Neuropathology of Rett syndrome

By Dr. D. Armstrong

Introduction

The features of the disorder that has become known as Rett syndrome were observed first in 1977 in Austria by Andres Rett^[1] and in 1983 in Sweden by Hagberg and his European colleagues.^[2] It is now recognized worldwide, usually presenting in a small girl with loss of speech and of purposeful hand use and the presence of stereotopies and ataxia. Some girls cannot walk, and many have epilepsy. Jellinger and Seitelberger made the first report of a series of eight Rett syndrome cases and observed decreased brain weight and decreased pigmentation of the substantia nigra pars compacta.^[3] Riederer and colleagues described the altered chemistry associated with Rett syndrome.^[4] Since these early reports, clinician-scientists have come to better understand this rare disorder in large part because the determination and encouragement of parents have prompted progress in the field. Even so, defining each aspect of the clinical-pathologic features of Rett syndrome has been difficult.

For example, it has been a challenge to determine that Rett syndrome is a genetic disorder. It affects girls and seems to be primarily a sporadic disease, with few familial cases. The identification of the X-linked MECP2 as the causative gene required careful collaborations among laboratories, extensive study, and sharing of data.^[5]Mutations in the MECP2 gene in exons 1 to 4 or DNA deletions have now been identified in more than 90% of Rett syndrome cases.^[5–8] Abnormalities in MECP2 have also been detected in some boys with an infantile encephalopathy,^[9] in some children with an Angelman syndrome phenotype,^[10] and in some boys with mental retardation.^[11] Three animal models are now being used to clarify the relationship of MECP2 to the pathogenesis of Rett syndrome ( Table 1 ).^[12–14]

The definition of the clinical features of Rett syndrome has also been evasive. Classic and variant forms have been defined. Genotype-phenotype correlations are now being made, and international data registries have been created to facilitate these associations.^[15]

The patient with Rett syndrome is small, and studies of growth have identified disorders of swallowing mechanisms.^[16] The characterization of motor problems, absence of speech, loss of hand use, stereotopies, ataxia, dystonia, and hypertonicity or hypotonicity has been problematic.^[17] The cognitive aspects of Rett syndrome have evaded definition because of problems in testing patients who have no speech or hand use. Girls display abnormalities of breathing, heart rate, peripheral circulation, and emotional hyperexcitability that have to be distinguished from autonomic dysfunction and seizures.^[18–20] These complex clinical features in aggregate define a unique and demanding clinical situation requiring complete daily care by parents and innovative assistance from physicians and support groups. The relatively healthy patients with Rett syndrome survive into the fourth and fifth decades, although there is an increased incidence of sudden death in the adolescent years.^[21]

Neurochemistry

The neurochemistry of Rett syndrome ( Table 2 ) has been difficult to determine in patients.^[22–41] Studies on cerebrospinal fluid and brain tissue have analyzed transmitters, their receptors, and additional trophic factors. Abnormalities have been reported in most systems, including in acetylcholine, dopamine, serotonin, glutamate substance P, and new nerve growth factor. The age of the patient with Rett syndrome^[29] and the severity of the symptoms^[31]influence measurements. Wenk concluded that the studies of neurotransmitters and peptides were variable except for acetylcholine, whose markers were consistently reduced.^[35]Kaufmann concluded in his review that dopamine is involved in the striatonigral system and in forebrain choline acetyltransferase expression, with a moderate reduction in cortical cholinergic innervation and an elevation of glutamate and of some glutamate receptor subtypes in younger subjects with Rett syndrome.^[32] The clearer documentation of the alterations in chemistry with relation to age is still needed. The changes in the neurotransmitters and other trophic factors are of great importance, early on for their role as neurotrophins in brain development and later for their role in normal functioning of the mature brain. It is hoped that animal models will allow for more controlled examinations of how these important chemicals are altered in Rett syndrome.

Pathology

In this article, I elaborate on the observations that have been made suggesting that neuronal maturation is defective in Rett syndrome and on the observations about MECP2 expression that coincide with these observations.

