The lack of detectable symptoms in female RTT patients until 6–18 months of age and the apparent phenotypic reversibility of some RTT phenotypes in MeCP2 knockout animals indicate that MeCP2 is not essential for early wiring of the nervous system but instead may only be required at late stages. It is possible that RTT patients have aberrant excitatory synaptic strength at very early stages, when the disease phenotype is not yet clearly observed. In fact, increasing evidence from clinical studies and mouse models indicates the presence of alterations during the so-called pre-symptomatic developmental phase (Charman et al., 2002
; De Filippis et al., 2009
; Kerr et al., 1987
; Picker et al., 2006
; Santos et al., 2007
To study human RTT neurons in culture, we derived iPSCs from RTT fibroblasts. RTT iPSCs are pluripotent and able to recapitulate X-inactivation upon neuronal differentiation. Even though the ratio of neurons expressing mutant MeCP2 due to X-inactivation was variable, the phenotypes described here for all RTT-derived neurons are similar. One interpretation could be that astrocytes, or other non-neuronal cells, carrying MeCP2 mutations present in our cultures could also affect neurons expressing the normal MeCP2 protein. In fact, the non-cell autonomous influence was recently described for RTT, indicating that glial cells carrying MeCP2 mutations can distress healthy neurons (Ballas et al., 2009
; Kishi and Macklis; Maezawa et al., 2009
Using human neurons carrying MeCP2 mutations, we showed that RTT glutamatergic neurons have a reduced number of synapses and dendritic spines when compared to non-affected controls. Moreover, electrophysiological recordings from RTT neurons showed a significant decrease in the frequency and amplitude of spontaneous synaptic currents compared to WT neurons. The reduced frequency in RTT neurons could reflect the presence of fewer release sites or a decreased release probability. The results of electrophysiology recordings are consistent with the decreased V-GLUT1 puncta observed in Map2-positive dendrites from RTT neurons. Also consistent with these findings, the frequency of intracellular calcium transients was decreased in RTT neurons when compared to controls. Our data indicate a potential imbalance in the neuronal networks associated with RTT pathology. The observations described here provide valuable information for RTT and, potentially, ASD patients, since they suggest that pre-symptomatic defects may represent novel biomarkers to be exploited as diagnostic tools and that early intervention may be beneficial.
Therapies aiming at earlier stages of development may attenuate the downstream consequences of MeCP2 mutations. Restoring protein levels may be challenging, since MeCP2 levels are tightly regulated and chronically overdosing neurons with the WT allele can be as harmful as a loss of expression (Collins et al., 2004
; Ramocki et al., 2009
; Van Esch et al., 2005
). Thus, we tested pharmacological treatment as a way to recover the RTT neuronal phenotype. We investigated the use of IGF1 in human neuronal cultures. Although it likely acts in a nonspecific manner, IGF1 is considered to be a candidate for pharmacological treatment of RTT and potentially other CNS disorders in a future clinical trial (Tropea et al., 2009
). While IFG1 treatment increased synapse number in some clones, it stimulated glutamatergic RTT neurons above normal levels. Our data indicate that the IGF1 dose and timing parameters need to be precisely tuned in future clinical trials to avoid side effects. In a different approach, we tested a read-through drug (gentamicin) to rescue neurons derived from iPSCs carrying a nonsense MeCP2 mutation. A lower dosage of gentamicin was enough to increase full-length MeCP2 levels in RTT neurons, rescuing glutamatergic synapses. New drugs with reduced toxicity and enhanced suppression of premature stop codon mutations might be good therapeutic candidates (Nudelman et al., 2009
; Welch et al., 2007
Control of glutamatergic synapse number and the other neuronal phenotypes analyzed here may be caused by loss of MeCP2 function in the cell. Alternatively, significant experimental and genomic variability in our system could be directly responsible for the RTT differences displayed in our data. Our gain and loss of function data strongly suggest that MeCP2 is indeed the causative agent of the cellular phenotypes reported here that might be relevant to the clinical features of RTT.
Our data indicate that iPSCs not only can recapitulate some aspects of a genetic disease but also can be used to better design and anticipate results from translational medicine. This cellular model has the potential to lead to the discovery of new compounds to treat RTT and other forms of ASD. Finally, other CNS diseases may be modeled in vitro using a similar approach.