Simulación en bucle cerrado

There are different overlapping terms for intellectual disability including mental handicap, mental retardation, developmental delay and learning disability. The term “mental retardation” has been widely used in the literature to describe individuals with significantly impaired intellectual functioning, but was seen as disparaging, and now “intellectual disability (ID)” is preferred by most researchers (Lai et al., 2012). The brains of individuals with ID do not function within the normal range for their ages. Affected individuals cannot meet the personal and social demands expected of them. They have a significantly reduced ability with regard to intellectual reasoning and adaptive skills (Huang Jichong et al., 2016a). School age children with ID may struggle with memory, problem solving, attention, reading, language, mental arithmetic and visual comprehension. They usually develop more slowly than their peers and soon fail to achieve developmental milestones in some, or all, of the developmental domains (Goharpey et al., 2013; Kok et al., 2016).

ID is a major health issue among young adults, with a worldwide estimated prevalence of about 2-3% (Maulik et al., 2011). It tends to be more common in developing countries due to the effect of environmental factors such as poor medical care, infection and malnutrition (Emerson and Hatton, 2007; Lakhan, 2015; Maris et al., 2013). ID is defined using three criteria: reduced intellectual ability, deficit in two or more adaptive behaviours and diagnosis before the age of 18 years (Carulla et al., 2011).

There has been a gender bias in the prevalence of ID and other neurodevelopmental disorders: more boys are diagnosed than girls by about 30- 50% (Abikoff et al., 2002; Polyak et al., 2015; Richardson S. A. et al., 1987). In a study of Taiwanese children aged 3-17 years between 2004 and 2010, it was found that 59.2% of ID patients were boys and 40.8% were girls (Lai et al., 2012). This difference could be partially due to the high contribution of X-linked disorders, as up to 10% of the known genetic causes of ID are located on the X

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chromosome (Gandomi et al., 2014; Niranjan et al., 2015). It is also reported that more boys are affected with Fragile X syndrome than girls (Polyak et al., 2015; Saldarriaga et al., 2014) but the alternative intact X-chromosome in females participates in compensating the deficiency (Raymond, 2006).

1.2.2 ID types

The Intelligence quotient (IQ) is the main measure that is used to quantify the severity of intellectual impairment. It is a statistical assessment of the thinking and reasoning ability of a person compared with individuals of the same age. Average IQ within each age group is set at 100, and a person with an IQ below 70 is recognised as having intellectual disability (Webb and Whitaker, 2012). Different systems have been used for IQ tests but the most commonly used ones are the Stanford-Binet intelligence scale and the Wechsler Adult Intelligence Scale (WAIS) (Silverman et al., 2010). Based on IQ score, the Diagnostic and Statistical Manual of Mental Disorders (DSM) divides ID into the following categories: mild, moderate, severe and profound (Carulla et al., 2011; Katz and Lazcano-Ponce, 2008; Schuchardt et al., 2010). Table 1 gives more details of each category.

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ID type IQ score Description of functional level and

support needed Approximate proportion

Mild 50-69 Intermittent support needed during

transitions or periods of uncertainty 85%

Moderate 36-49

Acquire some communication and self-help skills, require limited support in daily

situations

10%

Severe 20-35

Acquire only basic self-help and communication skills, require extensive

support for daily activities

3.5%

Profound <20

Require highly structured and supervised living conditions for every aspect of daily

routines

1.5%

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Sometimes, the mode of inheritance is also cited beside the ID to indicate its type. Although the genetics of ID is separately highlighted in a subsequent section (1.2.4), some of the genetics are mentioned here to explain the classification point. X-linked ID (XLID, OMIM 309530) is caused by mutations in genes on the X chromosome. Over 150 syndromes have been described (Lubs et al., 2012) but the most common example is fragile X syndrome (FXS), which is caused by an expanded trinucleotide repeat (CGG) in the FMR1 gene (Fragile X Mental Retardation Protein, OMIM 309550). Unaffected people have between 6 and 54 copies, whereas FXS patients have over 200 copies (Willemsen et al., 2011). Nance-Horan syndrome (NHS, OMIM: 302350) is one of the rare X-linked disorder which is thought to be under diagnosed and not fully evaluated (Toutain, 2003). The condition is characterized by congenital cataracts, teeth anomalies as well as intellectual disability, and it was first described by two independent studies (Horan and Billson, 1974; Nance et al., 1973). The bilateral cataract is normally severe in the affected males and would require an early surgical intervention in order to stop profound vision loss (Burdon et al., 2003). Other reported ocular abnormalities include small eyes (microphthalmia), small cornea (microcornea), recurrent eye movements (nystagmus) and absence of bilateral alignment during object focus (strabismus) (Hong et al., 2014).