The infant girl with Rett syndrome is nondysmorphic, with a head circumference that is normal at birth but that begins to decelerate in its growth at 2 to 3 months of age. The other growth parameters (ie, height, weight, hand, foot size) decline later.^[42] Autopsies of patients with Rett syndrome of various ages reveal that all organs, except the adrenal gland, are small for the age of the patient but not for the height. The brain, however, is an exception. The average brain weight is 900 g, the brain weight of a 1-year-old infant. Thus, the weight of the Rett syndrome brain is significantly less than that of the brain of non–Rett syndrome controls for age and for height (Figure 1). The identification of this preferential involvement of the brain suggested that Rett syndrome was a disease of the nervous system. This has been endorsed by the observations that MECP2 is expressed primarily in neurons.^[43] The weight of the Rett syndrome brain does not decrease significantly with age, so atrophy does not account for the small size, an observation that has been confirmed by imaging studies of patients with Rett syndrome.^[32]

The decrease in brain weight is not generalized. The cerebral hemispheres are affected more than the cerebellum. The brainstem weights have not been compared with those of non–Rett syndrome brains because of the various techniques of brainstem dissection (Figure 2). Imaging studies define alterations in brain volumes in prefrontal, posterior frontal, and anterior temporal regions, with relative preservation in the posterior temporal and posterior occipital regions.^[44,45]

With routine examinations, the Rett syndrome brain reveals no obvious alteration in the blood vessels, cranial nerves, gyri, white matter, basal ganglia, cerebellum, brain stem, or spinal cord, although these have not been evaluated quantitatively in detail. Single reports of cerebellar and spinal cord neuronal loss with time have been recorded,^[46,47] and there have been no consistent reports of changes in glia or in the accumulation of lipofuschin. Kaufmann found some increase in glial fibrillary acidic protein expression but could not determine if this is a primary or a secondary phenomenon.^[32] It has been generally concluded that no recognizable degenerative, demyelinating, or gross malformative process explains the Rett syndrome phenotype.

Studies of the Cerebral Cortex

To study the decreased size of the Rett syndrome brain, the cerebral cortex was examined with the Golgi technique. Pyramidal neurons of layers III and V of the frontal, motor, occipital, superior temporal, and inferior temporal cortices and layers II and IV of the subicular cortex and the neurons of Ca1 of the hippocampus were evaluated using camera lucida drawings and the Scholl analysis. There was a significant decrease in the dendritic territories in Rett syndrome of the frontal, motor, and subicular cortices when compared with non–Rett syndrome cases and trisomy 21 cases.^[48,49] The decreased dendritic territories in selected regions of the Rett syndrome brain correlated with other observations that defined in the cortex, thalamus, basal ganglia, amygdala, and hippocampus a decreased neuronal size and in the hippocampus an increased neuronal packing density.^[50] Casanova and colleagues studied cortical minicolumns in areas 9, 21, and 22 in five Rett syndrome brains. The minicolumns are reduced in size, significantly so, in area 21. The minicolumn is defined as the smallest functional unit of the cerebral cortex, and its diminished size in Rett syndrome could reflect the decreased dendritic branching of neurons that was observed in the Golgi studies or other undefined associations.^[51] Belichenko and colleagues made quantitative studies observing a decreased number of neurons in layers II and III of the frontal and temporal cortices,^[52] and in the speech cortex a normal number of neurons, a preservation of the size differences between the dominant and nondominant hemispheres, and reduced numbers of parvalbumin-positive interneurons and synaptic sites.^[53] Belichenko and colleagues observed decreased numbers of dendritic spines and afferent axons in the frontal cortex of four patients with Rett syndrome.^[54,55]

