS fibroblasts. The targeting of SRSF1 has been considered as a valid and useful therapeutic approach for patients in order to modulate the splicing by inhibiting protein kinase SRPK1, but many drug efforts showing dangers and toxicity (Egesipe et al. 2016). Though, metformin, an antidiabetic drug, was found to modulate SRSF1 expression. Authors then analyzed its effects on HGPS defects/abnormalities and the progerin content within HGPS fibroblasts. The results showed that the application of metformin led to decreased progerin expression in HGPS fibroblasts and reduced the abnormalities in nuclear shape, showing its therapeutic potential in the future (Egesipe et al. 2016).
Figure 1. Normal and Abnormal Nuclei. Shows shapes of wild type, HGPS, and treated HGPS cellular nuclei. (Egesipe et al. 2016)
Modulation of DNA Repair
In HGPS cells, and understandably with cells lacking Zmpste24, the processing enzyme responsible for cleavage, DNA damage remains unable to be unrepaired. It is believed that the cell is unable to recruit the checkpoint response and repair proteins that are meant to do the job of DNA repair within the cell. Though, it still remains unknown exactly how this process of DNA repair is compromised.
When DNA is damaged, heterochromatin undergoes nucleosome remodeling by ATM-mediated KAP-1 phosphorylation, which then is able to decondense the chromatin, making it readily available to the proteins responsible for DNA repair in cells (Liu et al. 2013). ATM deficiency leads to accumulated foci associated with H3K9me3, a heterochromatin mark. SUV39h1/2 is the main methyltransferase responsible for H3K9me3 and has the job of rescuing the defects in heterochromatin remodeling and DNA repair which can occur due to ATM deficiency (Liu et al. 2013). So, researchers decided to question whether the defective DNA repair mechanism in the presence of progerin is due to abnormal H3K9me3 mediated heterochromatin modeling. It was discovered that lamin A interacts with methyltransferase SUV39h1/2, which protects it from being degraded by the proteasome, whereas progerin binds exceptionally well and is therefore a target for cell degradation. This leads to increased levels of SUV39H1 and H3Kme3 in the cell. Therefore, if SUV39h1/2 is removed, the DNA repair mechanism can be rescued. In addition, cells without Zmpste24 are less likely to undergo death. This can lead to therapeutic effects in HGPS patients in regards to regulating heterochromatin marked with H3K9me3 and for targeting SUV39H1 (Liu et al. 2013).