Discussion
Our main finding was that the variant R allele of the MSTN K153R polymorphism is associated with exceptional longevity in the two independent cohorts we studied, with a significantly higher proportion of this allele in centenarians than in controls. In fact, the frequency of the R allele in our centenarians (7.1% in Spaniards and 7.6% in Italians) was approximately twofold higher than that previously reported in the literature for European Caucasians, i.e., 3-4% (Corsi et al., 2002; Ferrell et al., 1999; Kostek et al., 2009). Despite the complexity of the ageing process and the fact that exceptional longevity is likely a poly(Lawlor et al., 2011) X-linked myotubular myopathy is a severe form of congenital myopathy that often manifests with severe perinatal weakness and respiratory failure. An extension of lifespan (+30%) was also observed after therapy with follistatin (a natural antagonist of myostatin) in a mouse model of spinal muscular atrophy (Rose et al., 2009). Variants of the MSTN gene are associated with muscle hypertrophy phenotypes in a range of mammalian species (Grobet et al., 1997; McPherron et al., 1997; McPherron and Lee, 1997; Mosher et al., 2007). Myostatin expression increases might contribute to muscle wasting, as suggested by research in animal models such as immobilized mice (Carlson et al., 1999) and human models such as HIV infection (Gonzalez-Cadavid et al., 1998), critical illness (Constantin et al., 2011), or cachexia (Dasarathy et al., 2004). One study showed that serum myostatin levels increased with age, suggesting an association between myostatin and age-associated sarcopenia (Yarasheski et al., 2002). However, others have shown that myostatin expression does not increase (Kawada et al., 2001; Welle et al., 2002) and can even decrease (Baumann et al., 2003) in aging-induced sarcopenia in humans or rodents, thus questioning its involvement in muscle mass wasting during aging. In fact, the R allele has not been associated with higher muscle mass/function in old Caucasian people (aged <80 years) (see Garatachea and Luca (2013) for a review). Carriage of one R allele does not seem to exert a major influence on the muscle phenotypes of nonagenarians (Gonzalez-Freire et al., 2010), whereas, surprisingly, the only two reported Caucasian old people with the RR genotype (i.e., a Spanish woman aged 96 years and an Italian person with assumed age <80 years) seemed to have lower muscle strength than their age-matched peers (Corsi et al., 2002; Gonzalez-Freire et al., 2010). Nonetheless, given the low frequency of the R allele, further research with larger population samples is needed. In summary, the variant R allele of the MSTN K153R polymorphism was significantly more frequent in centenarians than in the control group. Although more research is needed, this genetic variation could be a potential contributor to exceptional longevity.

Acknowledgments
This study was funded by the Fondo de Investigaciones Sanitarias (FIS, ref. # PS09/00194).

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