Research Notes: Rett Syndrome

Childhood neurodevelopmental disorder characterized by normal early development followed by loss of purposeful use of the hands, distinctive hand movements, slowed brain and head growth, gait abnormalities, seizures, and mental retardation. Hypotonia is usually the first symptom. As the syndrome progresses, the child loses purposeful use of her hands and the ability to speak. Other early symptoms may include problems crawling or walking and diminished eye contact. The loss of functional use of the hands is followed by compulsive hand movements such as wringing and washing. The onset of this period of regression is sometimes sudden. Another symptom, apraxia - the inability to perform motor functions - is perhaps the most severely disabling feature of Rett syndrome, interfering with every body movement, including eye gaze and speech. Individuals with Rett syndrome often exhibit autistic-like behaviors in the early stages. Other symptoms may include toe walking; sleep problems; wide-based gait; teeth grinding and difficulty chewing; slowed growth; seizures; cognitive disabilities; and breathing difficulties while awake such as hyperventilation, apnea (breath holding), and air swallowing.

Caused by mutations in the MeCP2 gene found on the X chromosome. MECP2 contains instructions for the synthesis of a protein called methyl cytosine binding protein 2 (MeCP2), which acts as one of the many biochemical switches that tell other genes when to turn off and stop producing their own unique proteins. Because the MECP2 gene does not function properly in those with Rett syndrome, insufficient amounts or structurally abnormal forms of the protein are formed. The absence or malfunction of the protein is thought to cause other genes to be abnormally expressed, but this hypothesis has not yet been confirmed. Seventy to 80 percent of girls given a diagnosis of Rett syndrome have the MECP2 genetic mutation detected by current diagnostic techniques. It is believed the remaining 20 to 30 percent of cases may be caused by partial gene deletions, by mutations in other parts of the gene, or by genes that have not yet been identified.

Hum Mol Genet. 2007 Mar 15.
FXYD1 is an MeCP2 target gene overexpressed in the brains of Rett syndrome patients and Mecp2-null mice.
Deng V, Matagne V, Banine F, Frerking M, Ohliger P, Budden S, Pevsner J, Dissen GA, Sherman LS, Ojeda SR.
Division of Neuroscience, Oregon National Primate Research Center, Beaverton, OR, USA.

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder linked to heterozygous de novo mutations in the MeCP2 gene. MECP2 encodes methyl-CpG-binding protein 2 (MeCP2), which represses gene transcription by binding to 5-methylcytosine residues in symmetrically positioned CpG dinucleotides. Direct MeCP2 targets underlying RTT pathogenesis remain largely unknown. Here, we report that FXYD1, which encodes a transmembrane modulator of Na(+), K(+) -ATPase activity, is elevated in frontal cortex (FC) neurons of RTT patients and Mecp2-null mice. Increasing neuronal FXDY1 expression is sufficient to reduce dendritic arborization and spine formation, hallmarks of RTT neuropathology. Mecp2-null mouse cortical neurons have diminished Na(+),K(+)-ATPase activity, suggesting that aberrant FXYD1 expression contributes to abnormal neuronal activity in RTT. MeCP2 represses Fxyd1 transcription through direct interactions with sequences in the Fxyd1 promoter that are methylated in FC neurons. FXYD1 is therefore a MeCP2 target gene whose de-repression may directly contribute to RTT neuronal pathogenesis.

Hum Mol Genet. 2007 Mar 15.
15q11-13 GABAA receptor genes are normally biallelically expressed in brain yet are subject to epigenetic dysregulation in autism-spectrum disorders.
Hogart A, Nagarajan RP, Patzel KA, Yasui DH, Lasalle JM.
Medical Microbiology and Immunology, Rowe Program in Human Genetics, School of Medicine, University of California, Davis, CA, USA.

