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Rett syndrome is a neurodevelopmental disorder that is classified as an autism spectrum disorder by the DSM-IV. It was first described by Austrian pediatrician Andreas Rett in 1966. The clinical features include a deceleration of the rate of head growth (including microcephaly in some) and small hands and feet. Repetitive hand movements such as mouthing or wringing are also noted. Girls with Rett syndrome are prone to gastrointestinal disorders and up to 80% have seizures.[1] They typically have no verbal skills, and about 50% of females are not ambulatory. Scoliosis, growth failure, and constipation are very common and can be problematic. Some argue[citation needed] that it is misclassified as an autism spectrum disorder, just as it would be to include such disorders as fragile X syndrome, tuberous sclerosis, or Down syndrome where one can see autistic features.[2] The signs of this disorder are most easily confused with those of Angelman syndrome, cerebral palsy and autism.
[edit] CauseGenetically Rett syndrome (symbolized RTT) is caused by mutations in the gene MECP2 located on the X chromosome and can arise (1) sporadically or (2) from germline mutations. [edit] Sporadic mutationsRett syndrome is usually caused (95% or more) by a de novo mutation in the child, and not inherited from either parent. Parents are generally genotypically normal, without a MECP2 mutation. In sporadic cases of Rett syndrome, it is thought that the mutated MECP2 is usually derived from the male copy of the X chromosome. [3] It is not yet known what causes the sperm to mutate, and such mutations are rare. [edit] Germline mutationsIt can also be inherited from phenotypically normal mothers who have a germline mutation in the gene encoding methyl-CpG-binding protein-2, MECP2.[4] MECP2 is found near the end of the long arm of the X chromosome at Xq28. An atypical form of Rett syndrome, characterized by infantile spasms or early onset epilepsy, can also be caused by a mutation to the gene encoding cyclin-dependent kinase-like 5 (CDKL5). Rett syndrome affects one in every 12,500 female live births by age 12 years. [edit] Locus ceruleus and MECP2Brain levels of norepinephrine are lower in people with Rett syndrome.[5] The genetic loss of MECP2 changes the properties of cells in the locus ceruleus the exclusive source of noradrenergic innervation to the cerebral cortex and hippocampus. These changes include hyperexcitability and decreased functioning of its noradrenergic innervation.[6] Researchers have concluded that "Because these neurons are a pivotal source of norepinephrine throughout the brainstem and forebrain and are involved in the regulation of diverse functions disrupted in Rett syndrome, such as respiration and cognition, we hypothesize that the locus ceruleus is a critical site at which loss of MECP2 results in CNS dysfunction. Restoration of normal locus ceruleus function may therefore be of potential therapeutic value in the treatment of Rett syndrome."[6] [edit] Gender and Rett syndromeIt almost exclusively affects girls—male fetuses with the disorder rarely survive to term. Development is typically normal until 6–18 months, when language and motor milestones regress, purposeful hand use is lost and acquired deceleration in the rate of head growth (resulting in microcephaly in some) is seen. Hand stereotypies are typical and breathing irregularities such as hyperventilation, breathholding, or sighing are seen in many. Early on, autistic-like behavior may be seen. Because the disease-causing gene is located on the X chromosome, a female born with a MECP2 mutation on her X chromosome has another X chromosome with an ostensibly normal copy of the same gene, while a male with the mutation on his X chromosome has no other X chromosome, only a Y chromosome; thus, he has no normal gene. Without a normal gene to provide normal proteins in addition to the abnormal proteins caused by a MECP2 mutation, the XY karyotype male fetus is unable to check the development of the disease, hence the failure of many male fetuses with a MECP2 mutation to survive to term. Females with a MECP2 mutation, however, have a non-mutant chromosome that provides them enough normal protein to survive at least to birth. Research shows that males with Rett syndrome almost all have Klinefelter's syndrome as well (in which the male has an XXY karyotype).[7] Thus, a non-mutant MECP2 gene is necessary for a Rett's-affected embryo to survive in most cases, and the embryo, male or female, must have another X chromosome. There have, however, been several cases of 46,XY Karyotype males with a MECP2 mutation (associated with classical Rett syndrome in females) carried to term, who were affected by neonatal encephalopathy and died before 2 years of age.[8] The incidence of Rett syndrome in males is unknown, partly due to low survival of male fetuses with the Rett syndrome associated MECP2 mutations, and partly to differences between signs caused by MECP2 mutations and those caused by Rett's.