The characteristic teeth anomalies which have been described in Nance-Horan syndrome include screwdriver or cone-shaped incisors, supernumerary maxillary incisors (mesiodens) and increased gap between teeth (diastema) (Stevenson et al., 2012; Walpole et al., 1990). About 30-50% of individuals with NHS have intellectual disability (Brooks et al., 2010). In some of the cases, additional specific facial phenotypes have also been reported like inclination in the external part of the ear (anteverted pinnae), shortening in any of the five bones of the hand (metacarpals), and long face (Coccia et al., 2009). The severity of the above- mentioned symptoms would significantly differ from person to person even within members of one affected family. Carrier heterozygous females have been reported with some milder forms of these related manifestations but intellectual disability tends to less likely to occur in those females (Ding et al., 2009; Tug et al., 2013; Zhu D et al., 1990). The reported NHS mutations are, so far, not less

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than 33 and most of them are either nonsense mutations or InDels (insertions and deletions) but CNVs (copy number variations), missense and splice site mutations are few (Tian et al., 2017).

The autosomal recessive intellectual disability (ARID, OMIM 611093) forms have not been well studied as the search for the underlying causes has been largely hindered by the scarcity of big consanguineous families in the Western communities (Kuss et al., 2011). Joubert Syndrome (JBTS, OMIM 213300) with related disorders (JSRD) (Brancati et al., 2010) and Bardet-Biedl syndrome (BBS1, OMIM 209900) (Forsythe and Beales, 2013) are examples of autosomal recessive ID. JSRD are a group of developmental disabilities and various congenital abnormalities but an essential diagnostic hallmark to distinguish this group is the presence of molar tooth sign (MTS), which is a unique cerebellar and brainstem marker visible on brain imaging (Brancati et al., 2010; Romani et al., 2013). Bardet-Biedl syndrome (BBS) is characterized by defects in multiple organ systems but its main features are rod-cone dystrophy, obesity, hypogonadism, postaxial polydactyly, renal dysfunction and intellectual disability (Zaghloul and Katsanis, 2009).

The autosomal dominant ID (ADID, OMIM 614563) are not commonly seen due to the fact that patients with intellectual disability rarely reproduce (Winnepenninckx et al., 2003). The conditions of autosomal dominant ID are mainly reported with de novo mutations (Khan Muzammil Ahmad et al., 2016) and one example is chromosome 2q23.1 deletion syndrome (Van Bon et al., 2010). In fact, duplication of this 2q23.1 region has also been described (Mullegama et al., 2014) to cause a similar effect. Although the autosomal dominant Marfan Syndrome (MFS, OMIM 154700) (Hofman et al., 1988) is a disorder of mainly connective tissue with involvement of cardiovascular, skeletal, ocular and pulmonary systems, some cases of intellectual debility have also been described (Ades et al., 2006; Hoffjan, 2012).

There is also a different ID classification based on whether other clinical features are also present. When low IQ is the sole clinical feature and the only

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manifestation of ID, this is classified as non-syndromic ID. Syndromic ID is characterised by learning disabilities in the presence of other, co-morbid clinical features, such as large, protruding ears, long face and adult macro-orchidism of Fragile X syndrome, or the stunted growth, atypical fingerprints and eyelid crease of Down syndrome (DS, OMIM 190685). Other examples of syndromic ID include Rett syndrome (RTT, OMIM 312750), Angelman syndrome (AS, OMIM 105830) and Prader-Willi syndrome (PWS, OMIM 176270). Distinguishing between syndromic and non-syndromic forms of ID is sometimes difficult because the co- morbid features may be subtle (Miclea et al., 2015). Some researchers believe that the same genes are sometimes involved in both syndromic and non- syndromic ID, suggesting that there might be no strict boundaries between these forms (Beaulieu et al., 2013; Birk et al., 2010; Frints et al., 2002; Tejada et al., 2011).