Human chromosome 15q11-13 is a complex locus containing imprinted genes as well as a cluster of three GABA(A) receptor subunit (GABR) genes - GABRB3, GABRA5 and GABRG3. Deletion or duplication of 15q11-13 GABR genes occurs in multiple human neurodevelopmental disorders including Prader-Willi syndrome (PWS), Angelman syndrome (AS) and autism. GABRB3 protein expression is also reduced in Rett syndrome (RTT), caused by mutations in MECP2 on Xq28. Although GABRB3 is biallelically expressed in mouse brain, conflicting data exist regarding the imprinting status of the 15q11-13 GABR genes in humans. Using coding single nucleotide polymorphisms we show that all three GABR genes are biallelically expressed in 21 control brain samples, demonstrating that these genes are not imprinted in normal human cortex. Interestingly, four of eight autism and one of five RTT brain samples showed monoallelic or highly skewed allelic expression of one or more GABR genes, suggesting that epigenetic dysregulation of these genes is common to both disorders. Quantitative real-time RT-PCR analysis of PWS and AS samples with paternal and maternal 15q11-13 deletions revealed a paternal expression bias of GABRB3, while RTT brain samples showed a significant reduction in GABRB3 and UBE3A. Chromatin immunoprecipitation and bisulfite sequencing in SH-SY5Y neuroblastoma cells demonstrated that MeCP2 binds to methylated CpG sites within GABRB3. Our previous studies demonstrated that homologous 15q11-13 pairing in neurons was dependent on MeCP2 and was disrupted in RTT and autism cortex. Combined, these results suggest that MeCP2 acts as a chromatin organizer for optimal expression of both alleles of GABRB3 in neurons.

Epigenetics. 2006 Oct.
Reduced MeCP2 expression is frequent in autism frontal cortex and correlates with aberrant MECP2 promoter methylation.
Nagarajan RP, Hogart AR, Gwye Y, Martin MR, Lasalle JM.
Medical Microbiology and Immunology, Rowe Program in Human Genetics, School of Medicine, One Shields Ave, University of California, Davis, CA, USA.
[ Free full text ]

Mutations in MECP2, encoding methyl CpG binding protein 2 (MeCP2), cause most cases of Rett syndrome (RTT), an X-linked neurodevelopmental disorder. Both RTT and autism are "pervasive developmental disorders" and share a loss of social, cognitive and language skills and a gain in repetitive stereotyped behavior, following apparently normal perinatal development. Although MECP2 coding mutations are a rare cause of autism, MeCP2 expression defects were previously found in autism brain. To further study the role of MeCP2 in autism spectrum disorders (ASDs), we determined the frequency of MeCP2 expression defects in brain samples from autism and other ASDs. We also tested the hypotheses that MECP2 promoter mutations or aberrant promoter methylation correlate with reduced expression in cases of idiopathic autism. MeCP2 immunofluorescence in autism and other neurodevelopmental disorders was quantified by laser scanning cytometry and compared with control postmortem cerebral cortex samples on a large tissue microarray. A significant reduction in MeCP2 expression compared to age-matched controls was found in 11/14 autism (79%), 9/9 RTT (100%), 4/4 Angelman syndrome (100%), 3/4 Prader-Willi syndrome (75%), 3/5 Down syndrome (60%), and 2/2 attention deficit hyperactivity disorder (100%) frontal cortex samples. One autism female was heterozygous for a rare MECP2 promoter variant that correlated with reduced MeCP2 expression. A more frequent occurrence was significantly increased MECP2 promoter methylation in autism male frontal cortex compared to controls. Furthermore, percent promoter methylation of MECP2 significantly correlated with reduced MeCP2 protein expression. These results suggest that both genetic and epigenetic defects lead to reduced MeCP2 expression and may be important in the complex etiology of autism.

