Invecchiamento e mutazioni di splicing in FC / Ageing and splice mutations in cystic fibrosis

The RNA is an intermediate molecule between DNA and proteins. The information contained within genes (DNA segments) is initially read and transcribed to RNA intermediates, called precursor messenger RNAs (pre-mRNA), which undergo several modifications in the nucleus to reach functional maturity (mRNA). Then they are exported to the cytoplasm and translated to proteins

Not all the pre-mRNA sequence contain information for the protein synthesis: these coding segments, called exons, are separated by larger introns, as the non-coding sequences are known. The removal of the introns and consequent exon joining is a key pre-mRNA processing mechanism called splicing. This cute and paste reaction is performed by by a large nuclear multiproteic complex know as the spliceosome, which recognizes some signals at the intron/exon boundaries and is aided by a large set of splicing regulatory factors (SRF) of diverse nature(David & Manley, 2008; Nilsen & Graveley, 2010).

Some exons are always included in the mature mRNA; however, others can be included or excluded due to a differential use of splicing signals, thus creating alternative transcripts and consequently different proteins from the same pre-mRNA, with likely distinct catalytic activity. This mechanism, known as alternative splicing (AS) is believed to greatly contribute to tissue differentiation and specificity, embriogenesis and aging (Hallegger, Llorian, & Smith, 2010; Nilsen & Graveley, 2010). Therefore, AS decisions are highly regulated to ensure proper protein isoforms synthesis. Any mutation affecting the splicing signals could be mis interpreted and result in production of an aberrant protein, a imbalance in protein isoforms or no protein synthesis at all (Garcia-blanco, Baraniak, & Lasda, 2004; Pajares et al., 2007). Any of these possible scenarios may have pathological consequences, and mutations affecting splicing are nowadays considered to be responsible for a great fraction of genetic diseases (Lopez-Bigas, Audit, Ouzounis, Parra, & Guigo, 2005). For example, approximately 12% of the registered mutations for CFTR gene are believed to affect adequate splicing processing. A striking example of the importance of AS is the Hutchison Gilford progeria syndrome, a rare condition, remarkable because its symptoms strongly resemble normal human aging, but occur in young children (Pollex & Hegele, 2004).A splicing mutation (G608G) on the lamin A gene obstructs normal splicing of the mRNA transcript and leads to the formation of a mutant protein, progerin. Progerin is not able to support nuclear architecture as normal lamin A does, leading to a global disregulation of gene expression (Pegoraro et al., 2009; Pollex & Hegele, 2004).

Early reports have demonstrated that the production of distinct protein isoforms produced by AS changes with age (Kaneko et al., 1991; Pagani et al., 1991). The precise mechanisms underlying these changes are still unclear. It has been proposed that changes in the levels of SRF could contribute to ageing-regulated AS events. As a matter of fact, differential levels of SRF such as U2AF65, PTB/hnRNP I, PRP22 have been found in aged mice (Meshorer & Soreq, 2002a) and recent findings suggesting that some subset of genes undergo clear age-related changes in AS events, (Harries et al., 2011; de Magalhães, Curado, & Church, 2009).

In other genetic diseases, such as epydermolysis bullosa with pyloric atresia (PA-JEB) a general improvement with ageing has been reported in some patients carrying a splicing mutation in the integrin beta-4 gene (Chavanas et al., 1999). As mentioned before, age-related variations in SFR levels and/or spliceosome components levels may be responsible for the age-related AS alterations (Meshorer & Soreq, 2002b). Indeed, a recent work reported that the pre-mRNA processing is the main biological pathway affected by age (Harries et al., 2011).

So far, there are no evidences on age-related changes in CFTR pre-mRNA processing. Follow-up of CF patients carrying splicing mutations could provide evidence about CFTR splicing modulation with age. However, the progressive deterioration of immune response with age may influence CF development. Meta-analysis of age-related gene expression profiles indicated that inflammation and immune response genes are overexpressed in aged tissue (Harries et al., 2011; de Magalhães et al., 2009; Zahn et al., 2007).

Franco Pagani, Eugenio Fernandez-Alanis
Human Molecular Genetics, International Centre for Genetic Engineering and Biotechnology, Padriciano 99, 34149 Trieste, Italy

References

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