1.2.3 Environmental causes of ID

ID can have various causes, but genetic factors are thought to be responsible for up to 50% of cases (Karam et al., 2015; Kaufman et al., 2010). A wide range of environmental insults occurring during pregnancy, childbirth or infancy may account for the other half of cases. Exposure of the fetus to drugs or alcohol is the most common cause of ID during pregnancy. A significant contributor to the non-genetic causes of ID, with a general prevalence of 1-2 in every 1000 babies, is Fetal Alcohol Syndrome (FAS), in which the fetal brain is affected by alcohol crossing the placenta (Aronson and Hagberg, 1998; Cesconetto et al., 2016; Miller, 2014; Senturias and Asamoah, 2014). A wider range of neuropsychological impairments is associated with Fetal Alcohol Spectrum Disorder (FASD), another consequence of fetal exposure to alcohol. FASD is a major contributor to ID in the Western world and the different levels of FASD affect about 2-5% of the population (May et al., 2013). In the UK, there are 7000 babies born every year at high risk of FAS and FASD (Larcher and Brierley, 2014). The behaviour of these babies is determined by different factors, including the amount of consumed alcohol, maternal metabolism and developmental stage of the fetus during exposure (Basavarajappa and Subbanna, 2016).

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Environmental causes of ID that might take place during birth are preterm birth, oxygen deprivation and brain trauma. There have been various reports describing different degrees of risk of ID and attention deficit in preterm babies (Anderson Peter J, 2014; Hutchinson et al., 2013; Litt et al., 2005; Soleimani et al., 2014). A UK study by Johnson and coworkers (Johnson et al., 2016), involving 161 children born extremely preterm (EP) and 153 term-born controls, showed that the prevalence of ID was 47% in EP children compared with only 4.6% in the controls. ID is known to occur as a result of hypoxia or oxygen shortage (Sigelman and Rider, 2014), due to inadequate delivery of oxygen to cells. In cases of hypoxia, lysophosphatidic acid (LPA) receptors are activated, leading to overactivation of signalling pathways. As a result, the mitotic neural progenitor cells (NPCs) are driven to migrate from their normal specific regions within the developing brain into more superficial areas and thus fail to grow properly (Herr et al., 2011).

Brain trauma occurring before, during or after birth can also cause long- lasting/permanent harm to the brain (Carone, 2014; Foreman, 2009). A retrospective study was carried out to investigate the intellectual capability of 63 children aged less than 15 years, who had experienced severe traumatic brain injury (TBI) (Chevignard et al., 2016). IQ tests carried out 7-8 years after the injury showed that the children with TBI performed significantly worse than age and ethnicity matched healthy controls (p=0.016).

Other environmental causes of ID that can occur in early childhood include severe malnutrition and some infections. The diet is known to have an important effect on brain function and cognition (Gillette‐Guyonnet et al., 2013; Mangialasche et al., 2013). Deficiencies in any of the essential micronutrients have been associated with the development of various functional deficits (Kuper et al., 2015). For example, Waber and colleagues reported impaired IQ in adulthood as a result of malnutrition during infancy (Waber et al., 2014), while Swaminathan and co- workers reported that reduced cognitive and physical performance in Indian children was related to a shortage in vitamin B and folate intake (Swaminathan

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et al., 2013). The mechanism by which nutrition affects the development of the brain is complex. The processes of metabolism, enzyme production, energy control, neurogenesis, neurotransmission and synaptic plasticity are all affected by diet (Dauncey, 2014). Certain prenatal infections, like rubella, cytomegalovirus or Toxoplasma gondii, can disturb development of the fetus, and are also plausible risk factors for brain damage (Spreen et al., 1995). Various studies suggest that such infections can cause cognitive impairment and autistic spectrum disorders but the evidence is inconsistent (Bilder et al., 2013; Meyer et al., 2007). A study of a big cohort of individuals born in Sweden between 1984 and 2007 to mothers who were diagnosed with infection during pregnancy provides evidence that infections during pregnancy increase the risk of children having Autism spectrum disorder (ASD, OMIM 209850) (Lee Brian K. et al., 2015). Another study which checked children with ASD and ID for CMV infection concluded that congenital CMV infection is a potential contributor to ASD, particularly if the condition is also combined with ID (Engman et al., 2015).