Studies in Other Brain Regions

In the midbrain and substantia nigra, the large neurons of pars compacta have less melanin and tyrosine hydroxylase staining than controls.^[3] Imaging studies record decreased volumes of the midbrain, caudate, putamen, and thalamus.^[44] The neurons of the globus pallidus were observed on Golgi studies to have altered contours and appendages in the region of the upper extremity.^[56] In the cerebellum, a progressive loss of neurons with increasing age was observed in autopsy tissue,^[46] and by magnetic resonance imaging.^[57] In the spinal cord, there is a report of anterior horn neuronal loss and gliosis in the corticospinal tracts of two patients.^[47]

The clinical expression of breathing irregularities, heart rate variability, small cold feet, constipation, and difficulties in swallowing suggests autonomic impairment. Functional studies of the vagus nerve define an abnormality of vagal tone.^[58] Limited brainstem studies suggest abnormalities in serotonin receptors and in substance P content.^[41,59]

In the peripheral nerve, a mild distal axonal neuropathy was observed in 7 of 12 cases by Haas^[60] and in 4 of Jellinger and colleagues' cases.^[61] Muscle biopsies have shown variable Results, including type II atrophy, type I atrophy, myopathic changes (in a rare case), alterations in mitochondrial morphology, and a reduction in mitochondrial enzyme levels.^[61] Continuing reports of altered chemistry and mitochondrial enzymes suggest either primary or secondary involvement.^[62–64]

In summary, the major morphologic change in the Rett syndrome brain is a decrease in the size of the brain and of individual neurons, which show less dendritic arborization and spines than the non–Rett syndrome neurons. There is no obvious atrophy or degeneration, so the neuropathologic process in Rett syndrome has been hypothesized to be one of a failure of development.

Studies Related to MECP2

MECP2 modifies gene expression by inhibiting transcription, and this function would seem to be significant in the genetic control of brain development. The distribution of MECP2 protein has been studied in the developing human brain and in Rett syndrome.^[65] The earliest normal expression of MECP2 at 10 gestational weeks is in the nuclei of isolated cells of the cerebral subventricular zone, of the brain stem, and of the subcortical and Cajal-Retzius neurons of the cerebral cortex. The expression of MECP2 appears next in the thalamus, midbrain, and basal ganglia. The hippocampus and the cerebellum show less expression early in development, but with maturation, most neurons, but not glia, express MECP2. The cerebral cortical neurons continue to show increasing amounts of MECP2-immunopositive neurons into the adolescent years.^[43]

The interpretation of the immunochemical marking for MECP2 has been complicated by the availability of consistent antibodies, by antibodies that identify various epitopes, and by the observed variability of intensity of staining at various ages and within the same types of neurons. Most staining appears in the nucleus, but there is slight cytoplasmic staining in some neurons whose cytoplasm can be recognized by routine histology. LaSalle and colleagues, using laser scanning cytometry, were able to quantitate the expression of MECP2 in the human and murine brain and confirmed the presence of both a low and a high expression of MECP2 immunoreactivity.^[66]They studied the ontogeny of MECP2 in the developing mouse brain and showed that near the time of birth (E10–E19), MECP2 of the low expression type is abundant in the medulla and thalamus. The expression decreases after birth and then gradually increases (after 5 days) to increasing levels of high MECP2 expression in all neurons in the brain (3–40 weeks).^[67] The Ronnet laboratory showed a similar pattern of variable MECP2 expression in the neurons of the olfactory epithelium and identified MECP2 expression in the immature and mature olfactory receptor neurons prior to synaptogenesis in those neurons. They demonstrated in the mouse olfactory epithelium that when synaptogenesis was blocked by axonotomy, the expression of MECP2 was decreased.^[68] These observations draw attention to the role of MECP2 in synaptogenesis. Kaufmann's study on the location of MECP2 revealed that it is present not only in the nucleus but also in the postsynaptic compartment of the neuron, where it is available to react in response to neuronal activity.^[69] Ronnett's continuing examination of the effect of MECP2 deficiency on the olfactory epithelium in the MECP2-null mouse,^[70] and in the olfactory epithelium of patients with Rett syndrome showed that their olfactory receptor neurons exhibit a delay in maturation. In addition, the axonal projections of the MECP2-null animals are disorganized, resulting in a reduced size of the olfactory glomeruli.^[71]

