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Adult Neural Function Requires MeCP2

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Science  08 Jul 2011:
Vol. 333, Issue 6039, pp. 186
DOI: 10.1126/science.1206593

Abstract

Rett syndrome (RTT) is a postnatal neurological disorder caused by mutations in MECP2, encoding the epigenetic regulator methyl-CpG-binding protein 2 (MeCP2). The onset of RTT symptoms during early life together with findings suggesting neurodevelopmental abnormalities in RTT and mouse models of RTT raised the question of whether maintaining MeCP2 function exclusively during early life might protect against disease. We show by using an inducible model of RTT that deletion of Mecp2 in adult mice recapitulates the germline knock-out phenotype, underscoring the ongoing role of MeCP2 in adult neurological function. Moreover, unlike the effects of other epigenetic instructions programmed during early life, the effects of early MeCP2 function are lost soon after its deletion. These findings suggest that therapies for RTT must be maintained throughout life.

Rett syndrome (RTT) is a postnatal neurological disorder characterized by autistic symptoms, cognitive and motor abnormalities, as well as decreased brain growth during childhood (1). RTT is due to mutations in MECP2, which encodes the epigenetic regulator methyl-CpG-binding protein 2 (MeCP2). The onset of RTT symptoms during a critical period of brain development suggests that the function of MeCP2 in the maturing nervous system is critical for establishing normal adult neurological function. Although recent evidence (2) has shown that reexpression of MeCP2 in symptomatic mice that lack Mecp2 rescues several features of disease, it remains unknown whether providing MeCP2 function exclusively during early postnatal life might be sufficient to prevent or mitigate disease in adult animals. In other words, if the nervous system establishes a normal epigenetic program during early life, would neurological function be protected after later loss of MeCP2?

To address this question, we developed an adult onset model of RTT by crossing mice harboring a floxed Mecp2 allele [Mecp2flox (3)] and a tamoxifen-inducible CreER allele [CAGGS-CreER (4)] to delete Mecp2 when animals are fully mature (postnatal day 60 or older). Thus, MeCP2 expression is eliminated only during adult life. Tamoxifen given daily intraperitoneally at 100 mg/kg for 20 days effectively reduces whole-brain MeCP2 levels in Mecp2flox/y; CreER+/− mice (Fig. 1, A and B). Vehicle-treated Mecp2flox/y; CreER+/− mice did not experience substantial recombination (fig. S1, A and B).

Fig. 1

Adult deletion of Mecp2 recapitulates germline knock-out. (A and B) MeCP2 is depleted in AKO mice by Western blot of brain lysates [(A), N = 3 to 4 mice per genotype] and by immunofluorescence in cerebellum (B). Scale bar indicates 50 μm. (C) AKO mice display symptoms of disease. N = 6 to 12 per genotype. (D) AKO mice develop motor and learning impairments similar to those of germline Mecp2null/y (KO) mice. N = 10 to 26 per genotype. (E) Sst and Grin2a mRNA levels are altered in AKO mice. N = 4 to 12 per genotype. (F) AKO mice die prematurely (left) similar to KO mice (right). N = 10 to 26 per genotype. Data presented as mean ± SEM. *P < 0.05; **P < 0.01; ***P < 0.001. WT, wild type; DAPI, 4′,6′-diamidino-2-phenylindole.

Mice lacking Mecp2 as adults (AKO) develop symptoms of disease and behavioral deficits similar to germline null (KO) mice. By 10 weeks after dosing, AKO mice are less active, have abnormal gait, and develop hind-limb clasping, similar to 10- to 11-week-old KO mice (Fig. 1C). AKO mice also develop motor abnormalities and impaired nesting ability, as observed in KO mice (Fig. 1D). In addition, both AKO and KO mice show impaired learning and memory (Fig. 1D).

Adult deletion of Mecp2 also demonstrates that some genes whose expression levels are sensitive to MeCP2 abundance are altered in its absence (5). In total, we tested 10 genes whose expression levels are known to be altered in KO mice (Sst, Grin2a, Htr1a, Oprk1, Tac1, Nxph4, Bdnf, Gal, Lphn2, and Odz3), and 60% are significantly altered in AKO animals compared with that in wild-type controls (P < 0.05) (Fig. 1E and fig. S2). However, four of these altered genes (Htr1a, Oprk1, Tac1, and Nxph4) are also significantly altered in control Mecp2flox mice (P < 0.05), suggesting increased sensitivity of these loci to MeCP2 function (fig. S2).

Lastly, both AKO and KO mice died prematurely with similar median time to death [13 weeks after dosing period (n = 20) versus 13.3 weeks of life (n = 13), respectively] (Fig. 1F).

Taken together, multiple features of disease in a mouse model of RTT can be recapitulated after adult deletion of Mecp2, including disease symptoms, behavioral deficits, gene expression changes, and premature death, indicating that expression of MeCP2 during early life provides little if any protection against the disease. Therefore, unlike the effects of some long-lasting epigenetic instructions that are programmed during early life (6), the effects of MeCP2 on gene expression and neurological function appear to be lost soon after deletion. Moreover, this result argues that the temporal association of disease with the postnatal period of neurodevelopment may be unrelated to any “developmental” or stage-restricted function of MeCP2, at least in mouse models. The interpretation of MeCP2 function presented here is consistent with the findings of Guy et al. (2). However, the previous study does not exclude the possibility that rescue following reexpression of MeCP2 could have been achieved in part by reinvigorating stalled neurodevelopmental processes, which would have predicted that AKO mice should be partly protected against disease. Our results rule out this possibility and decisively show the dependence of the mature brain on MeCP2 function. Lastly, these findings suggest that therapies for RTT, like MeCP2 function, must be continuously maintained.

Supporting Online Material

www.sciencemag.org/cgi/content/full/science.1206593/DC1

Materials and Methods

Figs S1 and S2

References

References and Notes

  1. Acknowledgments: We thank S. Baker, Y. Lee, A. Flora, and L. Chen for discussions and NIH (grants NS057819, HD024064 to H.Z.; F31-NS073317 to C.M.; T32-NS043124 to R.S.), Baylor Research Advocates for Student Scientists (C.M.), International Rett Syndrome Foundation, Simons Foundation, and Rett Syndrome Research Trust (H.Z.) for support.
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