Hum Mol Genet. 2004 Mar 15.
Multiple pathways regulate MeCP2 expression in normal brain development and exhibit defects in autism-spectrum disorders.
Samaco RC, Nagarajan RP, Braunschweig D, LaSalle JM.
Medical Microbiology and Immunology, Rowe Program in Human Genetics, School of Medicine, University of California, Davis, USA.
[ Free full text ]

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in MECP2, encoding methyl-CpG-binding protein 2 (MeCP2). Although MECP2 is ubiquitously transcribed, MeCP2 expression is developmentally regulated and heterogeneous in neuronal subpopulations, defined as MeCP2(lo) and MeCP2(hi). To test the hypothesis that pathways affecting MeCP2 expression changes may be defective in RTT, autism and other neurodevelopmental disorders without MECP2 mutations, a high-throughput quantitation of MeCP2 expression was performed on a tissue microarray containing frontal cortex samples from 28 different patients with neurodevelopmental disorders and age-matched controls. Combined quantitative analyses of MeCP2 protein and alternatively polyadenylated transcript levels were performed by laser scanning cytometry and tested for significant differences from age-matched controls. Normal cerebral samples showed an increase in total MeCP2 expression and the percentage of MeCP2(hi) cells with age that could be explained by increased MECP2 transcription within the MeCP2(hi) population. A significant decrease in the relative usage of the long transcript in the MeCP2(lo) population was observed in postnatal compared to fetal brain, but alternate polyadenylation did not correlate with MeCP2 expression changes at the single cell level. Brain samples from several related neurodevelopmental disorders, including autism, pervasive developmental disorder, Prader-Willi and Angelman syndromes showed significant differences in MeCP2 expression from age-matched controls by apparently different transcriptional and post-transcriptional mechanisms. These results suggest that multiple pathways regulate the complex developmental expression of MeCP2 and are defective in autism-spectrum disorders in addition to RTT.

My abstract :-)

Rett syndrome is a neurodevelopmental disorder that presents with cognitive impairment and autistic behavior and is caused by mutations in the MECP2 gene (on chromosome X) which carries the instructions for making methyl-CpG-binding protein 2 (MeCP2). The MeCP2 protein, which is found throughout both fetal and adult brains, has two different forms - a short form which contains a 1.9 kb transcript from the MECP2 grene and a long form which contains a 10 kb-long transcript that includes the 1.9 kb coding region of MECP2 as well as an exceptionally long untranslated region. Too make things more complicated, some brain neurons have relatively high levels of the MeCP2 protein and are called MeCP2(hi) neurons, while other neurons have relatively low levels of MeCP2 and are called MeCP2(lo) neurons.

MeCP2 expression is developmentally regulated, that is, there are changes in the total amount of MeCP2 protein in the brain according to age, as well as changes in the amounts of short and long transcript forms of MeCP2 and changes in the number of MeCP2(lo) and MeCP2(hi) neurons. During normal brain development, the total amount of MeCP2 increases and there is a progressive increase in the number of MeCP2(hi) neurons from infancy to adulthood. At the same time, the amount of long-transcript MeCP2 falls relative to the amount of short-transcript MeCP2, even though juvenile and adult MeCP2(hi) neurons have more long-transcript MeCP2 than juvenile and adult MeCP2(lo) neurons.

In PWS, there are significantly higher MeCP2 levels in the MeCP2(hi) neurons compared to control subjects, even though total MECP2 levels are significantly lower. In addition, there are higher levels of both short and long-transcript MECP2 proteins in PWS. In other words, PWS brains do not shift to more MeCP2(hi) neurons during growth, nor does the normal fall in the amount of long-transcript MeCP2 occur. That means that the loss of expression of the genetic information on the paternal chromosome 15 gene that occurs in PWS somehow affects the expression of the MECP2 gene on chromosome X.

J Child Neurol. 1999 Mar.
Rett syndrome: randomized controlled trial of L-carnitine.
Ellaway C, Williams K, Leonard H, Higgins G, Wilcken B, Christodoulou J.
Western Sydney Genetics Program, the Royal Alexandra Hospital for Children, Westmead, Australia.