LaSalle studied the expression of MECP2 in female mice heterozygous for MECP2. The MECP2-negative cells were found to be uniformly distributed, and using tissue microarrays, she observed that the percentage of MECP2-negative cells was less than the expected 50%, in the range of 20% to 39%. This was interpreted to mean that an unbalanced X-chromosome inactivation pattern favors inactivation of the mutant allele. The MECP2 expression pattern in the cells that do express MECP2, however, differed from the Wt expression of MECP2, exhibiting more cells of the low expression type and suggesting that mutant MECP2 can influence the Rett syndrome phenotype by both cell-autonomous and non–cell-autonomous mechanisms. Similar observations were made on tissue microarrays of frontal cortex of three patients with Rett syndrome.^[67]

Discussion of MECP2 in Neuronal Maturation and in Rett Syndrome

In Rett syndrome, there is a low brain weight, a reduced branching of dendrites, and a decrease in dendritic spines. The studies on the ontogeny of MECP2 expression reveal that MECP2 is a marker of the "maturing neuron"^[43,72,73] being expressed in neurons as they mature and take up critical locations for the continuing development of the brain. The increase in MECP2 expression in the brain after the prenatal period, when most apparent brain development occurs, suggests that MECP2 might also be required for neuronal maintenance and function.

On the basis of these observations, and the fact that the child with Rett syndrome has a deceleration in growth of the head circumference during the time that rapid synaptogenesis is occurring, it has been postulated that synaptogenesis is deficient in Rett syndrome. This is supported by the observations that in mice MECP2 is expressed prior to synaptogenesis so that, presumably, the MECP2-deficient Rett syndrome brain would have fewer synapses. The animal models suggest a hypothesis that addresses the deficiency of synapses.

In normal early embryonic life, MECP2 is expressed in two developmentally important regions: the medulla and the thalamus. In normal brain development, the neurons of the medulla express critical trophic catecholamines (noradrenaline and serotonin)^[74] and send nonspecific afferents to the developing cortex, where they influence migration and organization of neuronal systems. MECP2 deficiency in brainstem neurons, subplate, and Cajal-Retzius neurons (and all neurons required for critical stages of development) would alter the efficacy of these neurons by interfering with their maturation, causing a reduced production of trophic factors,^[74] receptors,^[75] early response genes,^[76] and structural proteins^[77] and disorganization of cortical columns,^[51] dendrite growth, and synapse formation.^[48]

Studies of MECP2 in the olfactory epithelium of mice have shown that MECP2 is expressed prior to synapse formation and that interfering with synapse formation reduces the amount of MECP2 expression.^[68,70] LaSalle's model suggests that in the normal maturation of neurons, the neurons that express a low level of MECP2 are modified by a non–cell-autonomous mechanism (possibly by synaptic stimuli from surrounding neurons) to express MECP2 at a high level. Thus, through cell-autonomous and non–cell-autonomous regulation, the availability of MECP2 in individual neurons will influence its role in the genetic regulation of maturation. In the Rett syndrome brain, with fewer synapses, the expression of MECP2 is less than optimum. This direct role of MECP2 in response to neuronal activity has been suggested by two other studies. Kaufmann and colleagues' identification ofMECP2 in the postsynaptic compartment of neurons places it in a site where it is available to respond to neuronal activity.^[78] Chen and colleagues also demonstrated that calcium entry into the cell causes phosphorylation of MECP2 and releases its inhibitory role, enabling the transcription of brain-derived neurotrophic factor.^[79]

The abnormalities that we have observed in the anatomy, chemistry, and clinical manifestations of Rett syndrome all suggest that MECP2 is essential for neuronal maturation. The signal for the initial expression of MECP2 in a neuronal precursor cell can be regulated by a cell-autonomous mechanism. Its increasing expression in fully mature neurons can then be regulated in part by external factors, such as synaptic input. Proteomic studies of olfactory epithelium at the age at which synaptogenesis is taking place demonstrate that MECP2 deficiency alters some of the proteins required for neuronal outgrowth, path finding, and cell connections. It is also required for proteins involved in mitochondrial activity and stress management.^[80] Thus, MECP2 appears to be required for both the structural and the functional maturity of neurons (Figure 3). To promote neuronal maturation in children with Rett syndrome, it will be necessary now to determine how MECP2 deficiency can be treated.