1.2.4 ID genetics

The precise proportion of ID cases caused by gene mutations is unknown, but the literature states proportions ranging from 15 to 50% (Karam et al., 2015; Kaufman et al., 2010; Moeschler and Shevell, 2006). It is not unexpected for genes to have such a significant contribution in ID aetiology, as mutations could affect any of the numerous genes that are crucial for early brain development. The chromosomal aberration in Down’s syndrome (DS), trisomy 21, is considered to be the leading genetic cause of ID, as it alone accounts for up to 20% of worldwide cases (Lakhan and Kishore, 2016). Other chromosomal abnormalities frequently associated with ID include trisomy 13 and trisomy 18 (Miclea et al., 2015). The triplet repeat expansion (CGG) in the FMR1 gene causing fragile X syndrome (FXS) is the major monogenic cause of ID, accounting for 5% of cases (Inaba et al., 2014; Moeschler et al., 2014).

There have been various monogenic genetic causes identified for ID and they are generally related to dysfunction in neurodevelopmental processes or in synaptic

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organisation. The processes that regulate neural development during the embryonic stage work in a tightly controlled and organised cascade, resulting in the proliferation of neurons from neuronal precursor. The process of neural proliferation and production of fully differentiated neurons is called neurogenesis, a sophisticated process involving many transcription factors and regulatory genes (Busskamp et al., 2014; Stiles and Jernigan, 2010). Neurogenesis begins very early during embryogenesis, with the neuronal tube being formed about 15 days after fertilization. Neurogenesis continues in most brain regions until early postnatal stages, but the dynamic production of new neurons carries on into adulthood in the ventricular-subventricular zone and the subgranular zone of the dentate gyrus. Adult hippocampal neurogenesis is important in the formation of new memories (Sahay et al., 2011; Urbán and Guillemot, 2014).

Disruption within any steps of neurogenesis could have a significant impact on the cognitive ability of an individual. For example, defects in genes involved in controlling the production of neurons could lead to an imbalance between forming more progenitor cells and forming end differentiated neurons giving rise to microcephaly (OMIM 152950) which is characterized by reduced head circumference at birth and various degrees of ID (Chavali et al., 2014). Microcephaly is a heterogeneous neurodevelopmental disorder, in which mutations have been found in the following genes: MCPH1 (Microcephalin, OMIM 607117), WDR62 (WD Repeat-Containing Protein 62, OMIM 613583), CDK5RAP2 (CDK5 Regulatory Subunit-Associated Protein 2, OMIM 608201), CASC5 (Cancer Susceptibility Candidate Gene 5 Protein, OMIM 609173), ASPM (Abnormal Spindle-Like, Microcephaly-Associated, OMIM 605481), CENPJ (Centromeric Protein J, OMIM 609279), STIL (Scl/Tal1-Interrupting Locus, OMIM 181590), CEP135 (Centrosomal Protein, 135-KD, OMIM 611423), CEP152 (Centrosomal Protein, 152-KD, OMIM 613529), ZNF335 (Zinc Finger Protein 335, OMIM 610827), PHC1 (Polyhomeotic-Like 1, OMIM 602978) and CDK6 (Cyclin-Dependent Kinase 6, OMIM 603368) (Faheem et al., 2015). Problems associated with neuronal migration, restricting the ability of nerve precursor cells to travel from their origin into the cortical plate, have been reported to cause psychomotor disorders, including epilepsy and ID. Disorders that occur

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due to mutations in genes involved in neural migration include lissencephaly, polymicrogyria and periventricular heterotopia (Dyment et al., 2013; Filippi, 2015).

The human brain contains a quadrillion (1015_{) synapses (Srivastava and} Schwartz, 2014), which provide communication between neurons. Synapses are very complex junctions that are stabilised by adhesion molecules. Each synapse is made up of three main components: a presynaptic terminal, a post-synaptic terminal, and a cleft, measuring 20-25 nm, between them (Ho Victoria M. et al., 2011b). Communication between neurons through the synapses is activated when an action potential initiates an electrical impulse within the presynaptic terminal, which in turn triggers the release of various neurotransmitters. Neuron to neuron communication is achieved by these neurotransmitters, which diffuse through the synaptic cleft until they reach the post-synaptic terminal, where they attach to specific receptors. If the structure of synapses is disrupted, this could lead to dysfunction of the signalling network, as has been found in many brain disorders (Finnema et al., 2016). An illustration of the extreme complexity of the synapse, with some proteins encoded by genes implicated in ID pathogenicity, is shown in Figure 1.3. Miclea et al. (2015) have categorised ID genes based on their involvement in mechanisms and pathways, such as metabolic pathways, neurogenesis, neuronal migration, synaptic function, intracellular signalling, or epigenetic regulation of transcription. Examples of the different genes found in each group are given in Table 2.

