Endophenotype Approach to Developmental Dyslexia: Neurogenetic, Neurobiological, and Neurocognitive

Year 2018, Volume: 1 Issue: 2, 62 - 91, 01.08.2018

Sema Acar

Abstract

Developmental dyslexia is a neurodevelopmental disorder characterized by unexpected problems in learning to read, fluent reading, and reading comprehension in spite of age-appropriate IQ level, education, and environmental opportunities (Démonet, Taylor & Chaix, 2004; Vellutino, Fletcher, Snowling & Scanlon, 2004). Fluent reading and reading comprehension require rapid, successive and/or parallel activation and effective communication of multiple brain circuits. Brain “reading” circuits are sustained by multiple and distant neuronal networks which are also involved in language, visual, and orthographic processing, in addition to attention, 1 Öğr. Gör., Başkent Üniversitesi, Odyoloji Bölümü, [e-mail] functions, and higher level cognition processing (Norton & Wolf, 2012). However, genetically driven structural and functional differences in these brain circuits might cause problems in these neurocognitive abilities in developmental dyslexia. Therefore, individuals diagnosed with developmental dyslexia or with familial risk might have problems in reading related cognitive, sensorial, and language abilities such as phonological working memory, processing of rapid auditory/visual stimuli, phonological awareness, and rapid automatized naming (see Vellutino et. al., 2004, for a review). In addition, problems in motor coordination and balance are commonly reported (Rochelle & Talcott, 2006). As a consequence of heterogeneity in the clinical profile, various theories have been postulated to explain the underlying mechanisms of the disorder: The Phonological Deficit Theory suggests that problems in phonological awareness, rapid automatized naming, and phonological working memory are indeed the underlying reasons of developmental dyslexia, since these abilities are fundamental to reading development. The theory suggests that the problems in these abilities might be due to the perisylvian cortex dysfunction (Kovelman et. al., 2012). In addition, some researchers propose Double-Deficit Hypothesis. According to which, rapid automatized naming and phonological abilities are closely related with each other but mediated by independent sub-mechanisms. Therefore, individuals having disorders in both might have more severe reading problems (Nelson, 2015). On the one hand, Magnocelluar Theory suggests that the underlying reason of the developmental dyslexia is the synchronization and integration of the rapid visual and auditory stimuli due to medial thalamic and geniculate nucleus abnormalities (Stein & Walsh, 1997). On the other hand, Cerebellar Deficit Theory claims that developmental dyslexia is due to cerebellum disorders (Nicolson, Fawcett & Dean, 2001). Finally, some researchers focus on developmental dyslexia comorbidities such as dysgraphia, dyscalculia, and attention deficit and hyperactivity disorder; therefore, present A Multiple Cognitive Deficits Model (Peterson & Pennington, 2015). By focusing on different underlying mechanism of the disorder, all these theories contribute to the multifactorial nature of developmental dyslexia. However, among them, only the endophenotype approach could encompass the all underlying aspects of the disorder in a holistic manner (Kendler & Neale, 2010). As developmental dyslexia is a neurodevelopmental disorder with genetic, neurologic, and cognitive origins, endophenotype approach to developmental dyslexia proposes to study underlying reasons of developmental dyslexia within the triangle of neurogenetic, neurobiological, and neurocognitive evidence. Accordingly, this approach suggests using neuroimaging endophenotypes (i.e., an assessable neuroanatomical trait considered as the closest link to the biological basis of developmental dyslexia) as an intermediate level to build a link between genotypes (i.e., a genetic organization of an individual) and phenotypes (i.e., qualitative traits such as reading skills). 

Keywords

developmental dyslexia, the endophenotypic approach, dyslexia-candidate genes, the dorsal-ventralanterior reading brain regions, phonological awareness, rapid automatized naming, working memory

Gelişimsel Disleksi ve Endofenotipik Yaklaşım: Nörogenetik, Nörobiyolojik ve Nörobilişsel Temeller

Abstract

Gelişimsel disleksi, genetik, nörolojik ve bilişsel temellere sahip nörogelişimsel bir bozukluktur. Altta yatan mekanizmalarını açıklamak üzere çeşitli teoriler ortaya atılmıştır. Bunlar arasında, gelişimsel disleksinin tüm boyutlarını bütüncül olarak ele alan endofenotipik yaklaşım, genetik faktörler ile okuma bozuklukları arasındaki dolaylı ve karmaşık ilişkiyi nörobiyolojik belirteçler aracılığıyla açıklamaktadır. Bir başka deyişle, endofenotipik yaklaşım gelişimsel disleksi aday genlerinin beyin fonksiyon ve yapısındaki muhtemel etkilerini ve bu etkilerin bilişsel, duyusal, dil ve okuma becerileri ile olan karşılıklı ve zincirleme ilişkisini incelemektedir. Bu derlemenin amacı, gelişimsel disleksiyi endofenotipik yaklaşım çerçevesinde ele alarak bozukluğun nörogenetik, nörobiyolojik ve nörobilişsel boyutlarını incelemektir. Gelişimsel disleksinin altta yatan mekanizmalarını bütüncül ve multidisipliner bakış açısıyla ele alan bu derlemenin, gelişimsel disleksi erken dönem risk faktörlerinin belirlenmesi, tarama, değerlendirme ve terapi yöntemlerinin geliştirilmesine ışık tutacak nitelikte olması hedeflenmiştir.

Keywords

gelişimsel disleksi, endofenotipik yaklaşım, disleksi aday genleri, dorsal-ventral-anterior okuma ağları, fonolojik farkındalık, hızlı otomatik isimlendirme, işleyen bellek

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