References

1. Rett A. Uber ein zerebral-atrophisches Syndrome bein Hyperammonamie Wein Med Wochenschr 1966;116:723–726.
2. Hagberg B, Bacardi J, Dias K, Romos O. A progressive syndrome of autism, dementia, ataxia and loss of purposeful hand use in girls. Report of 35 cases. Ann Neurol 1983;14:471–479. .
3. Jellinger K, Seitelberger F. Neuropathology of Rett syndrome Am J Med Genet 1986;24:259–288.
4. Riederer P, Brucke T, Sofic E, et al. Neurochemical aspects of the Rett syndrome Brain Dev 1985;07:351–360.
5. Amir RE, Van Den Veyver I, Wan M, et al. Rett's syndrome is caused by mutations in X-linked MECP2 encoding methyl-CpG-binding protein Nat Genet 1999;23:185–188.
6. Wraby S. Discovery of a new protein isoform of MECP2 and exon 1 mutations causing Rett syndrome Clin Genet 2004;666:108–110.
7. Mnatzakanian GN, Lohi H, Munteanu I, et al. A previously unidentified MECP2 open reading frame defines a new protein isoform relevant to Rett syndrome Nat Genet 2004;36:339–341.
8. Shabazian MD, Zoghbi HY. Molecular genetics of Rett syndrome and clinical spectrum of MECP2 mutations Curr Opin Neurol 2001;14:171–176.
9. Schanen NC, Kurczynski TW, Brunelle D, et al. Neonatal encephalopathy in two boys in families with recurrent Rett syndrome J Child Neurol 1998;13:229–231.
10. Watson P, Black G, Ramsden S, et al. Angelmann syndrome phenotype associated with mutations in MECP2, a gene encoding a methyl CpG binding protein J Med Genet 2001;28:224–228.
11. Kudo S, NomuraY, Segawa M, et al. Functional characterization of MECP2 mutations found in male patients with X linked mental retardation J Med Genet 2002;39:132–136.
12. Guy J, Enrich B, Martin JE, Bird A. A mouse MECP2-null mutation causes neurologic symptoms that mimic Rett syndrome Nat Genet 2001;27:322–326.
13. Chen RZ, Akbarian S, Tudor M, Jaenisch R. Deficiency of methyl-CpG binding protein-2 in CNS neurons Results in a Rett-like phenotype in mice Nat Genet 2001;27:327–331.
14. Shahbazian MD, Young JI, Yuva-Paylor LA, et al. Mice with truncated MECP2 recapitulate many Rett syndrome features and display hyperacetylation of histone H3 Neuron 2002;35:243–254.
15. FyfeYS, Cream M, deKlerk N, et al. International Rett Syndrome Association database for Rett syndrome J Child Neurol 2003;18:709–713.
16. Motil KJ, Schultz RJ, Browning AB, et al. Oral pharyngeal and gastroesophageal dysmotility are present in girls and women with Rett syndrome J Pediatr Gastroenterol Nutr 1999;29:31–37.
17. Segawa M, Nomura Y. The Proceedings of the Symposium on the Rett Syndrome, Tokyo, Nov 3–4, 1990 Brain Dev 1992;14(Suppl):31–37.
18. Glaze DG, Frost JD, Zoghbi HY, Percy AK. Rett syndrome Correlation of electroencephalographic characteristics with clinical staging 1987;44:1053–1056.
19. Glaze DG, Frost JD, Zoghbi HY, Percy AK. Rett's syndrome. Characterization of respiratory patterns and sleep. Ann Neurol 1987;21:377–382. .
20. Kerr AM. A review of the respiratory disorder in Rett syndrome Brain Dev 1992;114:S43–S45.
21. Kerr A, Engerstrom IW. , Oxford, UK, Oxford University Press, 2001, 1–27: Oxford; UK. The clinical background to the Rett disorder, in Kerr A, Engerström IW (eds): Rett Disorder and the Developing Brain; p. S43-S45..