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Figure 1.3. Synaptic Complexity and some ID involved proteins. This cartoon represents the presynaptic terminal, the cleft and the postsynaptic components of the glutamatergic synapse in the brain. It shows multiple proteins which are available to propagate the messages through connecting the transmembrane and membrane- associated protein complexes with the underlying actin cytoskeleton. These include some adhesion molecules (like Neurexin, Neuroligin and Cadherins), and scaffolding proteins (like PSD-95, Cask and Shank). Many of the illustrated proteins (such as Neurexin, Cask and Shank) have also been implicated in other neuropsychiatric disorders like ASD. Disruption in any of these proteins would result in a failure to transmit the signals efficiently. (Adapted from Banerjee et al. (2014), no permission required).

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Pathway Genes encoding proteins implicated in ID

Metabolic Pathways

Organic acid metabolism (ALDH5A1, L2HGDH) Polysaccharide metabolism (NAGLU, SGSH) Protein glycosylation (POMGNT1, POMT1, POMT2, FKTN,

FKRP, LARGE) Purine metabolism (ADSL)

Monocarboxylate transporter (SLC16A2) Creatine transporter (SLC6A8)

Neurogenesis Mitotic spindle regulation (ASPM, CDK5RAP2, CENPJ) DNA repair and mitotic arrest (MCPH1)

Intracellular Signalisation

Ras-MAPK-ERK pathway (SOS1, RAF1, BRAF, SHOC2, HRAS, KRAS, PTPN11, SPRED1, MAP2K1, MAP2K2,

NF1, DYRK1A, RPS6KA3)

PI3K-AKT-mTOR pathway (TSC1, TSC2, PTEN)

Neuronal Migration Microtubule subunits (TUBA1A, TUBB2B) Microtubule regulation (DCX)

Microtubule associated proteins (PAFAH1B1) Transcription factors (ARX)

Presynaptic Function Vesicle traffic (RAB3GAP1, STXBP1, GDI1, RAB39B) Exocytosis inhibition (IL1RAPL1, CASK)

Adhesion between pre- and postsynaptic membranes (NRXN1, CDH15)

Postsynaptic Function

Adhesion between pre- and postsynaptic membranes (CNTNAP2, NLGN3, NLGN4)

Neurotransmitter receptor interaction with membrane proteins (SHANK2, SHANK3) Regulation of postsynaptic proteins (UBE3A,

UBE2A, UBR1; HUWE1, CUL4B) Subunits of NMDA receptor (GRIN2A, GRIN2B) Transport of mRNA from the nucleus to the cytoplasm (FMR1)

Epigenetic Regulation of Transcription

Histone acetyl-transferase (CREBBP, EP300) Histone deacetylase (HDAC4) Histone methyltransferase (NSD1, EHMT1, MLL2) Histone demethylase (KDM5C) Transcription factors (TCF4, RAI1, ZNF711, ZNF41,

ZNF674, ZNF81, PHF6, PHF8) DNA replication (SETBP1)

DNA methyltransferase (DNMT3B) Chromatin modification (ATRX, BRWD3) Repression of transcription factors (BCOR, MECP2)

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Based on the fact that ID is more common in males than in females, it was hypothesised that X-linked ID would account for at least 25% but data suggests only 10-20% (Ropers H. and Hamel, 2005; Winnepenninckx et al., 2003). So far, mutations in more than 100 genes located on the X chromosome have been implicated in ID (Lubs et al., 2012), which is consistent with the fact that the human X chromosome has a large amount of brain-specific transcripts that are crucial for learning and memory (Ross et al., 2005). The full figure of X-linked ID genes remains unknown but studying more of the large sample cohorts would help investigating all the other genes reside on the X chromosome. On the other hand, it is believed that the number of genes behind the autosomal forms of ID should not be less than 800-850 genes (Ropers H, 2010). In fact, a more recent study (Maris et al., 2013) suggests that total number of autosomal genes involved in ID could run into thousands, but about 400 of them have already been linked to ID (Lubs et al., 2012; Raymond, 2006). Based on the various performed studies on the non-syndromic ID, 51 loci and 34 genes have been identified with the majority of them residing on either chromosome 6 or 19 (Khan Muzammil Ahmad et al., 2016).

The completion of the human genome sequencing project (Little et al., 2003) has been the milestone which revolutionized the work in biology and medicine. Although it took more than 10 years and 3 billion USD to elucidate the first human genome (Von Bubnoff, 2008), the introduction of next generation sequencing