The genetic mechanism that links Hutchinson-Gilford progeria syndrome to physiological aging

Abstract

Aging is a complex process that is not completely understood. The study of segmental progeroid syndromes such as Hutchinson-Gilford progeria syndrome (HGPS) has allowed us to connect the common genetic mechanisms that occur in normal physiological aging, with the cellular alterations presented by this severe premature aging syndrome. Since the identification of mutations in the lamin A/C coding LMNA gene that cause HGPS and other laminopathies, there has been an increasing interest in the potential role of lamins in the normal aging process. Progerin, a mutant form of lamin A, has attracted particular great attention. LMNA mutations in HGPS activate a cryptic splice site, leading to an aberrant splicing of lamin A, which results in a lamin AΔ150 transcript and progerin. Recent research data, including the results presented in this thesis, provide support for the possibility of a shared mechanism between natural physiological aging and pathological aging occurring in HGPS. This shared mechanism could contribute to solving part of the aging puzzle. The overall aim of this thesis was to gain an increased understanding of potential common genetic mechanisms behind Hutchinson-Gilford progeria syndrome and normal physiological aging. For this purpose, the research was primarily designed to study the expression of LMNA and the global genome differential splicing in aging cells to investigate the potential relationships that link the genetic mechanisms found in HGPS to those occurring in normal physiological aging.

In paper I, we develop an absolute quantification method to determine the overall expression levels of the LMNA gene transcripts lamin A, lamin C and lamin AΔ150 (progerin) during the in vitro cell aging of primary dermal fibroblasts from HGPS patients and from age-matched and parent controls. We show that lamin C is the most highly expressed transcript and that the lamin AΔ150 transcript is present in unaffected controls at an approximately 160-fold lower expression level compared with HGPS patients. While the levels of lamin A and lamin C transcripts remained unchanged during in vitro cell aging, the lamin AΔ150 transcript increased in late passage cells from HGPS patients and parental controls, suggesting a similar mechanism in HGPS patients and unaffected donors during cellular aging.

In paper II, we expand on the first study and continue to investigate the expression of progerin in unaffected cells from different age groups, evaluating progerin as an aging biomarker for cellular senescence. We utilize a newly developed progerin antibody and quantify the percentage of progerin-expressing cells in the early and late passages of cells aged in vitro. We found that well-defined nuclear expression of progerin in primary dermal fibroblasts that were aged in vitro is an extremely rare event (or below the detection limit of an immunofluorescence assay). Our results do not rule out the possibility of progerin being expressed during normal cellular aging but question progerin’s contribution to physiological cellular aging.

In paper III, we investigate if the LMNA gene presents differential allelic expression, which could help to explain the phenotypic variability observed among HGPS patients and laminopathies in general. We made use of the rs4641C/T LMNA coding single nucleotide polymorphism (SNP) and developed an allele-specific absolute quantification method for the lamin A and lamin C transcripts. The contribution of each allele to the total transcript level was quantified in dermal fibroblasts from HGPS patients and unaffected controls. We show that the C allele is more frequently expressed, corresponding to a 70% of the total lamin A/C transcripts, and that the most common HGPS mutation, LMNA c.1824C>T, is found in both the C and T alleles, which could account for the phenotypic variability observed among HGPS patients.

In paper IV, we investigate whether aberrant splicing events, such as that occurring both in HGPS and sporadically in unaffected individuals, become more frequent and widespread on a genome-wide level during normal physiological aging, hypothetically, as a result of a declined stringency of the splicing machinery. To analyze the effect of age on splicing, we used exon microarrays to investigate the global genome differential exon expression and detect alternatively spliced genes in tissues of normal aging mice and in cells from a HGPS mouse model. We show that aging affects splicing via a considerable number of genes displaying significant differential and increased splicing with age. The most significantly enriched biological functions with alternative spliced genes during normal aging included RNA processing and the spliceosome pathway. Additionally, progeria cells had primarily differential splicing of extra cellular matrix genes, and explorative network enrichment analysis identified the NF-κB complex as a potential common network node for HGPS and normal tissue aging.