rett syndrome rett syndrome rett syndrome (rtt) is a genetic disorder that typically becomes apparent after 6–18 months of age and almost exclusively in females. symptoms include impairments in language and coordination, and repetitive movements. those affected often have slower growth, difficulty walking, and a smaller head size. complications of rett syndrome can include seizures, scoliosis, and sleeping problems. the severity of the condition is variable. not to be confused with tourette syndrome. rett syndromeother namescerebroatrophic hyperammonemia (obsolete), dementia, ataxia, and loss of purposeful hand use syndromea girl with rett syndrome smiling at the camera.specialtypsychiatry, pediatrics, neurologysymptomsimpairments in language and coordination, and repetitive movements, slower growth, smaller headcomplicationsseizures, scoliosis, sleeping problemsusual onsetafter 6–18 months of agedurationlifelongcausesmutation in the mecp2 genediagnostic methodbased on symptoms, genetic testingdifferential diagnosisangelman syndrome, autism, cerebral palsy, childhood disintegrative disorder, various neurodegenerative disorderstreatmentspecial education, physiotherapy, bracesmedicationanticonvulsantsprognosislife expectancy for many is middle age.frequency1 in 8,500 femaleslethal in males, with extremely rare exceptions. rett syndrome is due to a genetic mutation in the mecp2 gene, on the x chromosome. it almost always occurs as a new mutation, with less than one percent of cases being inherited. it occurs almost exclusively in girls; boys who have a similar mutation typically die shortly after birth. diagnosis is based on the symptoms and can be confirmed with genetic testing. there is no known cure for rett syndrome. treatment is directed at improving symptoms. anticonvulsants may be used to help with seizures. special education, physiotherapy, and leg braces may also be useful depending on the needs of the child. many of those with the condition live into middle age. the condition affects about 1 in 8,500 females. in 1999, lebanese-american physician huda zoghbi discovered the mutation that causes the condition. signs and symptoms stage i stage i, called early-onset, typically begins between 6 and 18 months of age. this stage is often overlooked because symptoms of the disorder may be somewhat vague, and parents and doctors may not notice the subtle slowing of development at first. the infant may begin to show less eye contact and have reduced interest in toys. there may be delays in gross motor skills such as sitting or crawling. hand-wringing and decreasing head growth may occur, but not enough to draw attention. this stage usually lasts for a few months but can continue for more than a year. stage ii stage ii, or the rapid destructive stage, usually begins between ages 1 and 4 and may last for weeks or months. its onset may be rapid or gradual as the child loses purposeful hand skills and spoken language. characteristic hand movements such as wringing, washing, clapping, or tapping, as well as repeatedly moving the hands to the mouth often begin during this stage which is called mouthing. the child may hold the hands clasped behind the back or held at the sides, with random touching, grasping, and releasing. the movements continue while the child is awake but disappear during sleep. breathing irregularities such as episodes of apnea and hyperventilation may occur, although breathing usually improves during sleep. some girls also display autistic-like symptoms such as loss of social interaction and communication. walking may be unsteady and initiating motor movements can be difficult. slowed head growth is usually noticed during this stage. stage iii stage iii, or the plateau or pseudo-stationary stage, usually begins between ages 2 and 10 and can last for years. apraxia, motor problems, and seizures are prominent during this stage. however, there may be improvement in behavior, with less irritability, crying, and autistic-like features. in stage iii there may be more interest in the surroundings and alertness, attention span, and communication skills may improve. many girls remain in this stage for most of their lives. stage iv stage iv, or the late motor deterioration stage, can last for years or decades. prominent features include reduced mobility, curvature of the spine, and muscle weakness, rigidity, spasticity, and increased muscle tone with abnormal posturing of an arm or leg. girls who were previously able to walk may stop walking. cognition, communication, or hand skills generally do not decline in stage iv. repetitive hand movements may decrease and eye gaze usually improves. variants the signs and symptoms of the typical form of the rett syndrome are well described. in addition to the classical form of rett syndrome, several atypical forms have been described over the years; the main groups are: congenital variant (rolando variant): in this severe subtype of rett syndrome, the development of the patients and their head circumference are abnormal from birth. the typical gaze of rett syndrome patients is usually absent; zappella variant of rett syndrome or preserved speech variant: in this subtype of rett syndrome the patients acquire some manual skills and language is partially recovered around the age of 5 years (that is after the regression phase). height, weight and head circumference are often in the normal range, and a good gross motor function can be observed. the zappella variant is a milder form of rett syndrome; hanefeld variant or early epilepsy variant. in this form of rett syndrome, the patients have epilepsy before 5 months of age. the definition itself of the rett syndrome has been refined over the years: as the atypical forms subsist near to the classical form (hagberg & gillberg, 1993), the "rett complex" terminology has been introduced. cause genetically, rett syndrome (rtt) is caused by mutations in the gene mecp2 located on the x chromosome (which is involved in transcriptional silencing and epigenetic regulation of methylated dna), and can arise sporadically or from germline mutations. in less than 10% of rtt cases, mutations in the genes cdkl5 or foxg1 have also been found to resemble it. rett syndrome is initially diagnosed by clinical observation, but the diagnosis is definitive when there is a genetic defect in the mecp2 gene. it has been argued that rett syndrome is in fact a neurodevelopmental condition as opposed to a neurodegenerative condition. one piece of evidence for this is that mice with induced rett syndrome show no neuronal death, and some studies have suggested that their phenotypes can be partially rescued by adding functional mecp2 gene back when they are adults. this information has also helped lead to further studies aiming to treat the disorder. sporadic mutations in at least 95% of rett syndrome cases, the cause is a de novo mutation in the child. that is, it is not inherited from either parent. parents are generally genotypically normal, without a mecp2 mutation. in cases of the sporadic form of rtt, the mutated mecp2 is thought to derive almost exclusively from a de novo mutation on the male copy of the x chromosome. it is not yet known what causes the sperm to mutate, and such mutations are rare. 