Rett syndrome

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Rett syndrome/disorder
ICD-10 F84.2
ICD-9 330.8
OMIM 312750
DiseasesDB 29908
MeSH C10.574.500.775

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]

Overview

Rett syndrome (also called Rett disorder) is a neurodevelopmental disorder that is classified as a pervasive developmental disorder by the DSM-IV. It was first described by Austrian neurologist 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. Stereotypic, repetitive hand movements such as mouthing or wringing are also noted. Symptoms of the disorder include cognitive impairment and problems with socialization, the latter during the regression period. Socialization typically improves by the time they enter school. 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. Many argue that it is misclassified as a pervasive developmental 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 symptoms of this disorder are most easily confused with those of Angelman syndrome, cerebral palsy and autism.

Cause

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.

Sporadic mutations

Rett syndrome is usually caused (95% or more) by a de novo mutation in the child (so it is inherited from a genotypically normal mother, i.e. one 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]

Germline mutations

It 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.

Gender and Rett syndrome

It 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.

Most individuals with Rett syndrome are female. 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's syndrome almost all have Klinefelter's syndrome as well (in which the male has an XXY karyotype).[5] 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.[6] 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 symptoms caused by MECP2 mutations and those caused by Rett's.[7][8]

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 (as they only have one X chromosome).

Development and symptoms

The 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 brain stem. 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.

Symptoms of Rett syndrome that are similar to autism:

  • screaming fits
  • panic attack
  • inconsolable crying
  • avoidance of eye contact
  • lack of social/emotional reciprocity
  • general lack of interest
  • markedly impaired use of nonverbal behaviors to regulate social interaction
  • loss of speech
  • Balance and coordination problems, including losing the ability to walk in many cases

Symptoms 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 should rarely be made):

Symptoms 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 requires corrective surgery in about 10%. Those who remain ambulatory tend to have less progression of scoliosis.

Differential diagnosis

Rett syndrome must be differentiated from other diseases that cause neurological manifestations in infants.

Diseases Type of motor abnormality Clinical findings Laboratory findings and diagnostic tests Radiographic findings
Spasticity Hypotonia Ataxia Dystonia
Leigh syndrome - - + +
Niemann-Pick disease type C - - + +
  • Abnormal liver function tests
  • Fibroblast cell culture with filipin staining
Infantile Refsum disease - + + - Elevated plasma VLCFA levels --
Adrenoleukodystrophy + - - -
  • Elevated plasma VLCFA levels
  • Molecular genetic testing for mutations in the ABCD1 gene
--
Zellweger syndrome - + - - --
Pyruvate dehydrogenase deficiency + + + -
  • Elevated lactate and pyruvate levels in blood and CSF
  • Abnormal PDH enzymatic activity in cultured fibroblasts
--
Arginase deficiency + - - - --
Holocarboxylase synthetase deficiency - + - - Elevated levels of:
  • Beta-hydroxyisovalerate
  • Beta-methylcrotonylglycine
  • Beta-hydroxypropionate
  • Methylcitrate
  • Tiglylglycine
--
Glutaric aciduria type 1 - - - + Elevated levels of:
Ataxia telangiectasia - - + - --
Pontocerebellar hypoplasias - + - - Genetic testing for PCH gene mutations
Metachromatic leukodystrophy - + + -
  • Deficient arylsulfatase A enzyme activity in leukocytes or cultured skin fibroblasts
--
Pelizaeus-Merzbacher + - + -
Angelman syndrome - - + -
  • Methylation studies and chromosome microarray to detect chromosome 15 anomalies and UBE3A mutations
--
Rett syndrome + - - +
  • Occurs almost exclusively in females
  • Normal development during first six months followed by regression and loss of milestones
  • Loss of speech capability
  • Stereotypic hand movements
  • Seizures
  • Autistic features
  • Clinical diagnosis
  • Genetic testing for MECP2 mutations
--
Lesch-Nyhan syndrome + - - + --
Miller-Dieker lissencephaly + + - -
  • Cytogenetic testing for 17p13.3 microdeletion
--
Dopa-responsive dystonia + - - +
  • Onset in early childhood
  • Symptoms worsen with fatigue and exercise
  • Positive response to a trial of levodopa
--


Treatment and prognosis

Currently there is no cure for Rett syndrome, although there have been some promising results with gene therapy in mice.[9]

Treatment of Rett syndrome includes:

The challenge of developing therapies for MECP2 disorders[10]

The recent studies 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 symptoms 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.

Mortality

Males 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:

  • spontaneous brainstem dysfunction
  • cardiac arrest
  • seizures
  • cardiac conduction abnormalities - abnormally prolonged QT interval on ECG
  • gastric perforation

Notes

  1. "Predictors of Seizure Onset in Rett Syndrome" Le Jian et al. [1]
  2. "Is Rett Syndrome a Subtype of Pervasive Developmental Disorders" Tsai, L.Y., Journal of Autism and Developmental Disorders
  3. Trappe R, Laccone F, Cobilanschi J; et al. (2001). "MECP2 mutations in sporadic cases of Rett syndrome are almost exclusively of paternal origin". American journal of human genetics. 68 (5): 1093–101. doi:10.1086/320109. PMC 1226090. PMID 11309679. Unknown parameter |month= ignored (help)
  4. "Rett syndrome is caused by mutations in X-linked MECP2, encoding methyl-CpG-binding protein 2" Amir, R. et al.
  5. Schwartzman, J.S., et al. Rett Syndrome in a Boy with 47,XXY Karyotype Confirmed by a Rare Mutation on the MECP2 Gene. 2001. Neuropediatrics 32:162-164
  6. Hardwick, S.A. et.al. Delineation of large deletions of the MECP2 gene in Rett syndrome patients, including a familial case with a male proband. 2007. European Journal of Human Genetics. 15(12):1218-29
  7. "New Study Reveals Rett Syndrome Can Strike Males" ScienceDaily, August 12, 2006
  8. Moog, U., et al. Neurodevelopmental disorders in males related to the gene causing Rett syndrome in females (MECP2). 2003. European Journal of Paediatric Neurology 07:5-12
  9. "Autism-like disorder 'reversible'", BBC News, 8 February 2007.
  10. "The Story of Rett Syndrome: From Clinic to Neurobiology." Chahrour et al. Neuron, Vol 56, 422-437, 08 November 2007

References

Further reading

  • Ben Zeev Ghidoni B (2007). "Rett syndrome". Child Adolesc Psychiatr Clin N Am. 16 (3): 723–43. doi:10.1016/j.chc.2007.03.004. PMID 17562589.
  • Bittner JG; et al. (2008). "Rett Syndrome and gastric perforation". Am Surg. 74 (4): 315–317.

Rett Disorder and the Developing Brain. edited by Alison Kerr & Ingegerd Witt Engerstrom Oxford University Press ISBN 0-19-856815-0, 2005


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