Rett syndrome is a severe neurodevelopmental disorder of unknown etiology, occurring almost exclusively in female patients. The etiology and functional significance of plasma carnitine deficiency seen in some patients with Rett syndrome is unknown. To investigate whether L-carnitine might be of benefit in Rett syndrome, a randomized, placebo-controlled, double-blind crossover trial of L-carnitine has been completed in 35 subjects. Eight-week treatment phases were completed for both a placebo and L-carnitine. Outcome was measured by parents/caregivers and at medical follow-up using three established tools: the Rett Syndrome Motor Behavioral Assessment, the Hand Apraxia Scale, and the Patient Well-Being Index. Analysis comparing change between baseline and week 8 of treatment for L-carnitine and the placebo showed that both parents/caregivers and medical follow-up detected improvements in the subjects' well-being. In addition, medical review showed an improvement on the Hand Apraxia Scale for a higher proportion of girls on L-carnitine. Identification of predictors of clinical improvement has been limited by the power of the study. These findings suggest that L-carnitine is of benefit in some patients with Rett syndrome. While L-carnitine did not lead to major functional changes in ability, the type of changes reported could still have a substantial impact on the girls and their families. Information is still needed, however, to determine if only subgroups of girls with the disorder are responsive to L-carnitine and the appropriate duration of therapy.

Klin Padiatr. 1996 May-Jun.
Carnitine deficiency and carnitine therapy in a patient with Rett syndrome. [Article in German]
Plochl E, Sperl W, Wermuth B, Colombo JP.
Klinische Genetik am Kinderspital der Landeskrankenstalten Salzburg.

Background: Rett syndrome can be diagnosed only clinically. Several biochemical abnormalities are known, but none of them is characteristic. To our knowledge only one study on carnitine deficiency and one case of successful carnitine therapy have been reported. Patient: A five years old girl with normal milestones in the first months of life became retarded in the second year with muscle hypotonia of unknown cause and loss of known abilities. Later on recurrent washing movements of the hands, hyperventilation and microcephaly were observed and the diagnosis of Rett syndrome was established. Method: A muscle biopsy was performed for the determination of enzymes of the respiratory chain and polarographic respirometry in permeabilized muscle fibres at the age of 3 1/2 years. Carnitine in plasma and urine was determined before and during a therapy with carnitine. Results: The activities of some enzymes of the respiratory chain were slightly decreased as was oxygen consumption in the permeabilized muscle fibres. However muscle morphology and histochemistry were normal. With normal carnitine in the muscle, plasma carnitine was clearly decreased but showed a normal ratio of acylcarnitine to free carnitine. Carnitine substitution [supplementation?] was started at the age of 3 1/2 years with 75 mg/kg/day and was later increased to 150 mg/kg/day. The treatment showed not only a normalisation of plasma carnitine but also an improvement of physical activity, muscle hypotonia, communication and sleep time. A wash out for one month and resumption of therapy confirmed the efficacy of this regime. Conclusions: The reason for the carnitine deficiency in the patient with Rett syndrome is not known. A primary carnitine deficiency is excluded by normal muscle carnitine. An explanation for the efficacy of the carnitine therapy is not known, although one could speculate that carnitine provides a transport system for acetyl groups, stimulates acetylcholine formation in the brain and in this way improves the disturbance of the cholinergic system.

South Med J. 1993 Dec.
L-carnitine as a treatment for Rett syndrome.
Plioplys AV, Kasnicka I.
Marklund Children's Home, Mercy Hospital and Medical Center, Chicago, IL.

A 17-year-old girl with Rett syndrome, who was taking no other medications, was treated with L-carnitine (50 mg/kg/day). Within 2 months of initiation of treatment, she became much more alert, developed good eye contact, started reaching for objects with both hands, and answered simple questions with one or two words. L-carnitine was discontinued and within 1 week she lapsed into her pretreatment condition of lethargy with no interest in her environment, not reaching for objects, poor eye contact, and not speaking. One week after L-carnitine was resumed, she again became alert, started reaching for objects, and saying one or two words. Her serum carnitine levels (free and total) were within normal limits before and after L-carnitine treatment, but were higher while she was taking L-carnitine. Her serum ammonia was within normal limits prior to starting L-carnitine. L-carnitine appears to be an effective treatment for this girl with advanced Rett syndrome.