22. Zoghbi HY, Percy AK, Glaze DG, et al. Reduction of biogenic aminic levels in the Rett syndrome N Engl J Med 1985;313:921–924.
23. Zoghbi HY, Milstien S, Butler IJN, et al. Cerebrospinal fluid biogenic amines and biopterin in Rett syndrome Ann Neurol 1989;25:56–60.
24. Brucke T, Sofic E, Killian W, et al. Reduced concentrations and increased metabolism of biogenic amines in a single case of Rett syndrome. A post mortem brain study. J Neural Transm 1987;68:315–324. .
25. Wenk GL. Naidu S, Casanova MF Altered neurochemical markers in Rett syndrome 1991;41:1753–1756.
26. Wenk G. Selective changes in Rett syndrome neurochemistry. Findings of normal dopaminergic and decreased cholinergic function. Eur Child Adolesc Psychiatry 1997;6(Suppl 1):87–88. .
27. Wenk GL, Hauss-Wegrzyniak B. Altered cholinergic function in the basal forebrain of girls with Rett syndrome Neuropediatrics 1999;30:125–129.
28. Perry TL, Dunn HG, Ho HH, Crichton JU. Cerebrospinal fluid values for monoamine metabolites, gamma aminobutyric acid and other amino compounds for Rett syndrome J Pediatr 1988;112:234–238.
29. Lekman A, Engerström IW, Holmber B, et al. CSF and urine biogenic amine metabolites in Rett syndrome Clin Genet 1990;37:173–178.
30. Nielsen JB, Bertelson ZA, Lou HC. Low CSF HVA levels in Rett syndrome. A reflection of restricted synapse formation?. Brain Dev 1992;(Suppl):S63–S65. .
31. Percy AK. Neurochemistry of the Rett syndrome Brain Dev 1992;14(Suppl):S57–S62.
32. Kaufmann WE. , 41907 Oxford: Oxford University Press; UK. .
33. Budden SS, Myer EC, Butler IJ. Cerebrospinal fluid studies in the Rett syndrome. Biogenic amines and beta-endorphins. Brain Dev 1990;12:81–84. .
34. Percy AK, Glaze DG, Schultz RJ, et al. Rett syndrome. Controlled study of an oral opiate antagonist, naltrexone. Ann Neurol 1994;35:464–470. .
35. Wenk GL. Rett syndrome. neurobiologic changes underlying specific symptoms. Progr Neurobiol 1997;51:383–391. .
36. Hamberger A, Gillberg C, Palm A, Hagberg B. Elevated CSF glutamate in Rett syndrome Neuropediatrics 1992;23:212–213.
37. Lappalainen R, Riikonen RS. High levels of cerebrospinal glutamate in Rett syndrome Pediatr Neurol 1996;15:213–216.
38. Lappalainen R, Lindholm D, Riikonen RS. Low levels of nerve growth factor in cerebrospinal fluid of children with Rett syndrome J Child Neurol 1996;11:296–300.
39. Riikonen R, Vanhala R. Levels of cerebrospinal fluid nerve-growth factor differ in infantile autism and Rett syndrome Dev Med Child Neurol 1999;41:148–152.
40. Matsuishi T, Nagamitsu S, Yamashita Y, et al. Decreased cerebrospinal fluid levels of substance P in patients with Rett syndrome Ann Neurol 1997;42:978–981.
41. Deguchi K, Antalffy B, Twohill LJ, et al. Decreased substance P immunoreactivity in Rett syndrome Pediatr Neurol 2000;22: 259–266.
42. Schultz RJ, Glaze DG, Motil KJ, et al. The pattern of growth failure in Rett syndrome Am J Dis Child 1993;147:633–637.
43. Shahbazian MD, Antalffy B, Armstrong DL, Zoghbi HY. Insight into Rett syndrome. MECP2 levels display tissue and cell specific differences and correlate with neuronal maturation. Hum Mol Genet 2002;11:115–124. .
