The ultimate goal of RTT research is to sufficiently understand the disease to design methods of treatment. To date, only three controlled clinical trials in RTT individuals have been reported, and all studies showed some, but no dramatic, effects. These trials involved the administration of the opioid antagonist naltrexone (Percy et al., 1994
), the amino acid L-carnitine important in fatty acid metabolism (Ellaway et al., 1999
), and folate-betaine which increases the available pool of methyl-donors (Glaze et al., 2009
). Here, we discuss some areas of research in the field that could potentially lead to the development of promising beneficial therapies.
Work in MeCP2 mouse models have been instrumental not only in understanding the function of MeCP2, but in suggesting possible therapeutic options by identifying neurochemical and molecular changes that occur in the absence of MeCP2 function. For example, mice lacking MeCP2 have decreased expression of BDNF and genetically increasing BDNF levels improves survival in these animals (Chang et al., 2006
), which has lead to the exploration of treating animals with BDNF. To pharmacologically increase BDNF, Mecp2-
null animals treated with the ampakine drug CX546, which increases BDNF levels in the nodose cranial sensory ganglia, caused an improvement in respiratory frequency and minute volume (Ogier et al., 2007
). Also with the notion of increasing BDNF levels, administration of a tripeptide form of IGF1, which can regulate BDNF levels, caused global improvements in the overall condition and longevity of Mecp2Jae
mice (Tropea et al., 2009
). Because serotonin levels are decreased in people with RTT and animal models (Samaco et al., 2009
), the role of serotonin receptor 1a agonist 8-OH-DPAT combined with a GABA reuptake inhibitor NO-711 was tested. This combination improved the breathing abnormalities in Mecp2-
deficient animals (Abdala et al., 2010
). Lastly, chronic L-dopa administration, with the goal to correct the dopamine and norepinephrine deficits (Samaco et al., 2009
), improved the motor deficits in Mecp2-
null mice (Panayotis et al., 2011
Another possible approach in treating the symptoms of RTT includes strategies to increase the levels of MeCP2. Post-natal reactivation of MeCP2 restored the general health condition, LTP defects and viability in mice, raising the possibility that gene therapy approaches to increase MeCP2 levels in RTT individuals may be beneficial (Guy et al., 2007
). However, the observations that over-expression as well as the partial loss of function of MeCP2 in cells can be detrimental has raised concerns about the viability of gene therapy in RTT, given that affected females have a proportion of cells that still express a normal version and amount of MeCP2 due to XCI (Collins et al., 2004
; Samaco et al., 2008
Attempts to control the increase in expression of MeCP2 in a mosaic population of wild-type and mutant MeCP2-expressing cells could be challenging. Therefore, an indirect approach to increase MeCP2 expression may be necessary, perhaps through decreasing the levels of miRNA that target MeCP2. Thus far, several miRNA have been demonstrated to target MeCP2, including miR-132, miR-212, mir-802 and miR-155 (Klein et al., 2007
; Kuhn et al., 2010
; Im et al., 2010
; Wada et al., 2010
). The long 3′UTR of MeCP2 harbors over 50 putative miRNA binding sites (Hon and Zhang, 2007
). By reducing the expression of such miRNAs in wild-type MeCP2-expressing neurons of Mecp2−/+
animals, one could test the hypothesis that increasing the levels of MeCP2 protein may partially compensate for the loss of MeCP2 function in mutant MeCP2-expressing neurons. The non-cell-autonomous increase in MeCP2 expression may not, however, be beneficial in a mosaic population of cells. Furthermore, given that neuronal phenotypes are sensitive to MeCP2 levels, such an approach must be well controlled to normalize MeCP2 expression.
Recent studies have also shown that a number of miRNA are dysregulated in Mecp2
-null animals (Urdinguio et al., 2010
; Wu et al., 2010
). Although the biological significance of these changes may also reflect the transcriptional noise that occurs in the absence of MeCP2 as a transcriptional dampener, it is possible that a few miRNA may cause a cascade of transcriptional changes relevant to disease pathogenesis. One could envision that designing therapies that target the downstream targets of affected miRNA in these studies could possibly improve RTT phenotypes.
Another method to increase the expression of MeCP2 relies on the previously described “read-through” compounds that suppress nonsense mutations and allow the production of full-length protein. Because a significant fraction of RTT disease-causing mutations in MECP2
are nonsense mutations, this strategy could prove valuable. Aminoglycosides (AG) such as gentamycin are capable of suppressing nonsense mutations. Recent work has demonstrated that AG can suppress such nonsense mutations in MECP2
both in cell culture and in fibroblasts derived from a mouse expressing an allele of Mecp2
containing a nonsense mutation (Brendel et al., 2011
). Among the unfortunate side-effects of AG, however, is nephrotoxicity and ototoxicity (Houghton et al., 2010
). Further work will be needed to determine whether these or other novel compounds with reduced toxicity but conserved read-through properties can work sufficiently well in vivo
to rescue the phenotypic abnormalities of this mouse model.
As detailed in vivo
work on the role of MeCP2 progresses, one of the striking observations is the variation in the cellular phenotypes observed depending on both the specific cell studied as well as the overall cellular milieu. For example, although somatosensory cortical neurons show decreased firing rates and decreased miniature excitatory postsynaptic currents (mEPSCs), neurons within the brainstem such as the locus ceruleus and neurons in the nucleus of the solitary tract show increased firing rates and increased mEPSCs (Dani et al., 2005
; Kline et al., 2010
). These differential findings raise a note of caution both for the over-generalization of specific cellular phenotypes observed in one neuronal population and, importantly, for the consideration of possible therapeutic strategies. For example, one might postulate that because there appears to be a decrease in the excitatory/inhibitory balance within the sensory cortex that therapies should be targeted towards decreasing inhibitory signaling or increasing excitatory signaling. However, while such a strategy might prove beneficial for cortical functioning, it might be detrimental for critical brainstem functions such as breathing.