[9][10] The severity of Rett syndrome in females can vary depending on the type and position of the mutation of MECP2 and the pattern of X-chromosome inactivation. It is generally assumed that 50% of a female's cells use the maternal X chromosome while the other 50% uses the paternal X chromosome (see X-inactivation). However, if most cells in the brain activate the X chromosome with the functional MECP2 allele, the individual will have very mild Rett syndrome; likewise, if most neurons activate the X chromosome with the mutated MECP2 allele, the individual will have very severe Rett syndrome just as males with MECP2 mutations do[citation needed] (as they only have one X chromosome). [edit] Development and signsThe infant with Rett syndrome often avoids detection until 6–18 months due to a relatively normal appearance and some developmental progress. However closer scrutiny reveals disturbance of the normal spontaneous limb and body movements that are thought to be regulated in the brainstem. The brief period of developmental progress is followed by stagnation and regression of previously acquired skills. During regression some features are similar to those of autism. It is, hence, easy to mistakenly diagnose Rett syndrome for autism. Signs of Rett syndrome that are similar to autism:
Signs of Rett syndrome that are also present in cerebral palsy (regression of the type seen in Rett syndrome would be unusual in cerebral palsy; this confusion could rarely be made):
Signs may stabilize for many decades, particularly for interaction and cognitive function such as making choices. Anti-social behavior may change to highly social behavior. Motor functions may slow as rigidity and dystonia appear. Seizures may be problematic, with a wide range of severity. Scoliosis occurs in most and may require corrective surgery. Those who remain ambulatory tend to have less progression of scoliosis. [edit] Treatment and prognosis
Currently there is no cure for Rett syndrome, but studies have shown that restoring MECP2 function may lead to a cure.[11] One area of research is in the use of Insulin-like Growth Factor 1 (IGF-1), which has been shown to partially reverse signs in MeCP2 mutant mice.[12] Such a treatment works because the neuronal cells have not atrophied, but rather are in an immature state. Treatment of Rett syndrome includes:
There is an association of the disease with brain-derived neurotrophic factor (BDNF).[13] [edit] The challenge of developing therapies for MECP2 disorders[14]The recent studies (funded by the International Rett Syndrome Foundation) demonstrating that neurological deficits resulting from loss of MeCP2 can be reversed upon restoration of gene function are quite exciting because they show that neurons that have suffered the consequences of loss of MeCP2 function are poised to regain functionality once MeCP2 is provided gradually and in the correct spatial distribution. This provides hope for restoring neuronal function in patients with RTT. However, the strategy in humans will require providing the critical factors that function downstream of MeCP2 because of the challenges in delivering the correct MeCP2 dosage only to neurons that lack it, given that the slightest perturbation in MeCP2 level is deleterious. Thus, therapeutic strategies necessitate the identification of the molecular mechanisms underlying individual RTT phenotypes and picking out the candidates that can be therapeutically targeted. The next phase of research needs to assess how complete the recovery is. Clearly, lethality, level of activity, and hippocampal plasticity are rescued, but are the animals free of any other RTT signs such as social behavior deficits, anxiety, and cognitive impairments? Since postnatal rescue results in viability, it will be important to evaluate if even the subtler phenotypes of RTT and MECP2 disorders are rescued when protein function is restored postnatally. This is particularly important given emerging data about early neonatal experiences and their long-term effects on behavior in adults. [edit] MortalityMales with pathogenic MECP2 mutations usually die within the first 2 years from severe encephalopathy, unless they have an extra X chromosome (often described as Klinefelter syndrome), or have somatic mosaicism. Females can live up to 40 years or more. Laboratory studies on Rett syndrome may show abnormalities such as:
A high proportion of deaths are abrupt, but most have no identifiable cause; in some instances death is the result most likely of:
[edit] Depiction in popular culture
[edit] Notes
[edit] References
[edit] Further reading
Rett Disorder and the Developing Brain. edited by Alison Kerr & Ingegerd Witt Engerstrom Oxford University Press ISBN 0-19-856815-0, 2005 [edit] External links
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