44. Reiss AL, Faruque F, Naudy S, et al. Neuroanatomy of Rett syndrome. A volumetric imaging study. Ann Neurol 1993;34:227–243. .
45. Subramaniam B, Naidu S, Reiss AL. Neuroanatomy in Rett syndrome. Cerebral cortex and posterior fossa. Neurology 1997;48:399–407. .
46. Oldfors A, Sourander P, Armstrong DL, et al. Rett syndrome. Cerebellar pathology. Pediatr Neurol 1990;6:310–314. .
47. Oldfors A, Hagberg BA, Nordgren H, et al. Rett syndrome. Spinal cord pathology. Pediatr Neurol 1988;4:172–174. .
48. Armstrong D, Dunn K, Antalffy B, Trivedi R. Selective dendritic alterations in the cortex of Rett syndrome J Neuropathol Exp Neurol 1995;54:195–201.
49. Armstrong DD, Dunn K, Antalffy B. Decreased dendritic branching in frontal, motor, limbic cortex in Rett syndrome compared with trisomy 21 J Neuropathol Exp Neurol 1998;57: 1013–1017.
50. Bauman MI, Kemper MD, Arin DM. Pervasive neuroanatomic abnormalities of the brain in three cases of Rett's syndrome 1995;45:1581–1586.
51. Casanova MF, Buxhoeveden D, Switala A, Roy E. Rett syndrome as a minicolumnopathy Clin Neuropathol 2003;22:163–168.
52. Belichenko PA, Hagberg BA, Dahlstrom A. Morphologic study of neocortical areas in Rett syndrome Acta Neuropathol (Berl) 1997;93:50–61.
53. Belichenko PA. , 51357 Oxford: Oxford University Press; UK. .
54. Belichenko PV, Oldfors A, Hagberg B, Dahlstrom AS. Rett syndrome 3-D confocal microscopy of cortical pyramidal dendrites and afferents 1994;05:1509–1513.
55. Belichenko PV, Dahlstrom A. Studies on the 3-dimensional architecture of dendritic spines and varicosities in human cortex by confocal laser scanning microscopy and Lucifer yellow microinjections J Neurosci Methods 1995;57:55–61.
56. Belichenko PA, Leontovich T, Mukjine J, et al. Morphologic studies of neocortical areas and basal ganglia in Rett syndrome, abstract, in Hand in Hand with Rett Syndrome J Neurosci Methods 1995;57:55–61.
57. Murakiami KW, Courchesne E, Haas RH, et al. Cerebellar and cerebral abnormalities in Rett syndrome AJR Am J Roentgenol 1992;159:177–183.
58. Julu POO. , ; The central autonomic disturbance in Rett syndrome, in Kerr A, Engerström IW (eds): Rett Disorder and the Developing Brain In: ord; UK; Oxford University, Press; 2001; 13, editors Oxford, UK, Oxford University Press, 2001, 131–183: Oxford; UK. p. 177-183..
59. Armstrong DD, Kinney HC. , Oxford, UK, Oxford University Press, 2001, 57–84: Oxford; UK. The neuropathology of the Rett disorder, in Kerr A, Engerström IW (eds): Rett Disorder and the Developing Brain; p. 177-183..
60. Hass RR, Love S. Peripheral nerve findings in Rett syndrome Oxford, UK, Oxford University Press, 2001, 5 7–84; Oxford: Oxford– Oxford.
61. Jellinger K, Griswold W, Armstrong D, Rett A. Peripheral nerve involvement in the Rett syndrome Brain Dev 1990;12:109–114.
62. Eeg-Olofsson O, Al-Zuhail AGH, Teebi AS, Al-Essa MNM. Abnormal mitochondria in the Rett syndrome Brain Dev 1988;10:260–262.
63. Ruch A, Kurczy TW, Velasco ME. Mitochondrial alterations in Rett syndrome Pediatr Neurol 1989;05:320–323.
64. Coker SB, Melynk AR. Rett syndrome and mitochondrial enzyme deficiencies J Child Neurol 1991;06:164–166.
