Caso Karen Atala Riffo Vs Chile

2.3.6.1General information

In female mammalians, only one copy of X chromosomes is active and the other is transcriptionally silenced or inactivated. Such a phenomenon is known as X chromosome random inactivation (XCI), which leads to the mosaic expression of the X-linked genes in the cells of female animals. Thematically, the X chromosomes from maternal or paternal origin

share the same probability to be inactivated, which renders half of the cells to express maternal or paternal X chromosome randomly in the early embryogenesis. However, such XCI is not always balanced, so that the nonrandom or skewed XCI has been reported, especially in RTT patients [101, 102]. Although the classic law of inheritance cannot explain XCI and the genetic mechanism remains unclear so far, a mathematical model of genetically influenced choice was proposed to fit the XCI pattern distributions [103].

2.3.6.2Nonrandom X chromosome inactivation and the impact on phenotypes

RTT is mostly caused by the mutations of the X-linked Mecp2 gene in heterozygous females. Instead of a uniform expression, central neurons show mosaic patterns of MeCP2 expression in the Mecp2+/− mice due to the XCI, which vary among regions and animal ages [104]. The XCI was believed to impact the phenotypic outcome in human patients and female animal models [105]. In many cases, RTT girls show random XCI with equal numbers of MeCP2 negative or positive cells, which theoretically renders 50% of Mecp2+/− individuals to carry the mutated gene. However, nonrandom XCI has been reported to contribute to clinical symptom variations in some RTT patients as well [106]. Skewed XCI to the WT may lead to the milder phenotypes in the RTT mouse model [102]. Our previous study also suggests that only ~20% the Mecp2+/− mice developed breathing disorders [107], which is consistent with the skewed XCI in RTT. Thus, nonrandom or preferential XCI may play a role in the phenotypic and individual variations of RTT.

In addition, in heterozygous females, the MeCP2-negative neurons generally displays different morphology from MeCP2–positive cells as shorter dendritic length and smaller cell size [108]. The more skewed XCI from the WT allele, the more severe neuronal phenotype of the MeCP2-negative cells would be. On the other hand, the MeCP2-negative cells affect the development of surrounding WT cells in Mecp2+/− mice as well [109], Therefore, the nonrandom

XCI may contribute to the cross-interaction of the MeCP2-negative and –positive cells on structural and functional outcomes, depending on the skewed condition.

2.3.7 Locus Coeruleus Nuclei

2.3.7.1Intrinsic membrane properties and CO2 chemosensitivity

LCis located bilaterally in the dorsal area of the rostral pons, which is involved in many behaviors via the widespread projections, especially the regulating the autonomic function as breathing activity. Defects of LC neurons are involved in multiple neuropsychiatric disorders, including Parkinson's disease, Alzheimer's disease and posttraumatic stress disorder [110, 111]. Recent studies in our and other labs have shown that the LC neurons in Mecp2-null mice are defective as well, which underlie the pathology of some RTT-like phenotypes. The defect manifests itself as abnormal intrinsic membrane prosperities and impaired CO2 chemosensitivity [112, 113]. The intrinsic membrane properties of LC neurons were impaired by showing the shorter time constant, stronger inward rectification and smaller medium afterhyperpolarization (mAHP) amplitude, which may contribute to the LC dysfunction as excessive firing and reduced metabolic function [4, 10]. The CO2 central chemoreceptors (CCRs) were found in the brainstem, including the LC, which play a critical role in respiratory and cardiovascular controls. In Mecp2- null mice, the CO2 chemosensitivity of LC neurons was defective, showing the abnormal

response to mild hypercapnia but normal response to the severe hypercapnia. The overexpression of Kir4.1 channel, which reduced the pH sensitivity, in LC area may contribute, allowing the neurons detect CO2 until severe hypercapnia develops [8]. Such a defect in LC neurons contributes to the RTT-like breathing abnormalities, including high breathing frequency variation, apnea, and hyperventilation.

2.3.7.2Norepinephrine biosynthesis and the homeostasis

As the prominent NE recourses, LC produces ~70% NE throughout the CNS. In patients with RTT and mouse models, the monoamine level was significantly reduced, including NE [46], leading to the RTT-like phenotypes, such as abnormal breathing activity. It was confirmed by the later studies that the expression levels of tyrosine hydroxylase (TH) and dopamine beta

hydroxylase (DBH), the rate-limiting enzyme in the NE synthesis, were significantly reduced in

Mecp2-null LC neurons [114]. Desipramine, an inhibitor of NE reuptake, can improve respiratory rhythm activity, increases the number of NE containing neurons, and extends the lifespan of Mecp2-null mice [44, 47].

The cause of the NE deficit remains unknown. Mutations of the Mecp2 gene, the general transcriptional regulator, may affect the enzyme expressions. The persistent hyperexcitation of LC neurons may contribute as well. There may be a homeostatic state between LC neuronal excitability and NE biosynthesis, allowing a stable release of NE at synapses. Although high LC neuronal excitation may lead to more NE release, persistent hyperexcitability may have adverse effects. In Mecp2-null mice, the excessive firing of LC neurons may contribute to their metabolic dysfunction by disturbing the homeostasis of NE synthesis, the NE production and NE release from presynaptic terminals. The idea is supported by our recent study that further stimulation of NE-ergic terminals in a mouse model with Mecp2 null did not improve the NE modulation to their target nuclei [115]. Coincidently, the LC neurons in Mecp2-null mice showed the age- dependent deterioration [27] and the TH expression level was also progressively reduced [50], which is consistent with the homeostasis idea as well. Interestingly, our study has shown that the severity of breathing abnormalities increases with the increased LC firing rate in the

between the neuronal hyperexcitability and the declining metabolic function. Other studies also reported the tight linkage between the neuronal firing activity and the behaviors of the animals. LC cells usually active firing during awake and become silence during rapid eye movement (REM) sleep [116]. The altered metabolic function or NE synthesis and release of LC neurons may underlie such linkage. In general, in patients with RTT and mouse models, the metabolic function of LC was impaired, which may be due to the imbalanced neuronal firing activity and NE synthesis.

2.3.7.3Defects of cell communications

As the major NE-ergic nuclei, LC neurons receives signals from and projects to the broad brain regions and spinal cord, affecting diverse behaviors, such as breathing, cognition, attention, sleep, learning and memory [117]. The LC-NE system sends output to the medulla, where the respiratory centers are located, and regulates the breathing activities. The prefrontal cortex receives the LC-NE projection as well, which contributes to the cognitive functions, seizure and social behaviors [118]. Although previous studies indicate that the LC neurons may function as individual cells to innervate divergent brain area distinctively, a viral-genetic tracing study suggests LC-NE circuit receives convergent signals from many brain regions through axon projections, including cerebellar Purkinje cells, intervening the LC-NE modulation in the target regions and the associated behaviors [59]. GABA and glutamate are the major inhibitory and excitatory neurotransmitters regulating the LC-NE circuits. One study in our lab reported a local group of GABAergic neurons in the dorsomedial area of LC (dmLC) involves the direct or indirect regulation in LC-NE output [119]. In patients with RTT and mouse models, the

normal function and leads to multiple RTT-like symptoms. Improving the NE-LC circuits by innervating the inputs may benefit their target regions, leading to the alleviation of the associated abnormal behaviors in RTT.