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CLINICAL TRIALS IN RETT SYNDROME
Scientific Rationale and Clinical Significance Young Girl with Rett Syndrome Rett syndrome (RTT) is a disorder predominantly affecting females and is characterized by severe cognitive impairment, autistic behavior, stereotypic movements, respiratory irregularities and frequently seizures (Naidu, 1997). The majority of RTT cases are associated with mutations in the coding region of MeCP2, a gene located on the long arm of the X chromosome (Xq28) (Lee et al., 2001; Van den Veyver and Zoghbi, 2001). MeCP2 encodes a widely distributed 486 amino acid protein, which belongs to the family of methyl-binding (MBD) proteins, involved in transcriptional repression via recruitment of histone deacetylases (Nan et al., 1997; Ballestar and Wolfe, 2001).The identification of MeCP2 as the gene linked to most RTT cases (Amir et al., 1999) initiated a new era of research on this disorder. Studies of MeCP2 expression and localization in the CNS (Akbarian et al., 2001; Johnston et al., 2001; LaSalle et al., 2001; Shahbazian et al., 2002; Aber et al., 2003; Ronnett et al., 2003) have provided a better understanding of the neurologic bases of this disorder characterized by abnormal synaptic development (Kaufmann, 2001). Analyses of MeCP2 mutations have demonstrated the complex and limited association between genotype and phenotype in RTT (Amano et al., 2000; Amir et al., 2000; Bienvenu et al., 2000; Cheadle et al., 2000; Huppke et al., 2000; Auranen et al., 2001; Nielsen et al., 2001; Chae et al., 2002; Hoffbuhr et al., 2001; Huppke et al., 2002; Leonard et al., 2003; Naidu et al., 2003; Weaving et al., 2003). Clinical criteria for the delineation of the disorder (Hagberg et al., 1985; Rett Syndrome Diagnostic Criteria Working Group, 1988) have also been revised (Kerr et al., 2001; Hagberg et al., 2002) in light of the presence of 20-40% RTT (Miltenberger-Miltenyi and Laccone, 2003) cases without MeCP2 mutations and the description of mutations in individuals without RTT features (Shahbazian and Zoghbi, 2001; Hoffbuhr et al., 2002; Hammer et al., 2002; Zapella et al., 2003). Recent descriptions of a novel MeCP2 isoform, more abundant in brain than the previously described MeCP2A (Mnatzakanian et al., 2004; Kriaucionis and Bird, 2004) and associated with pathogenic mutations in exon 1 (Mnatzakanian et al., 2004), and of abnormal levels of MeCP2 in other developmental disorders (Samaco et al., 2004), further emphasize the complexity of the molecular genetics of RTT. Availability of genetic testing, which is incorporating the aforementioned findings of the genetics of RTT (Miltenberger-Miltenyi and Laccone, 2003), and recognition of the wide phenotypic spectrum associated with RTT have raised the possibility of early diagnosis and intervention (Kerr, 2002; Percy, 2002a).

Despite these genetic, neurobiologic, and clinical advances, treatment of RTT remains largely symptomatic and non-specific (reviewed by Percy, 2002b). To this date, only three completed clinical trials, two of them controlled, have been reported in the literature (Percy, 2002b). One of the controlled studies, an L-carnitine clinical trial (Ellaway et al., 1999), was based on the postulated mitochondrial dysfunction in RTT (Eeg-Olofsson et al., 1988) and on previous experience with L-carnitine clinical supplementation (Plioplys and Kasnicka, 1993; Plochl et al., 1996). Ellaway et al. (1999) reported a moderate improvement with respect to baseline, after eight weeks of treatment, on the Patient Well-Being Index and the Hand Apraxia Scale. An open label medium-term follow-up study showed improvements in sleep efficiency, energy level, and communication skills (Ellaway et al., 2001). The second randomized, placebo-controlled, double-blind crossover trial involved the oral opiate antagonist naltrexone (Percy et al., 1994). This trial was designed on the basis of neurobiologic studies of RTT, which have shown increased opioid levels/content in CNS tissues (Jellinger et al., 1988) and CSF (Brase et al., 1989; Budden et al., 1990; Myer et al., 1992), and experimental work demonstrating that intraventricular injection of endorphins induces RTT-like neurobehavioral abnormalities. An improvement in respiratory parameters was detected after a 4-month period (Percy et al., 1994). As reported by Percy in his 2002 review, as of 2001, the only ongoing clinical trial in RTT (at the Baylor College of Medicine and Univ. of Alabama) involved the use of methyl-group donors. This type of compound represents a new strategy aiming at environmentally inducing changes in gene expression without modifying DNA (Van den Veyver, 2002). In the case of RTT, by supplying methyl groups, folate and betaine would enhance deficient MeCP2 function and/or increase methylated DNA-binding by other MBD proteins. Considering the severity of its neurobehavioral manifestations and the considerable body of accumulated neurobiologic and genetic data, RTT is a disorder in which well-designed clinical trials are possible and needed. Not only pharmacologic interventions may be suitable for modifying neurologic development and function in RTT, but it appears that alternative approaches, such as music therapy (Yasuhara and Sugiyama, 2001), might also have a positive impact on motor and cognitive aspects.

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