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. in these cases, inheritance follows an x-linked dominant pattern and is seen almost exclusively in females, as most males die in utero or shortly after birth. mecp2 is found near the end of the long arm of the x chromosome at xq28. an atypical form of rtt, 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). as stated by aine merwick, margaret o'brien, and norman delanty in an article on gene disorders titled complex single gene disorders and epilepsy, "rett syndrome affects one in every 12,500 female live births by age 12 years." mechanism the location of the gene responsible for rett syndrome pontine noradrenergic deficits brain levels of norepinephrine are lower in people with rett syndrome (reviewed in). the genetic loss of mecp2 changes the properties of cells in the locus coeruleus, the exclusive source of noradrenergic innervation to the cerebral cortex and hippocampus. these changes include hyperexcitability and decreased functioning of its noradrenergic innervation. moreover, a reduction of the tyrosine hydroxylase (th) mrna level, the rate-limiting enzyme in catecholamine synthesis, was detected in the whole pons of mecp2-null male as well as in adult heterozygous (mecp2+/-) female mice. using immunoquantitative techniques, a decrease of th protein staining level, number of locus coeruleus th-expressing neurons and density of dendritic arborization surrounding the structure was shown in symptomatic mecp2-deficient mice. however, locus coeruleus cells are not dying, but are more likely losing their fully mature phenotype, since no apoptotic neurons in the pons were detected. 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 coeruleus is a critical site at which loss of mecp2 results in cns dysfunction." the restoration of normal locus coeruleus function may therefore be of potential therapeutic value in the treatment of rett syndrome. midbrain dopaminergic disturbances the majority of dopamine in the mammalian brain is synthesized by nuclei located in the mesencephalon. the substantia nigra pars compacta (snpc), the ventral tegmental area (vta) and the retrorubral field (rrf) contains dopaminergic neurons expressing tyrosine hydroxylase (th, i.e. the rate-limiting enzyme in catecholamine synthesis). the nigro-striatal pathway originates from snpc and irradiate its principal rostral target, the caudate-putamen (cpu) through the median forebrain bundle (mfb). this connection is involved in the tight modulation of motor strategies computed by a cortico-basal ganglia- thalamo-cortical loop. indeed, based on the canonical anatomofunctional model of basal ganglia, nigrostriatal dopamine is able to modulate the motor loop by acting on dopaminergic receptors located on striatal gabaergic medium spiny neurons. dysregulation of the nigrostriatal pathway is causative from parkinson disease (pd) in humans. toxic and/or genetic ablation of snpc neurons produces experimental parkinsonism in mice and primates. the common features of pd and pd animal models are motor impairments (hypotonia, bradykinesia, hypokinesia). rtt pathology, in some aspects, overlaps the motor phenotype observed in pd patients. several neuropathological studies on postmortem brain samples argued for an snpc alteration evidenced by neuromelanin hypopigmentation, reduction in the structure area, and even controversial, signs of apoptosis. in parallel, an hypometabolism was underlined by a reduction of several catecholamines (dopamine, noradrenaline, adrenaline) and their principal metabolic by-products. mouse models of rtt are available and the most studied are constitutively deleted mecp2 mice developed by adrian bird or katelyn mccormick laboratories. in accordance with the motor spectrum of the rtt phenotype, mecp2-null mice show motor abnormalities from postnatal day 30 that worsen until death. these models offer a crucial substrate to elucidate the molecular and neuroanatomical correlates of an mecp2-deficiency. recently (2008), it was shown that the conditional deletion of mecp2 in catecholaminergic neurons (by crossing of th-cre mice with loxp-flanked mecp2 ones) recapitulates a motor symptomatology, it was further documented that brain levels of th in mice lacking mecp2 in catecholaminergic neurons only are reduced, participating to the motor phenotype. however, the most studied model for the evaluation of therapeutics is the mecp2-null mouse (totally devoid of mecp2). in this context, a reduction in the number and soma size of th-expressing neurons is present from 5 weeks of age and is accompanied by a decrease of th immunoreactivity in the caudate-putamen, the principal target of dopaminergic neurons arising from the snpc. moreover, a neurochemical analysis of dopaminergic contents in microdissected midbrain and striatal areas revealed a reduction of dopamine at five and nine weeks of age. it is noteworthy that later on (at nine weeks), the morphological parameters remain altered but not worsen, whereas the phenotype progresses and behavioral deficits are more severe. the amount of fully activated th (serine40-phosphorylated isoform) in neurons that remain in the snpc is mildly affected at 5 weeks but severely impaired by 9 weeks. finally, using a chronic and oral l-dopa treatment on mecp2-deficient mice authors reported an amelioration of some of the motor deficits previously identified. altogether, these results argue for an alteration of the nigrostriatal dopaminergic pathway in mecp2-deficient animals as a contributor of the neuromotor deficits. there is an association of the disease with brain-derived neurotrophic factor (bdnf). molecular functions of mecp2 in rett syndrome pathology as reviewed by sharifi and yasui, mecp2 protein, encoded by the mecp2 gene binds to dna with a high affinity for cpg methylated dna sites