65. Armstrong DD, Deguchi K, Antalffy B. Survey of MECP2 in the Rett syndrome and the non-Rett syndrome brain J Child Neurol 2003;18:683–687.
66. LaSalle JM, Goldstine J, Balmer D, Greco CH. Quantitative localization of heterogeneous methyl-CpG-binding protein-2 (MECP2) expression phenotypes in normal and Rett syndrome brain by laser and scanning cytometry Hum Mol Genet 2001;10:1729–1740.
67. Braunschweig D, Simcox T, Samaco RC, LaSalle JM. X-chromosome inactivation rations affect wild-type MECP2 expression within Rett syndrome and MECP2-/+ mouse brain Hum Mol Genet 2004;13:1275–1286.
68. Cohen, DRS, Matarazzo V, Palmer AM, et al. Expression of MECP2 in olfactory receptor neurons is developmentally regulated and occurs before synaptogenesis Mol Cell Neurosci 2003;22:417–429.
69. Aber KM, Nori P, MacDonald SM, et al. Methyl-CpG-binding protein 2 is localized in the postsynaptic compartment. An immunochemical study of subcellular fractions. Neuroscience 2003;116:77–80. .
70. Matarazzo V, Cohen D, Palmer AM, et al. The transcriptional repressor MECP2 regulates terminal neuronal differentiation Mol Cell Neurosci 2004;27:44–58.
71. Ronnett GV, Leopold D, Cai X, et al. Olfactory biopsies demonstrate a defect in neuronal development in Rett's syndrome Ann Neurol 2003;54:206–218.
72. Kishi N, Macklis JD. MECP2 is progressively expressed in post-migratory neurons and is involved in neuronal maturation rather than cell fate decision Mol Cell Neurosci 2004;27:306–21.
73. Akbarian S, Chen RZ, Gribnau J, et al. Expression pattern of the Rett syndrome gene MECP2 in primate prefrontal cortex Neurobiol Dis 2001;08:784–791.
74. Sundstrom E, Kolare S, Souverbie F, et al. Neurochemical differentiation of human bulbospinal monoaminergic neurons during the first trimester. Brain Res Dev Brain Res 1993;7:1–12 .
75. Riikonen K. Neurotrophic factors in the pathogenesis in Rett syndrome J Child Neurol 2003;18:693–697.
76. Blue ME, Naidu S, Johnston MV. Development of amino acid receptors in frontal cortex from girls with Rett syndrome Ann Neurol 1999;45:541–545.
77. Kaufmann WE, Worley PF, Taylor CV, et al. Cyclooxygenase-2 expression during rat neocortical development and in Rett syndrome Brain Dev 1997;19:25–34.
78. Kaufmann WE, Naidu S, Budden S. Abnormalexpression of microtubule-associated protein 2 (MAP2) in the neocortex in Rett syndrome Neuropediatrics 1995;26:109–113.
79. Chen WG, Chang Q, Lin YL, et al. Derepression of BDNF transcription involves calcium-dependent phosphorylation of MECP2 Science 2003;31:885–889.
80. Matarazzo V, Ronnett GV. Temporal and regional differences in the olfactory proteome as a consequence of MECP2 deficiency Proc Natl Acad Sci U S A 2004;101:7763–7768.

Presented as part of the Rett Syndrome, Neurobiology of Disease in Children, National Institutes of Health Conference in Ottawa, ON, Canada, October 13–16, 2004, in conjunction with the 33rd annual meeting of the Child Neurology Society.Presented as part of the Rett Syndrome, Neurobiology of Disease in Children, National Institutes of Health Conference in Ottawa, ON, Canada, October 13–16, 2004, in conjunction with the 33rd annual meeting of the Child Neurology Society.