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,
executive 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). Evidence for
developmental dyslexia genotypes comes from molecular genetic-neuroimaging
studies. Nine developmental dyslexia candidate loci (i.e., from DYX1 to DYX9) and
up to ten individual developmental dyslexia associated candidate genes have
been reported (see, Becker et al., 2017). A great number of association studies
have suggested that genetic risk alleles in three most consistently replicated candidate
genes (DYX1C1, KIAA0319, and DCDC2) might be associated with developmental dyslexia
related phenotypes and brain regions involved in reading (see Mascheretti et.
al., 2017, for a review). In addition, structural and functional neuroimaging
studies comprise evidence for the endophenotypes of developmental dyslexia.
Various functional and structural neuroimaging studies have reported that
dorsal, ventral, and anterior brain regions might be involved in reading
related neurocognitive processes; as a result, disruptions in these areas might
be correlated with developmental dyslexia related phenotypes (see Ozernov-Palchik
& Gaab, 2016, for a review). Finally, the phenotypes of developmental
dyslexia are fluent reading and reading comprehension problems which are linked
to the problems in cognitive, sensorial, and language abilities such as
phonological working memory, processing of rapid auditory/visual stimuli,
phonological awareness, and rapid automatized naming. Given all the data within
the perspective of endophenotype approach, it could be concluded that the
genetic risk alleles in affected individuals might lead disruptions in neuronal
migration, which consequently affects brain functions and structures critical
for reading skills. The aim of the present review is to explain the
neurogenetic, neurobiological, and neurocognitive aspects of developmental
dyslexia within endophenotype approach. Firstly, endophenotype approach will be
conceptualized within Gottlieb's Epigenetic Psychobiological Systems
Perspective. Later, neurogenetic, neurobiological, and neurocognitive origins
of developmental dyslexia will be presented respectively. Finally, the
importance of endophenotype approach to neurodevelopmental disorders such as
developmental dyslexia will be highlighted. In this respect, It is targeted to
contribute to the theoretical understanding of early risk factors of
developmental dyslexia, as well as developing effective assessment and
intervention methods, which reflects the multifactorial profile of the
disorder.
developmental dyslexia, the endophenotypic approach, dyslexia-candidate genes, the dorsal-ventral-anterior reading brain regions, phonological awareness, rapid automatized naming, working memory
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