The molecular basis of HGPS remained unknown until 2003, when two research groups discovered that the disorder is caused by mutations in the LMNA gene (De Sandre-Giovannoli
et al., 2003; Eriksson et al., 2003). At least six mutations in the gene can cause HGPS, but
more than 90 % of patients carry a heterozygous substitution in exon 11 (1824C>T) (De Sandre-Giovannoli et al., 2003; Eriksson et al., 2003; Capell and Collins, 2006). The de novo mutation alters splicing of the LMNA transcript, resulting in the deletion of 150 base pairs, i.e., 50 amino acids, near the carboxyl-terminus of Lamin A, leaving Lamin C unaffected (De Sandre-Giovannoli et al., 2003; Eriksson et al., 2003). Crucially, the truncated part of the protein contains an endoproteolytic cleavage site, which is used by the protease ‘Zinc Metallopeptidase STE24’ (ZMPSTE24) to remove a farnesyl residue added to pre-Lamin A during post- translational processing (Figure 5) (Sinensky et al., 1994; Bergo et al., 2002; Pendás et al., 2002). Similar to Ras proteins, pre-Lamin A, Lamin B1 and B2 usually undergo farnesylation by protein farnesyltransferase at a C-terminal CAAX motif (C = cysteine, A = an aliphatic amino acid, X = any amino acid), a modification facilitating their interaction with the hydrophobic nuclear membrane (Zhang and Casey, 1996; Capell and Collins, 2006; Young et al., 2013). Because this residue cannot be removed during the processing of mutant pre-Lamin A in HGPS, the protein, commonly referred to as ‘Progerin’, remains permanently farnesylated and attached to the nuclear lamina. There, its accumulation leads to a characteristic molecular phenotype that involves lobulation and wrinkling of the nuclear envelope (Eriksson et al., 2003; Goldman et al., 2004; Scaffidi and Misteli, 2005) (Figure 6). Intriguingly, Progerin is also expressed in cultured normal fibroblasts (Scaffidi and Misteli, 2006; Cao et al., 2007) and in aging skin (McClintock et al., 2007), thus suggesting that the mutant protein might have role in physiological aging, as well.
1. Introduction 1.3 The progeroid disease Hutchinson-Gilford Progeria Syndrome (HGPS)
Although very few patient-derived cell lineages are available for studying, some success has been made with regard to characterizing the expression of Progerin in different cell types. Mesodermal lineages including MSCs, fibroblasts, vascular smooth muscle cells (VSMCs) and endothelial cells exhibit high levels of the protein, whereas iPSCs and neural progenitors express only low amounts (Liu et al., 2011; Zhang et al., 2011). These variations result from the inherent tissue-related differences in the expression of A-type lamins (Zhang et al., 2011), the general downregulation of A-type lamins in pluripotent cell types (Constantinescu et al., 2006) and, as mentioned before, the expression of lineage-specific ncRNAs regulating LMNA transcript levels (Nissan et al., 2012). How they lead to the severe pathologies observed at the organismal level is a matter of ongoing research. The exhaustion of mesenchymal stem cell pools, as a consequence of altered Wnt signaling and increased hypoxia sensitivity, has been suggested to thwart the substitution of cells in affected tissues, for example (Halaschek-Wiener and Brooks-Wilson, 2007; Meshorer and Gruenbaum, 2008; Hernandez et al., 2010; Zhang et
al., 2011; Kubben et al., 2016). Additionally, with regard to atherosclerosis in particular, the
Figure 5: Post-translational processing of Lamin A and Progerin. Pre-lamin A is farnesylated by farnesyltransferase at its C-terminal CAAX motif (the polypeptide contains the C-terminal amino acids cysteine (C), serine (S), isoleucine (I) and methionine (M)). Subsequently, the three terminal amino acids are cleaved off by ZMPSTE24, and the farnesylated cysteine undergoes carboxymethylation. A second cleavage step by the same enzyme removes the 15 C-terminal amino acids plus the farnesyl group. This final step cannot occur in the processing of mutant Pre-lamin A, as aberrant splicing results in the absence of 50 amino acids that contain the endoproteolytic cleavage site (in red).
1. Introduction 1.3 The progeroid disease Hutchinson-Gilford Progeria Syndrome (HGPS) Progerin-related premature death of VSMCs has recently been demonstrated to constitute a central driving force (Zhang, Xiong and Cao, 2014; Hamczyk, del Campo and Andrés, 2018).
At the cellular level, the dominant-negative role of Progerin at the nuclear lamina has been linked to a multitude of disease-related changes (Broers et al., 2006; Vidak and Foisner, 2016; Kubben and Misteli, 2017). Most prominent are characteristic malformations of the nuclear envelope (Eriksson et al., 2003; Goldman et al., 2004; Scaffidi and Misteli, 2006), but other structural aberrations have been demonstrated in HGPS nuclei. They include a clustering of nuclear pores (Goldman et al., 2004), altered nuclear mechanical properties such as increased stiffness and thickening of the lamina (Dahl et al., 2006), as well as higher susceptibility to physical stress (Verstraeten et al., 2008; Zhang et al., 2011). Molecularly, these are accompanied by an impaired mobility of the nuclear envelope component ‘Sad1 And UNC84 Domain Containing 1’ (SUN1), a decrease in the levels of Lamin B1, and a loss of the nucleoplasmic fraction of Lamin A/C, all of which have been reported in Progerin-expressing nuclei (Scaffidi and Misteli, 2005; Shimi et al., 2011; Chen et al., 2014; Vidak et al., 2015).
Aside from structural alterations, HGPS fibroblasts are characterized by abnormal
Figure 6: Nuclear malformation and characteristic epigenetic alterations in Progerin- expressing cells. Lamin A-staining (in red) reveals characteristic nuclear lobulation in HGPS fibroblasts. H3K9me3 (A, B) and H3K27me3 (C, D) levels are reduced in HGPS nuclei, whereas H4K20me3 (E, F) levels are elevated (all in green). The arrowhead indicates the inactive X chromosome, which is enriched in H3K27me3. Scale bar = 10 µm. Adapted from
1. Introduction 1.3 The progeroid disease Hutchinson-Gilford Progeria Syndrome (HGPS) chromosome segregation and mitotic defects (Goldman et al., 2004; Cao et al., 2007), as well as a compromised DNA damage response. The latter involves the persistence of DNA damage foci marked by phosphorylated histone H2AX, the delayed recruitment of the repair factors 53BP1 and Rad51, and a mislocalization of the nucleotide excision repair factor ‘Xeroderma Pigmentosum Group A’ (XPA) to DNA double-strand breaks (Liu et al., 2005, 2008; Scaffidi and Misteli, 2006). Disrupted DNA damage signaling in HGPS cells is especially prominent at telomeres, triggering chromosomal aberrations, increased telomere shortening and premature senescence (Allsopp et al., 1992; Decker et al., 2009; Gonzalez-Suarez et al., 2009; Benson, Lee and Aaronson, 2010; Kan Cao et al., 2011; Wheaton et al., 2017). These changes are aggravated by heightened oxidative stress, as Progerin reduces antioxidant expression by sequestering ‘nuclear factor erythroid 2-like 2’ (NRF2), a central transcriptional activator of antioxidant genes (Lewis et al., 2010), away from its target genes in the nuclear interior (Viteri, Chung and Stadtman, 2010; Datta, Snow and Paschal, 2014; Kubben et al., 2016). The damage to proteins by reactive oxygen species and the accumulation of Progerin place additional stress on the proteasome, whose activity is decreased in HGPS cells (Viteri, Chung and Stadtman, 2010; Kubben et al., 2016).